Embed Size (px)
Citation preview
The Facts On File
DICTIONARYof
CHEMISTRY
The Facts On File
DICTIONARYof
CHEMISTRY
Edited byJohn Daintith
Fourth Edition
The Facts On File Dictionary of ChemistryFourth Edition
Copyright © 2005, 1999 by Market House Books Ltd
All rights reserved. No part of this book may be reproduced or utilized in anyform or by any means, electronic or mechanical, including photocopying,recording, or by any information storage or retrieval systems, withoutpermission in writing from the publisher. For information contact:
Facts On File, Inc.132 West 31st StreetNew York NY 10001
For Library of Congress Cataloging-in-Publication Data, please contact Facts On File, Inc.
ISBN 0-8160-5649-8
Facts On File books are available at special discounts when purchased in bulkquantities for businesses, associations, institutions, or sales promotions. Please callour Special Sales Department in New York at (212) 967-8800 or (800) 322-8755.
You can find Facts On File on the World Wide Web athttp://www.factsonfile.com
Compiled and typeset by Market House Books Ltd, Aylesbury, UK
Printed in the United States of America
MP PKG 10 9 8 7 6 5 4 3 2 1
This book is printed on acid-free paper.
PREFACE
This dictionary is one of a series designed for use in schools. It is intended for stu-dents of chemistry, but we hope that it will also be helpful to other science studentsand to anyone interested in science. Facts On File also publishes dictionaries in avariety of disciplines, including biology, physics, mathematics, forensic science,weather and climate, marine science, and space and astronomy.
The Facts On File Dictionary of Chemistry was first published in 1980 and the thirdedition was published in 1999. This fourth edition of the dictionary has been exten-sively revised and extended. The dictionary now contains over 3,000 headwords cov-ering the terminology of modern chemistry. A totally new feature of this edition isthe inclusion of over 1,700 pronunciations for terms that are not in everyday use. Anumber of appendixes have been included at the end of the book containing usefulinformation, including a list of chemical elements and a periodic table. There is alsoa list of Web sites and a bibliography. A guide to using the dictionary has also beenadded to this latest version of the book.
We would like to thank all the people who have cooperated in producing this book.A list of contributors is given on the acknowledgments page. We are also grateful tothe many people who have given additional help and advice.
v
ACKNOWLEDGMENTS
Contributors
Julia Brailsford B.Sc.John Clark B.Sc.John Connor B.Sc.Derek Cooper Ph.D. F.R.I.C.Rich Cutler B.Sc.D.E. Edwards B.Sc. M.Sc.Richard Hadfield B.Sc.Valerie Illingworth B.Sc. M.Phil.Alan Isaacs B.Sc. Ph.D.Elizabeth Martin M.A.P.R. Mercer B.A. T.C.R.S. Smith B.Sc. C.Chem. M.R.I.C.Derek Stefaniw B.Sc. Ph.D.Elizabeth Tootil M.Sc.J. Truman B.Sc.David Eric Ward B.Sc. M.Sc. Ph.D.
Pronunciations
William Gould B.A.
NoteUnless otherwise stated, the melting and boiling points given in the dictionary areat standard pressure. Relative densities of liquids are at standard pressure with theliquid at 20°C relative to water at 4°C. Relative densities of gases are relative toair, both gases being at standard temperature and pressure.
The following abbreviations are used in the text:
p.n. proton number(atomic number)
r.a.m. relative atomic mass(atomic weight)
vi
CONTENTS
Preface v
Acknowledgments vi
Guide to Using the Dictionary viii
Pronunciation Key x
Entries A to Z 1
Appendixes
I. Carboxylic Acids 297
II. Amino Acids 299
III. Sugars 302
IV. Nitrogenous Bases 303and Nucleosides
V. The Chemical Elements 305
VI. The Periodic Table 307
VII. The Greek Alphabet 308
VIII. Web Sites 309
Bibliography 310
vii
GUIDE TO USING THE DICTIONARY
The main features of dictionary entries are as follows.
HeadwordsThe main term being defined is in bold type:
acid A substance that gives rise to hy-drogen ions when dissolved in water.
PluralsIrregular plurals are given in brackets after the headword.
quantum (pl. quanta) A definite amountof energy released or absorbed in a process.
VariantsSometimes a word has a synonym or alternative spelling. This is placed in brackets afterthe headword, and is also in bold type:
promoter (activator) A substance thatimproves the efficiency of a catalyst.
Here, ‘activator’ is another word for promoter. Generally, the entry for the synonym con-sists of a simple cross-reference:
activator See promoter.
AbbreviationsAbbreviations for terms are treated in the same way as variants:
electron spin resonance (ESR) A simi-lar technique to nuclear magnetic reso-nance, but applied to unpaired electrons ...
The entry for the synonym consists of a simple cross-reference:
ESR See electron spin resonance.
FormulasChemical formulas are also placed in brackets after the headword. These are in normaltype:
potassium iodide (KI) A white ionicsolid....
Here, KI is the chemical formula for potassium iodide.
viii
Multiple definitionsSome terms have two or more distinct senses. These are numbered in bold type
abundance 1. The relative amount of agiven element among others; for example,the abundance of oxygen in the Earth’scrust is approximately 50% by mass.2. The amount of a nuclide (stable or ra-dioactive) relative to other nuclides of thesame element in a given sample.
Cross-referencesThese are references within an entry to other entries that may give additional useful in-formation. Cross-references are indicated in two ways. When the word appears in the de-finition, it is printed in small capitals:
boron nitride (BN) A compound formedby heating BORON in nitrogen…
In this case the cross-reference is to the entry for `boron’.
Alternatively, a cross-reference may be indicated by ‘See’, ‘See also’, or ‘Compare’, usu-ally at the end of an entry:
boron trifluoride (BF3) A colorless fum-ing gas made by… See boron trichloride.
Hidden entriesSometimes it is convenient to define one term within the entry for another term:
charcoal An amorphous form of carbonmade by… Activated charcoal is charcoalheated to…
Here, ‘activated charcoal’ is a hidden entry under charcoal, and is indicated by italic type.The entry for ‘activated charcoal’ consists of a simple cross-reference:
activated charcoal See charcoal.
PronunciationsWhere appropriate pronunciations are indicated immediately after the headword, en-closed in forward slashes:
deuteride /dew-ter-ÿd/ A compound ofdeuterium…
Note that simple words in everyday language are not given pronunciations. Also head-words that are two-word phrases do not have pronunciations if the component words arepronounced elsewhere in the dictionary.
ix
/a/ as in back /bak/, active /ak-tiv//ă/ as in abduct /ăb-dukt/, gamma /gam-ă//ah/ as in palm /pahm/, father /fah-ther/,/air/ as in care /kair/, aerospace /air-ŏ-
spays//ar/ as in tar /tar/, starfish /star-fish/, heart
/hart//aw/ as in jaw /jaw/, gall /gawl/, taut /tawt//ay/ as in mania /may-niă/ ,grey /gray//b/ as in bed /bed//ch/ as in chin /chin//d/ as in day /day//e/ as in red /red//ĕ/ as in bowel /bow-ĕl//ee/ as in see /see/, haem /heem/, caffeine
/kaf-een/, baby /bay-bee//eer/ as in fear /feer/, serum /seer-ŭm//er/ as in dermal /der-măl/, labour /lay-ber//ew/ as in dew /dew/, nucleus /new-klee-ŭs//ewr/ as in epidural /ep-i-dewr-ăl//f/ as in fat /fat/, phobia /foh-biă/, rough
/ruf//g/ as in gag /gag//h/ as in hip /hip//i/ as in fit /fit/, reduction /ri-duk-shăn//j/ as in jaw /jaw/, gene /jeen/, ridge /rij//k/ as in kidney /kid-nee/, chlorine /klor-
een/, crisis /krÿ-sis//ks/ as in toxic /toks-ik//kw/ as in quadrate /kwod-rayt//l/ as in liver /liv-er/, seal /seel//m/ as in milk /milk//n/ as in nit /nit/
/ng/ as in sing /sing//nk/ as in rank /rank/, bronchus /bronk-ŭs//o/ as in pot /pot//ô/ as in dog /dôg//ŏ/ as in buttock /but-ŏk//oh/ as in home /hohm/, post /pohst//oi/ as in boil /boil//oo/ as in food /food/, croup /kroop/, fluke
/flook//oor/ as in pruritus /proor-ÿ-tis//or/ as in organ /or-găn/, wart /wort//ow/ as in powder /pow-der/, pouch
/powch//p/ as in pill /pil//r/ as in rib /rib//s/ as in skin /skin/, cell /sel//sh/ as in shock /shok/, action /ak-shŏn//t/ as in tone /tohn//th/ as in thin /thin/, stealth /stelth//th/ as in then /then/, bathe /bayth//u/ as in pulp /pulp/, blood /blud//ŭ/ as in typhus /tÿ-fŭs//û/ as in pull /pûl/, hook /hûk//v/ as in vein /vayn//w/ as in wind /wind//y/ as in yeast /yeest//ÿ/ as in bite /bÿt/, high /hÿ/, hyperfine /hÿ-
per-fÿn//yoo/ as in unit /yoo-nit/, formula /form-yoo-lă//yoor/ as in pure /pyoor/, ureter /yoor-ee-ter//ÿr/ as in fire /fÿr/
x
Bold type indicates a stressed syllable. In pronunciations, a consonant is sometimes dou-bled to prevent accidental mispronunciation of a syllable resembling a familiar word; forexample, /ass-id/ (acid), rather than /as-id/ and /ul-tră- sonn-iks/ (ultrasonics), rather than/ul-tră-son-iks/. An apostrophe is used: (a) between two consonants forming a syllable, asin /den-t’l/ (dental), and (b) between two letters when the syllable might otherwise be mis-pronounced through resembling a familiar word, as in /th’e-ră-pee/ (therapy) and /tal’k/(talc). The symbols used are:
Pronunciation Key
AAS See atomic absorption spec-troscopy.
absolute alcohol Pure alcohol (ethanol).
absolute configuration A particularmolecular configuration of a CHIRAL mole-cule, as denoted by comparison with a ref-erence molecule or by some sequence rule.There are two systems for expressing ab-solute configuration in common use: theD–L convention and the R–S convention.See optical activity.
absolute temperature Symbol: T Atemperature defined by the relationship:
T = θ + 273.15where θ is the Celsius temperature. The ab-solute scale of temperature was a funda-mental scale based on Charles’ law appliedto an ideal gas:
V = V0(1 + αθ)where V is the volume at temperature θ, V0the volume at 0, and α the thermal expan-sivity of the gas. At low pressures (whenreal gases show ideal behavior) α has thevalue 1/273.15. Therefore, at θ = –273.15the volume of the gas theoreticallybecomes zero. In practice, of course, sub-stances become solids at these tempera-tures. However, the extrapolation can beused for a scale of temperature on which–273.15°C corresponds to 0° (absolutezero). The scale is also known as the ideal-gas scale; on it temperature intervals werecalled degrees absolute (°A) or degreesKelvin (°K), and were equal to the Celsiusdegree. It can be shown that the absolutetemperature scale is identical to the ther-modynamic temperature scale (on whichthe unit is the kelvin).
absolute zero The zero value of ther-
modynamic temperature; 0 kelvin or–273.15°C.
absorption A process in which a gas istaken up by a liquid or solid, or in which aliquid is taken up by a solid. In absorption,the substance absorbed goes into the bulkof the material. Solids that absorb gases orliquids often have a porous structure. Theabsorption of gases in solids is sometimescalled sorption. Compare adsorption.
absorption indicator (adsorption indica-tor) An indicator used for titrations thatinvolve a precipitation reaction. Themethod depends upon the fact that at theequivalence point there is a change in thenature of the ions absorbed by the precipi-tate particles. Fluorescein – a fluorescentcompound – is commonly used. For exam-ple, in the titration of sodium chloride so-lution with added silver nitrate, silverchloride is precipitated. Sodium ions andchloride ions are absorbed in the precipi-tate. At the end point, silver ions and ni-trate ions are in slight excess and silver ionsare then absorbed. If fluorescein is present,negative fluorescein ions absorb in prefer-ence to nitrate ions, producing a pink com-plex.
absorption spectrum See spectrum.
abundance 1. The relative amount of agiven element among others; for example,the abundance of oxygen in the Earth’scrust is approximately 50% by mass.2. The amount of a nuclide (stable or ra-dioactive) relative to other nuclides of thesame element in a given sample. The nat-ural abundance is the abundance of a nu-clide as it occurs naturally. For instance,chlorine has two stable isotopes of masses
1
A
accelerator
2
35 and 37. The abundance of 35Cl is75.5% and that of 37Cl is 24.5%. For someelements the abundance of a particular nu-clide depends on the source.
accelerator A catalyst added to increasethe rate of cross-linking reactions in poly-mers.
acceptor /ak-sep-ter, -tor/ The atom orgroup to which a pair of electrons is do-nated in a coordinate bond. Pi-acceptorsare compounds or groups that accept elec-trons into pi, p or d orbitals.
accumulator (secondary cell; storage bat-tery) An electric cell or battery that canbe charged by passing an electric currentthrough it. The chemical reaction in the cellis reversible. When the cell begins to rundown, current in the opposite directionwill convert the reaction products backinto their original forms. The most com-mon example is the LEAD-ACID ACCUMULA-TOR, used in vehicle batteries.
acenaphthene /as-ĕ-naf-th’een, -nap-/(C12H10) A colorless crystalline deriva-tive of naphthalene, used in producingsome dyes.
acetal /ass-ĕ-tal/ A type of organic com-pound formed by addition of an alcohol toan aldehyde. Addition of one alcohol mol-ecule gives a hemiacetal. Further additionyields the full acetal. Similar reactionsoccur with ketones to produce hemiketalsand ketals.
acetaldehyde /ass-ĕ-tal-dĕ-hÿd/ Seeethanal.
acetamide /ass-ĕ-tam-ÿd, -id; ă-set-ĕ-mÿd,-mid/ See ethanamide.
acetate /ass-ĕ-tayt/ See ethanoate.
acetic acid /ă-see-tik, ă-set-ik/ Seeethanoic acid.
acetone /ass-ĕ-tohn/ See propanone.
acetonitrile /ass-ĕ-toh-nÿ-trăl, ă-see-toh-,-tril, -trÿl/ See methyl cyanide.
acetophenone /ass-ĕ-toh-fee-nohn, ă-see-toh-/ See phenyl methyl ketone.
acetylation /ă-set’l-ay-shŏn/ See acylation.
acetyl chloride /ass-ĕ-t’l, ă-see-t’l/ Seeethanoyl chloride.
acetylene /ă-set-ă-leen, -lin/ See ethyne.
acetyl group See ethanoyl group.
acetylide /ă-set-ă-lÿd/ See carbide.
acetylsalicylic acid /ass-ĕ-t’l-sal-ă-sil-ik,ă-see-t’l-/ See aspirin.
Acheson process /ach-ĕ-s’n/ See carbon.
achiral /ă-kÿr-ăl/ Describing a moleculethat does not exhibit optical activity.
acid A substance that gives rise to hy-drogen ions when dissolved in water.Strictly, these ions are hydrated, known ashydroxonium or hydronium ions, and areusually given the formula H3O
+. An acid insolution will have a pH below 7. This def-inition does not take into account the com-petitive behavior of acids in solvents and itrefers only to aqueous systems. The Lowry–Brønsted theory defines an acid as a sub-stance that exhibits a tendency to release aproton, and a base as a substance thattends to accept a proton. Thus, when anacid releases a proton, the ion formed is theconjugate base of the acid. Strong acids(e.g. HNO3) react completely with waterto give H3O
+, i.e. HNO3 is stronger thanH3O
+ and the conjugate base NO3– is
weak. Weak acids (e.g. CH3COOH andC6H5COOH) are only partly dissociatedbecause H3O
+ is a stronger acid than the
1 2
Acenaphthene
free acids and the ions CH3COO– andC6H5COO– are moderately strong bases.The Lowry–Brønsted theory is named forthe English chemist Thomas Martin Lowry(1874–1936) and the Danish physicalchemist Johannes Nicolaus Brønsted(1879–1947). See also Lewis acid.
acid anhydride A type of organic com-pound of general formula RCOOCOR′,where R and R′ are alkyl or aryl groups.They are prepared by reaction of an acylhalide with the sodium salt of a carboxylicacid, e.g.:
RCOCl + R′COO–Na+ → RCOOCOR′+ NaCl
Like the acyl halides, they are very reactiveacylating agents. Hydrolysis is to car-boxylic acids:
RCOOCOR′ + H2O → RCOOH +R′COOH
See also acylation.
acid–base indicator An indicator thatis either a weak base or a weak acid andwhose dissociated and undissociated formsdiffer markedly in color. The color changemust occur within a narrow pH range. Ex-amples are methyl orange and phenolph-thalein.
acid dyes The sodium salts of organicacids used in the dyeing of silks and wool.They are so called because they are appliedfrom a bath acidified with dilute sulfuric orethanoic acid.
acid halide See acyl halide.
acidic Having a tendency to release aproton or to accept an electron pair from adonor. In aqueous solutions the pH is a
measure of the acidity, i.e. an acidic solu-tion is one in which the concentration ofH3O
+ exceeds that in pure water at thesame temperature; i.e. the pH is lower than7. A pH of 7 is regarded as being neutral.
acidic hydrogen A hydrogen atom in amolecule that enters into a dissociationequilibrium when the molecule is dissolvedin a solvent. For example, in ethanoic acid(CH3COOH) the acidic hydrogen is theone on the carboxyl group, –COOH.
acidic oxide An oxide of a nonmetalthat reacts with water to produce an acidor with a base to produce a salt and water.For example, sulfur(VI) oxide (sulfur triox-ide) reacts with water to form sulfuric(VI)acid:
SO3 + H2O → H2SO4and with sodium hydroxide to producesodium sulfate and water:
SO3 + NaOH → Na2SO4 + H2OSee also amphoteric; basic oxide.
acidimetry /ass-ă-dim-ĕ-tree/ Volumet-ric analysis or acid-base titration in whicha standard solution of an acid is added tothe unknown (base) solution plus the indi-cator. Alkalimetry is the converse, i.e. thebase is in the buret.
acidity constant See dissociation con-stant.
acid rain See pollution.
acid salt (acidic salt) A salt in whichthere is only partial replacement of theacidic hydrogen of an acid by metal orother cations. For polybasic acids the for-mulae are of the type NaHSO4 (sodium hy-drogensulfate) and Na3H(CO3)2.2H2O(sodium sesquicarbonate). For monobasicacids such as HF the acid salts are of theform KHF2 (potassium hydrogen fluoride).Although the latter were at one time for-mulated as a normal salt plus excess acid(i.e. KF.HF) it is preferable to treat these ashydrogen-bonded systems of the type K+
(F–H–F)–.
3
acid salt
R’C O
O
C OR
Acid anhydride structure
acid value A measure of the free acidpresent in fats, oils, resins, plasticizers, andsolvents, defined as the number of mil-ligrams of potassium hydroxide requiredto neutralize the free acids in one gram ofthe substance.
acridine /ak-ri-deen/ (C12H9N) A color-less crystalline heterocyclic compoundwith three fused rings. Derivatives of acri-dine are used as dyes and biological stains.
Acrilan /ak-ră-lan/ (Trademark) A syn-thetic fiber that consists of a copolymer of1-cyanoethene (acrylonitrile, vinylcyanide) and ethenyl ethanoate (vinyl ac-etate).
acrolein /ă-kroh-lee-in/ See propenal.
acrylic acid /ă-kril-ik/ See propenoicacid.
acrylic resin A synthetic resin made bypolymerizing an amide or ester derivativeof 2-propenoic acid (acrylic acid). Exam-ples of acrylic materials are Acrilan (frompropenonitrile) and Plexiglas (polymethyl-methacrylate). Acrylic resins are also usedin paints.
acrylonitrile /ak-ră-loh-nÿ-trăl, -tril, -trÿl,ă-kril-oh-/ See propenonitrile.
actinic radiation /ak-tin-ik/ Radiationthat can cause a chemical reaction; for ex-ample, ultraviolet radiation is actinic.
actinides /ak-tă-nÿdz/ See actinoids.
actinium /ak-tin-ee-ŭm/ A soft silvery-white radioactive metallic element that isthe first member of the actinoid series. Itoccurs in minute quantities in uranium
ores and the metal can be obtained by re-ducing the trifluoride with lithium. It canbe produced by neutron bombardment ofradium and is used as a source of alphaparticles. The metal glows in the dark; it re-acts with water to produce hydrogen.
Symbol: Ac; m.p. 1050±50°C; b.p.3200±300°C; r.d. 10.06 (20°C); p.n. 89;most stable isotope 227Ac (half-life 21.77years).
actinoids /ak-tă-noidz/ (actinides) Agroup of 15 radioactive elements whoseelectronic configurations display filling ofthe 5f level. As with the lanthanoids, thefirst member, actinium, has no f electrons(Ac [Rn]6d17s2) but other members alsoshow deviations from the smooth trend off-electron filling expected from simple con-siderations, e.g. thorium Th [Rn]6d27s2,berkelium Bk [Rn]5f86d17s2. The actinoidsare all radioactive and their chemistry isoften extremely difficult to study. In gen-eral, artificial methods using high-energybombardment are used to generate them.See also transuranic elements.
activated charcoal See charcoal.
activated complex The partiallybonded system of atoms in the transitionstate of a chemical reaction.
activation energy Symbol: Ea The min-imum energy a particle, molecule, etc.,must acquire before it can react; i.e. the en-ergy required to initiate a reaction regard-less of whether the reaction is exothermicor endothermic. Activation energy is oftenrepresented as an energy barrier that mustbe overcome if a reaction is to take place.See Arrhenius equation.
activator See promoter.
active mass See mass action; law of.
active site 1. a site on the surface of acatalyst at which catalytic activity occursor at which the catalyst is particularly ef-fective.
acid value
4
NNN
Acridine
2. The position on the molecule of an en-zyme that binds to the substrate when theENZYME acts as a catalyst.
activity 1. Symbol: a Certain thermody-namic properties of a solvated substanceare dependent on its concentration (e.g. itstendency to react with other substances).Real substances show departures fromideal behavior and a corrective concentra-tion term – the activity – has to be intro-duced into equations describing realsolvated systems.2. Symbol: A The average number of atomsdisintegrating per unit time in a radioactivesubstance.
activity coefficient Symbol: f A mea-sure of the degree of deviation from ideal-ity of a solvated substance, defined as:
a = fcwhere a is the activity and c the concentra-tion. For an ideal solute f = 1; for real sys-tems f can be less or greater than unity.
acyclic /ay-sÿ-klik, -sik-lik/ Describing acompound that is not cyclic (i.e. a com-pound that does not contain a ring in itsmolecules).
acyl anhydride /ass-ăl, ay-săl/ See acidanhydride.
acylating agent /ass-ă-layt-ing/ See acy-lation.
acylation /ass-ă-lay-shŏn/ A reactionthat introduces the acyl group (RCO–).Acylating agents are acyl halides (R.CO.X)and acid anhydrides (R.CO.O.CO.R),which react with such nucleophiles asH2O, ROH, NH3, and RNH2. In thesecompounds a hydrogen atom of a hydroxylor amine group is replaced by the RCO–group. In acetylation the acetyl group(CH3CO–) is used. In benzoylation thebenzoyl group (C6H5CO–) is used. Acyla-tion is used to prepare crystalline deriva-tives of organic compounds to identifythem (e.g. by melting point) and to protect–OH groups in synthetic reactions.
acyl group The group of atoms RCO–.
acyl halide (acid halide) A type of or-ganic compound of the general formulaRCOX, where X is a halogen (acyl chlo-ride, acyl bromide, etc.).
Acyl halides can be prepared by the re-action of carboxylic acid with a halogenat-ing agent. Commonly, phosphorus halidesare used (e.g. PCl5) or a sulfur dihalideoxide (e.g. SOCl2):
RCOOH + PCl5 → RCOCl + POCl3+ HCl
RCOOH + SOCl2 → RCOCl + SO2+ HCl
The acyl halides have irritating vaporsand fume in moist air. They are very reac-tive to the hydrogen atom of compoundscontaining hydroxyl (–OH) or amine(–NH2) groups. See acylation.
addition polymerization See polymer-ization.
addition reaction A reaction in whichadditional atoms or groups of atoms are in-troduced into an unsaturated compound,such as an alkene or ketone. A simple ex-ample is the addition of bromine across thedouble bond in ethene:H2C:CH2 + Br2 → BrH2CCH2Br
Addition reactions can be induced ei-ther by electrophiles or by nucleophiles.See also electrophilic addition; nucleophilicaddition.
adduct /ă-dukt/ See coordinate bond.
adenine /ad-ĕ-neen, -nin, -nÿn/ A ni-trogenous base found in DNA and RNA. Itis also a constituent of certain coenzymesand when combined with the sugar ribose
5
adenine
O
CR Cl
Acyl halide: an acyl chloride
it forms the nucleoside adenosine found inAMP, ADP, and ATP. Adenine has apurine ring structure.
adenosine /ă-den-ŏ-seen, -sin, ad-’n-ŏ-seen/ (adenine nucleoside) A NUCLEOSIDE
formed from adenine linked to D-ribosewith a β-glycosidic bond. It is widely foundin all types of cell, either as the free nucle-oside or in combination in nucleic acids.Phosphate esters of adenosine, such as ATP,are important carriers of energy in bio-chemical reactions.
adenosine diphosphate /dÿ-fos-fayt/ SeeADP.
adenosine monophosphate /mon-oh-fos-fayt/ See AMP.
adenosine triphosphate /trÿ-fos-fayt/See ATP.
adiabatic change /ad-ee-ă-bat-ik/ Achange during which no energy enters orleaves the system.
In an adiabatic expansion of a gas, me-chanical work is done by the gas as its vol-ume increases and the gas temperature falls.For an ideal gas undergoing a reversibleadiabatic change it can be shown that
pVγ = K1Tγp1–γ = K2
and TVγ–1 = K3where K1, K2, and K3 are constants and γ isthe ratio of the principal specific heat ca-pacities. Compare isothermal change.
adipic acid /ă-dip-ik/ See hexanedioicacid.
ADP (adenosine diphosphate) A NUCLEO-TIDE consisting of adenine and ribose withtwo phosphate groups attached. See alsoATP.
adsorbate /ad-sor-bayt, -zor-/ A sub-stance that is adsorbed on a surface. Seeadsorption.
adsorbent /ad-sor-bĕnt, -zor-/ The sub-stance on whose surface ADSORPTION takesplace.
adsorption /ad-sorp-shŏn, -zorp-/ Aprocess in which a layer of atoms or mole-cules of one substance forms on the surfaceof a solid or liquid. All solid surfaces takeup layers of gas from the surrounding at-mosphere. The adsorbed layer may be heldby chemical bonds (chemisorption) or byweaker van der Waals forces (physisorp-tion). Compare absorption.
adsorption indicator See absorptionindicator.
aerobic /air-oh-bik/ Describing a bio-chemical process that takes place only inthe presence of free oxygen. Compareanaerobic.
aerosol See sol.
AES See atomic emission spectroscopy.
affinity The extent to which one sub-stance is attracted to or reacts with an-other.
afterdamp See firedamp.
agate /ag-it, -ayt/ A hard microcrys-talline form of the mineral chalcedony (avariety of quartz). Typically it has greenishor brownish bands of coloration, and isused for making ornaments. Moss agate isnot banded, but has mosslike patterns re-sulting from the presence of iron and man-ganese oxides. Agate is used in instrumentbearings because of its resistance to wear.
agent orange A herbicide consisting ofa mixture of two weedkillers (2,4-D and2,4,5-T), which was formerly used in war-fare to defoliate trees where an enemy maybe hiding or to destroy an enemy’s crops. Italso contains traces of the highly toxicchemical dioxin, which may cause cancersand birth defects.
air The mixture of gases that surroundsthe Earth. The composition of dry air, byvolume, is:
nitrogen 78.08% oxygen 20.95% argon 0.93%
adenosine
6
carbon dioxide 0.03% neon 0.0018% helium 0.0005% krypton 0.0001% xenon 0.00001%Air also contains a variable amount of
water vapor, as well as particulate matter(e.g. dust and pollen), and small amountsof other gases.
air gas See producer gas.
alabaster A mineral form of gypsum(CaSO4.2H2O).
alanine /al-ă-neen, -nÿn/ See aminoacids.
albumen /al-byoo-mĕn/ The white of anegg, which consists mainly of albumin. Seealbumin.
albumin /al-byoo-min/ A soluble proteinthat occurs in many animal fluids, such asblood serum and egg white.
alchemy An ancient pseudoscience thatwas the precursor of chemistry, datingfrom early Christian times until the 17thcentury. It combined mysticism and exper-imental techniques. Many ancient al-chemists searched for the Philosopher’sstone – a substance that could transmutebase metals into gold and produce theelixir of life, a universal remedy for all ills.
alcohol A type of organic compound ofthe general formula ROH, where R is a hy-drocarbon group. Examples of simple alco-hols are methanol (CH3OH) and ethanol(C2H5OH).
Alcohols have the –OH group attachedto a carbon atom that is not part of an aro-matic ring: C6H5OH, in which the –OHgroup is attached to the ring, is thus a phe-nol. Phenylmethanol (C6H5CH2OH) doeshave the characteristic properties of alco-hols.
Alcohols can have more than one –OHgroup; those containing two, three or moresuch groups are described as dihydric, tri-hydric, and polyhydric respectively (as op-posed to those containing one –OH group,
which are monohydric). Examples areethane-1,2-diol (ethylene glycol; (HOCH2-CH2OH) and propane-1,2,3-triol (glyc-erol; HOCH2CH(OH)CH2OH).
Alcohols are further classified accord-ing to the environment of the –C–OHgrouping. If the carbon atom is attached totwo hydrogen atoms, the compound is aprimary alcohol. If the carbon atom is at-tached to one hydrogen atom and twoother groups, it is a secondary alcohol. Ifthe carbon atom is attached to three othergroups, it is a tertiary alcohol. Alcohols canbe prepared by:1. Hydrolysis of HALOALKANES using aque-
ous potassium hydroxide:RI + OH– → ROH + I–
2. Reduction of aldehydes by nascent hy-drogen (from sodium amalgam in water):
RCHO +2[H] → RCH2OHThe main reactions are:1. Oxidation by potassium dichromate(VI)
in sulfuric acid. Primary alcohols give
7
alcohol
primary (ethanol)
secondary (propan-2-ol)
tertiary (2-methylpropan-2-ol)
C
H OH
HCH3
C
H3C OH
HH3C
C
H3C OH
CH3H3C
Alcohols
aldehydes, which are further oxidized tocarboxylic acids:
RCH2OH → RCHO → RCOOH1. Secondary alcohols are oxidized to ke-
tones.2. Formation of esters with acids. The re-
action, which is reversible, is catalyzedby H+ ions:ROH + R′COOH ˆ R′COOR + H2O
3. Dehydration over hot pumice (400°C)to alkenes:
RCH2CH2OH – H2O → RCH:CH24. Reaction with sulfuric acid. Two types
of reaction are possible. With excessacid at 160°C dehdyration occurs togive an alkene:
RCH2CH2OH + H2SO4 → H2O +RCH2CH2.HSO4
RCH2CH2.HSO4 → RCH:CH2 +H2SO4
4. With excess alcohol at 140°C an ether isformed:
2ROH → ROR + H2OSee also acylation.
aldehyde /al-dĕ-hÿd/ A type of organiccompound with the general formulaRCHO, where the –CHO group (the alde-hyde group) consists of a carbonyl groupattached to a hydrogen atom. Simple ex-amples of aldehydes are methanal(formaldehyde, HCHO) and ethanal (ac-etaldehyde, CH3CHO).
Aldehydes are formed by oxidizing aprimary alcohol. In the laboratory potas-sium dichromate(VI) is used in sulfuricacid. They can be further oxidized to car-boxylic acids. Reduction (using a catalystor nascent hydrogen from sodium amal-gam in water) produces the parent alcohol.
Aldehydes undergo a number of reac-tions:1. They act as reducing agents, being oxi-
dized to carboxylic acids in the process.These reactions are used as tests foraldehydes using such reagents asFehling’s solution and Tollen’s reagent(silver-mirror test).
2. They form addition compounds withhydrogen cyanide to give ‘cyanohy-drins’. For example, propanal gives 2-hydroxybutanonitrile:
C2H5CHO + HCN →C2H5CH(OH)CN
3. They form addition compounds (bisul-fite addition compounds) with the hy-drogensulfate(IV) ion (hydrogensulfite;HSO3
–):RCHO + HSO3
– → RCH(OH)(HSO3)4. They undergo CONDENSATION REACTIONS
with such compounds as hydrazine, hy-droxylamine, and their derivatives.
5. With alcohols they form hemiacetalsand ACETALS.
6. They polymerize readily. Polymethanalor methanal trimer can be formed frommethanal depending on the conditions.Ethanal gives ethanal trimer or ethanaltetramer.See also Cannizzaro reaction; ketone.
aldohexose /al-doh-heks-ohs/ An aldoseSUGAR with six carbon atoms.
aldol /al-dol, -dohl/ See aldol reaction.
aldol reaction A reaction in which twomolecules of aldehyde combine to give analdol – i.e. a compound containing bothaldehyde and alcohol functional groups.The reaction is base-catalyzed; the reactionof ethanal refluxed with sodium hydroxidegives:2CH3CHO → CH3CH(OH)CH2CHOThe mechanism is similar to that of theCLAISEN CONDENSATION: the first step is re-moval of a proton to give a carbanion,which subsequently attacks the carbon ofthe carbonyl group on the other molecule:CH3CHO + OH– → –CH2CHO + H2O.
aldopentose /al-doh-pen-tohs/ An al-dose SUGAR with five carbon atoms.
aldose /al-dohs/ A SUGAR containing analdehyde (CHO) or potential aldehydegroup.
aldehyde
8
O
HCR
Aldehyde
alginic acid /al-jin-ik/ (algin;(C6H8O6)n) A yellow-white organic solidthat is found in brown algae. It is a com-plex polysaccharide and produces, in evenvery dilute solutions, a viscous liquid. Al-ginic acid has various uses, especially in thefood industry as a stabilizer and textureagent.
alicyclic compound /al-ă-sÿ-klik, -sik-lik/ An aliphatic cyclic compound, suchas cyclohexane.
aliphatic compound /al-ă-fat-ik/ Anorganic compound with properties similarto those of the alkanes, alkenes, andalkynes and their derivatives. Mostaliphatic compounds have an open chainstructure but some, such as cyclohexaneand sucrose, have rings. The term is used indistinction to aromatic compounds, whichare similar to benzene. Compare aromaticcompound.
alizarin /ă-liz-ă-rin/ (1,2-dihydroxyan-thraquinone) An important orange-redorganic compound used in the dyestuffs in-dustry to produce red lakes. It occurs natu-rally in the root of the plant madder andmay also be synthesized from an-thraquinone.
alkali /al-kă-lÿ/ A water-soluble strongbase. Strictly the term refers to the hydrox-ides of the alkali metals (group 1) only, butin common usage it refers to any solublebase. Thus borax solution may be de-scribed as mildly alkaline.
alkali metals (group 1 elements) Agroup of soft reactive metals, each repre-senting the start of a new period in the pe-riodic table and having an electronicconfiguration consisting of a rare-gasstructure plus one outer electron. The al-kali metals are lithium (Li), sodium (Na),potassium (K), rubidium (Rb), cesium (Cs),and francium (Fr). They formerly wereclassified in subgroup IA of the periodictable.
The elements all easily form positiveions M+ and consequently are highly reac-tive (particularly with any substrate that is
oxidizing). As the group is descended thereis a gradual decrease in ionization potentialand an increase in the size of the atoms; thegroup shows several smooth trends whichfollow from this. For example, lithium re-acts in a fairly controlled way with water,sodium ignites, and potassium explodes.There is a general decrease in the follow-ing: melting points, heats of sublimation,lattice energy of salts, hydration energy ofM+, ease of decomposition of nitrates andcarbonates, and heat of formation of the ‘-ide’ compounds (fluoride, hydride, oxide,carbide, chloride).
Lithium has the smallest ion and there-fore the highest charge/size ratio and is po-larizing with a tendency towards covalentcharacter in its bonding; the remaining ele-ments form typical ionic compounds inwhich ionization, M+X–, is regarded ascomplete. The slightly anomalous positionof lithium is illustrated by the similarity ofits chemistry to that of magnesium. For ex-ample, lithium hydroxide is much less sol-uble than the hydroxides of the othergroup 1 elements; lithium perchlorate issoluble in several organic solvents. Becauseof the higher lattice energies associatedwith smaller ions lithium hydride and ni-tride are fairly stable compared to NaH,which decomposes at 345°C. Na2N, K3Netc., are not obtained pure and decomposebelow room temperature.
The oxides also display the trend inproperties as lithium forms M2O with onlytraces of M2O2, sodium forms M2O2 andat high temperatures and pressures MO2,potassium, rubidium, and cesium formM2O2 if oxygen is restricted but MO2 ifburnt in air. Hydrolysis of the oxides or di-rect reaction of the metal with water leadsto the formation of the hydroxide ion.
Salts of the bases MOH are known forall acids and these are generally white crys-talline solids. The ions M+ are hydrated inwater and remain unchanged in most reac-tions of alkali metal salts.
Because of the ease of formation of theions M+ there are very few coordinationcompounds of the type MLn
+ apart fromsolvated species of very low correlationtimes. The group 1 elements form a varietyof organometallic compounds; the bonding
9
alkali metals
in lithium alkyls and aryls is essentially co-valent but the heavier elements form ioniccompounds. Organo-alkali metal com-pounds – particularly the lithium com-pounds – are widely used in syntheticorganic chemistry.
Francium is formed only by radioactivedecay and in nuclear reactions; all the iso-topes of francium have short half-lives, thelongest of which is 21 minutes (francium-223). The few chemical studies which havebeen carried out indicate that it would havesimilar properties to those of the other al-kali metals.
alkalimetry /al-kă-lim-ĕ-tree/ See acidime-try.
alkaline-earth metals (group 2 ele-ments) A group of moderately reactivemetals, harder and less volatile than the al-kali metals. They were formerly classifiedin subgroup IIA of the periodic table. Theterm alkaline earth strictly refers to the ox-ides, but is often used loosely for the ele-ments themselves. The electronicconfigurations are all those of a rare-gasstructure with an additional two electronsin the outer s orbital. The elements areberyllium (Be), magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba), and ra-dium (Ra). The group shows an increasingtendency to ionize to the divalent stateM2+. The first member, beryllium has amuch higher ionization potential than theothers and the smallest atomic radius.Thus it has a high charge/size ratio andconsequently the bonding in berylliumcompounds is largely covalent. The chem-istry of the heavier members of the group islargely that of divalent ions.
The group displays a typical trend to-wards metallic character as the group isdescended. For example, beryllium hy-droxide is amphoteric; magnesium hydrox-ide is almost insoluble in water and isslightly basic; calcium hydroxide is spar-ingly soluble and distinctly basic; andstrontium and barium hydroxides are in-creasingly soluble in water and stronglybasic. The group also displays a smoothtrend in the solubilities of the sulfates(MgSO4 is soluble, CaSO4 sparingly solu-
ble, and BaSO4 very insoluble). The trendto increasing metallic character is alsoshown by the increase in thermal stabilitiesof the carbonates and nitrates with increas-ing relative atomic mass.
The elements all burn in air (berylliummust be finely powdered) to give the oxideMO (covalent in the case of beryllium) andfor barium the peroxide, BaO2 in additionto BaO. The heavier oxides, CaO, SrO, andBaO, react with water to form hydroxides,M(OH)2; magnesium oxide reacts only athigh temperatures and beryllium oxide notat all. The metals Ca, Sr, and Ba all reactreadily with water to give the hydroxide:
M + 2H2O → M2+ + 2OH– + H2In contrast, magnesium requires diluteacids in order to react (to the salt plus hy-drogen), and beryllium is resistant to acidattack. A similar trend is seen in the directreaction of hydrogen: under mild condi-tions calcium, strontium, and barium giveionic hydrides, high pressures are requiredto form magnesium hydride, and berylliumhydride can not be prepared by direct com-bination.
Because of its higher polarizing power,beryllium forms a range of complexes inwhich the beryllium atom should betreated as an electron acceptor (i.e. the va-cant p orbitals are being used). Complexessuch as etherates, acetylethanoates, and thetetrafluoride (BeF4
2–) are formed, all ofwhich are tetrahedral. In contrast Mg2+,Ca2+, Sr2+, and Ba2+ have poor acceptorproperties and form only weak complexes,even with donors such as ammonia or edta.Magnesium forms Grignard reagents(RMgX), which are important in organicsynthesis, and related compoundsR2Mg.MgX2 and R2Mg are known. Thefew organic compounds of Ca, Sr, and Baare ionic. All isotopes of radium are ra-dioactive and radium was once widely usedfor radiotherapy. The half-life of 226Ra(formed by decay of 238U) is 1600 years.
alkaloid /al-kă-loid/ One of a group ofnatural organic compounds found inplants. They contain oxygen and nitrogenatoms; most are poisonous. However, theyinclude a number of important drugs withcharacteristic physiological effects, e.g.
alkalimetry
10
morphine, codeine, caffeine, cocaine, andnicotine.
alkane /al-kayn/ A type of hydrocarbonwith general formula CnH2n+2. Alkanes aresaturated compounds, containing no dou-ble or triple bonds. Methane (CH4) andethane (C2H6) are typical examples. Thealkanes are fairly unreactive (their formername, the paraffins, means ‘small affin-ity’). In ultraviolet radiation they reactwith chlorine to give a mixture of substitu-tion products. There are a number of waysof preparing alkanes:1. From a sodium salt of a carboxylic acid
treated with soda lime:RCOO–Na+ + NaOH → RH + Na2CO3
2. By reduction of a haloalkane withnascent hydrogen from the action ofethanol on a zinc–copper couple:
RX + 2[H] → RH + HX3. By the Wurtz reaction – i.e. sodium in
dry ether on a haloalkane:2RX + 2Na → 2NaX + RR
4. By the Kolbé electrolytic method:RCOO– → RR
5. By refluxing a haloalkane with magne-sium in dry ether to form a Grignardreagent:
RI + Mg → RMgI5. With acid this gives the alkane:
RMgI + H → RHThe main source of lower molecular
weight alkanes is natural gas (for methane)and crude oil.
alkene /al-keen/ A type of hydrocarbonwith the general formula CnH2n. Thealkenes (formerly called olefins) are unsat-urated compounds containing double car-bon–carbon bonds. They can be obtainedfrom crude oil by cracking alkanes. Impor-tant examples are ethene (C2H4) andpropene (C3H6), both of which are used inplastics production and as starting materi-als for the manufacture of many other or-ganic chemicals.
The methods of synthesizing alkenesare:1. The elimination of HBr from a
haloalkane using an alcoholic solutionof potassium hydroxide:
RCH2CH2Br + KOH → KBr + H2O +RCH:CH2
2. The dehydration of an alcohol by pass-ing the vapor over hot pumice (400°C):
RCH2CH2OH → RCH:CH2 + H2OThe reactions of alkenes include:1. Hydrogenation using a catalyst (usually
nickel at about 150°C):RCH:CH2 + H2 → RCH2CH3
2. Addition reactions with halogen acids togive haloalkanes:
RCH:CH2 + HX → RCH2CH2XThe addition follows Markovnikoff’srule.
3. Addition reactions with halogens, e.g.RCH:CH2 + Br2 → RCHBrCH2Br
4. Hydration using concentrated sulfuricacid, followed by dilution and warming:
RCH:CH2 + H2O → RCH(OH)CH35. Oxidation by cold potassium perman-
ganate solutions to give diols:RCH:CH2 + H2O + [O] →
RCH(OH)CH2OHEthene can be oxidized in air using asilver catalyst to the cyclic compoundepoxyethane (C2H4O).
6. Polymerization to polyethene (by theZiegler or Phillips process).
See also oxo process; ozonolysis.
alkoxide /al-koks-ÿd/ An organic com-pound containing an ion of the type RO–,where R is an alkyl group. Alkoxides aremade by the reaction of metallic sodium onan alcohol. Sodium ethoxide (C2H5O
–Na+)is a typical example.
alkoxyalkane /al-koks-ee-al-kayn/ (Di-ethyl ether.) See ether.
alkylbenzene /al-kăl-ben-zeen/ A typeof organic hydrocarbon containing one ormore alkyl groups substituted onto a ben-zene ring. Methylbenzene (C6H5CH3) and1,3-dimethylbenzene are simple examples.Alkylbenzenes can be made by a Friedel-Crafts reaction or by the Fittig reaction. In-dustrially, large quantities of methyl-benzene are made by the hydroforming ofcrude oil.
Substitution of alkylbenzenes can occurat the benzene ring. The alkyl group directsthe substituent into the 2- or 4-position.
11
alkylbenzene
Substitution of hydrogen atoms on thealkyl group can also occur.
alkyl group /al-kăl/ A group obtainedby removing a hydrogen atom from analkane or other aliphatic hydrocarbon.
alkyl halide See haloalkane.
alkyl sulfide A thioether with the gen-eral formula RSR′, where R and R′ arealkyl groups.
alkyne /al-kÿn/ A type of hydrocarbonwith the general formula CnH2n–2. Thealkynes are unsaturated compounds con-taining triple carbon–carbon bonds. Thesimplest member of the series is ethyne(C2H2), which can be prepared by the ac-tion of water on calcium dicarbide.
CaC2 + 2H2O → Ca(OH)2 + C2H2The alkynes were formerly called theacetylenes.
In general, alkynes can be made by thecracking of alkanes or by the action of ahot alcoholic solution of potassium hy-droxide on a dibromoalkane, for example:
BrCH2CH2Br + KOH → KBr +CH2:CHBr + H2O
CH2:CHBr + KOH → CHCH + KBr +H2O
The main reactions of the alkynes are:1. Hydrogenation with a catalyst (usually
nickel at about 150°C):C2H2 + H2 → C2H4C2H4 + H2 → C2H6
2. Addition reactions with halogen acids:C2H2 + HI → H2C:CHI
H2C:CHI + HI → CH3CHI23. Addition of halogens; for example, with
bromine in tetrachloromethane:C2H2 + Br2 → BrHC:CHBr
BrHC:CHBr + Br2 → Br2HCCHBr24. With dilute sulfuric acid at 60–80°C and
mercury(II) catalyst, ethyne formsethanal (acetaldehyde):
C2H2 + H2O → H2C:C(OH)HThis enol form converts to the alde-hyde:
CH3COH5. Ethyne polymerizes if passed through a
hot tube to produce some benzene:3C2H2 → C6H6
6. Ethyne forms unstable dicarbides(acetylides) with ammoniacal solutionsof copper(I) and silver(I) chlorides.
allotropy /ă-lot-rŏ-pee/ The ability ofcertain elements to exist in more than onephysical form. Carbon, sulfur, and phos-phorus are the most common examples.Allotropy is more common in groups 14,15, and 16 of the periodic table than inother groups. See also enantiotropy;monotropy.
alloy A mixture of two or more metals(e.g. bronze or brass) or a metal with smallamounts of non-metals (e.g. steel). Alloysmay be completely homogeneous mixturesor may contain small particles of one phasein the other phase.
allyl group /al-ăl/ See propenyl group.
Alnico /al-mă-koh/ (Trademark) Any ofa group of very hard brittle alloys used tomake powerful permanent magnets. Theycontain nickel, aluminum, cobalt, and cop-per in various proportions. Iron, titanium,and niobium can also be present. Theyhave a high remanence and coercive force.
alpha particle A He2+ ion emitted withhigh kinetic energy by a radioactive sub-stance. Alpha particles are used to causenuclear disintegration reactions.
alternating copolymer See polymer-ization.
alum A type of double salt. Alums aredouble sulfates obtained by crystallizingmixtures in the correct proportions. Theyhave the general formula:
M2SO4.M′2(SO4)3.24H2OWhere M is a univalent metal or ion,
and M′ is a trivalent metal. Thus, alu-minum potassium sulfate (called potashalum, or simply alum) is
K2SO4.Al2(SO4)3.24H2OAluminum ammonium sulfate (called
ammonium alum) is(NH4)2SO4.Al2(SO4)3.24H2O
The name ‘alum’ originally came fromthe presence of Al3+ as the trivalent ion, but
alkyl group
12
is also applied to other salts containingtrivalent ions, thus, Chromium(III) potas-sium sulfate (chrome alum) is
K2SO4.Cr2(SO4)3.24H2O
alumina See aluminum oxide.
aluminate See aluminum hydroxide.
aluminosilicate /ă-loo-mă-noh-sil-ă-kayt/See silicates.
aluminum A soft moderately reactivemetal; the second element in group 3 of theperiodic table. It was formerly classified insubgroup IIIA. Aluminum has the elec-tronic structure of neon plus three addi-tional outer electrons. There are numerousminerals of aluminum; it is the most com-mon metallic element in the Earth’s crust(8.1% by weight) and the third in order ofabundance. Commercially important min-erals are bauxite (hydrated Al2O3), corun-dum (anhydrous Al2O3), cryolite(Na3AlF6), and clays and mica (aluminosil-icates).
The metal is produced on a massivescale by the Hall–Heroult method in whichalumina, a non-electrolyte, is dissolved inmolten cryolite and electrolyzed. Thebauxite contains iron, which would conta-minate the product, so the bauxite is dis-solved in hot alkali, the iron oxide isremoved by filtration, and the pure alu-mina then precipitated by acidification.Molten aluminum is tapped off from thebase of the cell and oxygen evolved at theanode. The aluminum atom is much biggerthan boron (the first member of group 3)and its ionization potential is not particu-larly high. Consequently aluminum formspositive ions Al3+. However, it also hasnon-metallic chemical properties. Thus, itis amphoteric and also has a number of co-valently bonded compounds.
Unlike boron, aluminum does not forma vast range of hydrides – AlH3 and Al2H6may exist at low pressures, and the onlystable hydride, (AlH3)n, must be preparedby reduction of aluminum trichloride. Theion AlH4
– is widely used in the form ofLiAlH4 as a vigorous reducing agent.
The reaction of aluminum metal withoxygen is very exothermic but at ordinarytemperatures an impervious film of theoxide protects the bulk metal from furtherattack. This oxide film also protects alu-minum from oxidizing acids. There is onlyone oxide, Al2O3 (alumina), but a varietyof polymorphs and hydrates are known. Itis relatively inert and has a high meltingpoint, and for this reason is widely used asa furnace lining and for general refractorybrick. Aluminum metal will react with al-kalis releasing hydrogen and producing ini-tially Al(OH)3 then Al(OH)4
–.Aluminum reacts readily with the halo-
gens; in the case of chlorine thin sheets willburst into flame. The fluoride has a highmelting point (1290°C) and is ionic. Theother halides are dimers in the vapor phase(two halogen bridges). Aluminum alsoforms a sulfide (Al2S3), nitride (AlN), andcarbide (Al4C), the latter two at extremelyhigh temperatures.
Because of aluminum’s ability to ex-pand its coordination number and ten-dency towards covalence it forms a varietyof complexes such as AlF6
2– and AlCl4–. A
number of very reactive aluminum alkylsare also known, some of which are impor-tant as polymerization catalysts.
Symbol: Al; m.p. 660.37°C; b.p.2470°C; r.d. 2.698 (20°C); p.n. 13; r.a.m.26.981539.
aluminum acetate See aluminumethanoate.
aluminum bromide (AlBr3) A whitesolid soluble in water and many organicsolvents.
aluminum chloride (AlCl3) A white co-valent solid that fumes in moist air:
AlCl3 + 3H2O → Al(OH)3 + 3HClIt is prepared by heating aluminum in drychlorine or dry hydrogen chloride. Vapor-density measurements show that its struc-ture is a dimer; it consists of Al2Cl6molecules in the vapor. The AlCl3 structurewould be electron-deficient. Aluminumchloride is used in Friedel–Crafts reactionsin organic preparations.
13
aluminum chloride
aluminum ethanoate (aluminum acetate;Al(OOCCH3)3) A white solid soluble inwater. It is usually obtained as the dibasicsalt, basic aluminum ethanoate,Al(OH)(CH3COO)2. It is prepared by dis-solving aluminum hydroxide in ethanoicacid and is used extensively as a mordant indyeing and as a size for paper and card-board products. The solution is hydrolyzedand contains various complex aluminum-hydroxyl species and colloidal aluminumhydroxide.
aluminum fluoride (AlF3) A whitecrystalline solid that is slightly soluble inwater but insoluble in most organic sol-vents. Its primary use is as an additive tothe cryolite (Na3AlF6) electrolyte in theproduction of aluminum.
aluminum hydroxide (Al(OH)3) Awhite powder prepared as a colorlessgelatinous precipitate by adding ammoniasolution or a small amount of sodium hy-droxide solution to a solution of an alu-minum salt. It is an amphoteric hydroxideand is used as a foaming agent in fire ex-tinguishers and as a mordant in dyeing.
Its amphoteric nature causes it to dis-solve in excess sodium hydroxide solutionto form the aluminate ion (systematic nametetrahydroxoaluminate(III)):
Al(OH)3 + OH– → Al(OH)4– + H2O
When precipitating from solution, alu-minum hydroxide readily absorbs coloredmatter from dyes to form lakes.
aluminum nitrate (Al(NO3)3.9H2O) Ahydrated white crystalline solid preparedby dissolving freshly prepared aluminumhydroxide in nitric acid. It cannot be pre-pared by the action of dilute nitric acid onaluminum since the metal is rendered pas-sive by a thin surface layer of oxide.
aluminum oxide (alumina; Al2O3) Awhite powder that is almost insoluble inwater. Because of its amphoteric nature itwill react with both acids and alkalis. Alu-minum oxide occurs naturally as bauxite,corundum, and white sapphire; it is manu-factured by heating aluminum hydroxide.It is used in the extraction by electrolysis of
aluminum, as an abrasive (corundum), infurnace linings (because of its refractoryproperties), and as a catalyst (e.g. in the de-hydration of alcohols).
aluminum potassium sulfate (potashalum; Al2(SO4)3.K2SO4.24H2O) A whitesolid, soluble in water but insoluble in al-cohol, prepared by mixing equimolecularquantities of solutions of ammonium andaluminum sulfate followed by crystalliza-tion. It is used as a mordant for dyes, as awaterproofing agent, and as a tanning ad-ditive.
aluminum sulfate (Al2(SO4)3.18H2O)A white crystalline solid. It is used as a sizefor paper, a precipitating agent in sewagetreatment, a foaming agent in fire control,and as a fireproofing agent. Its solutionsare acidic by hydrolysis, containing suchspecies as Al(H2O)5(OH)2+.
aluminum trimethyl /trÿ-meth-ăl/ Seetrimethylaluminum.
amalgam /ă-mal-găm/ An alloy of mer-cury with one or more other metals. Amal-gams may be liquid or solid. An amalgamof sodium (Na/Hg) with water is used as asource of nascent hydrogen.
amatol /am-ă-tol, -tohl/ A high explo-sive that consists of a mixture of ammo-nium nitrate and TNT (trinitrotoluene).
ambidentate See isomerism.
americium /am-ĕ-rish-ee-ŭm/ A highlytoxic radioactive silvery element of theactinoid series of metals. A transuranic ele-ment, it is not found naturally on Earth butis synthesized from plutonium. The ele-ment can be obtained by reducing the tri-fluoride with barium metal. It reacts withoxygen, steam, and acids. 241Am has beenused in gamma-ray radiography.
Symbol: Am; m.p. 1172°C; b.p.2607°C; r.d. 13.67 (20°C); p.n. 95; moststable isotope 243Am (half-life 7.37 × 103
years).
amethyst A purple form of the mineral
aluminum ethanoate
14
quartz (silicon(IV) oxide, SiO2) used as asemiprecious gemstone. The color comesfrom impurities such as oxides of iron.
amide /am-ÿd, -id/ 1. A type of organiccompound of general formulae RCONH2(primary), (RCO)2NH (secondary), and(RCO)3N (tertiary). Amides are white,crystalline solids and are basic in nature,some being soluble in water. Amides can beformed by reaction of ammonia with acidchlorides or anhydrides:
(RCO)2O + 2NH3 → RCONH2 +RCOO–NH4
+
Reactions of amides include: 1. Reaction with hot acids to give car-
boxylic acids:RCONH2 + HCl + H2O → RCOOH +
NH4Cl 2. 2. Reaction with nitrous acid to give car-
boxylic acids and nitrogen:RCONH2 + HNO2 → RCOOH + N2 +
H2O 3. 3. Dehydration by phosphorus(V) oxide to
give a nitrile:RCONH2 – H2O → RCN
See also Hofmann degradation.2. An inorganic salt containing the NH2
–
ion. They are formed by the reaction ofammonia with certain metals (such assodium and potassium). See sodamide.
amination /am-ă-nay-shŏn/ The intro-duction of an amino group (–NH2) into anorganic compound. An example is the con-version of an aldehyde or ketone into anamide by reaction with hydrogen and am-monia in the presence of a catalyst:
RCHO + NH3 + N2 → RCH2NH2 +H2O
amine /ă-meen, am-in/ A compound con-taining a nitrogen atom bound to hydrogenatoms or hydrocarbon groups. They havethe general formula R3N, where R can behydrogen or an alkyl or aryl group. Aminescan be prepared by reduction of amides ornitro compounds.
An amine is classified according to thenumber of organic groups bonded to thenitrogen atom: one, primary; two, sec-ondary; three, tertiary. Since amines arebasic in nature they can form the quater-
nium ion, R3NH+. All three classes, plus aquaternium salt, can be produced by theHofmann reaction (which occurs in asealed vessel at 100°C):
RX + NH3 → RNH3+ X–
RNH3+ X– + NH3 ˆ RNH2 + NH4X
RNH2 + RX → R2NH2+ X–
R2NH2+ X– + NH3 ˆ R2NH + NH4X
R2NH + RX → R3NH+ X–
R3NH+ X– + NH3 ˆ R3N + NH4XR3N + RX → R4N
+X–
Reactions of amines include:1. Reaction with acids to form salts:
R3N + HX → R3NH+X–
2. Reaction with acid chlorides to give N-substituted acid amides (primary andsecondary amines only):RNH2 + R′COCl → R′CONHR + HX
amine salt A salt similar to an ammo-nium salt, but with organic groups at-tached to the nitrogen atom. For example,
15
amine salt
primary (ethylamine)
secondary (diethylamine)
tertiary (triethylamine)
C2H5
H
H
N
C2H5
C2H5
H
N
C2H5
C2H5
C2H5
N
Amines
triethylamine ((C2H5)3N) will react withhydrogen chloride to give triethylammo-nium chloride:
(C2H5)3N + HCl → (C2H3)3NH+ Cl–
Sometimes amine salts are named using thesuffix ‘-ium’. For instance, aniline(C6H5NH2) forms anilinium chlorideC6H5NH3
+ Cl–. Often insoluble alkaloidsare used in medicine in the form of theiramine salt (sometimes referred to as the‘hydrochloride’).
It is also possible for amine salts of thistype to have four groups on the nitrogenatom. For example, with chloroethane,tetraethylammonium chloride can beformed:
(C2H5)3N + C2H5Cl → (C2H5)4N+Cl–
amino acids /ă-mee-noh, am-ă-/ Deriva-tives of carboxylic acids in which a hydro-gen atom in an aliphatic acid has beenreplaced by an amino group. Thus, fromethanoic acid, the amino acid 2-aminoethanoic acid (glycine) is formed. Allare white, crystalline, soluble in water (butnot in alcohol), and with the sole exceptionof the simplest member, all are optically ac-tive.
In the body the various proteins are as-sembled from the necessary amino acidsand it is important therefore that all the
amino acids should be present in sufficientquantities. In humans, twelve of the twentyamino acids can be synthesized by the bodyitself. Since these are not required in thediet they are known as nonessential aminoacids. The remaining eight cannot be syn-thesized by the body and have to be sup-plied in the diet. They are known asessential amino acids.
The amino acids that occur in proteinsall have the –NH2 group and the –COOHgroup attached to the same carbon atom.They are thus alpha amino acids, the car-bon atom being the alpha carbon. Theyhave complex formulae and are usually re-ferred to by their common names, ratherthan systematic names: alanine CH3CH(NH2)COOH arginine NH2C(NH)NH(CH2)3CH(NH2)-
COOH asparagine NH2COCH2CH(NH2)COOH aspartic acidCOOHCH2CH(NH2)COOH cysteine SHCH2CH(NH2)COOH cystine [HOOCCH(NH2)CH2S]2glutamic acid COOH(CH2)2CH(NH2)-
COOH glutamine NH2CH(CH2)2(CONH2)-
COOH glycine CH2(NH2)COOH histidine C3H3N2CH2CH(NH2)COOH isoleucine (CH3)CH2CH(CH3)CH(NH2)-
COOH leucine (CH3)2CHCH2CH(NH2)COOH lysine NH2(CH2)4CH(NH2)COOH methionine CH3S(CH2)2CH(NH2)COOH phenylalanine C6H5CH2CH(NH2)COOH proline NH(CH2)3CHCOOH serine CH2OHCH(NH2)COOH threonine CH3CHOHCH(NH2)-
COOH tryptophan C6H4NHC2HCH2CH(NH2)-
COOH tyrosine C6H4OHCH2CH(NH2)COOH valine (CH3)2CHCH(NH2)COOH
Note that proline is in fact a cyclicimino acid, with the nitrogen atom bondedto the alpha carbon.
See also optical activity.
aminobenzene /ă-mee-noh-ben-zeen, am-ă-/ See aniline.
amino acids
16
H O
ONH3+
R CCC CCC
OH
R CCC C
NH2 O Hun-ionized
zwitterion
Amino acid
aminoethane /a-mee-noh-eth-ayn, am-ă-/See ethylamine.
amino group The group –NH2.
aminotoluine /ă-mee-noh-tol-yoo-een,am-ă-/ See toluidine.
ammine /am-een, ă-meen/ A complex inwhich ammonia molecules are coordinatedto a metal ion; e.g. [Cu(NH3)4]
2+.
ammonia /ă-moh-nee-ă/ (NH3) A color-less gas with a characteristic pungent odor.On cooling and compression it forms a col-orless liquid, which becomes a white solidon further cooling. Ammonia is very solu-ble in water (a saturated solution at 0°Ccontains 36.9% of ammonia): the aqueoussolution is alkaline and contains a propor-tion of free ammonia. Ammonia is also sol-uble in ethanol. It occurs naturally to asmall extent in the atmosphere, and is usu-ally produced in the laboratory by heatingan ammonium salt with a strong alkali.Ammonia is synthesized industrially fromhydrogen and atmospheric nitrogen by theHaber process.
The compound does not burn readily inair but ignites, giving a yellowish-brownflame, in oxygen. It will react with atmos-pheric oxygen in the presence of platinumor a heavy metal catalyst – a reaction usedas the basis of the commercial manufactureof nitric acid, which involves the oxidationof ammonia to nitrogen monoxide andthen to nitrogen dioxide. Ammonia coordi-nates readily to form ammines and reactswith sodium or potassium to form inor-ganic amides and with acids to form am-monium salts; for example, it reacts withhydrogen chloride to form ammoniumchloride:
NH3(g) + HCl(g) → NH4Cl(g)Ammonia is also used commercially in
the manufacture of fertilizers, mainly am-monium nitrate, urea, and ammonium sul-fate. It is used to a smaller extent in therefrigeration industry. Liquid ammonia isan excellent solvent for certain substances,which ionize in the solutions to give ionicreactions similar to those occurring inaqueous solutions. Ammonia is marketed
as the liquid, compressed in cylinders (‘an-hydrous ammonia’), or as aqueous solu-tions of various strengths. See alsoammonium hydroxide.
ammoniacal /am-ŏ-nÿ-ă-kăl/ Describ-ing a solution in aqueous ammonia.
ammonia-soda process See Solvayprocess.
ammonium alum See alum.
ammonium carbonate (sal volatile;(NH4)2CO3) A white solid that crystal-lizes as plates or prisms. It is very soluble inwater and readily decomposes on heatingto ammonia, carbon dioxide, and water.The white solid sold commercially as am-monium carbonate is actually a double saltof both ammonium hydrogencarbonate(NH4HCO3) and ammoniumaminomethanoate (NH2CO2NH4). Thissalt is manufactured from ammoniumchloride and calcium carbonate. It decom-poses on exposure to air into ammoniumhydrogencarbonate and ammonia, and itreacts with ammonia to give the true am-monium carbonate. Commercial ammo-nium carbonate is used in baking powders,smelling salts, and in the dyeing and wool-scouring industries.
ammonium chloride (sal ammoniac;NH4Cl) A white crystalline solid with acharacteristic saline taste. It is very solublein water (37 g per 100 g of water at 20°C).Ammonium chloride can be manufacturedby the action of ammonia on hydrochloricacid. It sublimes on heating because of theequilibrium:
NH4Cl(s) ˆ NH3(g) + HCl(g)Ammonium chloride is used in galva-
nizing, as a flux for soldering, in dyeingand calico printing, and in the manufactureof Leclanché and ‘dry’ cells.
ammonium hydroxide (ammonia solu-tion; NH4OH) An alkali that is formedwhen ammonia dissolves in water. It prob-ably contains hydrated ammonia mol-ecules as well as some NH4
+ and OH– ions.A saturated aqueous solution of ammonia
17
ammonium hydroxide
has a relative density of 0.88 g cm–3, and isknown as 880 ammonia. Ammonia solu-tion is a useful reagent and cleansing agent.
ammonium ion The ion NH4+, formed
by coordination of NH3 to H+. See alsoquaternary ammonium compound.
ammonium nitrate (NH4NO3) A col-orless crystalline solid that is very solublein water (871 g per 100 g of water at100°C). It is usually manufactured by theaction of ammonia on nitric acid. It is usedin the manufacture of explosives and, be-cause of its high nitrogen content, as a fer-tilizer.
ammonium phosphate (triammoniumphosphate (V); (NH4)3PO4) A colorlesscrystalline salt made from ammonia andphosphoric(V) acid, used as a fertilizer toadd both nitrogen and phosphorus to thesoil.
ammonium sulfate ((NH4)2SO4) A col-orless crystalline solid that is soluble inwater. When heated carefully it gives am-monium hydrogensulfate, which onstronger heating yields nitrogen, ammonia,sulfur(IV) oxide (sulfur dioxide), andwater. Ammonium sulfate is manufacturedby the action of ammonia on sulfuric acid.It is the most important ammonium salt be-cause of its widespread use as a fertilizer.Its only drawback as a fertilizer is that ittends to leave an acidic residue in the soil.
amorphous /ă-mor-fŭs/ Describing asolid substance that has no ‘long-range’regular arrangement of atoms; i.e. is notcrystalline. Amorphous materials can con-sist of minute particles that possess orderover a very short distance. Glasses are alsoamorphous; the atoms in the solid have arandom arrangement. X-ray analysis hasshown that many substances that wereonce described as amorphous are com-posed of very small crystals. For example,charcoal, coke, and soot (all forms of car-bon) are made up of small graphite-likecrystals.
amount of substance Symbol: n Ameasure of the number of entities presentin a substance. See mole.
AMP (adenosine monophosphate) ANUCLEOTIDE consisting of adenine, ribose,and phosphate. See ATP.
ampere /am-pair/ Symbol: A The SI baseunit of electric current, defined as the con-stant current that, maintained in twostraight parallel infinite conductors of neg-ligible circular cross section placed onemeter apart in vacuum, would produce aforce between the conductors of 2 × 10–7
newton per meter. The unit is named forthe French physicist and mathematicianAndré Marie Ampère (1775–1836).
amphiprotic /am-fă-proh-tik/ See sol-vent.
ampholyte ion /am-fŏ-lÿt/ See zwitte-rion.
amphoteric /am-fŏ-te-rik/ A materialthat can display both acidic and basicproperties. The term is most commonly ap-plied to the oxides and hydroxides of met-als that can form both cations and complexanions. For example, zinc oxide dissolvesin acids to form zinc salts and also dis-solves in alkalis to form zincates,[Zn(OH)4]
2–.
amu /ay-em-yoo/ See atomic mass unit.
amyl group /am-ăl/ See pentyl group.
amyl nitrite (C5H11ONO) A palebrown volatile liquid organic compound; anitrous acid ester of 3-methylbutanol (iso-amyl alcohol). It is used in medicine as aninhalant to dilate the blood vessels (andthereby prevent pain) in patients withangina pectoris.
amylopectin /am-ă-lo-pek-tin/ The water-insoluble fraction of STARCH.
amylose /am-ă-lohs/ A polymer of GLU-COSE, a polysaccharide sugar that is foundin starch.
ammonium ion
18
anabolism /ă-nab-ŏ-liz-ăm/ All themetabolic reactions that synthesize com-plex molecules from more simple mole-cules. See also metabolism.
anaerobic /an-air-oh-bik/ Describing abiochemical process that takes place in theabsence of free oxygen. Compare aerobic.
analysis The process of determining theconstituents or components of a sample.There are two broad major classes ofanalysis, qualitative analysis – essentiallyanswering the question ‘what is it?’ – andquantitative analysis – answering the ques-tion ‘how much of such and such a compo-nent is present?’ There is a vast number ofanalytical methods which can be applied,depending on the nature of the sample andthe purpose of the analysis. These includegravimetric, volumetric, and systematicqualitative analysis (classical wet meth-ods); and instrumental methods, such aschromatographic, spectroscopic, nuclear,fluorescence, and polarographic tech-niques.
Andrews’ experiment An investigation(1861) into the relationship between pres-sure and volume for a mass of carbon di-oxide at constant temperature. Theresulting isothermals showed clearly theexistence of a critical point and led togreater understanding of the liquefactionof gases. The experiment is named for theIrish physical chemist Thomas Andrews(1813–1885).
ångstrom /ang-strŏm/ Symbol: Å A unitof length defined as 10–10 meter. Theångstrom is sometimes still used for ex-pressing wavelengths of light or ultravioletradiation or for the sizes of molecules, al-though the nanometer is preferred. Theunit is named for the Swedish physicist An-ders Jonas Ångstrom (1814–74).
anhydride /an-hÿ-drÿd/ A compoundformed by removing water from an acid or,less commonly, a base. Many non-metaloxides are anhydrides of acids: for exampleCO2 is the anhydride of H2CO3 and SO3 isthe anhydride of H2SO4. Organic anhy-drides are formed by removing H2O fromtwo carboxylic-acid groups, giving com-pounds with the functional group–CO.O.CO–. These form a class of organiccompounds called ACID ANHYDRIDES (oracyl anhydrides).
anhydrite /an-hÿ-drÿt/ See calcium sul-fate.
anhydrous /an-hÿ-drŭs/ Describing asubstance that lacks moisture, or a saltwith no water of crystallization. For exam-ple, on strong heating, blue crystals of cop-per(II) sulfate pentahydrate, CuSO4.5H2O,form white anhydrous copper(II) sulfate,CuSO4.
aniline /an-ă-lin, -lÿn/ (aminobenzene;phenylamine; C6H5NH2) A colorless oilysubstance made by reducing nitrobenzene(C6H5NO2). Aniline is used for making
19
aniline
OC
H3C
HO
OC
HO
H3C
ethanoic acid
OC
H3C
O
OC
H3C
ethanoic anhydride
Anhydride
dyes, pharmaceuticals, and other organiccompounds.
anion /an-ÿ-ŏn, -on/ A negativelycharged ion, formed by addition of elec-trons to atoms or molecules. In electrolysisanions are attracted to the positive elec-trode (the anode). Compare cation.
anionic detergent /an-ÿ-on-ik/ See de-tergent.
anionic resin An ION-EXCHANGE mater-ial that can exchange anions, such as Cl–
and OH–, for anions in the surroundingmedium. Such resins are used for a widerange of analytical and purification pur-poses.
They are often produced by addition ofa quaternary ammonium group(–N(CH3)3
+) or a phenolic group (–OH–)to a stable polyphenylethene resin. A typi-cal exchange reaction is:
resin–N(CH3)3+Cl– + KOH ˆ
resin–N(CH3)3+OH– + KCl
Anionic resins can be used to separatemixtures of halide ions. Such mixtures canbe attached to the resin and recovered sep-arately by elution.
anisotropic /an-ÿ-sŏ-trop-ik/ A term de-scriptive of certain substances which haveone or more physical properties that differaccording to direction. Most crystals areanisotropic.
annealing /ă-neel-ing/ A type of heattreatment applied to metals to change theirphysical properties. The metal is heated to,and held at, an appropriate temperaturebefore being cooled at a suitable rate toproduce the desired grain structure. An-nealing is most commonly used to removethe stresses that have arisen during rolling,to increase the softness of the metal, and tomake it easier to machine. Objects made ofglass can also be annealed to removestrains.
annulene /an-yŭ-leen/ A ring compoundcontaining alternating double and singleC–C bonds. The compound C8H8, havingan 8-membered ring of carbon atoms, is
the first annulene larger than benzene. It isnot an AROMATIC COMPOUND because it isnot planar and does not obey the Hückelrule. C8H8 is called cyclo-octatetrane.Higher annulenes are designated by thenumber of carbon atoms in the ring. [10]-annulene obeys the Hückel rule but is notaromatic because it is not planar as a resultof interactions of the hydrogen atoms in-side the ring. There is some evidence that[18]-annulene, which is a stable red solid,has aromatic properties.
anode /an-ohd/ In electrolysis, the elec-trode that is at a positive potential with re-spect to the cathode. In any electricalsystem, such as a discharge tube or elec-tronic device, the anode is the terminal atwhich electrons flow out of the system.
anode sludge See electrolytic refining.
anodizing /an-ŏ-dÿz-ing/ An industrialprocess for protecting aluminum with anoxide layer formed in an electrolytic cellcontaining an oxidizing acid (e.g. sulfu-ric(VI) acid). The layer of Al2O3 is porousand can be colored with certain dyes.
anomer /an-ŏ-mer/ Either of two iso-meric forms of a cyclic form of a sugar thatdiffer in the disposition of the –OH groupon the carbon next to the O atom of thering (the anomeric carbon). Anomers arediastereoisomers. They are designated α–or β– according to whether the –OH isabove or below the ring respectively.
anthracene /an-thră-seen/ (C14H10) Awhite crystalline solid used extensively inthe manufacture of dyes. Anthracene isfound in the heavy- and green-oil fractionsof crude oil and is obtained by fractionalcrystallization. Its structure is benzene-like,having three six-membered rings fused to-
anion
20
Anthracene
gether. The reactions are characteristic ofAROMATIC COMPOUNDS.
anthracite /an-thră-sÿt/ The highestgrade of coal, with a carbon content of be-tween 92% and 98%. It burns with a hotblue flame, gives off little smoke and leaveshardly any ash.
anthraquinone /an-thră-kwi-nohn, an-thră-kwin-ohn/ (C6H4(CO)2C6H4) A col-orless crystalline quinone used inproducing dyestuffs such as alizarin.
antibonding orbital See orbital.
anti-isomer /an-tee-ÿ-sŏ-mer/ See iso-merism.
antiknock agent /an-tee-nok/ A sub-stance added to gasoline to inhibit preigni-tion or ‘knocking’. A common example islead tetraethyl.
antimonic /an-tă-mon-ik/ Designatingan antimony(IV) compound.
antimonous /an-tă-moh-nŭs/ Designat-ing an antimony(III) compound.
antimony /an-tă-moh-nee/ A metalloidelement existing in three allotropic forms;the most stable is a brittle silvery metal.Antimony belongs to group 15 (formerlyVB) of the periodic table. It is found inmany minerals, principally stibnite (Sb2S3).It is used in alloys – small amounts of anti-mony can harden other metals. It is alsoused in semiconductor devices.
Symbol: Sb; m.p. 630.74°C; b.p.1635°C; r.d. 6.691; p.n. 51; r.a.m. 112.74.
antimony(III) chloride (antimonytrichloride; SbCl3) A white deliquescentsolid, formerly known as butter of anti-mony. It is prepared by direct combinationof antimony and chlorine. It is readily hy-drolyzed by cold water to form a white pre-cipitate of antimony(III) chloride oxide(antimonyl chloride, SbOCl):
SbCl3 + H2O = SbOCl + 2HCl
antimony(III) chloride oxide See anti-mony(III) chloride.
antimonyl chloride /an-tă-mŏ-nil, an-tim-ŏ-nil/ See antimony(III) chloride.
antimony(III) oxide (antimony trioxide;Sb2O3) A white insoluble solid. It is anamphoteric oxide with a strong tendencyto act as a base. It can be prepared by di-rect oxidation by air, oxygen, or steam andis formed when antimony(III) chloride ishydrolyzed by excess boiling water.
antimony(V) oxide (antimony pentox-ide; Sb2O5) A yellow solid. It is usuallyformed by the action of concentrated nitricacid on antimony or by the hydrolysis ofantimony(V) chloride. Although an acidicoxide, it is only slightly soluble in water.
antimony pentoxide /pen-toks-ÿd/ Seeantimony(V) oxide.
antimony trichloride /trÿ-klor-ÿd, -kloh-rÿd/ See antimony(III) chloride.
antimony trioxide /trÿ-oks-ÿd/ See an-timony(III) oxide.
antioxidant /an-tee-oks-ă-dănt/ A sub-stance that inhibits oxidation. Antioxi-dants are added to such products as foods,paints, plastics, and rubber to delay theiroxidation by atmospheric oxygen. Somework by forming chelates with metal ions,thus neutralizing the catalytic effect of theions in the oxidation process. Other typesremove intermediate oxygen free radicals.Naturally occurring antioxidants can limittissue or cell damage in the body. These in-clude vitamin E and β-carotene.
antiparallel spins /an-tee-pa-ră-lel/ Spinsof two neighboring particles in which themagnetic moments associated with the spinare aligned in opposite directions.
apatite /ap-ă-tÿt/ A naturally occurringphosphate of calcium, CaF2.Ca3(PO4)3.
aprotic /ă-prot-ik, -proh-tik/ See solvent.
21
aprotic
aqua fortis /a-kwă for-tis/ An old namefor nitric acid, HNO3.
aqua regia /ree-jee-ă/ A mixture of con-centrated nitric acid and three to four partsof hydrochloric acid. It dissolves all metalsincluding gold, hence the name. The mix-ture contains chlorine and NOCl (nitrosylchloride).
aqueous /ay-kwee-ŭs, ak-wee-/ Describ-ing a solution in water.
aragonite /ă-rag-ŏ-nÿt, a-ră-gŏ-nÿt/ Ananhydrous mineral form of calcium car-bonate, CaCO3, which occurs associatedwith limestone and in some metamorphicrocks. It is also the main ingredient ofpearls. It is not as stable as calcite, intowhich it may change over time.
arene /ă-reen/ An organic compoundcontaining a benzene ring; i.e. an aromatichydrocarbon or derivative.
argentic oxide /ar-jen-tik/ See silver(II)oxide.
argentous oxide /ar-jen-tŭs/ See silver(I)oxide.
arginine /ar-jă-nÿn/ See amino acids.
argon /ar-gon/ An inert colorless odor-less monatomic element of the rare-gasgroup. It forms 0.93% by volume of air.Argon is used to provide an inert atmos-phere in electric and fluorescent lights, inwelding, and in extracting titanium and sil-icon. The element forms no known com-pounds.
Symbol: Ar; m.p. –189.37°C; b.p.–185.86°C; d. 1.784 kg m–3 (0°C); p.n. 18;r.a.m. 39.95.
aromatic compound An organic com-pound containing benzene rings in itsstructure. Aromatic compounds, such asbenzene, have a planar ring of atoms linkedby alternate single and double bonds. Thecharacteristic of aromatic compounds isthat their chemical properties are not thoseexpected for an unsaturated compound;
they tend to undergo nucleophilic substitu-tion of hydrogen (or other groups) on thering, and addition reactions only occurunder special circumstances.
The explanation of this behavior is thatthe electrons in the double bonds are delo-calized over the ring, so that the six bondsare actually all identical and intermediatebetween single bonds and double bonds.The pi electrons are thus spread in a mole-cular orbital above and below the ring. Theevidence for this delocalization in benzeneis that: The bond lengths between carbonatoms in benzene are all equal and inter-mediate between single and double bondlengths. Also, if two hydrogen atoms at-tached to adjacent carbon atoms are sub-stituted by other groups, the compoundhas only one structure. If the bonds weredifferent two isomers would exist. Benzenehas a stabilization energy of 150 kJ mol–1
over the Kekulé structure.The delocalization of the electrons in
the pi orbitals of benzene accounts for theproperties of benzene and its derivatives,which differ from the properties of alkenesand other aliphatic compounds. The phe-nomenon is called aromaticity. A defini-tion of aromaticity is that it occurs incompounds that obey the Hückel rule: i.e.that there should be a planar ring with atotal of (4n + 2) pi electrons (where n is anyinteger). Using this rule as a criterion cer-tain non-benzene rings show aromaticity.Such compounds are called nonbenzenoidaromatics. Other compounds that have aring of atoms with alternate double andsingle bonds, but do not obey the rule (e.g.cyclooctotetraene, which has a non-planarring of alternating double and singlebonds) are called pseudoaromatics. Therule is named for the German chemist ErichArmand Arthur Joseph Hückel (1896–1980).
Compare aliphatic compound. See alsoresonance.
aromaticity /ă-roh-mă-tis-ă-tee/ Seearomatic compound.
Arrhenius equation /ah-ren-ee-ŭs/ Anequation relating the rate constant of a
aqua fortis
22
chemical reaction and the temperature atwhich the reaction is taking place:
k = Aexp(–Ea/RT)where A is a constant, k the rate constant,T the thermodynamic temperature inkelvins, R the gas constant, and Ea the ac-tivation energy of the reaction.
Reactions proceed at different rates atdifferent temperatures, i.e. the magnitudeof the rate constant is temperature depen-dent. The Arrhenius equation is often writ-ten in a logarithmic form, i.e.
logek = logeA – E/2.3RTThis equation enables the activation en-
ergy for a reaction to be determined. Theequation is named for the Swedish physicalchemist Svante August Arrhenius(1859–1927).
arsenate(III) /ar-sĕ-nayt/ (arsenite) Asalt of the hypothetical arsenic(III) acid,formed by reacting arsenic(III) oxide withalkalis. Arsenate(III) salts contain the ionAsO3
3–. Copper arsenate(III) is used as aninsecticide.
arsenate(V) A salt of arsenic(V) acid,made by reacting arsenic(III) oxide, As2O3,with nitric acid. Arsenate(V) salts containthe ion AsO4
3–. Disodiumhydrogenarsen-ate(V) is used in printing calico.
arsenic /ar-sĕ-nik, ars-nik; adj. ars-sen-ik/ A toxic metalloid element existing inseveral allotropic forms; the most stable isa brittle gray metal. It belongs to group 15(formerly VA) of the periodic table. Ar-senic is found native and in several ores in-cluding mispickel (FeSAs), realgar (As4S4),and orpiment (As2S3). The element reactswith hot acids and molten sodium hydrox-ide but is unaffected by water and acidsand alkalis at normal temperatures. It isused in semiconductor devices, alloys, andgun shot. Various compounds are used inmedicines and agricultural insecticides andpoisons.
Symbol: As; m.p. 817°C (gray) at 3MPa pressure; sublimes at 616°C (gray);r.d. 5.78 (gray at 20°C); p.n. 33; r.a.m.74.92159.
arsenic(III) chloride (arsenious chloride;
AsCl3) A poisonous oily liquid. It fumesin moist air due to hydrolysis with watervapor:
AsCl3 + 3H2O = As2O3 + 6HClArsenic(III) chloride is covalent and ex-
hibits nonmetallic properties.
arsenic hydride See arsine.
arsenic(III) oxide (white arsenic; arse-nious oxide; As2O3) A colorless crys-talline solid that is very poisonous (0.1 gwould be a lethal dose). Analysis of thesolid and vapor states suggests a dimerizedstructure of As4O6. An amphoteric oxide,arsenic(III) oxide is sparingly soluble inwater, producing an acidic solution. It isformed when arsenic is burned in air oroxygen.
arsenic(V) oxide (arsenic oxide;As2O5) A white amorphous deliquescentsolid. It is an acidic oxide prepared by dis-solving arsenic(III) oxide in hot concen-trated nitric acid, followed bycrystallization then heating to 210°C.
arsenide /ar-sĕ-nÿd/ A compound of ar-senic and another metal. For example, withiron arsenic forms iron(III) arsenide,FeAs2, and gallium arsenide, GaAs, is animportant semiconductor.
arsenious chloride /ar-sen-ee-ŭs/ Seearsenic(III) chloride.
arsenious oxide See arsenic(III) oxide.
arsenite /ar-sĕ-nÿt/ See arsenate(III).
arsine /ar-seen, ar-seen, -sin/ (arsenic hy-dride; AsH3) A poisonous colorless gaswith an unpleasant smell. It decomposes toarsenic and hydrogen at 230°C. It is pro-duced in the analysis for arsenic (Marsh’stest).
artificial radioactivity Radioactivityinduced by bombarding stable nuclei withhigh-energy particles. For example:
1237Al + 0
1n → 12
14Na + 2
4Herepresents the bombardment of aluminumwith neutrons to produce an isotope of
23
artificial radioactivity
sodium. All the transuranic elements(atomic numbers 93 and above) are artifi-cially radioactive since they do not occur innature.
aryl group /a-răl/ An organic group de-rived by removing a hydrogen atom froman aromatic hydrocarbon or derivative.
asbestos /ass-best-ŏs/ A fibrous varietyof various rock-forming silicate minerals,such as the amphiboles and chrysotile. Ithas many uses that employ its properties ofheat-resistance and chemical inertness.Prolonged exposure to asbestos dust maycause asbestosis – a form of lung cancer.
asparagine /ă-spa-ră-jeen, -jin/ Seeamino acids.
aspartic acid /ă-spar-tik/ See aminoacids.
aspirator An apparatus for sucking agas or liquid from a vessel or body cavity.
aspirin (acetylsalicylic acid; C9H8O4) Acolorless crystalline compound made bytreating salicylic acid with ethanoyl hy-dride. It is used as an analgesic and an-tipyretic drug, and small doses areprescribed for patients at risk of heart at-tack or stroke. It should not be given toyoung children.
association The combination of mole-cules of a substance with those of anotherto form more complex species. An exampleis a mixture of water and ethanol (whichare termed associated liquids), the mole-cules of which combine via hydrogenbonding.
astatine /ass-tă-teen, -tin/ A radioactiveelement belonging to the halogen group. Itoccurs in minute quantities in uraniumores. Many short-lived radioisotopes areknown, all alpha-particle emitters.
Symbol: At; m.p. 302°C (est.); b.p.337°C (est.); p.n. 85; most stable isotope210At (half-life 8.1 hours).
asymmetric atom See chirality; iso-merism; optical activity.
atactic polymer See polymerization.
atmolysis /at-mol-ă-sis/ The separationof gases by using their different rates of dif-fusion.
atmosphere A unit of pressure definedas 101 325 pascals (atmospheric pressure).The atmosphere is used in chemistry onlyfor rough values of pressure; in particular,for stating the pressures of high-pressureindustrial processes.
atom The smallest part of an elementthat can exist as a stable entity. Atoms con-sist of a small dense positively charged nu-cleus, made up of neutrons and protons,with electrons in a cloud around this nu-cleus. The chemical reactions of an elementare determined by the number of electrons(which is equal to the number of protons inthe nucleus). All atoms of a given elementhave the same number of protons (the pro-ton number). A given element may havetwo or more isotopes, which differ in thenumber of neutrons in the nucleus.
The electrons surrounding the nucleusare grouped into shells – i.e. main orbitsaround the nucleus. Within these main or-bits there may be sub-shells. These corre-spond to atomic orbitals. An electron in anatom is specified by four quantum num-bers:1. The principal quantum number (n),
which specifies the main energy levels. ncan have values 1, 2, etc. The corre-sponding shells are denoted by letters K,L, M, etc., the K shell (n = 1) being thenearest to the nucleus. The maximumnumber of electrons in a given shell is2n2.
2. The orbital quantum number (l), whichspecifies the angular momentum. For agiven value of n, 1 can have possible val-ues of n–1, n–2, … 2, 1, 0. For instance,the M shell (n = 3) has three sub-shellswith different values of l (0, 1, and 2).Sub-shells with angular momentum 0, 1,2, and 3 are designated by letters s, p, d,and f.
aryl group
24
3. The magnetic quantum number (m).This can have values –l, –(l – 1) … 0 …+ (l + l), + 1. It determines the orienta-tion of the electron orbital in a magneticfield.
4. The spin quantum number (ms), whichspecifies the intrinsic angular momen-tum of the electron. It can have values+½ and –½.Each electron in the atom has four
quantum numbers and, according to thePauli exclusion principle, no two electronscan have the same set of quantum num-bers. This explains the electronic structureof atoms. See also Bohr theory.
atomic absorption spectroscopy (AAS)A technique in chemical analysis in whicha sample is vaporized and an absorptionspectrum is taken of the vapor. The ele-ments present are identified by their char-acteristic absorption lines.
atomic emission spectroscopy (AES)A technique in chemical analysis thatinvolves vaporizing a sample of material at high temperature. Atoms in excitedstates decay to the ground state, emittingelectromagnetic radiation at particular fre-quencies characteristic of that type ofatom.
atomic force microscope (AFM) Aninstrument used to investigate surfaces. Asmall probe consisting of a very small chipof diamond is held just above a surface ofa sample by a spring-loaded cantilever. Asthe probe is slowly moved over the surfacethe force between the surface and the tip ismeasured and the probe is automaticallyraised and lowered to keep this force con-stant. Scanning the surface in this way en-ables a contour map of the surface to begenerated with the help of a computer. Anatomic force microscope closely resemblesa SCANNING TUNNELLING MICROSCOPE (STM)in some ways, although it uses forces ratherthan electrical signals to investigate thesurface. Like a STM, it can resolve individ-ual molecules. Unlike a STM, it can be usedto investigate nonconducting materials, afeature that is useful in investigating bio-logical samples.
atomic heat See Dulong and Petit’s law.
atomicity /at-ŏ-mis-ă-tee/ The numberof atoms per molecule of an element. He-lium, for example, has an atomicity of one,nitrogen two, and ozone three.
atomic mass unit (amu; dalton) Symbol:u A unit of mass used for atoms and mol-ecules, equal to 1/12 of the mass of anatom of carbon-12. It is equal to 1.660 33× 10–27 kg.
atomic number See proton number.
atomic orbital See orbital.
atomic weight See relative atomic mass(r.a.m.).
ATP (adenosine triphosphate) The uni-versal energy carrier of living cells. Energyfrom respiration or, in photosynthesis,from sunlight is used to make ATP fromADP. It is then reconverted to ADP in var-ious parts of the cell by enzymes known asATPases, the energy released being used todrive three main cellular processes: me-chanical work (muscle contraction and cel-lular movement); the active transport ofmolecules and ions; and the biosynthesis ofbiomolecules. It can also be converted tolight, electricity, and heat.
ATP is a NUCLEOTIDE consisting of ade-nine and ribose with three phosphategroups attached. Hydrolysis of the termi-nal phosphate bond releases energy(30.6 kJ mol–1) and is coupled to an en-ergy-requiring process. Further hydrolysisof ADP to AMP sometimes occurs, releas-ing more energy.
atto- Symbol: a A prefix denoting 10–18.For example, 1 attometer (am) = 10–18
meter (m).
Aufbau principle /owf-bow/ A principlethat governs the order in which the atomicorbitals are filled in elements of successiveproton number; i.e. a statement of theorder of increasing energy. The order is asfollows:
25
Aufbau principle
1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2,4d10, 5p6, 6s2, 4f14, 5d10, 6p6, 7s2, 5f14,6d10
(the superscript indicates the maximumnumber of electrons for each level).Note1. Hund’s rule of maximum multiplicity
applies; i.e. degenerate orbitals are occu-pied singly before spin pairing occurs.
2. the unexpected position of the d-levels,which give rise to the first, second, andthird transition series.
3. the unusual position of the f-levels, giv-ing rise to the lanthanoids and actinoids.‘Aufbau’ is a German word meaning
‘building up’.
Auger effect /oh-zhay/ An effect inwhich an excited ion decays by emission ofan electron (rather than a photon). For ex-ample, if a substance is bombarded byhigh-energy electrons or gamma rays, anelectron from an inner shell may be ejected.The result is a positive ion in an excitedstate. This ion will decay to its ground stateby a transition of an outer electron to aninner shell. The energy released in the tran-sition may result in the emission of a pho-ton in the x-ray region of theelectromagnetic spectrum (this is x-ray flu-orescence). Alternatively, the energy maybe released in the form of a second electronejected from the atom to give a doublycharged ion. The emitted electron (knownas an Auger electron) has a characteristicenergy corresponding to the difference inenergy levels in the ion. The Auger effect isa form of autoionization. The effect isnamed for the French physicist PierreAuger (1899–93).
auric /ô-rik/ Designating a compound ofgold(III).
auric chloride /or-ik/ See gold(III) chlo-ride.
aurous /ôr-ŭs/ Designating a compoundof gold(I).
autocatalysis /aw-toh-kă-tal-ă-sis/ Seecatalyst.
autoclave /aw-tŏ-klayv/ An apparatusconsisting of an airtight container whosecontents are heated by high-pressuresteam; the contents may also be agitated.Autoclaves are used for reactions betweengases under pressure in industrial process-ing and for sterilizing objects.
autoionization /aw-toh-ÿ-ŏ-ni-zay-shŏn/ The spontaneous ionization of ex-cited atoms, ions or molecules as in theAuger effect
Avogadro constant /ah-vŏ-gah-droh/(Avogrado’s number) Symbol: NA Thenumber of particles in one mole of a sub-stance. Its value is 6.022 52 × 1023 mol–1.The constant is named for the Italian physi-cist and chemist Lorenzo Romano AmedeoCarlo Avogadro, Count of Quaregna andCerreto (1776–1856).
Avogadro’s law The principle thatequal volumes of all gases at the same tem-perature and pressure contain equal num-bers of molecules. It is often calledAvogadro’s hypothesis. It is strictly trueonly for ideal gases.
Avogadro’s number See Avogadroconstant.
azeotrope /ă-zee-ŏ-trop-ik/ (azeotropicmixture) A mixture of liquids for whichthe vapor phase has the same compositionas the liquid phase. It therefore boils with-out change in composition and conse-quently without progressive change inboiling point.
The composition and boiling points ofazeotropes vary with pressure, indicatingthat they are not chemical compounds.Azeotropes may be broken by distillationin the presence of a third liquid, by chemi-cal reactions, adsorption, or fractionalcrystallization. See constant-boiling mix-ture.
azeotropic distillation A method usedto separate mixtures of liquids that cannotbe separated by simple distillation. Such amixture is called an azeotrope. A solvent isadded to form a new azeotrope with one of
Auger effect
26
the components, and this is then removedand subsequently separated in a secondcolumn. An example of the use ofazeotropic distillation is the dehydration of96% ethanol to absolute ethanol.Azeotropic distillation is not widely usedbecause of the difficulty of finding inex-pensive non-toxic non-corrosive solventsthat can easily be removed from the newazeotrope.
azeotropic mixture See azeotrope.
azide /az-ÿd, -id, ay-zÿd/ 1. An inorganiccompound containing the ion N3
–.2. An organic compound of general for-mula RN3.
azine /az-een, -in/ An organic hetero-cyclic compound that has a hexagonal ringcontaining carbon and nitrogen atoms.Pyridine (C5H5N) is the simplest example.
azo compound /az-oh, ay-zoh/ A typeof organic compound of the general for-
mula RN:NR′, where R and R′ are aro-matic groups. Azo compounds can beformed by coupling a diazonium com-pound with an aromatic phenol or amine.Most are colored because of the presenceof the azo group –N:N–.
azo dye An important type of dye usedin acid dyes for wool and cotton. The dyesare azo compounds; usually sodium saltsof sulfonic acids.
azo group See azo compound.
azoimide /az-oh-im-ÿd, -id, ay-zoh-/ Seehydrazoic acid.
azulene /az-yŭ-leen/ (C10H8) A bluecrystalline compound having a seven-mem-bered ring fused to a five-membered ring. Itconverts to naphthalene on heating.
azurite /azh-ŭ-rÿt/ See copper(II) car-bonate.
27
azurite
NNN NNN OH
Azo compound
28
Babo’s law /bah-bohz/ The principlethat if a substance is dissolved in a liquid(solvent) the vapor pressure of the liquid isreduced; the amount of lowering is propor-tional to the amount of solute dissolved.See also Raoult’s law. The law is named forthe German chemist Lambert HeinrichClemens von Babo (1818–99).
back e.m.f. An e.m.f. that opposes thenormal flow of electric charge in a circuitor circuit element. In some electrolytic cellsa back e.m.f. is caused by the layer of hy-drogen bubbles that builds up on the cath-ode as hydrogen ions pick up electrons andform gas molecules (i.e. as a result of po-larization of the electrode).
Bakelite /bay-kĕ-lÿt/ (Trademark) Acommon thermosetting synthetic polymerformed by the condensation of phenol andmethanal.
baking powder A mixture of sodiumhydrogencarbonate (sodium bicarbonate,baking soda) and a weak acid such as tar-taric acid. The addition of moisture orheating causes a reaction that producesbubbles of carbon dioxide gas, which makedough or cake mixture rise.
baking soda See sodium hydrogencar-bonate.
ball mill A device commonly used in thechemical industry for reducing the size ofsolid material. Ball mills usually haveslowly rotating steel-lined drums, whichcontain steel balls. The material is crushedby the tumbling action of the contents ofthe drum. Compare hammer mill.
Balmer series /bahl-mer/ A series of
lines in the spectrum of radiation emittedby excited hydrogen atoms. The lines cor-respond to the atomic electrons falling intothe second lowest energy level, emitting en-ergy as radiation. The wavelengths (λ) ofthe radiation in the Balmer series are givenby:
1/λ = R(1/22 – 1/n2)where n is an integer and R is the Rydbergconstant. The series is named for the Swissmathematician Johann Jakob Balmer(1825–98). See Bohr theory. See also spec-tral series.
banana bond (bent bond) 1. Instrained-ring compounds the angles as-sumed on the basis of hybridization areoften not equal to the angles obtained byjoining the atomic centers. In these cases itis sometimes assumed that the bonding or-bital is bent or banana-like in shape. Cy-clopropane is an example, in whichgeometric considerations imply a bondangle of 60° while sp3 hybridization im-plies an inter-orbital angle of around 100°,giving a banana bond.2. A multicentre bond of the type present insuch compounds as diborane (B2H6).
band spectrum A spectrum that ap-pears as a number of bands of emitted or
B
H2C
H2C CH2
Banana bond
absorbed radiation. Band spectra are char-acteristic of molecules. Often each bandcan be resolved into a number of closelyspaced lines. The bands correspond tochanges of electron orbit in the molecules.The close lines seen under higher resolu-tion are the result of different vibrationalstates of the molecule. See also spectrum.
bar A unit of pressure defined as 105
pascals. The millibar (mb) is more com-mon; it is used for measuring atmosphericpressure in meteorology.
Barft process A process formerly usedfor protecting iron from corrosion by heat-ing it in steam, to form a layer of tri-irontetroxide (Fe3O4).
barites /bă-rÿ-teez/ See barium sulfate.
barium /bair-ee-ŭm/ A dense, low-melt-ing reactive metal; the fifth member ofgroup 2 (formerly IIA) of the periodic tableand a typical alkaline-earth element. Theelectronic configuration is that of xenonwith two additional outer 6s electrons.Barium is of low abundance; it is found aswitherite (BaCO3) and barytes (BaSO4).The metal is obtained by the electrolysis ofthe fused chloride using a cooled cathodewhich is slowly withdrawn from the melt.Because of its low melting point barium isreadily purified by vacuum distillation.Barium metal is used as a ‘getter’, i.e., acompound added to a system to seek outthe last traces of oxygen; and as an alloyconstituent for certain bearing metals.
Barium has a low ionization potentialand a large radius. It is therefore stronglyelectropositive and its properties, andthose of its compounds, are very similar tothose of the other alkaline-earth elementscalcium and strontium. Notable differ-ences in the chemistry of barium from therest of the group are:1. The much higher stability of the carbon-
ate.2. The formation of the peroxide below
800°C. Barium peroxide decomposes onstrong heating to give oxygen and bar-ium oxide:
BaO2 ˆ BaO + O
This equilibrium was the basis of the to-tally obsolete Brin process for the commer-cial production of oxygen.
Barium is also notable for the very lowsolubility of the sulfate, which permits itsapplication to gravimetric analysis for ei-ther barium or sulfate. Barium compoundsgive a characteristic green color to flameswhich is used in qualitative analysis. Bar-ium salts are all highly toxic with the ex-ception of the most insoluble materials.Metallic barium has the body-centeredcubic structure.
Symbol: Ba; m.p. 729°C; b.p. 1640°C;r.d. 3.594 (20°C); p.n. 56; r.a.m. 137.327.
barium bicarbonate See barium hydro-gencarbonate.
barium carbonate (BaCO3) A white in-soluble salt that occurs naturally as themineral witherite. Barium carbonate canbe readily precipitated by adding an alkalicarbonate to a barium salt solution. Onheating it decomposes reversibly with theformation of the oxide and carbon dioxide:
BaCO3 ˆ BaO + CO2It is used as a rat poison.
barium chloride (BaCl2) A white solidthat can be prepared by dissolving bariumcarbonate in hydrochloric acid and crystal-lizing out the dihydrate (BaCl2.2H2O).Barium chloride is used as the electrolyte inthe extraction of barium, as a rat poison,and in the leather industry.
barium hydrogencarbonate (barium bi-carbonate; Ba(HCO3)2) A compoundthat occurs only in aqueous solution. It isformed by the action of cold water con-taining carbon dioxide on barium carbon-ate, to which it reverts on heating:
BaCO3 + CO2 + H2O = Ba(HCO3)2
barium hydroxide (baryta; Ba(OH)2) A white solid usually obtained as the oc-tahydrate, Ba(OH)2.8H2O. Barium hy-droxide is the most soluble of the group 2hydroxides and can be used in volumetricanalysis for the estimation of weak acidsusing phenolphthalein as an indicator.
29
barium hydroxide
barium oxide
30
barium oxide (BaO) A white powderprepared by heating barium in oxygen orby thermal decomposition of barium car-bonate. It has been used in the manufactureof lubricating-oil additives.
barium peroxide (BaO2) A dense off-white powder that can be prepared by care-fully heating barium oxide in oxygen.Barium peroxide is used for bleachingstraw and silk and in the laboratory prepa-ration of hydrogen peroxide.
barium sulfate (BaSO4) A white solidthat occurs naturally as the mineralbarytes. Barium sulfate is very insoluble inwater and can be prepared easily as a pre-cipitate by adding sulfuric acid to bariumchloride. Barium sulfate is an important in-dustrial chemical. Under the name of‘blanc fixe’ it is used as a pigment extenderin surface coating compositions. It is alsoused in the glass and rubber industries andmedically (taken orally) to allow radi-ographs to be taken of the digestive system.
barn Symbol: b A unit of area defined as10–28 square meter. The barn is sometimesused to express the effective cross-sectionsof atoms or nuclei in the scattering or ab-sorption of particles.
barrel A measurement of volume oftenused in the chemical industry. It is equal to159 liters (about 29 US gallons).
baryta /bă-rÿ-tă/ See barium hydroxide.
barytes /bă-rÿ-teez/ (heavy spar) A min-eral form of barium sulfate (BaSO4).
basalt /bă-sawlt, bass-awlt/ A dark-col-ored basic igneous rock derived from solid-ified volcanic lava. It consists mainly offine crystals of pyroxine and plagioclasefeldspar.
base A compound that releases hydroxylions, OH–, in aqueous solution. Basic solu-tions have a pH greater than 7. In theLowry-Brønsted treatment a base is a sub-stance that tends to accept a proton. ThusOH– is basic as it accepts H+ to form water,
but H2O is also a base (although somewhatweaker) because it can accept a furtherproton to form H3O+. In this treatment theions of classical mineral acids such asSO4
2– and NO3– are weak conjugate bases
of their respective acids. Electron-pairdonors, such as trimethylamine and pyri-dine, are examples of organic bases.
base-catalyzed reaction A reactioncatalyzed by bases. Typical base-catalyzedreactions are the Claisen condensation andthe aldol reaction, in which the first step isabstraction of a proton to give a carbanion.
base metal A metal such as iron, lead,or copper, distinguished from a noblemetal such as gold or silver.
base unit A unit that is defined in termsof reproducible physical quantities. Seealso SI units.
basic Having a tendency to release OH–
ions. Thus any solution in which the con-centration of OH– ions is greater than thatin pure water at the same temperature isdescribed as basic; i.e. the pH is greaterthan 7.
basic aluminum ethanoate See alu-minum ethanoate.
basic oxide An oxide of a metal that re-acts with water to form a base, or with anacid to form a salt and water. For example,calcium oxide reacts with water to formcalcium hydroxide:
CaO + H2O → Ca(OH)2and with hydrochloric acid to produce cal-cium chloride and water:
CaO + 2HCl → CaCl2 + H2OSee also acidic oxide.
basic oxygen process See Bessemerprocess.
basic salt A compound intermediate be-tween a normal salt and a hydroxide oroxide. The term is often restricted to hydroxyhalides (such as Pb(OH)Cl,Mg2(OH)3Cl2.4H2O, and Zn(OH)F) andhydroxy-oxy salts (for example, 2PbCO3.
Pb(OH)2, Cu(OH)2.CuCl2, Cu(OH)2.CuCo3). The hydroxyhalides are essen-tially closely packed assemblies of OH–
with metal or halide ions in the octahedralholes. The hydroxy-oxy salts have morecomplex structures than those implied bythe formulae.
basic slag See slag.
batch process A manufacturing processin which the reactants are fed into theprocess in fixed quantities (batches), ratherthan in a continuous flow. At any particu-lar instant all the material, from its prepa-ration to the final product, has reached adefinite stage in the process. Baking a cakeis an example of a batch process. Suchprocesses present problems of automationand instrumentation and tend to be waste-ful of energy. For this reason, batch pro-cessing is only economically operated onan industrial scale when small quantities ofvaluable or strategic materials are re-quired, e.g. drugs, titanium metal. Com-pare continuous process.
battery A number of similar units, suchas electric cells, working together. Manydry ‘batteries’ used in radios, flashlights,etc., are in fact single cells. If a number ofidentical cells are connected in series, thetotal e.m.f. of the battery is the sum of thee.m.f.s of the individual cells. If the cells arein parallel, the e.m.f. of the battery is thesame as that of one cell, but the currentdrawn from each is less (the total current issplit among the cells).
bauxite // A mineral hydrated form ofaluminum hydroxide; the principal ore ofaluminum.
b.c.c. See body-centered cubic crystal.
Beckmann rearrangement /bek-măn/ A type of reaction in which theOXIME of a ketone is converted into anamide using a sulfuric acid catalyst. Firstdiscovered by the German chemist ErnstOtto Beckmann (1853–1923), it is used inthe manufacture of polyamides.
Beckmann thermometer /bek-măn/ Atype of thermometer designed to measuredifferences in temperature rather than scaledegrees. Beckmann thermometers have alarger bulb than common thermometersand a stem with a small internal diameter,so that a range of 5°C covers about 30 cen-timeters in the stem. The mercury bulb isconnected to the stem in such a way thatthe bulk of the mercury can be separatedfrom the stem once a particular 5° rangehas been attained. The thermometer canthus be set for any particular range ofworking temperature. It is named for theGerman chemist Ernst Otto Beckmann(1853–1923).
becquerel /bek-ĕ-rel/ Symbol: Bq The SIunit of activity equal to the activity of a ra-dioactive substance that has one sponta-neous nuclear change per second; 1 Bq = 1s–1. The unit is named for the French physi-cist Antoine Henri Becquerel (1852–1908).
beneficiation /ben-ĕ-fish-ee-ay-shŏn/ Theprocess of separating an ore into a usefulcomponent and waste material (known asgangue). The process is sometimes de-scribed as ore dressing.
bent bond See banana bond.
bentonite /ben-tŏ-nÿt/ A type of claythat is used as an adsorbent in makingpaper. The gelatinous suspension it formswith water is used to bind together thesand for making iron castings. Chemicallybentonite is an aluminosilicate of variablecomposition.
bent sandwich compound See sand-wich compound.
benzaldehyde /ben-zal-dĕ-hÿd/ See ben-zenecarbaldehyde.
benzene /ben-zeen, ben-zeen/ (C6H6) Acolorless liquid hydrocarbon with a char-acteristic odor. Benzene is a highly toxiccompound and continued inhalation of thevapor is harmful. It was originally isolatedfrom coal tar and for many years this wasthe principal source of the compound.
31
benzene
Contemporary manufacture is fromhexane; petroleum vapor is passed overplatinum at 500°C and at 10 atmospherespressure:
C6H14 → C6H6 + 4H2Benzene is the simplest aromatic hydro-
carbon. It shows characteristic elec-trophilic substitution reactions, which aredifficult to explain assuming a simple un-saturated structure (such as Kekulé’s(1865) or Dewar’s (1867) formulae). Thisanomolous behavior can now be explainedby assuming that the six pi electrons are de-localized and that benzene is, therefore, aresonance hybrid. It consists of two Kekuléstructures and three Dewar structures, theformer contributing 80% and the latter20% to the hybrid. Benzene is usually rep-resented by either a Kekulé structure or ahexagon containing a circle (which denotesthe delocalized electrons).
benzenecarbaldehyde /ben-zeen-kar-bal-dĕ-hÿd/ (benzaldehyde; C6H5CHO) Ayellow organic oil with a distinct almond-like odor. Benzenecarbaldehyde undergoesthe reactions characteristic of aldehydesand may be synthesized in the laboratory
by the usual methods of aldehyde synthe-sis. It is used as a food flavoring and in themanufacture of dyes and antibiotics, andcan be readily manufactured by the chlori-nation of methylbenzene and the subse-quent hydrolysis of (dichloromethyl)benzene:
C6H5CH3 + Cl2 → C6H5CHCl2C6H5CHCl2 + 2H2O → C6H5CH(OH)2
+ 2HClC6H5CH(OH)2 → C6H5CHO + H2O
benzenecarbonyl chloride /ben-zeen-kar-bŏ-nil/ (benzoyl chloride; C6H5COCl)A liquid acyl chloride used as a benzoylat-ing agent.
benzenecarbonyl group (benzoylgroup) The group C6H5.CO–.
benzenecarboxylic acid /ben-zeen-kar-boks-il-ik/ (benzoic acid; C6H5COOH) A white crystalline carboxylic acid. It isused as a food preservative. The carboxylgroup (–COOH) directs further substitu-tion onto the benzene ring in the 3 posi-tion.
benzenecarbaldehyde
32
Kekulé structures
Dewar structures
Benzene structures
benzene-1,2-dicarboxylic acid (ph-thalic acid; C6H4(COOH)2) A whitecrystalline aromatic acid. On heating itloses water to form phthalic anhydride,which is used to make dyestuffs and poly-mers for plasticizers and plastics.
benzene-1,4-dicarboxylic acid (tereph-thalic acid; C6H4(COOH)2) A colorlesscrystalline organic acid used to produceDacron and other polyesters.
benzene-1,3-diol (resorcinol; C6H4(OH)2)A white crystalline phenol used in the man-ufacture of dyestuffs and celluloid.
benzene-1,4-diol (hydroquinone, quinol;C6H4(OH)2) A white crystalline phenolused in making dyestuffs. See also quinone.
benzene ring The cyclic hexagonalarrangement of six carbon atoms that arecharacteristic of AROMATIC COMPOUNDS,i.e. of BENZENE and its derivatives.
benzenesulfonic acid /ben-zeen-sul-fon-ik/ (C6H5SO2OH) A white crystallineacid made by sulfonation of benzene. Anyfurther substitution onto the benzene ringis directed into the 3 position.
benzfuran /benz-fyoo-ran, -fyoo-ran/(coumarone; C8H6O) A crystalline com-pound having a benzene ring fused to afuran ring.
benzil See Benzilic acid rearrangement.
benzilic acid rearrangement /ben-zil-ik/ A reaction in which benzil (1,2-diphenylethan-1,2-dione) is treated withhydroxide and then with acid to give ben-zilic acid (2-hydroxy-2,2-diphenylethanoicacid):
C6H5.CO.CO.C6H5 →(C6H5)2C(OH).COOH
The reaction, which involves migration ofa phenyl group (C6H5–) from one carbonatom to another, was the first rearrange-ment reaction to be described (by Justusvon Liebig in 1828).
benzoic acid /ben-zoh-ik/ See ben-zenecarboxylic acid.
benzole /ben-zohl/ A mixture of mainlyaromatic hydrocarbons obtained fromcoal. See also benzene.
benzopyrene /ben-zoh-pÿ-reen/ Seebenzpyrene.
benzoquinone /ben-zoh-kwi-nohn/ Seequinone.
benzoylation /ben-zoh-ă-lay-shŏn/ Seeacylation.
benzoyl chloride /ben-zoh-ăl/ See ben-zenecarbonyl chloride.
benzoyl group See benzenecarbonylgroup.
33
benzoyl group
CHCH
CH
CHCH
CH
O
COH
CHCH
CH
CHCH
O
OH
CH2CH
CH
CHCH
CH
benzyl alcohol
benzaldehyde
benzoic acid
Benzene compounds
benzyl alcohol
benzaldehyde
benzoic acid
benzpyrene /benz-pÿ-reen/ (benzopyrene;C20H12) A cyclic aromatic hydrocarbonwith a structure consisting of five fusedbenzene rings. It occurs in coal tar and to-bacco smoke and has strong carcinogenicproperties.
benzpyrrole /benz-pi-rohl/ See indole.
benzyl alcohol /ben-zăl/ See phenyl-methanol.
benzyl group The group C6H5CH2–.
benzyne /ben-zÿn/ (C6H4) A short-livedintermediate present in some reactions.The ring of six carbon atoms contains twodouble bonds and one triple bond (the sys-tematic name is 1,2-didehydrobenzene).
Bergius process /ber-gee-ŭs/ A processformerly used for making hydrocarbonfuels from coal. A powdered mixture ofcoal, heavy oil, and a catalyst was heatedwith hydrogen at high pressure. Theprocess is named for the German industrialchemist Friedrich Karl Rudolph Bergius(1884–1949).
berkelium /ber-klee-ŭm, ber-kee-lee-ŭm/ A silvery radioactive transuranic ele-ment of the actinoid series of metals, notfound naturally on Earth. Several radioiso-topes have been synthesized. The metal re-acts with oxygen, steam, and acids.
Symbol: Bk; m.p. 1050°C; p.n. 97; r.d.14.79 (20°C); most stable isotope 247Bk(half-life 1400 years).
beryl A mineral, 3BeO.Al2O3.6SiO2,used as a source of beryllium. There areseveral varieties including the gemstonesemerald (colored green by traces ofchromium oxide) and aquamarine (a blue-green color).
beryllate /bĕ-ril-ayt/ See beryllium;beryllium hydroxide.
beryllium /bĕ-ril-ee-ŭm/ A light metallicelement, similar to aluminum but some-what harder; the first element in group 2(formerly IIA) of the periodic table. It has
the electronic configuration of helium withtwo additional outer 2s electrons.
Beryllium occurs in a number of miner-als such as beryllonite (NaBePO4),chrysoberyl (Be(AlO2)2), bertrandite(4BeO.2SiO2), and beryl (3BeO.Al2O3.6SiO2). The element accounts for only0.0006% by mass of the Earth’s crust. Themetal is obtained by conversion of the oreto the sulfate at high temperature and pres-sure with concentrated sulfuric acid, thento the chloride, followed by electrolysis ofthe fused chloride. Alternatively, extrac-tion by hydrogen fluoride followed by elec-trolysis of the fused fluoride may beemployed. The metal has a much lowergeneral reactivity than lithium or other ele-ments in group 2. It is used as an antioxi-dant and hardener in some alloys, such ascopper and phosphor bronzes.
Beryllium has the highest ionization po-tential of group 2 and the smallest size.Consequently it is less electropositive andmore polarizing than other members of thegroup. Thus, Be2+ ions do not exist as suchin either solids or solutions, and even withthe most electronegative elements there ispartial covalent character in the bonds.The metal reacts directly with oxygen, ni-trogen, sulfur, and the halogens at variouselevated temperatures, to form the oxideBeO, nitride Be3N2, sulfide BeS, andhalides BeX2, all of which are covalent.Beryllium does not react directly with hy-drogen but a polymeric hydride (BeH2)ncan be prepared by reduction of (CH3)2Beusing lithium tetrahydridoaluminate.
Beryllium is amphoteric forming beryl-late species, such as [Be(OH)4]2– and[Be(OH)]3
3+. The hydroxide is only weaklybasic. The element does not form a truecarbonate; the basic beryllium carbonate,BeCO3.Be(OH)2 is formed when sodiumcarbonate is added to solutions of beryl-lium compounds.
Beryllium hydride, chloride, and di-methylberyllium form polymeric bridgedspecies but, whereas the bridging in thechloride is via an electron pair on chlorineatoms and can be regarded as an electron-pair donor bond, the bonding in the hy-dride and in the methyl compound involvestwo-electron three-centre bonds. Coordi-
benzpyrene
34
nation compounds are quite common withberyllium; some examples include[BeCl4]2–, (R2O)2BeCl2, and[Be(NH3)4]Cl2. Beryllium also forms anumber of alkyl compounds, some ofwhich can be stabilized by coordination.Beryllium is extremely toxic.
Symbol: Be; m.p. 1278±5°C; b.p.2970°C (under pressure); r.d. 1.85 (20°C);p.n. 4; r.a.m. 9.012182.
beryllium bicarbonate See berylliumhydrogencarbonate.
beryllium bronze See bronze.
beryllium carbonate (BeCO3) An un-stable solid prepared by prolonged treat-ment of a suspension of berylliumhydroxide with carbon dioxide. The result-ing solution is evaporated and filtered in anatmosphere of carbon dioxide.
beryllium chloride (BeCl2) A whitesolid obtained by passing chlorine over aheated mixture of beryllium oxide and car-bon. It is a poor conductor in the fusedstate and in the solid form consists of a co-valent polymeric structure. It is readily sol-uble in organic solvents but is hydrolyzedin the presence of water to give the hy-droxide. The anhydrous salt is used as acatalyst.
beryllium hydrogencarbonate (beryl-lium bicarbonate; Be(HCO3)2) A com-pound formed in solution by the action ofcarbon dioxide on a suspension of the car-bonate, to which it reverts on heating:
BeCO3 + CO2 + H2O = Be(HCO3)2
beryllium hydroxide (Be(OH)2) Asolid that can be precipitated from beryl-lium-containing solutions by an alkali. Inan excess of alkali, beryllium hydroxidedissolves to give a beryllate, Be(OH)4
2–. Inthis way, beryllium and aluminum hydrox-ides are similar and the reaction is an indi-cation that beryllium hydroxide isamphoteric.
beryllium oxide (BeO) A white solidformed by heating beryllium in oxygen or
by the decomposition of beryllium hydrox-ide or carbonate. Beryllium oxide is insolu-ble in water but it shows basic propertiesby dissolving in acids to form berylliumsalts:
BeO + 2H+ → Be2+ + H2OHowever, beryllium oxide also resem-
bles acidic oxides by reacting with alkalisto form beryllates:
BeO + 2OH– + H2O → Be(OH)42–
It is thus an amphoteric oxide. Beryl-lium oxide is used for the production ofberyllium and beryllium-copper refracto-ries, for high-output transistors, and forprinted circuits. The chemical properties ofberyllium oxide are similar to those of alu-minum oxide.
beryllium sulfate (BeSO4) An insolublesalt prepared by the reaction of berylliumoxide with concentrated sulfuric acid. Onheating, it breaks down to give the oxide.When heated in the presence of water, itreadily dissolves to give an acidic solution.
Bessemer process /bess-ĕ-mer/ Aprocess for making steel from pig iron. Avertical cylindrical steel vessel (converter)is used, lined with a refractory material.Air is blown through the molten iron andcarbon is oxidized and thus removed fromthe iron. Other impurities – silicon, sulfur,and phosphorus – are also oxidized. Ironscontaining large amounts of phosphorusare treated in a converter lined with a basicmaterial, so that a phosphate slag isformed. The required amount of carbon isthen added to the iron to produce the de-sired type of steel. The process is named forthe British inventor and engineer Sir HenryBessemer (1813–98).
beta particle An electron emitted by aradioactive substance as it decays.
bi- Prefix used formerly in naming acidsalts. The prefix indicates the presence ofhydrogen; for instance, sodium bisulfate(NaHSO4) is sodium hydrogensulfate, etc.
bicarbonate /bÿ-kar-bŏ-nayt,-nit/ Seehydrogencarbonate.
35
bicarbonate
bimolecular /bÿ-mŏ-lek-yŭ-ler/ De-scribing a reaction or step that involvestwo molecules, ions, etc. A common exam-ple of this type of reaction is the decompo-sition of hydrogen iodide,
2HI → H2 + I2
takes place between two molecules and istherefore a bimolecular reaction. Othercommon examples are:
H2O2 + I2 → OH– + HIOOH– + H+ → H2O
All bimolecular reactions are secondorder, but some second-order reactions arenot bimolecular.
binary compound A chemical com-pound formed from two elements; e.g.Fe2O3 or NaCl.
biochemical oxygen demand (BOD)The amount of oxygen taken from naturalwater by microorganisms that decomposeorganic waste matter in the water. It istherefore a measure of the quantity of or-ganic pollutants present. It is found bymeasuring the amount of oxygen in a sam-ple of water, keeping the sample and thenmaking the measurement again five dayslater.
biochemistry The study of chemicalcompounds and reactions occurring in liv-ing organisms.
biodegradable /bÿ-oh-di-gray-dă-băl/ Seepollution.
bioinorganic chemistry /bÿ-oh-in-or-gan-ik/ The study of biological moleculesthat contain metal atoms or ions. Many en-zymes have active metal atoms andbioinorganic compounds are important inother roles such as protein folding, oxygentransport, and electron transfer. Two im-portant examples of bioinorganic com-pounds are hemoglobin (containing iron)and chlorophyll (containing magnesium).
biosynthesis /bÿ-oh-sin-th’ĕ-sis/ The re-actions by which organisms obtain the var-ious compounds needed for life.
biphenyl /bÿ-fen-ăl, -fee-năl/ (C6H5C6H5)An organic compound having a structurein which two phenyl groups are joined by aC–C bond. See also polychlorinatedbiphenyl.
bipyridyl /bÿ-pÿ-ră-dăl/ See dipyridyl.
Birkeland–Eyde process /berk-lănd ÿ-dĕ/An industrial process for fixing nitrogen(as nitrogen monoxide) by passing airthrough an electric arc:
N2 + O2 → 2NOThe process is named for the Norwe-
gian physicist and chemist Kristian OlafBernhard Birkeland (1867–1917) and theNorwegian engineer and industrialistSamuel Eyde (1866–1940). See also nitro-gen fixation.
bismuth /biz-mŭth/ A brittle pinkishmetallic element belonging to group 15(formerly VA) of the periodic table. It oc-curs native and in the ores Bi2S3 and Bi2O3.The element does not react with oxygen orwater under normal temperatures. It canbe dissolved by concentrated nitric acid.Bismuth is widely used in alloys, especiallylow-melting alloys. The element has theproperty of expanding when it solidifies.Compounds of bismuth are used in cos-metics and medicines.
Symbol: Bi; m.p. 271.35°C; b.p.1560±5°C; r.d. 9.747 (20°C); p.n. 83;r.a.m. 208.98037.
bismuth(III) carbonate dioxide /dÿ-oks-ÿd, -id/ (bismuthyl carbonate; Bi2O2CO3)A white solid prepared by mixing solutionsof bismuth nitrate and ammonium carbon-ate. It contains the (BiO)+ ion.
bismuth(III) chloride (bismuth trichlo-ride; BiCl3) A white deliquescent solid. Itcan be prepared by direct combination ofbismuth and chlorine. Bismuth(III) chlo-ride dissolves in excess dilute hydrochloricacid to form a clear liquid, but if diluted itproduces a white precipitate ofbismuth(III) chloride oxide (bismuthylchloride, BiOCl):
BiCl3 + H2O = BiOCl + 2HClThis reaction is often used as an exam-
bimolecular
36
ple of a reversible reaction and as a confir-matory test for bismuth during quantita-tive analysis. Bismuth(V) chloride does notform.
bismuth(III) chloride oxide See bis-muth(III) chloride.
bismuth(III) nitrate oxide (bismuthyl ni-trate; BiONO3) A white insoluble solid,often referred to as bismuth subnitrate. It isprecipitated when bismuth(III) nitrate is di-luted and contains the (BiO)+ ion. Bis-muth(III) nitrate oxide is used inpharmaceutical preparations.
bismuth trichloride /trÿ-klor-ÿd, -kloh-rÿd/ See bismuth(III) chloride.
bismuthyl carbonate /biz-mŭ-thăl/ Seebismuth(III) carbonate dioxide.
bismuthyl chloride See bismuth(III)chloride.
bismuthyl compound A compoundcontaining the (BiO)+ ion or BiO grouping.
bismuthyl ion See bismuth(III) carbon-ate dioxide.
bismuthyl nitrate See bismuth(III) ni-trate oxide.
bisulfate /bÿ-sul-fayt/ See hydrogensul-fate.
bisulfite /bÿ-sul-fÿt/ See hydrogensulfite.
bisulfite addition compound See alde-hyde.
bittern /bit-ern/ The liquid that is leftafter sodium chloride has been crystallizedfrom sea water.
bitumen /bă-tew-mĕn, bich-û-mĕn/ Amixture of solid or semisolid hydrocarbonsobtained from coal, oil, etc. See tar.
bituminous coal /bă-tew-mĕ-nŭs/ Atype of second-grade coal, containing morethan 65% carbon but also quantities of
gas, coal tar, and water. It is the commontype of coal used for domestic and indus-trial purposes. See also anthracite.
biuret /bÿ-yû-ret, bÿ-yoo-ret/ (H2NCON-HCONH2) A colorless crystalline or-ganic compound made by heating UREA
(carbamide). It is used in a chemical test forPROTEINS.
bivalent /bÿ-vay-lĕnt, biv-ă-/ (divalent)Having a valence of two.
blackdamp See firedamp.
blanc fixe /blahng-feeks/ See bariumsulfate.
blast furnace A furnace for producingiron from iron(III) oxide. A mixture of theore with coke and a flux is heated by pre-heated air blown into the bottom of thefurnace. The flux is often calcium oxide(from limestone). Molten pig iron is run offfrom the bottom of the furnace.
bleach Any substance used to removecolor from materials such as cloth andpaper. All bleaches are oxidizing agents,and include chlorine, sodium chlorate(I)solution (NaClO, sodium hypochlorite),hydrogen peroxide, and sulfur(IV) oxide.Sunlight also has a bleaching effect.
bleaching powder A white solid thatcan be regarded as a mixture of calciumchlorate(I) (calcium hypochlorite), calciumchloride, and calcium hydroxide. It is pre-pared on a large scale by passing a currentof chlorine through a tilted cylinder downwhich is passed calcium hydroxide. Bleach-ing powder has been used for bleachingpaper pulps and fabrics and for sterilizingwater. Its bleaching power arises from theformation, in the presence of air containingcarbon dioxide, of the oxidizing agentchloric(I) acid (hypochlorous acid, HClO):
Ca(ClO)2.Ca(OH)2.CaCl2 + 2CO2 →2CaCO3 + CaCl2 + 2HClO
blende /blend/ A naturally occurring sul-fide ore.
37
blende
block copolymer See polymerization.
blue vitriol /vit-ree-ŏl/ Copper(II) sul-fate pentahydrate (CuSO4.5H2O).
boat conformation See conformation.
BOD See biochemical oxygen demand.
body-centered cubic crystal (b.c.c.) Acrystal structure in which the unit cell hasan atom, ion, or molecule at each corner ofa cube and at the center of the cube. In thistype of structure the coordination numberis 8. It is less close-packed than the face-centered cubic structure. The alkali metalsform crystals with body-centered cubicstructures.
bohrium /bor-ee-ŭm, boh-ree-/ A syn-thetic radioactive element first detected bybombarding a bismuth target withchromium nuclei. Only a small number ofatoms have ever been produced.
Symbol: Bh; p.n. 107; most stable iso-tope 262Bh (half life 0.1s).
Bohr magneton /bor, bohr/ See magne-ton.
Bohr theory A theory introduced in1911 to explain the spectrum of atomic hy-drogen. The model he used was that of anucleus with charge +e, orbited by an elec-tron with charge –e, moving in a circle ofradius r. If v is the velocity of the electron,the centripetal force, mv2/r is equal to theforce of electrostatic attraction, e2/4πε0r2.Using this, it can be shown that the totalenergy of the electron (kinetic and poten-tial) is –e2/8πε0r.
If the electron is considered to havewave properties, then there must be awhole number of wavelengths around theorbit, otherwise the wave would be a pro-gressive wave. For this to occur
nλ = 2πrwhere n is an integer, 1, 2, 3, 4, …. Thewavelength, λ, is h/mv, where h is thePlanck constant and mv the momentum.Thus for a given orbit:
nh/2π = mvr
This means that orbits are possible onlywhen the angular momentum (mvr) is anintegral number of units of h/2π. Angularmomentum is thus quantized. In fact, Bohrin his theory did not use the wave behaviorof the electron to derive this relationship.He assumed from the beginning that angu-lar momentum was quantized in this way.Using the above expressions it can beshown that the electron energy is given by
E = –me4/8ε02n2h2
Different values of n (1, 2, 3, etc.) cor-respond to different orbits with differentenergies; n is the principal quantum num-ber. In making a transition from an orbit n1to another orbit n2 the energy difference∆W is given by:
∆W = W1 – W2 = me4(1/n22 –
1/n12)/8ε0
2h2
This is equal to hv where v is the fre-quency of radiation emitted or absorbed.Since vλ = c, then
1/λ = me4(1/n12 – 1/n2
2)/8ε02ch3
The theory is in good agreement withexperiment in predicting the wavelengthsof lines in the hydrogen spectrum, al-though it is less successful for largerATOMS. Different values of n1 and n2 corre-spond to different spectral series, with linesgiven by the expression:
1/λ = R(1/n12 – 1/n2
2)R is the Rydberg constant. Its experi-
mental value is 1.096 78 × 107 m–1. Thevalue from Bohr theory (me4/8πε1
2ch2) is1.097 00 × 107 m–1.
The theory is named for the Danishphysicist Niels Bohr (1885–1962).
boiling The process by which a liquid isconverted into a gas or vapor by heating atits boiling point. At this temperature thevapor pressure of the liquid is equal to theexternal pressure, and bubbles of vaporcan form within the liquid.
boiling point The temperature at whichthe vapor pressure of a liquid is equal to at-mospheric pressure. This temperature is al-ways the same for a particular liquid at agiven pressure (for reference purposes usu-ally taken as standard pressure). See alsoelevation of boiling point.
block copolymer
38
boiling-point-composition diagram Adiagram for a two-component liquid sys-tem representing both the variation of theboiling point and the composition of thevapor phase as the liquid-phase composi-tion is varied. A mixture of A and B atcomposition L1 would have a boiling pointT1 and a vapor composition V1, whichwhen condensed would give a liquid at L2.L2 would boil at T2 to give vapor V2 equiv-alent to liquid of composition L3, and soon. Thus the whole process of either distil-lation or fractionation of this system willlead to progressive enrichment in compo-nent A.
For perfect solutions obeying Raoult’slaw the curves would coincide but in realcases there will be sufficient intermolecularattraction to cause deviation from this. Theseparation of the curves as well as the dif-ference in boiling points determines theperformance of fractionation columns. Seealso constant-boiling mixture.
Boltzmann constant /bohlts-măn, -mahn/Symbol: k The constant 1.380 54 J K–1,equal to the gas constant (R) divided by theAvogadro constant (NA). The constant isnamed for the Austrian theoretical physi-cist Ludwig Edward Boltzmann (1844–1906). See also degrees of freedom.
Boltzmann formula A fundamental re-sult in statistical mechanics stating that the
entropy S of a system is related to the num-ber W of distinguishable ways in which thesystem can exist by the equation: S = klnW, where k is the Boltzmann constant.This formula is a quantitative expressionof the idea that the entropy of a system is ameasure of its disorder. It was discoveredby Ludwig Boltzmann in the late 19th cen-tury in the course of his investigations intothe foundations of statistical mechanics.
bomb calorimeter A sealed insulatedcontainer, used for measuring energy re-leased during combustion of substances(e.g. foods and fuels). A known amount ofthe substance is ignited inside the calorime-ter in an atmosphere of pure oxygen, andundergoes complete combustion at con-stant volume. The resultant rise in temper-ature is related to the energy released bythe reaction. Such energy values (calorificvalues) are often quoted in joules per kilo-gram (J kg–1).
bond The interaction between atoms,molecules, or ions holding these entities to-gether. The forces giving rise to bondingmay vary from extremely weak intermole-cular forces, about 0.1 kJ mol–1, to verystrong chemical bonding, about 103 kJmol–1. See bond energy; bond length; coor-dinate bond; covalent bond; electrovalentbond; hydrogen bond; metallic bond.
bond dissociation energy See bond en-ergy.
bond energy The energy involved informing a bond. For ammonia, for in-stance, the energy of the N–H bond is onethird of the energy involved for the process
NH3 → N + 3HIt is thus one third of the heat of atomiza-tion. Strictly speaking bond enthalpyshould be used.
The bond dissociation energy is a dif-ferent quantity to the bond energy. It is theenergy required to break a particular bondin a compound, e.g.:
NH3 → NH2 + H
bond enthalpy See bond energy.
39
bond enthalpy
100%A Composition 100%B
Tem
per
atu
re
Temp
erature T1
T2
L1L2L3L4
T3 V3
V2
V1
Boiling point–composition diagram
bonding orbital See orbital.
bond length The length of a chemicalBOND, the distance between the centers ofthe nuclei of two atoms joined by a chemi-cal bond. Bond lengths may be measuredby electron or x-ray diffraction.
boracic acid /bŏ-rass-ik/ See boric acid.
boranes /bor-aynz, boh-raynz/ See boronhydrides.
borate /bor-ayt, boh-rayt/ See boron.
borax /bor-aks, boh-raks/ See disodiumtetraborate decahydrate.
borax-bead test A preliminary test inqualitative inorganic analysis that can be aguide to the presence of certain metals. Aborax bead is formed by heating a littleborax on a loop in a platinum wire. Aminute sample is introduced into the beadand the color observed in both the oxidiz-ing and reducing areas of a Bunsen-burnerflame. The color is also noted when thebead is cold.
boric acid /bor-ik, boh-rik/ (boracic acid;H3BO3) A white crystalline solid solublein water; in solution it is a very weak acid.Boric acid is used as a mild antiseptic eyelotion and was formerly used as a foodpreservative. It is used in glazes for enam-eled objects and is a constituent of Pyrexglass.
Trioxoboric(III) acid is the full system-atic name for the solid acid and it exists inthis form in its dilute solutions. However,in more concentrated solutions polymer-
ization occurs to give polydioxoboric(III)acid.
boride /bor-ÿd, boh-rÿd/ A compound ofboron, especially one with a more elec-tropositive element.
Born–Haber cycle /born hay-ber/ Acycle used in calculating the lattice energiesof solids. The steps involved are:Atomization of sodium:
Na(s) → Na(g) ∆H1Ionization of sodium:
Na(g) → Na+(g) ∆H2Atomization of chlorine:
Cl2(g) → 2Cl(g) ∆H3Ionization of chlorine:
Cl(g) + e– → Cl–(g) ∆H4Formation of solid:
Na+(g) + Cl–(g) → NaCl(s) ∆H5This last step involves the lattice energy
∆H5. The sum of all these enthalpies isequal to the heat of the reaction:
Na(s) + ½Cl2(g) → NaCl(s)The cycle is named for the German
physicist Max Born (1882–1970) and theGerman physical chemist Fritz Haber(1868–1934). See also Hess’s law.
borohydride ions /bor-oh-hÿ-drÿd, boh-roh-/ See boron hydride.
boron /bor-on, boh-ron/ A hard ratherbrittle metalloid element of group 3 (for-merly IIIA) of the periodic table. It has theelectronic structure 1s22s22p1. Boron is oflow abundance (0.0003%) but the naturalminerals occur in very concentrated formsas either borax (Na2B4O7.10H2O) or cole-manite (Ca2B6O11). The element is ob-tained by conversion to boric acid followed
bonding orbital
40
BORAX-BEAD COLORS (H = hot; C = cold)
Metal Oxidizing flame Reducing flamechromium green H+C green H+Ccobalt blue H+C blue H+Ccopper green H, blue C often opaqueiron brown-red H, yellow C green H+Cmanganese violet H+C colourless H+Cnickel red–brown C gray–black C
by dehydration to B2O3 then reductionwith magnesium. High-purity boron forsemiconductor applications is obtained byconversion to boron trichloride, which canbe purified by distillation, then reductionusing hydrogen. Only small quantities ofelemental boron are needed commercially;the vast majority of boron supplied by theindustry is in the form of borax or boricacid.
As boron has a small atom and has arelatively high ionization potential its com-pounds are predominantly covalent; theion B3+ does not exist. Boron does not reactdirectly with hydrogen to form boron hy-drides (boranes) but the hydrolysis of mag-nesium boride does produce a range ofboranes such as B4H10, B5H9, and B6H10.Thermal decomposition of these higher bo-ranes produces, among other things, thesimplest borane, B2H6 (diborane). Thespecies BH3 is only a short-lived reactionintermediate.
Finely divided boron burns in oxygenabove 600°C to give the oxide, B2O3, anacidic oxide which will dissolve slowly inwater to give boric acid (B(OH)3) andrapidly in alkalis to give borates such asNa2B4O7. A number of polymeric specieswith B–B and B–O links are known, e.g. alower oxide (BO)x, and a polymeric acid(HBO2)n. Although the parent acid isweak, many salts containing borate anionsare known but their stoichiometry gives lit-tle indication of their structure, many ofwhich are cyclic or linear polymers. Thesecontain both BO3 planar groups and BO4tetrahedra. Boric acid and the borates givea range of glassy substances on heating;these contain cross-linked B–O–B chainsand nets. In the molten state these materi-als react with metal ions to form borates,which on cooling give characteristic colorsto the glass. See borax-bead test.
Boron reacts with nitrogen on strongheating (1000°C) to give boron nitride, aslippery white solid with a layer structuresimilar to that of graphite, i.e., hexagonalrings of alternating B and N atoms. Thematerial has an extremely high meltingpoint and is thermally very stable but thereis sufficient bond polarity in the B–N linksto permit slow hydrolysis by water to give
ammonia. There is also a ‘diamond-like’form of B–N which is claimed to be evenharder than diamond.
Elemental boron reacts directly withfluorine and chlorine but for practical pur-poses the halides are obtained via the BF3route from boric acid:
B2O3 + 3CaF2 + 3H2SO4 → 3CaSO4 +3H2O + 2BF3
BF3 + AlCl3 → AlF3 + BCl3These halides are all covalent mol-
ecules, which are all planar and trigonal inshape. Boron halides are industrially im-portant as catalysts or promoters in a vari-ety of organic reactions includingpolymerization and Friedel–Crafts typealkylations. The decomposition of boronhalides in atmospheres of hydrogen at ele-vated temperatures is also used to deposittraces of pure boron in semiconductor de-vices.
Boron forms a range of compoundswith elements that are less electronegativethan itself, called borides. Borides such asZrB2 and TiB2 are hard refractory sub-stances, which are chemically inert andhave remarkably high electrical conductiv-ities. Borides have a wide range of stoi-chiometries, from M4B through to MB6,and can exist in close-packed arrays,chains, and two-dimensional nets. Naturalboron consists of two isotopes, 10B(18.83%) and 11B (81.17%). These per-centages are sufficiently high for their de-tection by splitting of infrared absorptionor by n.m.r. spectroscopy.
Both borax and boric acid are used asmild antiseptics and are not regarded astoxic; boron hydrides are however highlytoxic.
Symbol: B; m.p. 2300°C; b.p. 2658°C;r.d. 2.34 (20°C); p.n. 5; r.a.m. 10.811.
boron hydrides (boranes) Hydrides ofboron ranging from B2H6 to B20H16, whichhave complex molecular structures. Thesecompounds are notable for containingB–H–B bonds. Investigations have shownthat the boron hydrides are electron defi-cient and thus the bonding can only be ra-tionalized in terms of MULTICENTER BONDS.The compound B2H6 can be prepared byreacting boron trichloride with hydrogen.
41
boron hydrides
A complex ion, tetrahydridoborate(III)(borohydride, BH4
–), can be formed; it hasvery strong reducing properties. Other,more complex, borohydride ions exist.
boron nitride (BN) A compoundformed by heating BORON in nitrogen(1000°C). It has two crystalline forms.
boron oxide (B2O3) A glassy hygro-scopic solid that eventually forms boricacid. It forms various salts but also exhibitssome amphoteric properties.
boron tribromide (BBr3) A colorlessliquid. See boron trichloride.
boron trichloride /trÿ-klor-ÿd, -kloh-rÿd/(BCl3) A fuming liquid made by passingdry chlorine over heated boron. It israpidly hydrolysed by water:
BCl3 + 3H2O → 3HCl + H3BO3As there are only three pairs of shared
electrons in the outer shell of the boronatom, boron halides form very stable addi-tion compounds with ammonia by the ac-ceptance of a lone electron pair in acoordinate bond to complete a sharedoctet.
boron trifluoride (BF3) A colorlessfuming gas made by heating a mixture ofboron oxide, calcium fluoride, and concen-trated sulfuric acid. See boron trichloride.
borosilicates /bor-ŏ-sil-ă-kayts/ Com-plex compounds similar to silicates, butcontaining BO3 and BO4 units in additionto the SiO4 units. Certain crystallineborosilicate minerals are known. In addi-tion, borosilicate glasses can be made byusing boron oxide in addition tosilicon(IV) oxide. These tend to be tougherand more heat resistant than ‘normal’ sili-cate glass.
Bosch process /bosh/ The reactionCO + H2O → CO2 + H2
using water gas over a hot catalyst. It hasbeen used to make hydrogen for the Haberprocess. The process is named for the Ger-man industrial chemist Carl Bosch(1874–1940).
bowl classifier A device that separatessolid particles in a mixture of solids andliquid into fractions according to particlesize. Feed enters the center of a shallowbowl, which contains revolving blades.The coarse solids collect on the bottom,fine solids at the periphery.
Boyle’s law At a constant temperature,the pressure of a fixed mass of a gas is in-versely proportional to its volume: i.e.
pV = Kwhere K is a constant. The value of K de-pends on the temperature and on the na-ture of the gas. The law holds strictly onlyfor ideal gases. Real gases follow Boyle’slaw at low pressures and high tempera-tures. The law is named for the Britishchemist and physicist Robert Boyle(1627–91). See gas laws.
Brackett series /brak-it/ See hydrogenatom spectrum.
Brady’s reagent /bray-dee/ See 2,4-dini-trophenylhydrazine.
Bragg equation /brag/ An equationused to deduce the crystal structure of amaterial using data obtained from x-raysdirected at its surface. The conditionsunder which a crystal will reflect a beam ofx-rays with maximum intensity is:
nλ = 2dsinθwhere θ is the angle of incidence and re-flection (Bragg angle) that the x-rays makewith the crystal planes, n is a small integer,λ is the wavelength of the x-rays, and d isthe distance between the crystal planes.The equation is named for the Britishphysicist Sir William Lawrence Bragg(1890–1971).
branched chain See chain.
brass Any of a group of copper–zinc al-loys containing up to 50% of zinc. Thecolor of brass changes from red-gold togolden to silvery-white with increasing zinccontent. Brasses are easy to work and resistcorrosion well. Brasses with up to 35%zinc can be worked cold and are speciallysuited for rolling into sheets, drawing into
boron nitride
42
wire, and making into tubes. Brasses with35–46% zinc are harder and stronger butless ductile; they require hot working (e.g.forging). The properties of brass can be im-proved by the addition of other elements;lead improves its ability to be machined,while aluminum and tin increase its corro-sion resistance. See also bronze; nickel–sil-ver.
bremsstrahlung /brem-shtrah-lûng/ Seex-radiation.
brine A concentrated solution of sodiumchloride.
Brin process See barium.
bromic(I) acid /broh-mik/ (hypobromousacid; HBrO) A pale yellow liquid madeby reacting mercury(II) oxide withbromine water. It has strong bleachingpowers. Bromic(I) acid will donate protonsto only a small extent and hence is a weakacid in aqueous solution. It is a strong oxi-dizing agent. See also bromine water.
bromic(V) acid (HBrO3) A colorlessliquid made by the addition of dilute sulfu-ric acid to barium bromate. It dissociatesextensively in aqueous solution and is astrong acid.
bromide /broh-mÿd/ See halide.
bromination /broh-mă-nay-shŏn/ Seehalogenation.
bromine /broh-meen, -min/ A deep red,moderately reactive element belonging tothe halogens; i.e. group 17 (formerly VIIA)of the periodic table. Bromine is a liquid atroom temperature (mercury is the onlyother element with this property). It occursin small amounts in seawater, salt lakes,and salt deposits but is much less abundantthan chlorine. Bromine reacts with mostmetals but generally with less vigor thanchlorine. It has less oxidizing power thanchlorine and consequently can be releasedfrom solutions of bromides by reactionwith chlorine gas. The laboratory methodis the more convenient oxidation by man-
ganese dioxide. Industrial methods of pro-duction utilize oxidation by chlorine orelectrolysis with removal from the solutionby purging with air. Bromine and its com-pounds are used in pharmaceuticals, pho-tography, chemical synthesis, fumigants,and in significantly large quantities as 1,2-dibromoethane (which is added to gasolineto combine with lead produced from thedecomposition of the antiknock agent leadtetraethyl).
The electropositive elements form ionicbromides and the non-metals form fullycovalent bromides. Like chlorine, bromineforms oxides, Br2O and BrO2, both ofwhich are unstable. The related oxo-acidanions hypobromite (BrO–) and bromate(BrO3
–) are formed by the reaction ofbromine with cold aqueous alkali and hotaqueous alkali respectively, but thebromine analogs of chlorite and perchlo-rate are not known.
Bromine and the interhalogens arehighly toxic. Liquid bromine and brominesolutions are also very corrosive and gog-gles and gloves should always be wornwhen handling such compounds.
Symbol: Br; m.p. –7.25°C; b.p.58.78°C; r.d. 3.12 (20°C); p.n. 35; r.a.m.79.904.
bromine trifluoride (BrF3) A colorlessfuming liquid made by direct combinationof fluorine and bromine. It is a very reac-tive compound, its reactions being similarto those of its component halides.
bromine water A yellow solution ofbromine in water, which containsbromic(I) acid (hypobromous acid, HBrO).It is a weak acid and a strong oxidizingagent, and decomposes to give a mixture ofbromide (Br–) and bromate(V) (BrO3
–)ions.
bromobutyl /broh-moh-byoo-t’l/ Seebutyl rubber.
bromoethane /broh-moh-eth-ayn/ (ethylbromide; C2H5Br) A colorless volatilecompound, used as a refrigerant. It can bemade from ethene and hydrogen bromide.
43
bromoethane
bromoform /broh-mŏ-form/ See tribro-momethane.
bromomethane /broh-moh-meth-ayn/(methyl bromide; CH3Br) A colorlessvolatile compound used as a solvent. It canbe made from methane and bromine.
Brønsted–Lowry theory /bron-sted low-ree/ See acid.
bronze Any of a group of copper-tin al-loys usually containing 0.5–10% of tin.They are generally harder, stronger in com-pression, and more corrosion resistantthan brass. Zinc is often added, as in gun-metal (2–4% zinc), to increase strengthand corrosion-resistance; bronze coinsoften contain more zinc (2.5%) than tin(0.5%). The presence of lead improves itsmachining qualities.
Some copper-rich alloys containing notin are also called bronzes. Aluminumbronzes, for example, with up to 10% alu-minum, are strong, resistant to corrosionand wear, and can be worked cold or hot;silicon bronzes, with 1–5% silicon, havehigh corrosion-resistance; berylliumbronzes, with about 2% beryllium, arevery hard and strong.
brown coal See lignite.
Brownian movement (Brownian mo-tion) The random motion of small parti-cles in a fluid – for example, smokeparticles in air. The particles, which may belarge enough to be visible with a micro-scope, move because they are continuouslybombarded by the molecules of the fluid.Brownian movement is named for the Scot-tish botanist Robert Brown (1773–1858).
brown-ring test A qualitative test usedfor the detection of nitrate. A freshly pre-pared solution of iron(II) sulfate is mixedwith the sample and concentrated sulfuricacid is introduced slowly to the bottom ofthe tube using a dropping pipette so thattwo layers are formed. A brown ringformed where the liquids meet indicatesthe presence of nitrate. The brown color is
[Fe(NO)]SO4, which breaks down onshaking.
buckminsterfullerene /buk-min-ster-fûl-ĕ-reen/ An allotrope of carbon contain-ing clusters of 60 carbon atoms bound in ahighly symmetric polyhedral structure. TheC60 polyhedron has a combination of pen-tagonal and hexagonal faces similar to thepanels on a soccer ball. The molecule wasnamed for the American architect RichardBuckminster Fuller (1895–1983) becauseits structure resembles a geodesic dome (in-vented by Fuller). The C60 polyhedra areinformally called bucky balls. The originalmethod of making the allotrope was to firea high-power laser at a graphite target.This also produces less stable carbon clus-ters, such as C70. It can be produced moreconveniently using an electric arc betweengraphite electrodes in an inert gas. The al-lotrope is soluble in benzene, from which itcan be crystallized to give yellow crystals.This form of carbon is also known as ful-lerite.
The discovery of buckminsterfullereneled to a considerable amount of researchinto its properties and compounds. Partic-ular interest has been shown in trappingmetal ions inside the carbon cage to formenclosure compounds. Buckminster-fullerene itself is often simply calledfullerene. The term also applies to deriva-tives of buckminsterfullerene and to simi-
bromoform
44
Buckminsterfullerene
lar cluster (e.g. C70). Carbon structuressimilar to that in C60 can also form smalltubes, known as bucky tubes.
bucky ball /buk-ee/ See buckminster-fullerene.
bucky tube See buckminsterfullerene.
buffer A solution in which the pH re-mains reasonably constant when acids oralkalis are added to it; i.e. it acts as a bufferagainst (small) changes in pH. Buffer solu-tions generally contain a weak acid andone of its salts derived from a strong base;e.g. a solution of ethanoic acid and sodiumethanoate. If an acid is added, the H+ reactswith the ethanoate ion (from dissociatedsodium ethanoate) to form undissociatedethanoic acid; if a base is added the OH–
reacts with the ethanoic acid to form waterand the ethanoate ion. The effectiveness ofthe buffering action is determined by theconcentrations of the acid–anion pair:
K = [H+][CH3COO–]/[CH3COOH]where K is the dissociation constant.
Phosphate, oxalate, tartrate, borate,and carbonate systems can also be used forbuffer solutions.
bumping Violent boiling of a liquidcaused when bubbles form at a pressureabove atmospheric pressure.
Bunsen burner /bun-sĕn/ A gas burnerconsisting of a vertical metal tube with anadjustable air-inlet hole at the bottom. Gasis allowed into the bottom of the tube andthe gas–air mixture is burnt at the top.With too little air the flame is yellow andsooty. Correctly adjusted, the burner givesa flame with a pale blue inner cone of in-completely burnt gas, and an almost invis-ible outer flame where the gas is fullyoxidized and reaches a temperature ofabout 1500°C. The inner region is the re-ducing part of the flame and the outer re-gion the oxidizing part (see borax-beadtest). The Bunsen burner is named for theGerman chemist Wilhelm Bunsen(1811–99). He did not invent the Bunsenburner, but used it to great effect in pio-neering work on spectroscopy.
Bunsen cell A type of primary cell inwhich the positive electrode is formed bycarbon plates in a nitric acid solution andthe negative electrode consists of zincplates in sulfuric acid solution.
buret /byû-ret/ A piece of apparatusused for the addition of variable volumesof liquid in a controlled and measurableway. The buret is a long cylindrical gradu-ated tube of uniform bore fitted with astopcock and a small-bore exit jet, en-abling a drop of liquid at a time to beadded to a reaction vessel. Burets arewidely used for titrations in volumetricanalysis. Standard burets permit volumemeasurement to 0.005 cm3 and have a totalcapacity of 50 cm3; a variety of smaller mi-croburettes is available. Similar devices areused to introduce measured volumes of gasat regulated pressure in the investigation ofgas reactions.
buta-1,3-diene /byoo-tă dÿ-een/ (butadi-ene; CH2:CHCH:CH2) A colorless gasmade by catalytic dehydrogenation of bu-tane. It is used in the manufacture of syn-thetic rubber. Buta-1,3-diene is conjugatedand to some extent the pi electrons are de-localized over the whole of the molecule.
butanal /byoo-tă-nal/ (butyraldehyde;C3H7CHO) A colorless liquid aldehyde.
butane /byoo-tayn, byoo-tayn/ (C4H10) A gaseous alkane obtained either from thegaseous fraction of crude oil or by the‘cracking’ of heavier fractions. Butane iseasily liquefied and its main use is as aportable supply of fuel (bottle gas).
Butane is the fourth member of the ho-mologous series of alkanes.
butanedioic acid /byoo-tayn-dÿ-oh-ik/(succinic acid) A crystalline carboxylicacid, HOOC(CH2)2COOH, that occurs inamber and certain plants. It forms duringthe fermentation of sugar (sucrose).
butanoic acid /byoo-tă-noh-ik/ (butyricacid; C3H7COOH) A colorless liquidcarboxylic acid. Esters of butanoic acid arepresent in butter.
45
butanoic acid
butanols /byoo-tă-nolz, -nohlz/ (butyl al-cohols; C4H9OH) Two alcohols that arederived from butane: the primary alcoholbutan-1-ol (CH3(CH2)2CH2OH) and thesecondary alcohol butan-2-ol(CH3CH(OH)CH2CH3). Both are color-less volatile liquids used as solvents.
butanone /byoo-tă-nohn/ (methyl ethylketone; CH3COC2H5) A colorlessvolatile liquid ketone. It is manufacturedby the oxidation of butane and used as asolvent.
butenedioic acid /byoo-teen-dÿ-oh-ik/ Either of two isomers. Transbutene-dioic acid (fumaric acid) is a crystallinecompound found in certain plants. Cis-butenedioic acid (maleic acid) is used in themanufacture of synthetic resins. It can beconverted into the trans isomer by heatingat 120°C.
butyl alcohol /byoo-t’l/ See butanols.
butyl group The straight chain alkylgroup CH3(CH2)2CH2–.
butyl rubber A type of synthetic rubbermade by copolymerizing isobutylene (2-methylpropene, CH3:C(CH3)2) with smallamounts of isoprene (methylbuta-1,3-diene, CH3:C(CH3)CH:CH2).
Before the introduction of tubeless tiresbutyl rubber was used for inner tubes be-cause it is impervious to air. Subsequentlyhalogenated butyl rubbers were developed(halobutyls), which could be cured athigher temperature and vulcanized withother rubbers. Both chlorobutyls and bro-mobutyls are manufactured. These types ofrubber are used in tubeless tires bonded to
the inner surface of the tire. Other uses arein sealants, hoses, and pond liners.
butyraldehyde /byoo-t’l-al-dĕ-hÿd/ Seebutanal.
butyric acid /byoo-ti-rik/ See butanoicacid.
by-product A substance obtained dur-ing the manufacture of a main chemicalproduct. For example, propanone is notnow manufactured from propan-1-ol, butis obtained as a by-product in the manu-facture of phenol by the cumene process.Calcium chloride is a by-product of theSolvay process for making sodium carbon-ate. Some metals are obtained as by-prod-ucts in processes to extract other metals.Cadmium, for instance, is a by-product ofthe extraction of zinc.
butanols
46
HO.OC H
CO.OH
C
CH
H CO.OH
CO.OHC
C
Hcis (maleic)
trans (fumaric)
Butenedioic acids
47
cadmium /kad-mee-ŭm/ A transitionmetal obtained as a by-product during theextraction of zinc. It is used to protectother metals from corrosion, as a neutronabsorber in nuclear reactors, in alkali bat-teries, and in certain pigments. It is highlytoxic.
Symbol: Cd; m.p. 320.95°C; b.p.765°C; r.d. 8.65 (20°C); p.n. 48; r.a.m.112.411.
cadmium cell See Weston cadmiumcell.
caesium /see-zee-ŭm, see-see-/ See ce-sium.
cage compounds See clathrate.
calamine /kal-ă-mÿn, -min/ 1. A mineralconsisting of hydrated zinc silicate (2ZnO.SiO2.H2O). It is also known as hemi-morphite.2. Zinc carbonate (ZnCO3), which occursnaturally as a mineral also known as smith-sonite. The carbonate is used medicinallyin suspension as a soothing lotion for sun-burn and skin complaints. Formerly, thenatural mineral was used for this but nowmedicinal calamine is made by precipitat-ing a basic zinc carbonate from a solutionof a zinc salt. It is usually colored pink bythe addition of small quantities of iron(III)oxide.
calcination /kal-să-nay-shŏn/ The for-mation of a calcium carbonate depositfrom hard water.
calcite /kal-sÿt/ A mineral form of cal-cium carbonate occurring in limestone,chalk, and marble.
calcium /kal-see-ŭm/ A moderately soft,low-melting reactive metal; the third ele-ment in group 2 (formerly IIA) of the peri-odic table. The electronic configuration isthat of argon with an additional pair of 4selectrons.
Calcium is widely distributed in theEarth’s crust and is the third most abun-dant element. Large deposits occur aschalk or marble, CaCO3; gypsum, CaSO4.2H2O; anhydrite, CaSO4; fluorspar, CaF;and apatite, CaF2.Ca3(PO4)3. Howeversufficiently large quantities of calciumchloride are available as waste from theSolvay process to satisfy industrial require-ments for the metal, which is produced byelectrolysis of the fused salt. Large quanti-ties of lime, Ca(OH)2, and quicklime,CaO, are produced by decomposition ofthe carbonate for use in both building andagriculture. Several calcium minerals aremined as a source of other substances.Thus, limestone is a cheap source of carbondioxide, gypsum and anhydrite are used inthe manufacture of sulfuric acid, phos-phate rock for phosphoric acid, andfluorspar for a range of fluorochemicals.
Calcium has a low ionization potentialand a relatively large atomic radius. It istherefore a very electropositive element.The metal is very reactive and the com-pounds contain the divalent ion Ca2+. Cal-cium forms the oxide (CaO), a white ionicsolid, on burning in air, but for practicalpurposes the oxide is best prepared byheating the carbonate, which decomposesat about 800°C. Both the oxide and themetal itself react with water to give thebasic hydroxide (Ca(OH)2). On heatingwith nitrogen, sulfur, or the halogens, cal-cium reacts to form the nitride (Ca3N2),sulfide (CaS), or the halides (CaX2). Cal-
C
cium also reacts directly with hydrogen togive the hydride CaH2 and borides, ar-senides, carbides, and silicides can be pre-pared in a similar way. Both the carbonateand sulfate are insoluble. Calcium salts im-part a characteristic brick-red color toflames which is an aid to qualitative analy-sis. At ordinary temperatures calcium hasthe face-centered cubic structure with atransition at 450°C to the close-packedhexagonal structure.
Symbol: Ca; m.p. 839°C; b.p. 1484°C;r.d. 1.55 (20°C); p.n. 20; r.a.m. 40.0878.
calcium acetylide See calcium dicar-bide.
calcium bicarbonate See calcium hy-drogencarbonate.
calcium carbide See calcium dicarbide.
calcium carbonate (CaCO3) A whitesolid that occurs naturally in two crys-talline forms: calcite and aragonite. Theseminerals make up the bulk of such rocks asmarble, limestone, and chalk. Calcium car-bonate also occurs in the mineral dolomite(CaCO3.MgCO3). It is sparingly soluble inwater but dissolves in rainwater containingcarbon dioxide to form calcium hydrogen-carbonate, which causes temporary hard-ness of water. Calcium carbonate is a basicraw material in the Solvay process and isused for making glass, mortar, and cement.
calcium chloride (CaCl2) A white solidthat occurs in a number of hydrated formsand is readily available as a by-product ofthe Solvay process. It is readily soluble inwater and the solution, known as brine, isused in refrigerating plants. Other applica-tions that depend on its water-absorbingproperty and the low freezing point of theaqueous solution include the suppressionof dust on roads and in mines and the melt-ing of snow. Calcium chloride is used as theelectrolyte in the production of calcium.
calcium cyanamide (CaCN2) A solidprepared by heating calcium dicarbide to atemperature in excess of 800°C in an at-mosphere of nitrogen. It is used as a fertil-
izer because ammonia and calcium carbon-ate are slowly formed when water is added:
CaCN2 + 3H2O → CaCO3 + 2NH3Other uses include the defoliation of cot-ton plants and the production ofmelamine.
calcium dicarbide (calcium acetylide; cal-cium carbide; CaC2) A colorless solidwhen pure. In countries where electricity ischeap, calcium dicarbide is produced on alarge scale by heating calcium oxide withcoke at a temperature in excess of 2000°Cin an electric-arc furnace. Water is thenadded to give ETHYNE (C2H2), an importantindustrial organic chemical:
CaC2 + 2H2O → C2H2 + Ca(OH)2The structure of calcium dicarbide is in-
teresting because the carbon is present ascarbide ions (C2
2–).
calcium fluoride (CaF2) A white crys-talline compound found naturally as fluo-rite (fluorspar).
calcium-fluoride structure (fluoritestructure) A form of crystal structure inwhich each calcium ion is surrounded byeight fluorine ions arranged at the cornersof a cube and each fluorine ion is sur-rounded tetrahedrally by four calciumions.
calcium hydrogencarbonate (calciumbicarbonate, Ca(HCO3)2) A solid formedwhen water containing carbon dioxide dis-solves calcium carbonate:
CaCO3 + H2O + CO2 → Ca(HCO3)2Calcium hydrogencarbonate is a cause oftemporary hard water. The solid is un-known at room temperature. See hardness.
calcium hydroxide (slaked lime;Ca(OH)2) A white solid that dissolvessparingly in water to give the alkali knownas limewater. Calcium hydroxide is manu-factured by adding water to the oxide, aprocess known as slaking, which evolvesmuch heat. If just sufficient water is addedso that the oxide turns to a fine powder,the product is slaked lime. If more water isadded, a thick suspension called milk oflime is formed. Calcium hydroxide has
calcium acetylide
48
49
calomel electrode
many uses. As a base, it is used to neutral-ize acid soil and in industrial processessuch as the Solvay process. It is also used inthe manufacture of mortar, whitewash,and bleaching powder and for the soften-ing of temporary hard water.
calcium nitrate (Ca(NO3)2) A deliques-cent salt that is very soluble in water. It isusually crystallized as the tetrahydrateCa(NO3)2.4H2O. When the hydrate isheated, the anhydrous salt is first producedand this subsequently decomposes to givecalcium oxide, nitrogen dioxide, and oxy-gen. Calcium nitrate is used as a nitroge-nous fertilizer.
calcium octadecanoate /ok-tă-dek-ă-noh-ayt/ (calcium stearate; Ca(CH3(CH2)16-COO)2) An insoluble salt of octadecanoicacid. It is formed as ‘scum’ when SOAP, con-taining the soluble salt sodium octade-canoate, is mixed with hard watercontaining calcium ions.
calcium oxide (quicklime; CaO) Awhite solid formed by heating calcium inoxygen or, more widely, by the thermal de-composition of calcium carbonate. On alarge scale, limestone (calcium carbonate)is heated in a tall tower called a lime kiln toa temperature of 550°C. The reversible re-action:
CaCO3 ˆ CaO + CO2proceeds in a forward direction as the car-bon dioxide is carried away by the upwardcurrent through the kiln. Calcium oxide isused in extractive metallurgy to produce aslag with the impurities in metal ores; it isalso used as a drying agent and it is an in-termediate for the production of calciumhydroxide.
calcium phosphate (Ca3(PO4)2) Asolid that occurs naturally in the mineralapatite (CaF2.Ca3(PO4)3) and in rock phos-phate. It is the chief constituent of animalbones and is used extensively in fertilizers.
calcium silicate (Ca2SiO4) A white in-soluble crystalline compound found in var-ious cements and minerals. It is also a
component of the slag produced in smelt-ing iron and other metals in a blast furnace.
calcium stearate See calcium octade-canoate.
calcium sulfate (anhydrite; CaSO4) Awhite solid that occurs abundantly as themineral anhydrite and as the dihydrate(CaSO4.2H2O), known as gypsum or al-abaster. When heated, gypsum loses waterto form the hemihydrate (2CaSO4.H2O),which is plaster of Paris. If the water is re-placed, gypsum reforms and sets as a solid.Plaster of Paris is therefore used for takingcasts and for setting broken limbs. Calciumsulfate is sparingly soluble in water and isa cause of permanent hardness in water. Itis used in ceramics, paint, and in papermaking.
Calgon /kal-gon/ (Trademark) A sub-stance often added to detergents to removeunwanted chemicals that have dissolved inwater and would otherwise react with soapto form a scum. Calgon consists of compli-cated polyphosphate molecules, which ab-sorb dissolved calcium and magnesiumions. The metal ions become trappedwithin the Calgon molecules.
caliche /kah-lee-chay/ An impure com-mercial form of sodium nitrate.
californium /kal-ă-for-nee-ŭm/ A sil-very radioactive transuranic element of theactinoid series of metals, not found natu-rally on Earth. Several radioisotopes havebeen synthesized, including californium-252, which is used as an intense source ofneutrons in certain types of portable detec-tor and in the treatment of cancer.
Symbol: Cf; m.p. 900°C; p.n. 98; moststable isotope 251Cf (half-life 900 years).
calixarene /kă-liks-ă-reen/ See host–guest chemistry.
calomel /kal-ŏ-mel/ See mercury(I) chlo-ride.
calomel electrode A half cell having amercury electrode coated with mercury(I)
calorie
50
chloride (called calomel), in an electrolyteconsisting of potassium chloride and (satu-rated) mercury(I) chloride solution. Itsstandard electrode potential against the hy-drogen electrode is accurately known(–0.2415 V at 25°C) and it is a convenientsecondary standard.
calorie /kal-ŏ-ree/ Symbol: cal A unit ofenergy approximately equal to 4.2 joules.It was formerly defined as the energyneeded to raise the temperature of onegram of water by one degree Celsius. Be-cause the specific thermal capacity of waterchanges with temperature, this definition isnot precise. The mean or thermochemicalcalorie (calTH) is defined as 4.184 joules.The international table calorie (calIT) is de-fined as 4.1868 joules. Formerly the meancalorie was defined as one hundredth of theheat needed to raise one gram of waterfrom 0°C to 100°C, and the 15°C calorieas the heat needed to raise it from 14.5°Cto 15.5°C.
calorific value /kal-ŏ-rif-ik/ The energycontent of a fuel, defined as the energy re-leased in burning unit mass of the fuel.
calorimeter /kal-ŏ-rim-ĕ-ter/ A device orapparatus for measuring thermal proper-ties such as specific heat capacity, calorificvalue, etc. See bomb calorimeter.
calx /kal’ks/ A metal oxide obtained byheating an ore to high temperatures in air.
camphor /kam-fer/ (C10H16O) A natu-rally-occurring white organic compoundwith a characteristic penetrating odor. It isa cyclic compound and a ketone, formerlyobtained from the wood of the camphortree but now made synthetically. Camphoris used as a platicizer for celluloid and as aninsecticide against clothes moths.
candela Symbol: cd The SI base unit ofluminous intensity, defined as the intensity(in the perpendicular direction) of theblack-body radiation from a surface of1/600 000 square meter at the temperatureof freezing platinum and at a pressure of101 325 pascals.
cane sugar See sucrose.
Cannizzaro reaction /kahn-need-dzah-roh/ The reaction of aldehydes to give al-cohols and acid anions in the presence ofstrong bases. The aldehydes taking part inthe Cannizzaro reaction lack hydrogenatoms on the carbon attached to the alde-hyde group. For instance, in the presence ofhot aqueous sodium hydroxide:
NaOH + 2C6H5CHO →C6H5CH2OH + C6H5COO–Na+
This reaction is a disproportionation, in-volving both oxidation (to acid) and reduc-tion (to alcohol). It can also occur withmethanal to give methanol and meth-anoate ions.
NaOH + 2HCHO → CH3OH +HCOO–Na+
The reaction is named for the Italianchemist Stanislao Cannizzaro (1826–1910)who first described it in 1853.
canonical form See resonance.
caproic acid /kă-proh-ik/ See hexanoicacid.
caprolactam /kap-roh-lak-tăm/ (C6H11NO)A white crystalline substance used in themanufacture of NYLON.
caprylic acid /kă-pril-ik/ See octanoicacid.
carbamide /kar-bă-mÿd, -mid, kar-bam-ÿd, -id/ See urea.
carbanion /kar-ban-ÿ-ŏn, -on/ An inter-mediate in an organic reaction in whichone carbon atom is electron-rich and car-ries a negative charge. Carbanions are usu-ally formed by abstracting a hydrogen ionfrom a C–H bond using a base, e.g. fromethanal to form –CH2CHO, or fromorganometallic compounds in which thecarbon atom is bonded to an electroposi-tive metal.
carbazole /kar-bă-zohl/ (C12H9N) Awhite crystalline compound used in themanufacture of dyestuffs.
carbene /kar-been/ A transient species ofthe form RR′C:, with two valence electronsthat do not form bonds. The simplest ex-ample is methylene, H2C:. Carbenes areshort-lived intermediates in some organicreactions. They attack double bonds toform three-membered rings (i.e. cyclo-propane derivatives). They also attack sin-gle bonds in insertion reactions, forexample:
R–O–H + R2C: → R–O–C(R2)HR–H + R2C: → R–C(R2)–H
Carbenes are important ‘reagents’ in or-ganic synthesis and various methods existfor generating them in the reactionmedium.
carbenium ion /kar-bee-nee-ŭm/ Seecarbocation.
carbide /kar-bÿd/ A compound of car-bon with a more electropositive element.The carbides of the elements are classifiedinto:1. Ionic carbides, which contain the car-
bide ion C4–. An example is aluminumcarbide, Al4C3. Compounds of this typereact with water to give methane (theywere formerly also called methanides).The dicarbides are ionic carbon com-pounds that contain the dicarbide ion–C:C–. The best-known example is cal-cium dicarbide, CaC2, also known ascalcium carbide, or simply carbide.Compounds of this type give ethynewith water. They were formerly calledacetylides or ethynides. Ionic carbidesare formed with very electropositivemetals. They are crystalline.
2. Covalent carbides, which have giant-molecular structures, as in silicon car-bide (SiC) and boron carbide (B4C3).These are hard high-melting solids.Other covalent compounds of carbon(CO2, CS2, CH4, etc.) have covalentmolecules.
3. Interstitial carbides, which are intersti-tial compounds of carbon with transi-tion metals. Titanium carbide (TiC) is anexample. These compounds are all hardhigh-melting solids, with metallic prop-erties. Some carbides (e.g. nickel carbide
Ni3C) have properties intermediate be-tween those of interstitial and ionic car-bides.
carbocation An ion with a positivecharge in which the charge is mostly local-ized on a carbon atom. There are twotypes. Carbonium ions have five bonds tothe carbon atom and a complete outer shellof 8 electrons. The simplest example wouldbe the carbonium ion CH5
+, which couldbe regarded as formed by adding H+ tomethane, CH4, in the same way that theammonium ion, NH4
+, is formed from am-monia, NH3. There is, however, a differ-ence between ammonia and methane inthat ammonia has a lone pair of electrons,which it can donate in forming the NH4
+
ion. The carbonium ion CH5+ (and similar
ions) is a transient species, produced in thegas phase by electron bombardment of or-ganic compounds and detected in a massspectrum. Its shape is that of a carbonatom with three hydrogens in a plane andone hydrogen above and one below (a trig-onal bipyramid).
Carbenium ions have three bonds to thecentral carbon and are planar, with thebonds directed toward the corners of a tri-angle (sp2 hybridization). They have sixelectrons in outer shell of carbon and a va-cant p orbital. Carbenium ions are impor-tant intermediates in a number of organicreactions, notably the SN1 mechanism ofNUCLEOPHILIC SUBSTITUTION. It is possible toproduce stable carbenium ions in salts ofthe type (C6H5)3C+Cl–, which are orange-red solids. In these the triphenylmethylcation is stabilized by delocalization overthe three phenyl groups. It is also possibleto produce carbenium ions using SU-PERACIDS.
carbocyclic compound /kar-boh-sÿ-klik,-sik-lik/ A compound, such as benzene orcyclohexane, that contains a ring of carbonatoms in its structure.
carbohydrates /kar-boh-hÿ-dayts/ Aclass of compounds occurring widely in na-ture and having the general formula typeCx(H2O)y. (Note that although the namesuggests a hydrate of carbon these com-
51
carbohydrates
carbolic acid
52
pounds are in no way hydrates and have nosimilarities to classes of hydrates.) Carbo-hydrates are generally divided into twomain classes: SUGARS and POLYSACCHA-RIDES.
Carbohydrates are both stores of en-ergy and structural elements in living sys-tems; plants having typically 15%carbohydrate and animals about 1% car-bohydrate. The body is able to build uppolysaccharides from simple units (an-abolism) or break the larger units down tomore simple units for releasing energy (ca-tabolism).
carbolic acid /kar-bol-ik/ A formername for phenol (hydroxybenzene,C6H5OH).
carbon /kar-bŏn/ The first element ofgroup 14 (formerly IVA) of the periodictable. Carbon is a universal constituent ofliving matter and the principal deposits ofcarbon compounds are derived from livingsources; i.e., carbonates (chalk and lime-stone) and fossil fuels (coal, oil, and gas). Italso occurs in the mineral dolomite. The el-ement forms only 0.032% by mass of theEarth’s crust. Minute quantities of elemen-tal carbon also occur as the allotropesgraphite and diamond. A third allotrope,buckminsterfullerene (C60), also exists.
The industrial demand for graphite issuch that it is manufactured in large quan-
tities using the Acheson process in whichcoke and small amounts of asphalt or clayare raised to high temperatures. Largequantities of impure carbon are also con-sumed in the reductive extraction of met-als. Apart from the demand for diamond asa gemstone there is a large industrial de-mand for low-grade small diamonds fordrilling and grinding machinery. High-temperature studies show that graphiteand diamond can be inter-converted at3000°C and extremely high pressures butthe commercial exploitation would not beviable.
Carbon burns in oxygen to form carbondioxide and carbon monoxide, CO. Car-bon dioxide is soluble in water forming theweakly acidic carbonic acid, the parentacid of the metal carbonates. In contrastCO is barely soluble in water but it willreact with alkali to give the methanoate(formate) ion:
CO + OH– → HCO2–
Carbon will react readily with sulfur atred heat to form CS2 but it does not reactdirectly with nitrogen. Cyanogen, (CN)2,must be prepared by heating covalentmetal cyanides such as CuCN. Carbon willalso react directly with many metals at ele-vated temperatures to give carbides. Car-bides can also be obtained by heating themetal oxide with carbon or heating themetal with a hydrocarbon. There is a be-wilderingly wide range of metal carbides,
Graphite
Diamond
Carbon structures
both salt-like with electropositive elements(CaC2) and covalent with the metalloids(SiC), and there are also many interstitialcarbides formed with metals such as Cr,Mn, Fe, Co, and Ni.
The compounds with C–N bonds forma significant branch of inorganic chemistryof carbon, these are hydrogen cyanide(HCN) and the cyanides, cyanic acid(HNCO) and the cyanates, and thiocyanicacid (HNCS) and the thiocyanates.
Naturally occurring carbon has the iso-topic composition 12C (98.89%), 13C(1.11%) and 14C (minute traces in theupper atmosphere produced by slow neu-tron capture by 14N atoms). 14C is used forradiocarbon dating because of its longhalf-life of 5730 years.
Symbol: C; m.p. 3550°C; b.p. 4830°C(sublimes); C60 sublimes at 530°C; r.d.3.51 (diamond), 2.26 (graphite), 1.65 (C60)(all at 20°C); p.n. 6; r.a.m. 12.011.
See also buckminsterfullerene.
carbonate /kar-bŏ-nayt, -nit/ A salt ofcarbonic acid (containing the ion CO3
2–).
carbonation /kar-bŏ-nay-shŏn/ 1. Thesolution of carbon dioxide in a liquidunder pressure, as in carbonated softdrinks.2. The addition of carbon dioxide to com-pounds, e.g. the insertion of carbon diox-ide into Grignard reagents.
carbon black A finely divided form ofcarbon produced by the incomplete com-bustion of such hydrocarbon fuels as nat-ural gas or petroleum oil. It is used as ablack pigment in inks and as a filler forrubber in tire manufacture.
carbon cycle The circulation of carboncompounds in the environment, one of themajor natural cycles of an element. Carbondioxide in the air is used by green plants inphotosynthesis (in which it is combinedwith water to form sugars and starches).Plants are eaten by animals which exhalecarbon dioxide, or when plants and ani-mals die their remains decompose with theproduction of carbon dioxide. Some plantsare burned or converted to fossil fuels
which are burned, again with a formationof carbon dioxide.
carbon dating (radiocarbon dating) Amethod of dating – measuring the age of(usually archaeological) materials that con-tain matter of living origin. It is based onthe fact that 14C, a beta emitter of half-lifeapproximately 5730 years, is being formedcontinuously in the atmosphere as a resultof cosmic-ray action.
The 14C becomes incorporated into liv-ing organisms. After death of the organismthe amount of radioactive carbon de-creases exponentially by radioactive decay.The ratio of 12C to 14C is thus a measure ofthe time elapsed since the death of the or-ganic material.
The method is most valuable for speci-mens of up to 20 000 years old, though ithas been modified to measure ages up to70 000 years. For ages of up to about 8000years the carbon time scale has been cali-brated by dendrochronology; i.e. by mea-suring the 12C:14C ratio in tree rings ofknown antiquity.
carbon dioxide /dÿ-oks-ÿd, -id/ (CO2) A colorless odorless nonflammable gasformed when carbon burns in excess oxy-gen. It is also produced by respiration. Car-bon dioxide is present in the atmosphere(0.03% by volume) and is converted inplants to carbohydrates by photosynthesis.In the laboratory it is made by the action ofdilute acid on metal carbonates. Industri-ally, it is obtained as a by-product in cer-tain processes, such as fermentation or themanufacture of lime. The main uses are asa refrigerant (solid carbon dioxide, calleddry ice) and in fire extinguishers and car-bonated drinks. Increased levels of carbondioxide in the atmosphere from the com-bustion of fossil fuels are thought to con-tribute to the greenhouse effect.
Carbon dioxide is the anhydride of theweak acid carbonic acid, which is formedin water:
CO2 + H2O → H2CO3
carbon disulfide (CS2) A colorless poi-sonous flammable liquid made frommethane (natural gas) and sulfur. It is a sol-
53
carbon disulfide
vent. The pure compound is virtually odor-less, but CS2 usually has a revolting smellbecause of the presence of other sulfurcompounds.
carbon fibers Fibers of graphite, whichare used, for instance, to strengthen poly-mers. They are made by heating stretchedtextile fibers and have an orientated crystalstructure.
carbonic acid /kar-bon-ik/ (H2CO3) Adibasic acid formed in small amounts in so-lution when carbon dioxide dissolves inwater:
CO2 + H2O ˆ H2CO2
It forms two series of salts: hydrogencar-bonates (HCO3
–) and carbonates (CO32–).
The pure acid cannot be isolated.
carbonium ion /kar-boh-nee-ŭm/ Seecarbocation.
carbonize /kar-bŏ-nÿz/ (carburize) Toconvert an organic compound into carbonby incomplete oxidation at high tempera-ture.
carbon monoxide /mon-oks-ÿd, -id/(CO) A colorless flammable toxic gasformed by the incomplete combustion ofcarbon. In the laboratory it can be made bydehydrating methanoic acid with concen-trated sulfuric acid:
HCOOH – H2O → COIndustrially, it is produced by the oxida-tion of carbon or of natural gas, or by thewater-gas reaction. It is a powerful reduc-ing agent and is used in metallurgy.
Carbon monoxide is neutral and onlysparingly soluble in water. It is not the an-hydride of methanoic acid, although underextreme conditions it can react withsodium hydroxide to form sodiummethanoate. It forms metal carbonyls withtransition metals, and its toxicity is due toits ability to form a complex with hemo-globin.
carbon tetrachloride /tet-ră-klor-ÿd, -id,-kloh-rÿd, -rid/ See tetrachloromethane.
carbonyl /kar-bŏ-nil/ A complex inwhich carbon monoxide ligands are coor-dinated to a metal atom. A common exam-ple is tetracarbonyl nickel(0), Ni(CO)4.
carbonyl chloride (phosgene;COCl2) A colorless toxic gas with achoking smell. It is used as a chlorinatingagent and to make polyurethane plasticsand insecticides; it was formerly employedas a war gas.
carbonyl group The group –C=O. Itoccurs in aldehydes (RCO.H), ketones(RR′CO), carboxylic acids (RCO.OH),and in carbonyl complexes of transitionmetals.
carborundum /kar-bŏ-run-dŭm/ See sil-icon carbide.
carboxylate ion /kar-boks-ă-layt/ Theion –COO–, produced by ionization of acarboxyl group. In a carboxylate ion thenegative charge is generally delocalizedover the O–C–O grouping and the twoC–O bonds have the same length, interme-diate between that of a double C=O and asingle C–O.
carboxyl group /kar-boks-ăl/ The or-ganic group –CO.OH, present in car-boxylic acids.
carboxylic acid /kar-boks-il-ik/ A typeof organic compound with the general for-mula RCOOH. Many carboxylic acidsoccur naturally in plants and (in the formof esters) in fats and oils, hence the alter-native name fatty acids. Carboxylic acidswith one COOH group are called monoba-sic, those with two, dibasic, and those with
carbon fibers
54
O
OHC
R
Carboxylic acid
three, tribasic. The methods of preparationare:1. Oxidation of a primary alcohol or an
aldehyde:RCH2OH + 2[O] → RCOOH + H2O
2. Hydrolysis of a nitrile using dilute hy-drochloric acid:
RCN + HCl + 2H2O → RCOOH +NH4Cl
The acidic properties of carboxylicacids are due to the carbonyl group, whichattracts electrons from the C–O and O–Hbonds. The carboxylate ion formed,R–COO–, is also stabilized by delocaliza-tion of electrons over the O–C–O group-ing.
Other reactions of carboxylic acids in-clude the formation of esters and the reac-tion with phosphorus(V) chloride to formacyl halides.
carburizing /kar-bŭ-rÿz, -byŭ-/ See casehardening.
carbylamine reaction /kar-băl-ă-meen, -am-in/ See isocyanide test.
carcinogen /kar-sin-ŏ-jĕn/ An agent thatcauses cancer in animals, such as tobaccosmoke or ionizing radiation.
Carius method /kah-ree-ŭs/ A methodin quantitative analysis for identifyinghalogens, phosphorus, and sulfur in or-ganic compounds. The compound isheated with concentrated nitric acid, whichoxidizes the organic matter and leaves theelement in a form in which it can be de-tected by normal qualitative analysis. Themethod is named for the German chemistGeorg Ludwig Carius (1829–75).
carnallite /kar-nă-lÿt/ A mineral chlo-ride of potassium and magnesium,KCl.MgCl2.6H2O.
Carnot cycle /kar-noh/ The idealized re-versible cycle of four operations occurringin a perfect heat engine. These are the suc-cessive adiabatic compression, isothermalexpansion, adiabatic expansion, andisothermal compression of the workingsubstance. The cycle returns to its initial
pressure, volume, and temperature, andtransfers energy to or from mechanicalwork. The efficiency of the Carnot cycle isthe maximum attainable in a heat engine.The cycle is named for the French physicistNicolas Leonard Sadi Carnot (1796–1832). See Carnot’s principle.
Carnot’s principle (Carnot theorem) The efficiency of any heat engine cannot begreater than that of a reversible heat engineoperating over the same temperaturerange. It follows directly from the secondlaw of thermodynamics, and means that allreversible heat engines have the same effi-ciency, independent of the working sub-stance. If heat is absorbed at temperatureT1 and given out at T2, then the Carnot ef-ficiency is (T1 – T2)/T1.
Caro’s acid /kah-roh/ See peroxosulfu-ric(VI) acid.
carrier gas The gas used to carry thesample in gas chromatography.
case hardening Processes for increasingthe hardness of the surface or ‘case’ of thesteel used to make such components asgears and crankshafts. The oldest methodis carburizing, in which the carbon contentof the surface layer is increased by heatingin a carbon-rich environment. Nitriding in-volves the diffusion of nitrogen into thesurface layer of steel forming intenselyhard nitride particles in the structure. Acombination of both carburizing and ni-triding is sometimes employed.
casein /kay-seen, -see-in/ A phosphorus-containing protein that occurs in milk andcheese. It is easily digested by young mam-mals and is their major source of proteinand phosphorus. It has been used in themanufacture of paper and buttons.
cast iron Alloys of iron and carbonmade by remelting the crude iron producedin a blast furnace. The carbon content isusually between 2.4 and 4.0% and may bepresent as iron carbide (white cast iron) oras graphite (gray cast iron). Metals can beadded to improve the properties of the
55
cast iron
alloy. Additional elements such as phos-phorus, sulfur, and manganese are alsopresent as impurities. Cast iron is a cheapmetal with an extensive range of possibleproperties and it has been used on a verylarge scale.
catabolism /kă-tab-ŏ-liz-ăm/ All themetabolic reactions that break down com-plex molecules to simpler compounds. Thefunction of catabolic reactions is to pro-vide energy. See also metabolism.
catalyst /kat-ă-list/ A substance that al-ters the rate of a chemical reaction withoutitself being changed chemically in the reac-tion. The catalyst can, however, undergophysical change; for example, large lumpsof catalyst can, without loss in mass, beconverted into a powder. Small amounts ofcatalyst are often sufficient to increase therate of reaction considerably. A positivecatalyst increases the rate of a reaction anda negative catalyst reduces it. Homoge-neous catalysts are those that act in thesame phase as the reactants (i.e. in gaseousand liquid systems). For example, nitro-gen(II) oxide gas will catalyze the reactionbetween sulfur(IV) oxide and oxygen in thegaseous phase. Heterogeneous catalysts actin a different phase from the reactants. Forexample, finely divided nickel (a solid) willcatalyze the hydrogenation of oil (liquid).
The function of a catalyst is to providea new pathway for which the rate-deter-mining step has a lower activation energythan in the uncatalyzed reaction. A catalystdoes not change the products in an equilib-rium reaction and their concentration isidentical to that in the uncatalyzed reac-tion; i.e. the position of the equilibrium re-mains unchanged. The catalyst simplyincreases the rate at which equilibrium isattained.
In autocatalysis, one of the products ofthe reaction itself acts as a catalyst. In thistype of reaction the reaction rate increaseswith time to a maximum and finally slowsdown. For example, in the hydrolysis ofethyl ethanoate, the ethanoic acid pro-duced catalyzes the reaction.
catalytic converter /kat-ă-lit-ik/ A de-vice fitted to the exhaust system of gaso-line-fuelled vehicles to remove pollutantgases from the exhaust. It consists of a hon-eycomb structure (to provide maximumarea) coated with platinum, palladium,and rhodium catalysts. Such devices canconvert carbon monoxide to carbon diox-ide, oxides of nitrogen to nitrogen, and un-burned fuel to carbon dioxide and water.
catalytic cracking The conversion,using a catalyst, of long-chain hydrocar-bons from the refining of petroleum intomore useful shorter-chain compounds suchas those occurring in kerosene and gaso-line.
catechol /kat-ĕ-chohl, -kohl/ (1,2-dihy-droxybenzene) A colourless crystallinePHENOL containing two hydroxyl groups. Itis used in photographic developing.
catecholamine /kat-ĕ-chohl-ă-meen, -kohl-/Any of a group of important amines thatcontain a catechol ring in their molecules.They are neurotransmitters and hormones.Examples are epinephrine and norepineph-rine.
catenation /kat-ĕ-nay-shŏn/ The forma-tion of chains of atoms in molecules.
cathode /kath-ohd/ In electrolysis, theelectrode that is at a negative potentialwith respect to the anode. In any electricalsystem, such as a discharge tube or elec-tronic device, the cathode is the terminal atwhich electrons enter the system.
cation /kat-ÿ-ŏn, -on/ A positivelycharged ion, formed by removal of elec-trons from atoms or molecules. In electrol-ysis, cations are attracted to the negativelycharged electrode (the cathode). Compareanion.
cationic detergent /kat-ÿ-on-ik/ See de-tergent.
cationic resin An ION-EXCHANGE mater-ial that can exchange cations, such as H+
and Na+, for ions in the surrounding
catabolism
56
57
centigrade scale
medium. Such resins are used for a widerange of purification and analytical pur-poses.
They are often produced by adding asulfonic acid group (–SO3
–H+) or a car-boxylate group (–COO–H+) to a stablepolyphenylethene resin. A typical exchangereaction is:
resin–SO3–H+ + NaCl = resin–SO3
–Na+
+ HClThey have been used to great effect to sep-arate mixtures of cations of similar sizehaving the same charge. Such mixtures canbe attached to cationic resins and progres-sive elution will recover them in order ofdecreasing ionic radius. Promethium wasfirst isolated using this technique.
caustic lime See calcium hydroxide.
caustic potash See potassium hydrox-ide.
caustic soda See sodium hydroxide.
celestine /sel-ĕ-stÿn, -stin/ A mineral sul-fate of strontium, SrSO4.
cell A system having two plates (elec-trodes) in a conducting liquid (electrolyte).An electrolytic cell is used for producing achemical reaction by passing a currentthrough the electrolyte (i.e. by ELECTROLY-SIS). A voltaic (or galvanic) cell produces ane.m.f. by chemical reactions at each elec-trode. Electrons are transferred to or fromthe electrodes, giving each a net charge.
There is a convention for writing cell re-actions in voltaic cells. The DANIELL CELL,for instance, consists of a zinc electrode ina solution of Zn2+ ions connected (througha porous pot) to a solution of Cu2+ ions inwhich is placed a copper electrode. The re-actions at the electrodes are
Zn → Zn2+ + 2ei.e. oxidation of the zinc to zinc(II), and
Cu2+ + 2e → Cui.e. reduction of copper(II) to copper. A cellreaction of this type is written:
Zn|Zn2+(aq)|Cu2+(aq)|CuThe e.m.f. is the potential of a lead on theright minus the potential of a lead on theleft. Copper is positive in this case and the
e.m.f. of the cell is stated as +1.10 volts. Seealso accumulator; Leclanché cell.
cellulose /sel-yŭ-lohs/ A polysaccharide(C6H10O5)n of glucose, which is the mainconstituent of the cell walls of plants. It isobtained from wood pulp.
cellulose acetate (cellulose ethanoate) A polymeric substance made by acetylatingcellulose. It is used in plastics, in acetatefilm, and in acetate rayon.
cellulose trinitrate /trÿ-nÿ-trayt/ (guncotton; nitrocellulose) A highly flamma-ble substance made by treating cellulosewith a nitric–sulfuric acid mixture. Cellu-lose trinitrate is used in explosives and inlacquers. It is an ester of nitric acid (i.e. nota true nitro compound).
Celsius scale A TEMPERATURE SCALE inwhich the temperature of melting pure iceis taken as 0° and the temperature of boil-ing water 100° (both at standard pressure).The degree Celsius (°C) is equal to thekelvin. This was known as the centigradescale until 1948, when the present namebecame official. Celsius’ original scale wasinverted (i.e. had 0° as the steam tempera-ture and 100° as the ice temperature). Thescale is named for the Swedish astronomerAnders Celsius (1701–44).
cement A powdered mixture of calciumsilicates and aluminates, which is made byheating limestone (CaCO3) with clay, andgrinding the result. When mixed withwater, reactions occur with the water(hence the name hydraulic cement) and ahard solid aluminosilicate is formed.
cementite /sĕ-men-tÿt/ (Fe3C) A con-stituent of certain cast irons and steels. Thepresence of cementite increases the hard-ness of the metal.
centi- Symbol: c A prefix denoting 10–2.For example, 1 centimeter (cm) = 10–2
meter (m).
centigrade scale See Celsius scale.
centrifugal pump
58
centrifugal pump A device commonlyused for transporting fluids around achemical plant. Centrifugal pumps usuallyhave 6–12 blades rotating inside a fixedcircular casing. As the blades rotate, thefluid is impelled out of the pump along apipe. Centrifugal pumps do not producehigh pressures but they have the advantageof being relatively cheap because they aresimple in design, have no valves, and workat high speeds. In addition they are notdamaged if a blockage develops. Comparedisplacement pump.
centrifuge /sen-tră-fyooj/ An apparatusfor rotating a container at high speeds,used to increase the rate of sedimentationof suspensions or the separation of two im-miscible liquids. See also ultracentrifuge.
ceramics /sĕ-ram-iks/ Useful high-melt-ing inorganic materials. Ceramics includesilicates and aluminosilicates, refractorymetal oxides, and metal nitrides, borides,etc. Pottery and porcelain are examples ofceramics.
cerium /seer-ee-ŭm/ A ductile malleablegray element of the lanthanoid series ofmetals. It occurs in association with otherlanthanoids in many minerals, includingmonazite and bastnasite. The metal is reac-tive; it reacts with water and tarnishes inair. It is used in several alloys (especiallyfor lighter flints), as a catalyst, and in com-pound form in carbon-arc searchlights,etc., and in the glass industry.
Symbol: Ce; m.p. 799°C; b.p. 3426°C;r.d. 6.7 (hexagonal structure, 25°C); p.n.58; r.a.m. 140.15.
cermet /ser-met/ A synthetic compositematerial made by combining a ceramic anda sintered metal. Cermets have better tem-perature and corrosion resistance thanstraight ceramics. For example, achromium–alumina cermet is used to makeblades for gas-turbine engines.
cerussite /seer-ŭ-sÿt/ A naturally occur-ring form of lead(II) carbonate that is animportant lead ore. It forms orthorhombic
crystals and is often found together withgalena (PbS).
cesium /see-zee-ŭm, see-see-/ (caesium) A soft golden highly reactive low-meltingelement of the alkali-metal group. It isfound in several silicate minerals, includingpollucite (CsAlSi2O6). The metal oxidizesin air and reacts violently with water. Ce-sium is used in photocells, as a catalyst,and in the cesium atomic clock. The ra-dioactive isotopes 134Cs (half life 2.065years) and 137Cs (half life 30.3 years) areproduced in nuclear reactors and are po-tentially dangerous atmospheric pollu-tants.
Symbol: Cs; m.p. 28.4°C; b.p. 678.4°C;r.d. 1.873 (20°C); p.n. 55; r.a.m. 132.91.
cesium-chloride structure A form ofcrystal structure that consists of alternatelayers of cesium ions and chloride ionswith the center of the lattice occupied by acesium ion in contact with eight chlorideions (i.e. four chloride ions in the planeabove and four in the plane below).
CFC Chlorofluorocarbon. See halocar-bon.
c.g.s. system A system of units that usesthe centimeter, the gram, and the second asthe base mechanical units. Much early sci-entific work used this system, but it hasnow almost been abandoned.
chain When two or more atoms formbonds with each other in a molecule, achain of atoms results. This chain may be astraight chain, in which each atom is addedto the end of the chain, or it may be abranched chain, in which the main chain ofatoms has one or more smaller SIDE CHAINS
branching off it.
chain reaction A self-sustaining chemi-cal reaction consisting of a series of steps,each of which is initiated by the one beforeit. An example is the reaction between hy-drogen and chlorine:
Cl2 → 2Cl•H2 + Cl• → HCl + H•H• + Cl2 → HCl + Cl•
2H• → H22Cl• → Cl2
The first stage, chain initiation, is the dis-sociation of chlorine molecules into atoms;this is followed by two chain propagationreactions. Two molecules of hydrogenchloride are produced and the ejected chlo-rine atom is ready to react with more hy-drogen. The final steps, chain termination,stop the reaction.
Induced nuclear fission reactions de-pend on chain reactions; the fission reac-tion is maintained by the two or threeneutrons set free in each fission.
chair conformation See conformation.
chalcogens /chal-kŏ-jĕnz/ See group 16elements.
chalk A natural form of calcium carbon-ate (CaCO3) formed originally by marineorganisms. (Blackboard chalk is calciumsulfate, CaSO4.)
chamber process See lead-chamberprocess.
chaotic reaction A chemical reaction inwhich the concentrations of the reactantsshow chaotic behavior, i.e. the evolution ofthe reaction may become unpredictable.Reactions of this type ususally involve alarge number of complex interlinked steps.
charcoal An amorphous form of carbonmade by heating wood or other organicmaterial in the absence of air. Activatedcharcoal is charcoal that has been heated todrive off absorbed gas. It is used for ab-sorbing gases and for removing impuritiesfrom liquids.
Charles’ law /sharlz, charl-ziz/ For agiven mass of gas at constant pressure, thevolume increases by a constant fraction ofthe volume at 0°C for each Celsius degreerise in temperature. The constant fraction(α) has almost the same value for all gases– about 1/273 – and Charles’ law can bewritten in the form
V = V0(1 + αvθ)where V is the volume at temperature θ°Cand V0 the volume at 0°C. The constant αvis the thermal expansivity of the gas. For anideal gas its value is 1/273.15.
A similar relationship exists for thepressure of a gas heated at constant vol-ume:
p = p0(1 + αpθ)Here, αp is the pressure coefficient. For anideal gas
αp = αvalthough they differ slightly for real gases.It follows from Charles’ law that for a gasheated at constant pressure,
V/T = Kwhere T is the thermodynamic temperatureand K is a constant. Similarly, at constantvolume, p/T is a constant.
Charles’ volume law is sometimescalled Gay-Lussac’s law after its indepen-dent discoverer. The law is named for boththe French physicist and physical chemistJacques Alexandre César Charles(1746–1823) and for the French physicistJoseph-Louis Gay-Lussac (1778–1850).See also absolute temperature; gas laws.
chelate /kee-layt/ A metal coordinationcomplex in which one ligand coordinatesat two or more points to the same metalion. The resulting complex contains ringsof atoms that include the metal atom. Anexample of a chelating agent is 1,2-di-aminoethane (H2NCH2CH2NH2), whichcan coordinate both its amine groups to thesame atom. It is an example of a bidentateligand (having two ‘teeth’). Edta, whichcan form up to six bonds, is another exam-ple of a chelating agent. The word chelatecomes from the Greek word meaning‘claw’. See also sequestration.
chemical bond A link between atomsthat leads to an aggregate of sufficient sta-bility to be regarded as an independentmolecular species. Chemical bonds includecovalent bonds, electrovalent (ionic)bonds, coordinate bonds, and metallicbonds. Hydrogen bonds and van der Waalsforces are not usually regarded as truechemical bonds.
59
chemical bond
chemical combination, laws of Agroup of chemical laws developed duringthe late 18th and early 19th centuries,which arose from the recognition of the im-portance of quantitative (as opposed toqualitative) study of chemical reactions.The laws are:1. the law of conservation of mass (mat-
ter);2.the law of constant (definite) propor-
tions;3. the law of multiple proportions;4. the law of equivalent (or reciprocal) pro-
portions,These laws played a significant part in Dal-ton’s development of his atomic theory(1808). See conservation of mass, law of;constant proportions, law of; equivalentproportions, law of; multiple proportions,law of.
chemical dating A method of usingchemical analysis to find the age of an ar-chaeological specimen in which composi-tional changes have taken place over time.For example, the determination of theamount of fluorine in bone that has beenburied gives an indication of its age be-cause phosphate in the bone has graduallybeen replaced by fluoride ions fromgroundwater. Another dating techniquedepends on the fact that, in living organ-isms, amino acids are optically active.After death a slow racemization reactionoccurs and a mixture of L- and D-isomersforms. The age of bones can be accuratelydetermined by measuring the relativeamounts of L- and D-amino acids present.
chemical engineering The branch ofengineering concerned with the design andmaintenance of a chemical plant and itsability to withstand extremes of tempera-ture and pressure, corrosion, and wear. Itenables laboratory processes producinggrams of material to be converted into alarge-scale plant producing tonnes of mate-rial. Chemical engineers plan large-scalechemical processes by linking together theappropriate unit processes and by studyingsuch parameters as heat and mass transfer,separations, and distillations.
chemical equation A method of repre-senting a chemical reaction using chemicalformulas (see formula). The formulas ofthe reactants are given on the left-hand sideof the equation, with the formulas of theproducts on the right. The two halves areseparated by a directional arrow or arrows(on an equals sign). A number preceding aformula (called a stoichiometric coeffi-cient) indicates the number of molecules ofthat substance involved. The equationmust balance – that is, the number ofatoms of any one element must be the sameon both sides of the equation. A simple ex-ample is the equation for the reaction be-tween hydrogen and oxygen to form water:
2H2 + O2 → 2H2OA more complex equation represents thereaction between disodium tetraborate(borax) and aqueous hydrochloric acid togive boric acid and sodium chloride:
Na2B4O7 + 2HCl + 5H2O → 4H3BO3 +2NaCl
chemical equilibrium See equilibrium.
chemical formula See formula.
chemical potential Symbol: µ. For theith component of a mixture the chemicalpotential µi is defined by the partial deriva-tive of the Gibbs free energy G of the sys-tem with respect to the amount ni of thecomponent, when the temperature, pres-sure, and amounts of other componentsare constant, i.e. µi = ∂G/∂ni. If the chemi-cal potentials of components are equalthen the components are in equilibrium.Also, in a one-component system with twophases it is necessary for the chemical po-tentials to be equal in the two phases forthere to be equilibrium.
chemical reaction A process in whichone or more elements or chemical com-pounds (the reactants) react to produce adifferent substance or substances (theproducts).
chemical shift An effect of chemicalstructure on the position of a spectral line.See nuclear magnetic resonance.
chemical combination, laws of
60
61
chloric acid
chemical symbol A letter or pair of let-ters that stand for a chemical element, asused in chemical formulas and equations.See chemical equation; formula.
chemiluminescence /kem-ă-loo-mă-nes-ĕns/ The emission of light during a chem-ical reaction.
chemisorption /kem-ă-sorp-shŏn, -zorp-/See adsorption.
Chile saltpeter /chil-ee/ See sodium ni-trate.
china clay (kaolin) A white powder ob-tained from the natural decomposition ofgranites. It is used as a filler in paper-mak-ing, in the pottery industries, and in phar-maceuticals. See kaolinite.
chiral /kÿr-ăl/ Having the property ofchirality. For example, lactic acid is a chi-ral compound because it has two possiblestructures that cannot be superposed. Seeoptical activity.
chirality /kÿr-al-ă-tee/ The property ofexisting in left- and right-handed forms;i.e. forms that are not superposable inthree-dimensional space. In chemistry theterm is applied to the existence of opticalisomers. See optical activity.
chirality element A part of a moleculethat causes it to display chirality. The mostcommon type of element is a chirality cen-ter, which is an atom attached to four dif-ferent atoms or groups. This is alsoreferred to as an asymmetric atom. Lesscommonly a molecule may have a chiralityaxis, as in the case of certain substituted al-lenes of the type R1R2C=C=CR3R4. In thisform of compound the R1 and R2 groupsdo not lie in the same plane as the R3 andR4 groups because of the nature of the dou-ble bonds. The chirality axis lies along theC=C=C chain. It is also possible to havemolecules that contain a chirality plane.See optical activity.
chloral /klor-ăl, kloh-răl/ See trichloro-ethanal.
chloral hydrate See trichloroethanal.
chloramine /klor-ă-meen, kloh-ră-/(NH2Cl) A colorless liquid made by re-acting ammonia with sodium chlorate(I)(NaOCl). It is formed as an intermediate inthe production of hydrazine. Chloramine isunstable and changes explosively into am-monium chloride and nitrogen trichloride.
chlorate /klor-ayt, kloh-rayt/ A salt ofchloric(V) acid.
chloric(I) acid (hypochlorous acid;HClO) A colorless liquid produced whenchlorine is dissolved in water. It is a bleachand gives chlorine water its disinfectantproperties. To increase the yield of acid,the chlorine water can be shaken with asmall amount of mercury(II) chloride. TheCl–O bond is broken more easily than theO–H bond in aqueous solution; the acid isconsequently a poor proton donor andhence a weak acid.
chloric(III) acid (chlorous acid; HClO2)A pale yellow liquid produced by reactingchlorine dioxide with water. It is a weakacid and oxidizing agent.
chloric(V) acid (chloric acid; HClO3) Acolorless liquid with a pungent odor,formed by the action of dilute sulfuric acidon barium chlorate. It is a strong acid andhas bleaching properties. Chloric(V) acid isa strong oxidizing agent and in concen-trated solution it will ignite organic sub-stances, such as paper and sugar.
chloric(VII) acid (perchloric acid;HClO4) A colorless liquid that fumesstrongly in moist air. It is made by vacuumdistillation of a mixture of potassium per-chlorate and concentrated sulfuric acid. Incontact with organic material it is danger-ously explosive.
The hydrate (HClO4.H2O) of chlo-ric(VII) acid is a white crystalline solid atroom temperature and has an ionic latticestructure of the form (H3O)+(ClO4)–.
chloric acid /klor-ik, kloh-rik/ See chlo-ric(V) acid.
chloride /klor-ÿd, kloh-rÿd/ See halide;chlorine.
chlorination /klor-ă-nay-shŏn, kloh-ră-/1. Treatment with chlorine; for instance,the use of chlorine to disinfect water.2. See halogenation.
chlorine /klor-een, -in, kloh-reen, -rin/ Agreen reactive gaseous element belongingto the halogens; i.e. group 17 (formerlyVIIA) of the periodic table. It occurs in sea-water, salt lakes, and underground de-posits of halite, NaCl. It accounts for about0.055% of the Earth’s crust. Chlorine isstrongly oxidizing and can be liberatedfrom its salts only by strong oxidizingagents, such as manganese(IV) oxide,potassium permanganate(VII), or potas-sium dichromate; note that sulfuric acid isnot sufficiently oxidizing to release chlo-rine from chlorides. Industrially, chlorineis prepared by the electrolysis of brine andin some processes chlorine is recovered bythe high-temperature oxidation of wastehydrochloric acid. Chlorine is used in largequantities, both as the element, to producechlorinated organic solvents, and for theproduction of polyvinyl chloride (PVC),the major thermoplastic in use today, andin the form of hypochlorites for bleaching.
Chlorine reacts directly and often vig-orously with many elements; it reacts ex-plosively with hydrogen in sunlight to formhydrogen chloride, HCl, and combineswith the electropositive elements to formmetal chlorides. The metals of main groups1 and 2 form ionic chlorides but an in-crease in the metal charge/size ratio leadsto the chlorides becoming increasingly co-valent. For example, CsCl is totally ionic,AlCl3 has a layer lattice, and TiCl4 is es-sentially covalent. The electronegative ele-ments form volatile molecular chloridescharacterized by the single covalent bondto chlorine. With the exception of Pb2+,Ag+, and Hg2
2+, the ionic chlorides are sol-uble in water, dissolving to give the hy-drated metal ion and the chloride ion Cl–.Chlorides of metals other than the mostelectropositive are hydrolyzed if aqueoussolutions are evaporated; for example,
ZnCl2 + H2O → Zn(OH)Cl + HClFeCl3 + 3H2O → Fe(OH)3 + 3HCl
Chlorine forms four oxides, chlorinemonoxide, Cl2O; chlorine dioxide, ClO2;chlorine hexoxide, Cl2O6; and chlorineheptoxide, Cl2O7; all of which are highlyreactive and explosive. Chlorine dioxidefinds commercial application as an activeoxidizing agent but because of its explosivenature it is usually diluted by air or othergases. The chloride ion is able to functionas a ligand with a large variety of metalions forming such species as [FeCl4]–,[CuCl4]3–, and [Co(NH3)4Cl2]+. The for-mation of anionic chloro-complexes is ap-plied to the separation of metals byanion-exchange methods.
Because of the hydrolysis of many metalchlorides when solutions are evaporated,special techniques must be used to prepareanhydrous chlorides. These are:1. reaction of dry chlorine with the hot
metal;2. reaction of dry hydrogen chloride with
the hot metal (lower valences);3. reaction of dry hydrogen chloride on the
hydrated chloride.The solubility of inorganic metal chloridesis such that they are not an environmentalproblem unless the metal ion itself is toxicbut many organochlorine compounds aresufficiently stable for the accumulatedresidues of chlorine-containing pesticidesto present a severe problem in some areas.This arises because they can accumulate infood chains and concentrate in the tissuesof higher animals (see DDT). Chlorine andhydrogen chloride are both highly toxic.Thus chlorides that hydrolyze to releaseHCl should also be regarded as toxic. Asorganochlorine compounds are frequentlyeven more toxic, they should not be han-dled without gloves and precautionsshould be taken against inhalation.
Symbol: Cl; m.p. –100.38°C; b.p.–33.97°C; d. 3.214 kg m–3 (0°C); p.n. 17;r.a.m. 35.4527.
chlorine dioxide /dÿ-oks-ÿd, -id/(ClO2) An orange gas formed by the ac-tion of concentrated sulfuric acid on potas-sium chlorate. It is a powerful oxidizingagent and its explosive properties in the
chloride
62
63
chloroplatinic acid
presence of a reducing agent were used tomake one of the first matches. It is widelyused in the purification of water and as ableach in the flour and wood-pulp indus-try. On an industrial scale an aqueous so-lution of chlorine dioxide is made bypassing nitrogen dioxide up a towerpacked with a fused mixture of aluminumoxide and clay, down which a solution ofsodium chlorate flows.
chlorine monoxide /mon-oks-ÿd, -id/ See dichlorine oxide.
chlorine(I) oxide See dichlorine oxide.
chlorite /klor-ÿt, kloh-rÿt/ A chlorate(III)salt; i.e. a salt of chloric(III) acid (chlorousacid).
chloroacetic acid /klor-oh-ă-see-tik, -set-ik, kloh-roh-/ See chloroethanoic acid.
chlorobenzene /klor-oh-ben-zeen, klor-oh-ben-zeen, kloh-roh-/ (monochloroben-zene; C6H5Cl) A colorless liquid made bythe catalytic reaction of chlorine with ben-zene. It can be converted to phenol by re-action with sodium hydroxide underextreme conditions (300°C and 200 atmos-pheres pressure). It is also used in the man-ufacture of other organic compounds.
2-chlorobuta-1,3-diene /klor-ŏ-byoo-tă-dÿ-een, kloh-roh- / (chloroprene; H2C:CH.CCl:CH2) A colorless liquid derivative ofbutadiene used in the manufacture of neo-prene rubber.
chlorobutyl /klor-oh-byoo-t’l, kloh-roh-/See butyl rubber.
chloroethane /klor-oh-eth-ayn, kloh-roh-/(ethyl chloride; C2H5Cl) A gaseous com-pound made by the addition of hydrogenchloride to ethene. It is used as a refriger-ant and a local anesthetic.
chloroethanoic acid /klor-oh-eth-ă-noh-ik, kloh-roh-/ (chloroacetic acid;CH2ClCOOH) A colorless crystallinesolid made by substituting one of the hy-drogen atoms of the methyl group of
ethanoic acid with chlorine, using redphosphorus. It is a stronger acid thanethanoic acid because of the electron-with-drawing effect of the chlorine atom.Dichloroethanoic acid (dichloroaceticacid, CHCl2COOH) and trichloroethanoicacid (trichloroacetic acid, CCl3COOH) aremade in the same way. The acid strengthincreases with the number of chlorineatoms present.
chloroethene /klor-oh-eth-een, kloh-roh-/(vinyl chloride; H2C:CHCl) A gaseousorganic compound used in the manufac-ture of PVC (polyvinyl chloride). Chloro-ethene is manufactured by the reactionbetween ethyne and hydrogen chlorideusing a mercury(II) chloride catalyst:
C2H2 + HCl → H2C:CHClAn alternative source, making use of theready supply of ethene, is via dichloro-ethane:
H2C:CH2+ Cl2 → CH2Cl.CH2Cl →H2C:CHCl
chlorofluorocarbon /klor-ŏ-floo-ŏ-rŏ-kar-bŏn, kloh-roh-/ See halocarbon.
chloroform /klor-ŏ-form, kloh-rŏ-/ Seetrichloromethane.
chloromethane /klor-ŏ-meth-ayn, kloh-roh-/ (methyl chloride; CH3Cl)
chlorophylls /klor-ŏ-filz, kloh-rŏ-/ Thepigments present in plants that act as cata-lysts in the photosynthesis of carbohy-drates from carbon dioxide and water.There are four types, known as chloro-phylls a, b, c, and d.
chloroplatinic acid /klor-ŏ-plă-tin-ik,kloh-rŏ-/ (platinic chloride; H2PtCl6) Areddish compound prepared by dissolvingplatinum in aqua regia. When crystallizedfrom the resulting solution, crystals of thehexahydrate (H2PtCl6.6H2O) are ob-tained. The crystals are needle-shaped anddeliquesce on exposure to moist air.Chloroplatinic acid is a relatively strongacid, giving rise to the family of chloro-platinates.
chloroprene /klor-ŏ-preen, kloh-rŏ-/ See2-chlorobuta-1,3-diene.
chlorous acid /klor-ŭs, kloh-rŭs/ Seechloric(III) acid.
cholesteric crystal /kŏ-less-tĕ-rin/ Seeliquid crystal.
chromate /kroh-mayt/ Any oxygen-con-taining derivative of chromium. Usuallythe term is reserved for the chromate(VI)species.
chromatography /kroh-mă-tog-ră-fee/ A technique used to separate or analyzecomplex mixtures. A number of relatedtechniques exist; all depend on two phases:a mobile phase, which may be a liquid or agas, and a stationary phase, which is eithera solid or a liquid held by a solid. The sam-ple to be separated or analyzed is carriedby the mobile phase through the stationaryphase. Different components of the mix-ture are absorbed or dissolved to differentextents by the stationary phase, and conse-quently move along at different rates. Inthis way the components are separated.There are many different forms of chro-matography depending on the phases usedand the nature of the partition process be-tween mobile and stationary phases. Themain classification is into column chro-matography and planar chromatography.
A simple example of column chro-matography is in the separation of liquidmixtures. A vertical column is packed withan absorbent material, such as alumina(aluminum oxide) or silica gel. The sampleis introduced into the top of the columnand washed down it using a solvent. Thisprocess is known as elution; the solventused is the eluent and the sample being sep-arated is the eluate. If the components arecolored, visible bands appear down thecolumn as the sample separates out. Thecomponents are separated as they emergefrom the bottom of the column. In this par-ticular example of chromatography thepartition process is adsorption on the par-ticles of alumina or silica gel. Columnchromatography can also be applied tomixtures of gases. See gas chromatogra-
phy. In the other main type of chromato-graphy, planar chromatography, the sta-tionary phase is a flat sheet of absorbentmaterial. See paper chromatography; thin-layer chromatography.
Components of the mixture are heldback by the stationary phase either by ad-sorption (e.g. on the surface of alumina) orbecause they dissolve in it (e.g. in the mois-ture within chromatography paper).
chrome alum /krohm/ See alum.
chromic anhydride /kroh-mik/ Seechromium(VI) oxide.
chromic oxide See chromium(III)oxide.
chromite /kroh-mÿt/ (chrome iron ore;FeCr2O4) A mineral that consists ofmixed oxides of chromium and iron, theprincipal ore of chromium. It occurs asblack masses with a metallic luster.
chromium /kroh-mee-ŭm/ A transitionmetal that occurs naturally as chromite(FeO.Cr2O3), large deposits of which arefound in Zimbabwe. The ore is convertedinto sodium dichromate(VI) and then re-duced with carbon to chromium(III) oxideand finally to metallic chromium with alu-minum. Chromium is used in strong alloysteels and stainless steel and for plating ar-ticles. It is a hard silvery metal that resistscorrosion at normal temperatures. It reactsslowly with dilute hydrochloric and sulfu-ric acids to give hydrogen and bluechromium(II) compounds, which quicklyoxidize in air to green chromium(III) ions.The oxidation states are +6 in chromates(CrO4
2–) and dichromates (Cr2O72–), +3
(the most stable), and +2. In acidic solu-tions the yellow chromate(VI) ion changesto the orange dichromate(VI) ion. Dichro-mates are used as oxidizing agents in thelaboratory; for example as a test for sul-fur(IV) oxide (sulfur dioxide) and to oxi-dize alcohols.
Symbol: Cr; m.p. 1860±20°C; b.p.2672°C; r.d. 7.19 (20°C); p.n. 24; r.a.m.51.9961.
chloroprene
64
chromium(II) oxide (chromous oxide;CrO) A black powder prepared by theoxidation of chromium amalgam with di-lute nitric acid. At high temperatures(around 1000°C) chromium(II) oxide is re-duced by hydrogen.
chromium(III) oxide (chromic oxide;chromium sesquioxide; Cr2O3) A greenpowder that is almost insoluble in water. Itis isomorphous with iron(III) oxide andaluminum(III) oxide. Chromium(III) oxideis prepared by gently heatingchromium(III) hydroxide or by heatingammonium dichromate. Alternative prepa-rations include the heating of a mixture ofammonium chloride and potassiumdichromate or the decomposition ofchromyl chloride by passing it through ared-hot tube. Chromium(III) oxide is usedas a pigment in the paint and glass indus-tries.
chromium(IV) oxide (chromium diox-ide; CrO2) A black solid prepared byheating chromium(III) hydroxide in oxy-gen at a temperature of 300–350°C.Chromium(IV) oxide is very unstable.
chromium(VI) oxide (chromium triox-ide; chromic anhydride; CrO3) A redcrystalline solid formed when concentratedsulfuric acid is added to a cold saturatedsolution of potassium dichromate. Thelong prismatic needle-shaped crystals thatare produced are extremely deliquescent.Chromium(VI) oxide is readily soluble inwater, forming a solution that containsseveral of the polychromic acids. On heat-ing it decomposes to give chromium(III)oxide. Chromium(VI) oxide is used as anoxidizing reagent.
chromophore /kroh-mŏ-for, -fohr/ Agroup of atoms in a molecule that is re-sponsible for the color of the compound.
chromous oxide /kroh-mŭs/ Seechromium(II) oxide.
chromyl chloride /kroh-măl/(CrO2Cl2) A dark red covalent liquidprepared either by distilling a dry mixture
of potassium dichromate and sodium chlo-ride with concentrated sulfuric acid or bythe action of concentrated sulfuric acid onchromium(VI) oxide dissolved in concen-trated hydrochloric acid. Chromyl chlorideis hydrolyzed by water, and with solutionsof alkalis it undergoes immediate hydroly-sis to produce chromate ions. It is used asan oxidizing agent in organic chemistry.Chromyl chloride oxidizes methyl groupsat the ends of aromatic side chains to alde-hyde groupings (Étard’s reaction).
cinnabar /sin-ă-bar/ See mercury(II) sul-fide.
cinnamic acid /să-nam-ik, sin-ă-mik/ See 3-phenylpropenoic acid.
CIP system (Cahn–Ingold–Prelog system)A method of producing a sequence ruleused in the absolute description ofstereoisomers in the R–S convention (seeoptical activity) or the E-Z CONVENTION.The rule is to consider the atoms that arebound directly to a chiral center (or to adouble bond). The group in which thisatom has the highest proton number hasthe highest priority. So, for example, inHCClBr(NH2), the order of priority is Br >Cl > NH2 > H. If two atoms are the same,the substituents are considered, with thesubstituents of highest proton number tak-ing precedence. So in
C(NH2)(NO2)(CH3)(C2H6 )the order is NO2 > NH2 > C2H6 > CH3.The system is named after the Britishchemists Robert Cahn (1899–1981) andSir Christopher Ingold (1893–1970) andthe Bosnian–Swiss chemist Vladimir Prelog(1906– ).
cis- Designating an isomer with groupsthat are adjacent. See isomerism.
cis-trans isomerism /sis-tranz/ See iso-merism.
citric acid /sit-rik/ A white crystallinecarboxylic acid important in plant and an-imal cells. It is present in many fruits. Thesystematic name is 2-hydroxypropane-1,2,3-tricarboxylic acid. The formula is:
65
citric acid
HOOCCH2C(OH)(COOH)CH2COOH
Claisen condensation /klÿ-zĕn/ A reac-tion in which two molecules of ester com-bine to give a keto-ester – a compoundcontaining a ketone group and an estergroup. The reaction is base-catalyzed bysodium ethoxide; the reaction of ethylethanoate refluxed with sodium ethoxidegives:
2CH3.CO.OC2H5 →CH3.CO.CH2.CO.OC2H5 + C2H5OH
The mechanism is similar to that of thealdol reaction, the first step being forma-tion of a carbanion from the ester:
CH3COC2H5 + –OC2H5 →–CH2COC2H5 + C2H5OH
This attacks the carbon atom of the car-bonyl group on the other ester molecule,forming an intermediate anion that decom-poses to the keto-ester and the ethanoateion. The reaction is named for the Germanorganic chemist Ludwig Claisen (1851–1930).
Clark cell A type of cell formerly usedas a standard source of e.m.f. It consists ofa mercury cathode coated with mercurysulfate, and a zinc anode. The electrolyte iszinc sulfate solution. The e.m.f. producedis 1.4345 volts at 15°C. The Clark cell hasbeen superseded as a standard by the We-ston (Trademark) cadmium cell. The cell isnamed for the English engineer Josiah La-timer Clark (1822–98).
clathrate /klath-rayt/ (enclosure com-pound) A substance in which small(guest) molecules are trapped within thelattice of a crystalline (host) compound.Clathrates are formed when suitable hostcompounds are crystallized in the presenceof molecules of the appropriate size. Al-though the term ‘clathrate compound’ isoften used, they are not true compounds;no chemical bonds are formed, and theguest molecules interact by weak van derWaals forces. The clathrate is maintainedby the cagelike lattice of the host. The hostlattice must be broken down, for example,by heating or dissolution in order to releasethe guest. This should be compared withzeolites, in which the holes in the host lat-
tice are large enough to permit entrance oremergence of the guest without breakingbonds in the host lattice. Quinol formsmany clathrates, e.g. with SO2; water (ice)forms a clathrate with xenon.
clays Naturally occurring aluminosili-cates which form pastes and gels withwater.
cleavage The splitting of a crystal alongplanes of atoms, to form smooth surfaces.
close packing The arrangement of par-ticles (usually atoms) in crystalline solids inwhich each particle has 12 nearest neigh-bors: six in the same layer (or plane) as it-self and three each in the layer above andbelow. This arrangement provides themost economical use of space. The twoprincipal types are cubic close packing andhexagonal close packing.
cluster compound A type of compoundin which a cluster of metal atoms are joinedby metal–metal bonds. Cluster compoundsare formed by certain transition elements,such as molybdenum and tungsten.
coagulation /koh-ag-yŭ-lay-shŏn/ Theassociation of particles (e.g. in colloids)into clusters. See flocculation.
coal A black mineral that consistsmainly of carbon, used as a fuel and as asource of organic chemicals. It is the fos-silized remains of plants that grew in theCarboniferous and Permian periods andwere buried and subjected to high pres-sures underground. There are various typesof coal, classified according to their in-creasing carbon content. See anthracite; bi-tuminous coal; lignite.
coal gas A fuel gas made by heating coalin a limited supply of air. It consists mainlyof hydrogen and methane, with some car-bon monoxide (which makes the gas highlypoisonous). COAL TAR and coke are formedas by-products.
coal tar Tar produced by heating coal inthe absence of oxygen. It is a mixture of
Claisen condensation
66
many organic compounds (e.g. benzene,toluene, and naphthalene) and also con-tains free carbon.
cobalt /koh-bawlt/ A lustrous silvery-blue hard ferromagnetic transition metaloccurring in association with nickel. It isused in alloys for magnets, cutting tools,and electrical heating elements and in cata-lysts and some paints.
Symbol: Co; m.p. 1495°C; b.p.2870°C; r.d. 8.9 (20°C); p.n. 27; r.a.m.58.93320.
cobaltic oxide /koh-bawl-tik/ Seecobalt(III) oxide.
cobaltous oxide /koh-bawl-tŭs/ Seecobalt(II) oxide.
cobalt(II) oxide (cobaltous oxide;CoO) A green powder prepared by theaction of heat on cobalt(II) hydroxide inthe absence of air. Alternatively it may beprepared by the thermal decomposition ofcobalt(II) sulfate, nitrate, or carbonate,also in the absence of air. Cobalt(II) oxideis a basic oxide, reacting with acids to givesolutions of cobalt(II) salts. It is stable inair up to temperatures of around 600°C,after which it absorbs oxygen to form tri-cobalt tetroxide. Cobalt(II) oxide can bereduced to cobalt by heating in a stream ofcarbon monoxide or hydrogen. It is used inthe pottery industry and in the productionof vitreous enamels.
cobalt(III) oxide (cobaltic oxide;Co2O3) A dark gray powder formed bythe thermal decomposition of eithercobalt(II) nitrate or carbonate in air. Ifheated in air, cobalt(III) oxide undergoesfurther oxidation to give tricobalt tetrox-ide, Co3O4.
coenzymes Small organic molecules(compared to the size of an enzyme) thatenable enzymes to carry out their catalyticactivity. Examples include nicotinamideadenine dinucleotide (NAD) and ubi-quinone (coenzyme Q). Some coenzymesare capable of catalyzing reactions in theabsence of an enzyme but the rate of reac-
tion is never as high as when a catalyst ispresent. A coenzyme is not a true catalystbecause it undergoes chemical change dur-ing the reaction.
coherent units A system or sub-set ofunits (e.g. SI units) in which the derivedunits are obtained by multiplying or divid-ing together base units, with no numericalfactor involved.
coinage metals A group of malleablemetals forming group 11 of the periodictable. (They were formerly subgroup IB.)The metals all have an outer s1 electronicconfiguration but they differ from the al-kali metals (also outer s1) in having inner d-electrons. They are copper (Cu), silver(Ag), and gold (Au). The coinage metals allhave much higher ionization potentialsthan the alkali metals and high positivestandard electrode potentials and aretherefore much more difficult to oxidize. Afurther significant difference from the al-kali metals is the variety of oxidation statesobserved for the coinage metals. Thus cop-per in aqueous chemistry is the familiar(hydrated) blue divalent Cu2+ ion but col-orless copper(I) compounds can be pre-pared with groups that stabilize lowvalences such as CN–, e.g. CuCN. A fewcompounds of copper(III) have also beenprepared. The common form of silver isAg(I), e.g. AgNO3, with a few Ag(II) com-pounds stable in the solid only. The mostcommon oxidation state of gold is Au(III),although cyanide ion again stabilizes Au(I)compounds, e.g., [Au(CN)4]3–. The metalsall form a large number of coordinationcompounds (unlike group 1) and are gen-erally treated with the other elements of theappropriate transition series.
coke A dense substance obtained fromthe carbonization of coal. It is used as afuel and as a reducing agent.
colligative properties /kŏ-lig-ă-tiv/ Agroup of properties of solutions that de-pends on the number of particles present,rather than the nature of the particles. Col-ligative properties include:1. The lowering of vapor pressure.
67
colligative properties
2. The elevation of boiling point.3. The lowering of freezing point.4. Osmotic pressure.
Colligative properties are all basedupon empirical observation. The explana-tion of these closely related phenomena de-pends on intermolecular forces and thekinetic behavior of the particles, which isqualitatively similar to those used in deriv-ing the kinetic theory of gases.
collimator /kol-ă-may-ter/ An arrange-ment for producing a parallel beam of ra-diation for use in a spectrometer or otherinstrument. A system of lenses and slits isutilized.
colloid /kol-oid/ A heterogeneous sys-tem in which the interfaces betweenphases, though not visibly apparent, areimportant factors in determining the sys-tem properties. The three important attrib-utes of colloids are:1. They contain particles, commonly made
up of large numbers of molecules, form-ing the distinctive unit or disperse phase.
2. The particles are distributed in a contin-uous medium (the continuous phase).
3. There is a stabilizing agent, which has anaffinity for both the particle and themedium; in many cases the stabilizer is apolar group.
Particles in the disperse phase typicallyhave diameters in the range 10–6–10–4 mm.Milk, rubber, and emulsion paints are typ-ical examples of colloids. See also sol.
colorimetric analysis /kul-ŏ-ră-met-rik/ Quantitative analysis in which theconcentration of a colored solute is mea-sured by the intensity of the color. The testsolution can be compared against standardsolutions.
columbium /kŏ-lum-bee-ŭm/ The for-mer name (in the USA) for niobium.
column chromatography See chro-matography, gas chromatography.
combustion A reaction with oxygenwith the production of heat and light. Thecombustion of solids and liquids occurs
when they release flammable vapor, whichreacts with oxygen in the gas phase. Com-bustion reactions usually involve a com-plex sequence of free-radical chainreactions. The light is produced by excitedatoms, molecules, or ions. In highly lumi-nous flames it comes from small incandes-cent particles of carbon.
Sometimes the term is also applied toslow reactions with oxygen, and also to re-actions with other gases (for example, cer-tain metals ‘burn’ in chlorine). See alsospontaneous combustion.
common salt See sodium chloride.
complex (coordination compound) Atype of compound in which molecules orions form coordinate bonds with a metalatom or ion. The coordinating species(called ligands) have lone pairs of elec-trons, which they can donate to the metalatom or ion. They are molecules such asammonia or water, or negative ions such asCl– or CN–. The resulting complex may beneutral or it may be a complex ion. For ex-ample:
Cu2+ + 4NH3 → [Cu(NH3)4]2+
Fe3+ + 6CN– → [Fe(CN)6]3–
Fe2+ + 6CN– → [Fe(CN)6]4–
The formation of such coordinationcomplexes is typical of transition metals.Often the complexes contain unpairedelectrons and are paramagnetic and col-ored. See also chelate. See illustration over-leaf.
component /kŏm-poh-nĕnt/ One of theseparate chemical substances in a mixturein which no chemical reactions are takingplace. For example, a mixture of ice andwater has one component; a mixture of ni-trogen and oxygen has two components.When chemical reactions occur betweenthe substances in a mixture, the number ofcomponents is defined as the number ofchemical substances present minus thenumber of equilibrium reactions takingplace. Thus, the system: N2 + 3H2 ˆ2NH3 is a two-component system. See alsophase rule.
collimator
68
69
condensation
compound A chemical combination ofatoms of different elements to form a sub-stance in which the ratio of combiningatoms remains fixed and is specific to thatsubstance. The constituent atoms cannotbe separated by physical means; a chemicalreaction is required for the compounds tobe formed or to be changed. The existenceof a compound does not necessarily implythat it is stable. Many compounds havelifetimes of less than a second. See alsomixture.
concentrated Denoting a solution inwhich the amount of solute in the solvent isrelatively high. The term is always relative;for example, whereas concentrated sulfuricacid may contain 96% H2SO4, concen-trated potassium chlorate may contain aslittle as 10% KClO3.Compare dilute.
concentration The amount of sub-stance per unit volume or mass in a solu-tion. Molar concentration is amount ofsubstance (in moles) per cubic decimeter(liter). Mass concentration is mass of soluteper unit volume. Molal concentration isamount of substance (in moles) per kilo-gram of solute.
concerted reaction A reaction thattakes place in a single stage rather than asa series of simple steps. In a concerted re-action there is a transition state in whichbonds are forming and breaking at thesame time. An example is the SN2 mecha-nism in NUCLEOPHILIC SUBSTITUTION. Seealso pericyclic reaction.
condensation /kon-den-say-shŏn/ Theconversion of a gas or vapor into a liquidor solid by cooling.
X
X
X M
X
X
X
X
X
X
X M
X
XM
X
X
X
X
X
X
M
X
octahedral
square-planar
tetrahedral trigonal-bipyramid
Shapes of complexes
condensation polymerization
70
condensation polymerization See poly-merization.
condensation reaction A reaction inwhich addition of two molecules occursfollowed by elimination of a smaller mol-ecule, usually water. Condensation reac-tions (addition–elimination reactions) arecharacteristic of aldehydes and ketones re-acting with a range of nucleophiles. Thereis typically nucleophilic addition at the Catom of the carbonyl group followed byelimination of water.
conducting polymer A type of organicpolymer that conducts electricity like ametal. Conducting polymers are crystallinesubstances containing conjugated unsatu-rated carbon–carbon bonds. In principle,they provide lighter and cheaper alterna-tives to metallic conductors.
conductiometric titration /kŏn-duk-tee-oh-met-rik/ A titration in which measure-ment of the electrical conductance is madecontinuously throughout the addition ofthe titrant and well beyond the equivalence
R
NH2
OH
R’
NH3ammonia
H2NNH2hydrazine
C6H5NH NH2phenylhydrazine
HONH2hydroxlamine
R
R’NH2NC
H
C6H5
NNCR
R’
OH
NCR
R’
Nucleophile Product Name
C
phenylhydrazone
hydroxy amine
hydrazone
oxime
In the above the reactant is RCOR’
Condensation reactions with aldehydes and ketones
71
conjugate acid
point. This is in place of traditional end-point determination by indicators. The op-eration is carried out in a conductance cell,which is part of a resistance bridge circuit.The method depends on the fact that ionshave different ionic mobilities, H+ andOH– having particularly high values. Themethod is especially useful for weakacid–strong base and strong acid–weakbase titrations for which color-changetitrations are unreliable.
configuration /kŏn-fig-yŭ-ray-shŏn/ 1.The arrangement of electrons about thenucleus of an atom. Configurations arerepresented by symbols, which contain:1. An integer, which is the value of the
principal quantum number (shell num-ber).
2. A lower-case letter representing thevalue of the azimuthal quantum number(l), i.e.
s means l = 0, p means l = 1,d means l = 2, f means l = 3.
3. A numerical superscript giving the num-ber of electrons in that particular set; forexample, 1s2, 2p3, 3d5.
The ground state electronic configura-tion (i.e. the most stable or lowest energystate) may then be represented as follows,for example, He, 1s2; N, 1s22s22p5. Theseare identical to the ‘electron box’ models,which are often met. However, elementsare commonly abbreviated by using aninert gas to represent the ‘core’, e.g. Zr hasthe configuration [Kr]4d25s2.2. The arrangement of atoms or groups ina molecule.
conformation A particular shape ofmolecule that arises through the normalrotation of its atoms or groups about singlebonds. Any of the possible conformationsthat may be produced is called a con-former, and there will be an infinite num-ber of these possibilities, differing in theangle between certain atoms or groups onadjacent carbon atoms. Sometimes, theterm ‘conformer’ is applied more strictly topossible conformations that have mini-mum energies – as in the case of the boatand chair conformations of CYCLOHEXANE.
conjugate acid /kon-jŭ-git/ See acid.
chair conformation boat conformation
Conformations of cyclohexane
eclipsed staggered gauche
Conformations for rotation about a single bond
XXX
X
XX
Conformations
conjugate base See base.
conjugated /kon-jŭ-gay-tid/ Describingcompounds that have alternating doubleand single bonds in their structure. For ex-ample, but-1,3-ene (H2C:CHCH:CH2) is atypical conjugated compound. In suchcompounds there is delocalization of theelectrons in the double bonds.
conservation of energy, law of Enun-ciated by Helmholtz in 1847, this lawstates that in all processes occurring in anisolated system the energy of the system re-mains constant. The law does of coursepermit energy to be converted from oneform to another (including mass, since en-ergy and mass are equivalent).
conservation of mass, law of Formu-lated by Lavoisier in 1774, this law statesthat matter cannot be created or destroyed.Thus in a chemical reaction the total massof the products equals the total mass of thereactants (the term ‘mass’ must include anysolids, liquids, and gases – including air –that participate).
Constantan /kon-stăn-tan/ (Trademark)A copper-nickel (cupronickel) alloy con-taining 45% nickel. It has a high electricalresistivity and very low temperature coeffi-cient of resistance and is therefore used inthermocouples and resistors.
constant-boiling mixture A generalobservation for most liquids is that thevapor phase above a liquid is richer in themore volatile component (a deviation fromRaoult’s law). Consequently most liquidmixtures show a regular increase in the
boiling point as the liquid is progressivelydistilled. The boiling point-compositioncurve shows that distillation of the liquidof composition L1 gives a vapor richer in Aand represented by composition V1. Fur-ther distillation leads to the liquid compo-sition moving towards B. For certainmixtures in which there are strong inter-molecular attractions the boiling point-composition curves show minima ormaxima. Fractional distillation of the for-mer leads to initial changes in the vaporuntil a distillate of composition L2 isreached at which point a constant boilingmixture or AZEOTROPE distills over. Furtherattempts to fractionate the distillate do notlead to a change in composition. An exam-ple of an azeotropic mixture of minimumboiling point is water (b.p. 100°C) andethanol (b.p. 78.3°C), the azeotrope being4.4% water and boiling at 78.1°C.
Mixtures that display a maximum inthe boiling point-composition curve canlead to initial separation of pure A on frac-tionation but as the composition of the liq-uid moves towards B and reaches themaximum, a constant boiling mixture L3 isreached that will distill over unchanged.An example of an azeotropic mixture ofmaximum boiling point is water (b.p.100°C) and hydrogen chloride (b.p.–80°C), the azeotrope being 80% waterand boiling at 108.6°C.
constant composition, law of Seeconstant proportions; law of.
constant proportions, law of Formu-lated by Proust in 1779 after the analysis ofa large number of compounds, the princi-ple that the proportion of each element in
conjugate base
72
Tem
per
atu
re
Tem
per
atu
re
Tem
per
atu
re
100% A 100% B 100% A 100% B 100% A 100% B
V1
L1 L2 L3
Constant-boiling mixtures
73
copper(I) chloride
a compound is fixed or constant. It followsthat the composition of a pure chemicalcompound is independent of the method ofpreparation. It is also called the law of def-inite proportions and the law of constantcomposition.
contact process An industrial processfor the manufacture of sulfuric acid. Sul-fur(IV) oxide and air are passed over aheated catalyst (vanadium(V) oxide orplatinum) and sulfur(VI) oxide is pro-duced:
2SO2 + O2 → 2SO3The sulfur(VI) oxide is dissolved in sulfuricacid
SO3 + H2SO4 → H2S2O7The resulting oleum is diluted to give sul-furic acid.
continuous phase See colloid.
continuous process A manufacturingprocess in which the raw materials are con-stantly fed into the plant. These react asthey flow through the equipment to give acontinuing flow of product. At any point,only a small amount of material is at a par-ticular stage in the process but material atall stages of the reaction is present. Thefractional distillation of crude oil is an ex-ample of a continuous process. Suchprocesses are relatively easy to automateand can therefore manufacture a productcheaply. The disadvantages of continuousprocessing are that it usually caters for alarge demand and the plant is expensive toinstall and cannot normally be used tomake other things. Compare batchprocess.
continuous spectrum A SPECTRUM
composed of a continuous range of emittedor absorbed radiation. Continuous spectraare produced in the infrared and visible re-gions by hot solids.
converter See Bessemer process.
coordinate bond /koh-or-dă-nit/ (dativebond) A covalent bond in which thebonding pair is visualized as arising fromthe donation of a lone pair from one
species to another species, which behavesas an electron acceptor. The definition in-cludes such examples as the ‘donation’ ofthe lone pair of the ammonia molecule toH+ (an acceptor) to form NH4
+ or to Cu2+
to form [Cu(NH3)4]2+.The donor groups are known as Lewis
bases and the acceptors are either hydro-gen ions or Lewis acids. Simple combina-tions, such as H3N→BF3, are known asadducts. See also complex.
coordination compound /koh-or-dă-nay-shŏn/ See complex.
coordination number The number ofcoordinate bonds formed to a metal atomor ion in a complex.
copolymerization /koh-pol-ă-mer-i-zay-shŏn/ See polymerization.
copper /kopp-er/ A transition metal oc-curring in nature principally as the sulfide.It is extracted by roasting the ore in a con-trolled air supply and purified by electroly-sis of copper(II) sulfate solution usingimpure copper as the anode and pure cop-per as the cathode. Copper is used in elec-trical wires and in such alloys as brass andbronze.
The metal itself is golden-red in color;copper(I) compounds are white (except theoxide, which is red), and copper(II) com-pounds are blue in solution. Copper is un-reactive to dilute acids (except nitric acid).Copper(I) compounds are unstable in solu-tion and decompose to copper and cop-per(II) ions. Both copper(I) and copper(II)ions form complexes, copper(II) ions beingidentified by the dark blue complex[Cu(NH3)4]2+ formed with excess ammo-nia solution.
Symbol: Cu; m.p. 1083.5°C; b.p.2567°C; r.d. 8.96 (20°C); p.n. 29; r.a.m.63.546.
copper(II) carbonate (CuCO3) A greencrystalline compound that occurs in min-eral form as the basic salt in azurite andmalachite. See also verdigris.
copper(I) chloride (cuprous chloride;
CuCl) A white solid, insoluble in water,prepared by heating copper(II) chloride inconcentrated hydrochloric acid with excesscopper turnings. When the solution is col-orless, it is poured into air-free water (orwater containing sulfur(IV) oxide) and awhite precipitate of copper(I) chloride isobtained. On exposure to air this precipi-tate turns green due to the formation ofbasic copper(II) chloride. Copper(I) chlo-ride absorbs carbon monoxide gas. It isused as a catalyst in the rubber industry.The chloride is essentially covalent instructure. In the vapor phase both dimericand trimeric forms exist. It is used in or-ganic chemistry in the Sandmeyer reac-tions.
copper(II) chloride (cupric chloride;CuCl2) A compound prepared by dis-solving excess copper(II) oxide orcopper(II) carbonate in dilute hydrochloricacid. On crystallization, emerald greencrystals of the dihydrate (CuCl2.2H2O) areobtained. The anhydrous chloride may beprepared as a brown solid by burning cop-per in excess chlorine. Alternatively the di-hydrate may be dehydrated usingconcentrated sulfuric acid. Dilute solutionsof copper(II) chloride are blue, concen-trated solutions are green, and solutions inthe presence of excess hydrochloric acidare yellow.
copper(II) nitrate (cupric nitrate;Cu(NO3)2) A compound prepared bydissolving either excess copper(II) oxide orcopper(II) carbonate in dilute nitric acid.On crystallization, deep blue crystals of thetrihydrate (Cu(NO3)2.3H2O) are obtained.The crystals are prismatic in shape and areextremely deliquescent. On heating, cop-per(II) nitrate will decompose to give cop-per(II) oxide, nitrogen(IV) oxide, andoxygen. The white anhydrous salt is pre-pared by adding a solution of dinitrogenpentoxide in nitric acid to crystals of thetrihydrate.
copper(I) oxide (cuprous oxide; CuO) An insoluble red powder prepared by theheating of copper with copper(II) oxide orthe reduction of an alkaline solution of
copper(II) sulfate. Copper(I) oxide is easilyreduced by hydrogen when heated; it is ox-idized to copper(II) oxide when heated inair. It is used in the glass industry. Cop-per(I) oxide undergoes disproportionationin acid solutions, producing copper(II) ionsand copper. The oxide will dissolve in con-centrated hydrochloric acid due to the for-mation of the complex ion [CuCl2]–.Copper(I) oxide is a covalent solid.
copper(II) oxide (cupric oxide; CuO) Ablack solid prepared by the action of heaton copper(II) nitrate, hydroxide, or car-bonate. It is a basic oxide and reacts withdilute acids to form solutions of copper(II)salts. Copper(II) oxide can be reduced tocopper by heating in a stream of hydrogenor carbon monoxide. It can also be reducedby mixing with carbon and heating themixture. Copper(II) oxide is stable up to itsmelting point, after which it decomposes togive oxygen, copper(I) oxide, and eventu-ally copper.
copper(II) sulfate (cupric sulfate; CuSO4)A compound prepared as the hydrate bythe action of dilute sulfuric acid on cop-per(II) oxide or copper(II) carbonate. Oncrystallization, blue triclinic crystals of thepentahydrate (blue vitriol, CuSO4.5H2O)are formed. Industrially copper(II) sulfateis prepared by passing air through a hotmixture of dilute sulfuric acid and scrapcopper. The solution formed is recycleduntil the concentration of the copper(II)sulfate is sufficient. Copper(II) sulfate isreadily soluble in water. The monohydrate(CuSO4.H2O) is formed at 100°C and theanhydrous salt at 250°C. Anhydrous cop-per(II) sulfate is white; it is extremely hy-groscopic and turns blue on absorption ofwater. It decomposes on heating to givecopper(II) oxide and sulfur(VI) oxide.
Copper(II) sulfate is used as a woodpreservative, a fungicide (in Bordeaux mix-ture), and in the dyeing and electroplatingindustries.
coral A form of calcium carbonate thatis secreted by various invertebrate marineanimals (such as certain members of the
copper(II) chloride
74
class Anthozoa) for support and livingspace.
corn rule See optical activity.
corrosion Reaction of a metal with anacid, oxygen, or other compound with de-struction of the surface of the metal. Rust-ing is a common form of corrosion.
corundum /kŏ-run-dŭm/ (emery;Al2O3) A naturally occurring form ofaluminum oxide that sometimes containssmall amounts of iron and silicon(IV)oxide. Ruby and sapphire are impure crys-talline forms. It is used in various polishes,abrasives, and grinding wheels.
coulomb /koo-lom/ Symbol: C The SIunit of electric charge, equal to the chargetransported by an electric current of oneampere flowing for one second. 1 C = 1 As. The unit is named for the French physi-cist Charles Augustin de Coulomb(1736–1806).
coulombmeter /koo-lom-ĕ-ter/ (coulome-ter; voltameter) A device for determiningelectric charge or electric current usingelectrolysis. The mass m of material re-leased in time t is measured and this can beused to calculate the charge (Q) and thecurrent (I) from the electrochemical equiv-alent (z) of the element, using Q = m/z or I= m/zt.
coulometer /koo-lom-ĕ-ter/ See coulomb-meter.
coumarin /koo-mă-rin/ (1,2-benzopyrone;C9H6O2) A colorless crystalline com-pound with a pleasant odor, used in mak-ing perfumes. On hydrolysis with sodiumhydroxide it forms coumarinic acid.
coumarone /koo-mă-rohn/ See benzfu-ran.
coupling A chemical reaction in whichtwo groups or molecules join together. Anexample is the formation of azo com-pounds.
covalent bond /koh-vay-lĕnt/ A bondformed by the sharing of an electron pairbetween two atoms. The covalent bond isconventionally represented as a line, thusH–Cl indicates that between the hydrogenatom and the chlorine atom there is anelectron pair formed by electrons of oppo-site spin implying that the binding forcesare strongly localized between the twoatoms. Molecules are combinations ofatoms bound together by covalent bonds;covalent bonding energies are of the order103 kJ mol–1.
Modern bonding theory treats the elec-tron pairing in terms of the interaction ofelectron (atomic) orbitals and describes thecovalent bond in terms of both ‘bonding’and ‘anti-bonding’ molecular orbitals.
covalent crystal A crystal in which theatoms present are covalently bonded. Theyare sometimes referred to as giant latticesor macromolecules. The best known com-pletely covalent crystal is diamond.
covalent radius The radius an atom isassumed to have when involved in a cova-lent bond. For homonuclear diatomic mol-ecules (e.g. Cl2) this is simply half themeasured internuclear distance. For het-eroatomic molecules substitutional meth-ods are used. For example, the internucleardistance of bromine fluoride (BrF) is about180 pm, therefore using 71 pm for the co-valent radius of fluorine (from F2) we get109 pm for bromine. The accepted value is114 pm.
cracking A process whereby petroleumdistillates are thermally decomposed tolighter fractions. The process can be purelythermal, although catalytic cracking is byfar the most widely used method.
cream of tartar See potassium hydro-gentartrate.
creosote /kree-ŏ-soht/ A colorless oilyliquid containing phenols and distilledfrom wood tar, used as a disinfectant. Thename is also given to creosote oil, a darkbrown liquid distilled from coal tar andused for preserving timber. It also consists
75
creosote
cresols
76
of phenols, mixed with some methylphe-nols.
cresols /kree-solz, -sohlz/ See methylphe-nols.
critical point The conditions of temper-ature and pressure under which a liquidbeing heated in a closed vessel becomes in-distinguishable from the gas or vaporphase. At temperatures below the criticaltemperature (Tc) the substance can be liq-uefied by applying pressure; at tempera-tures above Tc this is not possible. For eachsubstance there is one critical point; for ex-ample, for carbon dioxide it is at 31.1°Cand 73.0 atmospheres.
critical pressure The lowest pressureneeded to bring about liquefaction of a gasat its critical temperature.
critical temperature The temperaturebelow which a gas can be liquefied by ap-plying pressure and above which noamount of pressure is sufficient to bringabout liquefaction. Some gases have criti-cal temperatures above room temperature(e.g. carbon dioxide 31.1°C and chlorine144°C) and have been known in the liquidstate for many years. Liquefaction provedmuch more difficult for those gases (e.g.oxygen –118°C and nitrogen –146°C) thathave very low critical temperatures.
critical volume The volume of onemole of a substance at its critical point.
crossed-beam reaction A chemical re-action performed with two molecularbeams intersecting at an angle. Crossed-beam reactions usually involve very simplereactions at low pressures. The productsare detected by mass spectroscopy and fun-damental information can be obtainedabout reaction mechanisms.
cross linkage An atom or short chainjoining two longer chains in a polymer.
crown ether A compound that has alarge ring composed of –CH2–CH2–O–units. For example, 18-crown-6 has the
formula C12H24O6 (six CH2CH2O units).The rings of these compounds are not pla-nar – the name comes from the shape of themolecule. The oxygen atoms of these cyclicethers can coordinate to central metal ionsor to other positive ions (e.g. NH4
+). Thecrown ethers have a number of uses inanalysis, separation of mixtures, and ascatalysts. Cryptands are similar com-pounds in which the ether chains are linkedby nitrogen atoms to give a three-dimen-sional cage structure. They are similar inaction to crown ethers but generally formmore strongly bound complexes. See alsohost–guest chemistry.
crude oil See petroleum.
cryohydrate /krÿ-oh-hÿ-drayt/ A EUTEC-TIC mixture of ice and certain salts.
cryolite /krÿ-ŏ-lÿt/ See sodium hexafluo-roaluminate.
cryoscopic constant /krÿ-ŏ-skop-ik/ Seedepression of freezing point.
cryptands /krip-tandz/ See crownethers.
crystal A solid substance that has a def-inite geometric shape. A crystal has fixedangles between its faces, which have dis-tinct edges. The crystal will sparkle if thefaces are able to reflect light. The constantangles are caused by the regular arrange-ments of particles (atoms, ions, or mol-ecules) in the crystal. If broken, a largecrystal will form smaller crystals.
In crystals, the atoms, ions, or mol-ecules of the substance form a distinct reg-ular array in the solid state. The faces andtheir angles bear a definite relationship tothe arrangement of these particles.
crystal-field theory A theory of theproperties of metal COMPLEXES. Originallyit was introduced by Hans Bethe in 1929 toaccount for the properties of transition ele-ments in ionic crystals. In the theory, theligands surrounding the metal atom or ionare thought of as negative charges, and theeffect of these charges on the energies of
77
crystallization
the d orbitals of the central metal ion isconsidered. Crystal-field theory was suc-cessful in describing the spectroscopic,magnetic, and other properties of com-plexes. It has been superseded by ligand-field theory, in which the overlap of the dorbitals on the metal ion with orbitals onthe ligands is taken into account.
crystal habit The shape of a crystal. Thehabit depends on the way in which thecrystal has grown; i.e. the relative rates ofdevelopment of different faces.
crystalline /kris-tă-lin, -lÿn/ Denoting asubstance that forms crystals. Crystallinesubstances have a regular internal arrange-ment of atoms, even though they may notexist as geometrically regular crystals. For
instance, lead (and other metals) are crys-talline. Such substances are composed ofaccumulations of tiny crystals. Compareamorphous.
crystallite /kris-tă-lÿt/ A small crystalthat has the potential to grow larger. It isoften used in mineralogy to describe speci-mens that contain accumulations of manyminute crystals of unknown chemical com-position and crystal structure.
crystallization /kris-tă-li-zay-shŏn/ Theprocess of forming crystals. When a sub-stance cools from the gaseous or liquidstate to the solid state, crystallization oc-curs. Crystals will also form from a solu-tion saturated with a solute.
Cubic crystal structures
face-centered
body-centered simple cubic
crystallography
78
crystallography /kris-tă-log-ră-fee/ Thestudy of the formation, structure, andproperties of crystals. See also x-ray crys-tallography.
crystalloid /kris-tă-loid/ A substancethat is not a colloid and which will there-fore not pass through a semipermeablemembrane. See colloid; semipermeablemembrane.
crystal structure The particular repeat-ing arrangement of atoms (molecules orions) in a crystal. ‘Structure’ refers to theinternal arrangement of particles, not theexternal appearance.
crystal system A classification of crys-tals based on the shapes of their unit cell. Ifthe unit cell is a parallelopiped with lengthsa, b, and c and the angles between theseedges are α (between b and c), β (betweena and c), and γ (between a and b), then theclassification is: cubic: a = b = c; α = β = γ= 90° tetragonal: a = b ≠ c; α = β = γ = 90°orthorhombic: a ≠ b ≠ c; α = β = γ = 90°hexagonal: a = b ≠ c; α = β = 90°; γ = 120°trigonal: a = b ≠ c; α = β = γ ≠ 90° mono-clinic: a ≠ b ≠ c; α = γ = 90° ≠ β triclinic: a≠ b ≠ c; α ≠ β ≠ γThe orthorhombic system is also called therhombic system.
CS gas ((2-chlorobenzylidine-)-malanoni-trile; C6H4ClCH:C(CN)2) A white or-ganic compound that is a nasal irritantused in powder form as a tear gas for riotcontrol.
cubic Denoting a crystal in which theunit cell is a cube. In a simple cubic crystalthe particles are arranged at the corners ofa cube. See also body-centered cubic crys-tal; face-centered cubic crystal; crystal sys-tem.
cubic close packed See face-centeredcubic crystal.
cumene process /kyoo-meen/ An indus-trial process for the manufacture of phenolfrom isopropylbenzene (cumene), which isitself made by passing benzene vapor and
propene over a phosphoric acid catalyst(250°C and 30 atmospheres):
C6H6 + CH2:CH(CH3) →C6H5CH(CH3)2
The isopropylbenzene is oxidized by air toa ‘hydroperoxide’:
C6H5C(CH3)2–O–O–HThis is hydrolyzed by dilute acid to phenol(C6H5OH) and propanone (CH3COCH3),which is a valuable by-product.
cupellation /kyoo-pe-lay-shŏn/ A tech-nique used for the separation of silverand/or gold from impurities by the use ofan easily oxidizable metal, such as lead.The impure metal is heated in a furnaceand a blast of hot air is directed upon it.The lead or other metal forms an oxide,leaving the silver or gold behind.
cupric chloride /kyoo-prik/ See cop-per(II) chloride.
cupric nitrate See copper(II) nitrate.
cupric oxide See copper(II) oxide.
cupric sulfate See copper(II) sulfate.
cuprite /kyoo-prÿt/ A red mineral formof copper(I) oxide (Cu2O), a principal oreof copper.
cupronickel /kyoo-prŏ-nik-ăl/ A seriesof strong copper-nickel alloys containingup to 45% nickel. Those containing 20%and 30% nickel are very malleable, can beworked cold or hot, and are very corro-sion-resistant. They are used, for example,in condenser tubes in power stations.Cupronickel with 25% nickel is widelyused in coinage. See also Constantan.
cuprous chloride /kyoo-prŭs/ See cop-per(I) chloride.
cuprous oxide See copper(I) oxide.
curie /kyoo-ree, kyoo-ree/ Symbol: Ci Aunit of radioactivity, equivalent to theamount of a given radioactive substancethat produces 3.7 × 1010 disintegrations
per second, the number of disintegrationsproduced by one gram of radium.
curium /kyoo-ree-ŭm/ A highly toxic ra-dioactive silvery element of the actinoid se-ries of metals. A transuranic element, it isnot found naturally on Earth but is synthe-sized from plutonium. Curium-244 andcurium-242 have been used in thermoelec-tric power generators.
Symbol: Cm; m.p. 1340±40°C; b.p. ≅3550°C; r.d. 13.3 (20°C); p.n. 96; moststable isotope 247Cm (half-life 1.56 × 107
years). The unit is named for the Frenchphysicist Pierre Curie (1859–1906).
cyanamide process /sÿ-an-ă-mÿd, -mid,sÿ-ă-nam-ÿd, -id/ An industrial processfor fixing nitrogen by heating calcium di-carbide in air.
CaC2 + N2 → CaCN2The product, calcium cyanamide, hy-drolyzes to ammonia and can be used as afertilizer. The process is expensive due tothe high cost of calcium dicarbide. See alsonitrogen fixation.
cyanide /sÿ-ă-nÿd, -nid/ 1. A salt of hy-drogen cyanide, containing the cyanide ion(CN–). See hydrocyanic acid.2. See nitrile.
cyanide process A technique used forthe extraction of gold from its ores. Aftercrushing the gold ore to a fine powder it isagitated with a very dilute solution ofpotassium cyanide. The gold is dissolvedby the cyanide to form potassium di-cyanoaurate(I) (potassium aurocyanide,KAu(CN)2). This complex is reduced withzinc, filtered, and melted to obtain the puregold metal.
cyanoferrate /sÿ-an-oh-fe-rayt/ A com-pound containing the ion [Fe(CN)6]4– (thehexacyanoferrate(II) ion) or the ion[Fe(CN)6]3– (the hexacyanoferrate(III)ion). These ions are usually encountered astheir potassium salts. Potassium hexa-cyanoferrate(II) (K4Fe(CN)6, potassiumferrocyanide) is a yellow crystalline com-pound. Potassium hexacyanoferrate(III)
(K3Fe(CN)6, potassium ferricyanide) is anorange crystalline compound. Its solutiongives a deep blue precipitate with iron(II)ions, and is used as a test for iron(II) (fer-rous) compounds. Prassian blue is a bluepigment containing hexacyanoferrate ions.
cyanogen /sÿ-an-ŏ-jen/ (C2N2) A toxicflammable gas prepared by heating mer-cury cyanide. See also pseudohalogens.
cyanohydrin /sÿ-an-oh-hÿ-drin/ An ad-dition compound formed between an alde-hyde or ketone and hydrogen cyanide. Thegeneral formula is RCH(OH)(CN) (froman aldehyde) or RR′C(OH)(CN) (from aketone). Cyanohydrins are easily hy-drolyzed to hydroxycarboxylic acids. Forinstance, the compound 2-hydrox-ypropanonitrile (CH3CH(OH)(CN)) is hy-drolyzed to 2-hydroxypropanoic acid(CH3CH(OH)(COOH)).
cyclic compound A compound con-taining a ring of atoms. If the atoms form-ing the ring are all the same the compoundis homocyclic; if different atoms are in-volved it is heterocyclic.
cyclization /sÿ-kli-zay-shŏn/ A reactionin which a straight-chain compound is con-verted into a cyclic compound.
cycloaddition /sÿ-kloh-ă-dish-ŏn/ Seepericyclic reaction.
cycloalkane /sÿ-kloh-al-kayn/ A satu-rated cyclic hydrocarbon comprising a ring
79
cycloalkane
R OH
CNH
OH
CN
R
R’
C
C
from aldehyde
from ketone
Cyanohydrins
of carbon atoms, each carrying two hydro-gen atoms, general formula CnH2n. Cyclo-propane (C3H6), and cyclobutane (C4H8)both have strained rings and are highly re-active. Other cycloalkanes have similarproperties to the alkanes, although they aregenerally less reactive.
cyclohexadiene-1,4-dione /sÿ-kloh-heks-ă-dÿ-een, -dÿ-ohn/ See quinone.
cyclohexane /sÿ-kloh-heks-ayn/ (C6H12)A colorless liquid alkane that is commonlyused as a solvent and in the production ofhexanedioic acid (adipic acid) for the man-ufacture of nylon. Cyclohexane, itself, ismanufactured by the reformation of longerchain hydrocarbons present in crude-oilfractions. It is also interesting from a struc-tural point of view, existing as a ‘puckered’six-membered ring, having all bonds be-tween carbon atoms at 109.9° (the tetrahe-dral angle). The molecule undergoes rapidinterconversion between two ‘chair-like’CONFORMATIONS, which are energeticallyequivalent, passing through a ‘boat-like’structure of higher energy. It is commonlyrepresented by a hexagon.
cyclonite /sÿ-klŏ-nÿt, sik-lŏ/ A high ex-plosive made from hexamine.
cyclo-octatetraene /sÿ-kloh-ok-tă-tet-ră-een/ See annulene.
cyclopentadiene /sÿ-kloh-pen-tă-dÿ-een/ A cyclic hydrocarbon made bycracking petroleum. The molecules have afive-membered ring containing two car-bon-carbon double bonds and one CH2
group. It forms the negative cyclopentadi-enyl ion C5H5
–, present in sandwich com-pounds, such as ferrocene.
cyclopentadienyl ion /sÿ-kloh-pen-tă-dÿ-en-ăl/ See cyclopentadiene.
cysteine /sis-tee-een/ See amino acids.
cytidine /sÿ-tă-din, -deen, -sit-ă-/ (cytosinenucleoside) A NUCLEOSIDE formed whencytosine is linked to D-ribose via a β-glyco-sidic bond.
cytosine /sÿ-tŏ-sin, -seen/ A nitrogenousbase found in DNA and RNA. Cytosinehas the pyrimidine ring structure.
cyclohexadiene-1,4-dione
80
81
dalton /dawl-tŏn/ See atomic mass unit.
Dalton’s atomic theory /dawl-tŏnz/ Atheory explaining the formation of com-pounds by elements first published by JohnDalton in 1803. It was the first modern at-tempt to describe chemical behavior interms of atoms. The theory was based oncertain postulates:1. All elements are composed of small par-
ticles, which Dalton called atoms.2. All atoms of the same element are iden-
tical.3. Atoms can neither be created nor de-
stroyed.4. Atoms combine to form ‘compound
atoms’ (i.e. molecules) in simple ratios.Dalton also suggested symbols for the
elements. The theory was used to explainthe law of conservation of mass and thelaws of chemical combination. The theoryis named for the British chemist and physi-cist John Dalton (1766–1844).
Dalton’s law (of partial pressures) Thepressure of a mixture of gases is the sum ofthe partial pressures of each individualconstituent. (The partial pressure of a gasin a mixture is the pressure that it wouldexert if it alone were present in the con-tainer.) Dalton’s law is strictly true only forideal gases.
Daniell cell /dan-yĕl/ A type of primarycell consisting of two electrodes in differentelectrolytes separated by a porous parti-tion. The positive eletrode is copper im-mersed in copper(II) sulfate solution. Thenegative electrode is zinc–mercury amal-gam in either dilute sulfuric acid or zincsulfate solution. The porous pot preventsmixing of the electrolytes, but allows ionsto pass. With sulfuric acid the e.m.f. is
about 1.08 volts; with zinc sulfate it isabout 1.10 volts.
At the copper electrode copper ions insolution gain electrons from the metal andare deposited as copper atoms:
Cu2+ + 2e– → CuThe copper electrode thus gains a posi-
tive charge. At the zinc electrode, zincatoms from the electrode lose electrons anddissolve into solution as zinc ions, leavinga net negative charge on the electrode
Zn → 2e– + Zn2+
The cell is named for the British chemistand meteorologist John Frederic Daniell(1790–1845).
darmstadtium /darm-stat-ee-ŭm. -shtat-/A radioactive metallic element that doesnot occur naturally on the Earth. It is madeeither by bombarding a lead target withnickel nuclei or by bombarding a pluto-nium target with sulfur nuclei. There areseveral isotopes; the most stable is 281Ds,with a half-life of about 1.6 minutes. Thechemical properties of darmstadtiumshould be similar to those of platinum. The element is named for Darmstadt, the place in Germany where it was dis-covered.
Symbol Ds; p.n. 110.
dative bond /day-tiv/ See coordinatebond.
Davy lamp /day-vee/ See safety lamp.
d-block elements The TRANSITION EL-EMENTS of the first, second, and third longperiods of the periodic table, i.e. Sc to Zn,Y to Cd, and La to Hg. They are so calledbecause in general they have inner d-levelswith configurations of the type (n – 1)dxns2
where x = 1–10.
D
DDT /dee-dee-tee/ (dichlorodiphenyl-trichloroethane; (ClC6H4)2CH(CCl3)) Acolorless crystalline organic compound,once widely used as an insecticide. Its use isnow restricted or banned in many coun-tries because it is stable and remains un-changed in the soil, and passes up the foodchain to accumulate in the fatty tissues ofcarnivorous animals.
deactivation A process in which the reac-tivity of a substance is lessened, or even to-tally removed . Usually this deactivation isunwanted, as in the poisoning of catalysts.
de Broglie wave /dĕ-broh-lyee/ A waveassociated with a particle, such as an elec-tron or proton. In 1924, Louis de Brogliesuggested that, since electromagneticwaves can be described as particles (pho-tons), particles of matter could also havewave properties. The wavelength (λ) hasthe same relationship to momentum (p) asin electromagnetic radiation:
λ = h/pwhere h is the Planck constant. The wave isnamed for the French physicist PrinceLouis Victor Pierre Raymond de Broglie(1882–1987). See also quantum theory.
debye /dĕ-bay-ĕ/ Symbol: D A unit of elec-tric dipole moment equal to 3.335 64 ×10–30 coulomb meter. It is used in express-ing the dipole moments of molecules. theunit is named for the Dutch–Americanphysicist Peter Joseph William Debye(1884–1966).
Debye–Hückel theory /dĕ-bay-ĕ hoo-kĕl/A theory to predict the conductivity of ionsin dilute solutions of strong ELECTROLYTES.It assumes that electrolytes in dilute solu-tion are completely dissociated into ionsbut takes into account interionic attractionand repulsion. Agreement between the the-ory and experiment occurs only with verydilute solutions (less than 10–3M). See dis-sociation.
deca- Symbol: da A prefix denoting 10.For example, 1 decameter (dam) = 10 me-ters (m).
decahydrate /dek-ă-hÿ-drayt/ A crys-talline solid containing ten molecules ofwater of crystallization per molecule ofcompound.
decant /di-kant/ To pour off the clear liq-uid above a sediment.
decay 1. The spontaneous breakdown ofa radioactive isotope. See half-life; radioac-tivity.2. The transition of excited atoms, ions,molecules, etc., to the ground state.
deci- Symbol: d A prefix denoting 10–1.For example, 1 decimeter (dm) = 10–1
meter (m).
decomposition A chemical reaction inwhich a compound is broken down intocompounds with simpler molecules.
decrepitation /di-krep-ă-tay-shŏn/ Heat-ing a crystalline solid so that it emits acrackling noise, usually as a result of loss ofwater of crystallization.
defect An irregularity in the orderedarrangement of particles in a crystal lattice.There are two main types of defect in crys-tals. Point defects occur at single latticepoints and there are two types. A vacancyis a missing atom; i.e. a vacant lattice point.Vacancies are sometimes called Schottkydefects (named for the Swiss–Germanphysicist Walter Schottky (1886–1976).An interstitial is an atom that is in a posi-tion that is not a normal lattice point. If anatom moves off its lattice point to an inter-stitial position the result (vacancy plus in-terstitial) is a Frenkel defect (named for theRussian mathematical physicist Jacov Il-l’ich Frenkel (1894–1952). All solids aboveabsolute zero have a number of point de-fects, the concentration of defects depend-ing on the temperature. Point defects canalso be produced by strain or by irradia-tion.
Dislocations (or line defects) are alsoproduced by strain in solids. They are ir-regularities extending over a number of lat-tice points along a line of atoms.
DDT
82
83
delta metal
definite proportions, law of See con-stant proportions; law of.
degassing Removal of dissolved or ab-sorbed gases from liquids or solids.
degenerate Describing different quantumstates that have the same energy. For in-stance, the five d orbitals in a transition-metal atom all have the same energy butdifferent values of the magnetic quantumnumber m. Differences in energy occur if amagnetic field is applied or if the arrange-ment of ligands around the atom is notsymmetrical. The degeneracy is then saidto be ‘lifted’.
degradation A chemical reaction involv-ing the decomposition of a molecule intosimpler molecules, usually in stages. TheHofmann degradation of amides is an ex-ample.
degrees of freedom 1. The independentways in which particles can take up energy.In a monatomic gas, such as helium orargon, the atoms have three translationaldegrees of freedom (corresponding to mo-tion in three mutually perpendicular direc-tions). The mean energy per atom for eachdegree of freedom is kT/2, where k is theBoltzmann constant and T the thermody-namic temperature; the mean energy peratom is thus 3kT/2.
A diatomic gas also has two rotationaldegrees of freedom (about two axes per-pendicular to the bond) and one vibra-tional degree (along the bond). Therotations also each contribute kT/2 to theaverage energy. The vibration contributeskT (kT/2 for kinetic energy and kT/2 forpotential energy). Thus, the average energyper molecule for a diatomic molecule is3kT/2 (translation) + kT (rotation) + kT(vibration) = 7kT/2.
Linear triatomic molecules also havetwo significant rotational degrees of free-dom; non-linear molecules have three. Fornon-linear polyatomic molecules, the num-ber of vibrational degrees of freedom is 3N– 6, where N is the number of atoms in themolecule.
The molar energy of a gas is the averageenergy per molecule multiplied by the Avo-gadro constant. For a monatomic gas it is3RT/2, etc.2. The independent physical quantities(e.g. pressure, temperature, etc.) that de-fine the state of a given system. See phaserule.
dehydration /dee-hÿ-dray-shŏn/ 1. Re-moval of water from a substance.2. Removal of the elements of water (i.e.hydrogen and oxygen in a 2:1 ratio) toform a new compound. An example is thedehydration of propanol to propene overhot pumice:
C3H7OH → CH3CH:CH2 + H2O
deionization /dee-ÿ-ŏ-ni-zay-shŏn/ Amethod of removing ions from a solutionusing ION EXCHANGE. The term is com-monly applied to the purification of tapwater; deionized water has superseded dis-tilled water in many chemical applications.
deliquescent /del-ă-kwess-ĕnt/ Describinga solid compound that absorbs water fromthe atmosphere, eventually forming a solu-tion. See also hygroscopic.
delocalization /dee-loh-kă-li-zay-shŏn/ Aspreading out of bonding electrons in amolecule over the molecule.
delocalized bond /dee-loh-kă-lÿzd/ (non-localized bond) A type of bonding in mol-ecules that occurs in addition to sigmabonding. The electrons forming the delo-calized bond are no longer regarded as re-maining between two atoms; i.e. theelectron density of the delocalized elec-trons is spread over several atoms and mayspread over the whole molecule.
The electron density of the delocalizedbond is spread by means of a delocalizedmolecular orbital and may be regarded as aseries of pi bonds extending over severalatoms, for example the pi bonds in butadi-ene and the C–O pi bonds in the carbonateion.
delta metal (delta brass) A strong alloy ofcopper and zinc, containing also a little
iron. Its main use is for making cartridgecases.
denaturation /dee-nay-chŭ-ray-shŏn/The changes in structure that occur when aprotein is heated. These changes are irre-versible and affect the properties of theprotein.
denaturing /dee-nay-chŭ-ring/ Theprocess of adding small amounts of conta-minants to ethanol to make it unfit fordrinking. Denatured alcohol (such asmethylated spirits) is not subject to the ex-cise duty on alcoholic drinks.
dendritic growth /den-drit-ik/ Growth ofcrystals in a branching (‘tree-like’) habit.
dendritic polymer See supramolecularpolymer.
denitrification /dee-nÿ-tră-fă-kay-shŏn/See nitrogen cycle.
density Symbol: ρ The mass per unit vol-ume of a given substance. The units are gdm–3, etc. See also relative density.
density functional theory A method ofcalculating the electronic structure of mol-ecules using the electron density distribu-tion in atoms. The theory was developedby Walter Kohn (1923– ) and his col-leagues in the mid-1960s and has been usedextensively in chemistry and solid statephysics.
deoxyribonucleic acid /dee-oks-ă-rÿ-boh-new-klee-ik/ See DNA.
deoxyribose /dee-oks-ă-rÿ-bohs/ See ri-bose.
depolarizer A substance used in a voltaiccell to prevent polarization. Hydrogenbubbles forming on the electrode can be re-moved by an oxidizing agent, such as man-ganese(IV) oxide.
depression of freezing point A colliga-tive property of solutions in which thefreezing point of a given solvent is lowered
by the presence of a solute. The amount ofthe reduction is proportional to the molalconcentration of the solute. The depressiondepends only on the concentration and isindependent of solute composition. Theproportionality constant, Kf, is called thefreezing point constant or sometimes thecryoscopic constant. ∆t = KfCM, where ∆t isthe lowering of the temperature and CM isthe molal concentration; the unit of Kf iskelvin kilogram mole–1 (K kg mol–1). Al-though closely related to the property ofboiling-point elevation, the cryogenicmethod can be applied to measurement ofrelative molecular mass with considerableprecision. A known weight of pure solventis slowly frozen, with stirring, in a suitablecold bath and the freezing temperaturemeasured using a Beckmann thermometer.A known weight of solute of known mol-ecular mass is introduced, the solventthawed out, and the cooling process andmeasurement repeated. The addition is re-peated several times and an average valueof Kf for the solvent obtained by plotting ∆tagainst CM. The whole process is then re-peated using the unknown solute and itsrelative molecular mass determined usingthe value of Kf previously obtained.
The effect is applied to more precisemeasurement of relative molecular mass byusing a pair of Dewar flasks (pure solventand solution) and measuring ∆t by means
denaturation
84
Beckmannthermometer
tube forintroductionof solute
containerfor freezingmixture
Apparatus for measuring depression of freezing point
of thermocouples. The theoretical explana-tion is similar to that for LOWERING OF
VAPOR PRESSURE. The freezing point of thesolvent is that point at which the curve rep-resenting the vapor pressure above the liq-uid phase intersects the curve representingthe vapor pressure above the frozen sol-vent. The addition of solute depresses theformer curve but as the solid phase thatseparates is always pure solvent (above theeutectic point), there is no attendant de-pression of the latter curve. Consequentlythe point of intersection is depressed, re-sulting in a lowering of the freezing point.
derivative A compound obtained by reac-tion from another compound. The term ismost often used in organic chemistry ofcompounds that have the same generalstructure as the parent compound.
derived unit A unit defined in terms ofbase units, and not directly from a stan-dard value of the quantity it measures. Forexample, the newton is a unit of force de-fined as a kilogram meter second–2 (kg ms–2). See also SI units.
desalination /dee-sal-ă-nay-shŏn/ Any ofvarious techniques for removing the salts(mainly sodium chloride) from sea water tomake it fit for drinking, irrigation, use inwater-cooled engines, and for makingsteam for steam turbines. There are variousmethods. Those based on distillation de-pend on a cheap source of heat energy, ofwhich solar energy is the most promising,especially in hot climates. Flash evapora-tion (evaporation under reduced pressure)and freezing to make pure ice are othermethods, as are electrodialysis, ion ex-change, reverse osmosis, and the use ofmolecular sieves.
desiccation /dess-ă-kay-shŏn/ Removal ofmoisture from a substance.
desiccator /dess-ă-kay-ter/ A laboratoryapparatus for drying solids or for keepingsolids free of moisture. It is a container inwhich is kept a hygroscopic material (e.g.calcium chloride or silica gel).
destructive distillation The process ofheating an organic substance such that itwholly or partially decomposes in the ab-sence of air to produce volatile products,which are subsequently condensed. The de-structive distillation of coal was theprocess for manufacturing coal gas andcoal tar. Formerly, methanol was made bythe destructive distillation of wood.
detergents A group of substances that im-prove the cleansing action of solvents, par-ticularly water. The majority of detergents,including soap, have the same basic struc-ture. Their molecules have a hydrocarbonchain (tail) that does not attract water mol-ecules. The tail is said to be hydrophobic(water hating). Attached to this tail is asmall group (head) that readily ionizes andattracts water molecules. It is said to be hy-drophilic (water loving). Detergents reducethe surface tension of water and thus im-prove its wetting power. Because the deter-gent ions have their hydrophilic headsanchored in the water and their hydropho-bic tails protruding above it, the water sur-face is broken up, enabling the water tospread over the material to be cleaned andpenetrate between the material and thedirt. With the assistance of agitation, thedirt can be floated off. The hydrophobictails of the detergent molecules ‘dissolve’ ingrease and oils. The protruding hy-drophilic heads repel each other causingthe oil to roll up and form a drop, whichfloats off into the water as an emulsion.More recently synthetic detergents, oftenderived from petrochemicals, have beendeveloped. Unlike SOAPS these detergentsdo not form insoluble scums with hardwater.
Synthetic detergents are of three types.Anionic detergents form ions consisting ofa hydrocarbon chain to which is attachedeither a sulfonate group, –SO2–O–, or asulfate group, –O–SO2–O–. The corre-sponding metal salts are soluble in water.Cationic detergents have organic positiveions of the type RNH3
+, in which R has along hydrocarbon chain. Non-ionic deter-gents are complex chemical compoundscalled ethoxylates. They owe their deter-gent properties to the presence of a number
85
detergents
of oxygen atoms in one part of the mol-ecule, which are capable of forming hydro-gen bonds with the surface watermolecules, thus reducing the surface ten-sion of the water.
deuterated compound A compound inwhich one or more 1H atoms have been re-placed by deuterium (2H) atoms.
deuteride /dew-ter-ÿd/ A compound ofdeuterium with other elements; i.e. a hy-dride in which the deuterium isotope is pre-sent rather than 1H. See hydride.
deuterium /dew-teer-ee-ŭm/ Symbol: D,2H A naturally occurring stable isotope ofhydrogen in which the nucleus containsone proton and one neutron. The atomicmass is thus approximately twice that of1H; deuterium is known as ‘heavy hydro-gen’. Chemically it behaves almost identi-cally to hydrogen, forming analogouscompounds, although reactions of deu-terium compounds are often slower thanthose of the corresponding 1H compounds.This is made use of in kinetic studies wherethe rate of a reaction may depend on trans-fer of a hydrogen atom (i.e. a kinetic iso-tope effect).
deuterium oxide (D2O) The chemicalname of heavy water. See deuterium.
deuteron /dew-ter-on/ The nucleus of thedeuterium atom, 2H+.
Dewar flask /dew-er/ (vacuum flask) Adouble-walled container of thin glass withthe space between the walls evacuated andsealed to stop conduction and convectionof energy through it. The glass is often sil-vered to reduce radiation.
Dewar structure A representation of thestructure of BENZENE in which there is a sin-gle bond between two opposite corners ofthe hexagonal ring and two double bondsat the sides of the ring. The Dewar struc-tures contribute to the resonance hybrid ofbenzene.
dextrin /deks-trin/ A polysaccharide sugarproduced by the action of amylase enzymeson or the chemical hydrolysis of starch.Dextrins are used as adhesives.
dextro-form /deks-troh-form/ See opticalactivity.
dextronic acid /deks-tron-ik/ See glu-conic acid.
dextrorotatory /deks-troh-roh-tă-tor-ee. -toh-ree/ See optical activity.
dextrose /deks-trohs/ (grape-sugar) Natu-rally occurring GLUCOSE belongs to thestereochemical series D and is dextrorota-tory, indicated by the symbol (+). Thus theterm dextrose is used to indicate D-(+)-glu-cose. As other stereochemical forms of glu-cose have no significance in biologicalsystems the term ‘glucose’ is often used in-terchangeably with dextrose in biology.
d-form See optical activity.
D-form See optical activity.
diagonal relationship There is a generaltrend in the periodic table for electronega-tivity to increase from left to right along aperiod and decrease down a group. Thus amove of ‘one across and one down’, i.e. adiagonal move in the table, gives rise to ef-fects that tend to cancel each other. Thereis a similar combined effect for size, whichdecreases along a period but increasesdown a group. This diagonal relationshipgives rise to similarities in chemical proper-ties, which are particularly noticeable forthe following pairs: Li–Mg; Be–Al; B–Si.Li–Mg: 1. both have carbonates that give CO2 on
heating; 2. both burn in air to give the normal oxide
only; 3. both form a nitride; 4. both form hydrated chlorides that hy-
drolyze slowly. Be–Al: 1. both give hydrogen with alkalis; 2. both give water-insoluble hydroxides
that dissolve in alkali;
deuterated compound
86
87
dibasic acid
3. both form complex ions of the typeMCl3–;
4. both have covalently bridged chlorides. B–Si: 1. both form acidic oxides of a giant cova-
lent-molecule type with high meltingpoints;
2. both form low-stability hydrides that ig-nite in air;
3. both form readily hydrolyzable chlo-rides that fume in air;
4. both have amorphous and crystallineforms and both form glasses with basicoxides, such as Na2O.
diamagnetism /dÿ-ă-mag-nĕ-tiz-ăm/ Seemagnetism.
1,6-diaminohexane /dÿ-am-ă-noh-heks-ayn/ (hexamethylene diamine; H2N(CH2)6-NH2) An organic compound used as astarting material in the production ofNYLON. It is manufactured from cyclo-hexane.
diamond An allotrope of CARBON. It is thehardest naturally occurring substance andis used for jewelry and, industrially, forcutting and drilling equipment. Each car-bon atom is surrounded by four equallyspaced carbon atoms arranged tetrahe-drally. The carbon atoms form a three-di-mensional network with eachcarbon–carbon bond equal to 0.154 nmand at an angle of 109.5° with its neigh-bors. In diamonds millions of atoms are co-valently bonded to form a giant molecularstructure, the great strength of which re-sults from the strong covalent bonds. Dia-monds can be formed synthetically fromgraphite in the presence of a catalyst andunder extreme temperature and pressure;although small, such diamonds are of ade-quate size for many industrial uses.
diastereoisomer See isomerism.
diatomaceous earth /dÿ-ă-tŏ-may-shŭs/See diatomite.
diatomic /dÿ-ă-tom-ik/ Describing a mol-ecule that consists of two atoms. Hydrogen
(H2), oxygen (O2), nitrogen (N2), and thehalogens are examples of diatomic ele-ments.
diatomite /dÿ-at-ŏ-mÿt/ (diatomaceousearth; kieselguhr) A whitish powderedmineral consisting mainly of silica (sili-con(IV) oxide) derived from the shells ofdiatoms. It is used to make fireproof ce-ments and as an absorbent in the manufac-ture of DYNAMITE.
diazine /dÿ-ă-zeen, -zin/ See pyrazine.
diazole /dÿ-ă-zohl, dÿ-ay-/ See pyrazole.
diazonium salt /dÿ-ă-zoh-nee-ŭm/ Acompound of the type RN2
+X–, where R isan aromatic group and X– a negative ion.Diazonium salts are made by diazotization.They can be isolated but are very unstable,and are usually prepared in solution. The–N2
+ group renders the benzene ring sus-ceptible to nucleophilic substitution (ratherthan electrophilic substitution). Typical re-actions are:1. Reaction with water on warming the so-
lution:RN2
+ + H2O → ROH + N2 + H+
2. Reaction with halogen ions (CuCl cata-lyst for chloride ions):
RN2+ + I– → RI + N2
Diazonium ions can also act as elec-trophiles and substitute other benzenerings (diazo coupling). See azo compound.
diazotization /dÿ-a-zŏ-ti-zay-shŏn/ Thereaction of an aromatic amine (e.g. aniline)with nitrous acid at low temperatures(below 5°C).
C6H5NH2 + HNO2 → C6H5N+N + OH–
+ H2OThe acid is prepared in situ by reaction be-tween nitric acid and sodium nitrite. Theresulting diazonium ion is susceptible to at-tack by nucleophiles and provides amethod of nucleophilic substitution ontothe benzene ring.
dibasic acid /dÿ-bay-sik/ An acid whichhas two active protons, such as sulfuricacid. Dibasic acids can give rise to two se-
1,2-dibromoethane
88
ries of salts. For example, sulfuric acid(H2SO4) forms sulfates (SO4
2–) and hydro-gensulfates (HSO–
4).
1,2-dibromoethane /dÿ-broh-moh-eth-ayn/ (ethylene dibromide; BrCH2CH2Br)A colorless volatile organic liquid, made byreacting bromine with ethene. It is used asa fuel additive to remove lead (as lead bro-mide, which is also volatile). See also leadtetraethyl.
dicarbide /dÿ-kar-bÿd/ See carbide.
dicarboxylic acid /dÿ-kar-boks-il-ik/ Anorganic acid that has two carboxyl groups(–COOH). An example is hexanedioicacid, HOOC(CH2)4COOH (adipic acid).
dichlorine oxide /dÿ-klor-een, -in, -kloh-reen, -rin/ (chlorine monoxide; Cl2O) Anorange gas made by passing chlorine overmercury(II) oxide. It is a strong oxidizingagent and dissolves in water to give chlo-ric(I) acid.
dichloroacetic acid /dÿ-klor-oh-ă-see-tik,-set-ik, -kloh-roh-/ See chloroethanoicacid.
dichloroethanoic acid /dÿ-kor-oh-eth-ă-noh-ik, -kloh-roh-/ See chloroethanoicacid.
dichromate(VI) /dÿ-kroh-mayt/ A saltcontaining the ion Cr2O7
–. Dichromatesare strong oxidizing agents. See potassiumdichromate.
dielectric constant /dÿ-i-lek-trik/ (rela-
tive permittivity) Symbol: εr A quantitythat characterizes how a medium reducesthe electric field strength associated with adistribution of electric charges. If twopoint charges Q1 and Q2 are a distance dapart in a medium with a permittivity ε thismeans that the force F between the chargesis given by F = (1/4πε)(Q1Q2/d2).The dielectric constant of the medium isgiven by εr = ε/ε0, where ε0 is the permittiv-ity of free space. The dielectric constant ofair is very slightly greater than 1, while thatof water is about 80. The value of the di-electric constant of a medium has impor-tant physical and chemical consequences,particularly for ions in solutions.
Diels–Alder reaction /deelz awl-der/ Anorganic addition reaction used to make six-membered ring compounds. A CONJUGATED
compound (a compound with two doublebonds separated by a single bond, such asa diene) adds to a compound with a singledouble bond to form the six-memberedring. The reaction is named for the Germanorganic chemists Otto Paul Herman Diels(1876–1954) and Kurt Alder (1902–58).
diene /dÿ-een/ An organic compound con-taining two carbon–carbon double bonds.
diesel fuel /dee-zĕl/ A petroleum fractionconsisting of various alkanes in the boilingrange 200–350°C, used as a fuel for diesel(compression-ignition) engines.
diethyl ether /dÿ-eth-ăl/ See ethoxy-ethane.
diffusion /di-fyoo-zhŏn/ Movement of a
butadienebutadienebutadiene maleic acidmaleic acidmaleic acid phthalic acidphthalic acidphthalic acid
HHH
CCC HHH
HHHCCC
HHH
HHHCCC
HHHCCC
COOH
COOHCCC
CCCH
H
H
COOH
COOHCCC
CCC
CCC
H
CCC
CCCCCCH
H
Diels–Alder reaction
gas, liquid, or solid as a result of the ran-dom thermal motion of its particles (atomsor molecules). A drop of ink in water, forexample, will slowly spread throughoutthe liquid. Diffusion in solids occurs veryslowly at normal temperatures. See alsoGraham’s law.
dihydrate /dÿ-hÿ-drayt/ A crystallinecompound with two molecules of water ofcrystallization per molecule of compound.
dihydric alcohol /dÿ-hÿ-drik/ See diol.
2,3-dihydroxybutanedioic acid /dÿ-hÿ-droks-ăl-byoo-tayn-dÿ-oh-ik/ See tartaricacid.
dihydroxypurine /dÿ-hÿ-droks-ă-poor-een, -in/ See xanthine.
diiodine hexachloride /dÿ-ÿ-ŏ-deenheks-ă-klor-ÿd, -id, -kloh-rÿd, -kloh-rid/(iodine trichloride; I2Cl6) A yellow crys-talline solid made by reacting excess chlo-rine with iodine. It is a strong oxidizingagent and at 70°C it dissociates into iodinemonochloride and chlorine.
dilead(II) lead(IV) oxide /dÿ-led/ (redlead; Pb3O4) A powder made by heatinglead(II) oxide or lead(II) carbonate hydrox-ide at 400°C. It is black when hot and redor orange when cold. On strong heating itdecomposes to lead(II) oxide and oxygen.Dilead(II) lead (IV) oxide is used as a pig-ment and in glass making. In practice ittends to have less oxygen than denoted inthe formula.
diluent /dil-yoo-ĕnt/ A solvent that isadded to reduce the strength of a solution.
dilute Denoting a solution in which theamount of solute is low relative to that ofthe solvent. The term is always relative andincludes dilution at trace level as well as thecommon term ‘bench dilute acid’, whichusually means a 2M solution. Compareconcentrated.
dimensionless units The radian andsteradian in SI units. See SI units.
dimer /dÿ-mer/ A compound (or mol-ecule) formed by combination or associa-tion of two molecules of a monomer. Forinstance, aluminum chloride (AlCl3) is adimer (Al2Cl6) in the vapor.
dimesoiodic (VII) acid /dÿ-mess-oh-ÿ-od-ik/ See iodic (VII) acid.
dimethylbenzene /dÿ-meth-ăl-ben-zeen,ben-zeen/ (xylene; C6H4(CH3)2) An or-ganic hydrocarbon present in the light-oilfraction of crude oil. It is used extensivelyas a solvent. There are three isomeric com-pounds with this name and formula,distinguished as 1,2-, 1,3-, and 1,4-di-methylbenzene according to the positionsof the methyl groups on the benzene ring.
dimorphism /dÿ-mor-fiz-ăm/ See poly-morphism.
dinitrogen oxide /dÿ-nÿ-trŏ-jĕn/ (nitrousoxide; N2O) A colorless gas with a faintlysweet odor and taste. It is appreciably sol-uble in water (1.3 volumes in 1 volume ofwater at 0°C) but more soluble in ethanol.It is prepared commercially by the carefulheating of ammonium nitrate:
NH4NO3(s) = N2O(g) + 2H2O(g)Dinitrogen oxide is fairly easily decom-
posed on heating to temperatures above520°C, giving nitrogen and oxygen. Thegas is used as a mild anesthetic in medicineand dentistry, being marketed in small steelcylinders. It is sometimes called laughinggas because it induces a feeling of elationwhen inhaled.
dinitrogen tetroxide /te-troks-ÿd/ (N2O4)A colorless gas that becomes a pale yellowliquid below 21°C and solidifies below–11°C. On heating, the gas dissociates tonitrogen dioxide molecules:
N2O4(g) = 2NO2(g)This dissociation is complete at 140°C.Liquid dinitrogen tetroxide has good sol-vent properties and is used as a nitratingagent.
2,4-dinitrophenylhydrazine /dÿ-nÿ-troh-fen-ăl-hÿ-dră-zeen/ (Brady’s reagent;
89
2,4-dinitrophenylhydrazine
diol
90
C6H6N4O4) An orange solid commonlyused in solution with methanol and sulfu-ric acid to produce crystalline derivativesby condensation with aldehydes and ke-tones. The derivatives, known as 2,4-dini-trophenylhydrazones, can easily bepurified by recrystallization and have char-acteristic melting points, used to identifythe original aldehyde or ketone.
diol /dÿ-ôl/ (dihydric alcohol; glycol) Analcohol that has two hydroxyl groups(–OH) per molecule of compound.
1,4-dioxan /dÿ-oks-ăn/ ((CH2)2O2) A col-orless liquid cyclic ether. It is an inert com-pound miscible with water used as asolvent.
dioxins /dÿ-oks-inz/ A related group ofhighly toxic chlorinated compounds. Par-ticularly important is the compound2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD), which is produced as a by-prod-uct in the manufacture of 2,4,5-T, and mayconsequently occur as an impurity in cer-tain types of weedkiller. The defoliantknown as agent orange used in Vietnamcontained significant amounts of TCDD.Dioxins cause a skin disease (chloracne)and birth defects. Dioxins have been re-leased into the atmosphere as a result of ex-plosions at herbicide manufacturingplants, most notably at Seveso, Italy, in1976.
diphosphane /dÿ-fos-fayn/ (diphosphine;P2H4) A yellow liquid that can be con-densed out from phosphine in a freezingmixture. It ignites spontaneously in air.
diphosphine /dÿ-fos-feen, -fin/ Seediphosphane.
dipolar bond /dÿ-pohl-er/ See coordinatebond.
dipole /dÿ-pohl/ A system in which twoequal and opposite electric charges are sep-arated by a finite distance. Polar moleculeshave permanent dipoles. Induced dipolescan also occur. See also dipole moment.
dipole moment Symbol: µ A quantitativemeasure of polarity in either a bond (bondmoment) or a molecule as a whole (molec-ular dipole moment). The unit is the debye(equivalent to 3.34 × 10–30 coulombmeter). Molecules such as HF, H2O, NH3,and C6H5NH2 possess dipole moments;CCl4, N2, C6H6, and PF5 do not.
The molecular dipole moment can beestimated by vector addition of individualbond moments if the bond angles areknown. The possession of a dipole momentpermits direct interaction with electricfields or interaction with the electric com-ponent of radiation.
dipyridyl /dÿ-pÿ-ră-dăl/ (bipyridyl) Acompound formed by linking two pyridinerings. There are various isomers, some ofwhich are used in herbicides.
direct dyes A group of dyes that aremostly azo-compounds derived from ben-zidene or benzidene derivatives. They areused to dye cotton, viscose rayon, andother cellulose fibers directly, using a neu-tral bath containing sodium chloride orsodium sulfate as a mordant.
disaccharide /dÿ-sak-ă-rÿd, -rid/ A SUGAR
with molecules composed of two monosac-charide units. Sucrose and maltose are ex-amples. These are linked by a –O– linkage(glycosidic link).
disconnection See retrosynthetic analy-sis.
dislocation See defect.
disodium hydrogen phosphate(V) /dÿ-soh-dee-ŭm/ (disodium hydrogen or-thophosphate; Na2HPO4) A white solidprepared by titrating phosphoric acid withsodium hydroxide solution using phe-nolphthalein as the indicator. On evapora-tion the solution yields efflorescentmonoclinic crystals of the dodecahydrate,Na2HPO4.12H2O. The effloresced saltcontains 7H2O. Disodium hydrogen phos-phate is used in the textile industry.
disodium oxide (sodium monoxide;
Na2O) A highly reactive whitish deliques-cent solid that combines violently withwater to form sodium hydroxide.
disodium tetraborate decahydrate(borax; Na2B4O7.10H2O) A white crys-talline solid, sparingly soluble in coldwater but readily soluble in hot water. Itoccurs naturally as salt deposits in dry lakebeds, especially in California. It is an im-portant industrial material, being used inthe manufacture of enamels and heat-resis-tant glass, as a paper glaze, etc.
Its full systematic name is sodium(I)heptaoxotetraborate(III)-10-water. In so-lution hydrolysis occurs:
B4O72– + 7H2O = 2OH– + 4H3BO3
The borax-bead test gives characteristiccolors with certain cations.
disperse dyes Water-insoluble dyes,which, when held in fine suspension, canbe applied to acetate rayon fabrics. Thedye, together with a dispersing agent, iswarmed to a temperature of 45–50°C andthe fabric added. By modifying the methodof application it is possible to dye poly-acrylic and polyester fibers. The yellow/or-ange shades are nitroarylamine derivativesand the green to bluish shades are deriva-tives of 1-amino anthraquinone. Certainazo compounds are disperse dyes and thesegive a range of colors.
disperse phase See colloid.
dispersing agent A compound used to as-sist emulsification or dispersion.
dispersion force A weak type of inter-molecular force. See van der Waals force.
displacement pump A commonly useddevice for transporting liquids and gasesaround chemical plants. It works on theprinciple of the bicycle pump: a pistonraises the pressure of the fluid and, when itis high enough, a valve opens and the fluidis discharged through an outlet pipe. As thepiston moves back the pressure falls andthe cycle continues. Displacement pumpscan be used to generate very high pressures(e.g. in the synthesis of ammonia) but be-
cause of the system of valves, they are moreexpensive than other types of pump. Com-pare centrifugal pump.
displacement reaction A chemical reac-tion in which an atom or group displacesanother atom or group from a molecule. Acommon example is the displacement ofhydrogen from acids by metals:
Zn + 2HCl → ZnCl2 + H2
disproportionation /dis-prŏ-por-shŏ-nay-shŏn/ A chemical reaction in whichthere is simultaneous oxidation and reduc-tion of the same compound. An example isthe reaction of copper(I) chloride to copperand copper(II) chloride:
2CuCl → Cu + CuCl2in which there is simultaneous oxidation(to Cu(II)) and reduction (to Cu(0)).
Another example is the reaction ofchlorine with water:
Cl2 + H2O → 2H+ + Cl– + ClO–
in which there is reduction to Cl– and oxi-dation to ClO–.
dissociation Breakdown of a moleculeinto two molecules, atoms, radicals, orions. Often the reaction is reversible, as inthe ionic dissociation of weak acids inwater:
CH3COOH + H2O ˆ CH3COO– +H3O+
dissociation constant The equilibriumconstant of a dissociation reaction. For ex-ample, the dissociation constant of a reac-tion:
AB ˆ A + Bis given by:
K = [A][B]/[AB]where the brackets denote concentration(activity).
Often the degree of dissociation is used– the fraction (α) of the original compoundthat has dissociated at equilibrium. For anoriginal amount of AB of n moles in a vol-ume V, the dissociation constant is givenby:
K = α2n/(1 – α)V Note that thisexpression is for dissociation into two
molecules.
91
dissociation constant
Acid dissociation constants (or acidityconstants, symbol: Ka) are dissociationconstants for the dissociation into ions insolution:
HA + H2O ˆ H3O+ + A– Theconcentration of water can be taken asunity, and the acidity constant is given
by:Ka = [H3O+][A–]/[HA]
The acidity constant is a measure of thestrength of the acid. Base dissociation con-stants (Kb) are similarly defined. The ex-pression:
K = α2n/(1 – α)Vapplied to an acid is known as Ostwald’sdilution law (named for the Germanchemist Friedrich Wilhelm Ostwald(1853–1932).. In particular if α is small (aweak acid) then K = α2n/V, or α = C√V,where C is a constant. The degree of disso-ciation is then proportional to the squareroot of the dilution.
distillation The process of boiling a liquidand condensing the vapor. Distillation isused to purify liquids or to separate com-ponents of a liquid mixture. See also de-structive distillation; fractional distillation;steam distillation; vacuum distillation.
distilled water Water that has been puri-fied by distillation, perhaps several times.
disulfur dichloride /dÿ-sul-fer/ (sulfurmonochloride; S2Cl2) A red fuming liquidwith a strong smell. It is prepared by pass-ing chlorine over molten sulfur and is usedto harden rubber.
diterpene See terpene.
dithionate /dÿ-th’ÿ-ŏ-nayt/ A salt ofdithionic acid.
dithionic acid /dÿ-th’ÿ-on-ik/ (sulfinicacid; hyposulfuric acid; H2S2O6) A strongsulfur oxyacid that decomposes slowly onstanding or heating.
dithionite /dÿ-th’ÿ-ŏ-nÿt/ (sulfinate) Asalt of dithionous acid. Dithionites arepowerful reducing agents.
dithionous acid /dÿ-th’ÿ-ŏ-nŭs/ (sulfinicacid; hyposulfurous acid; H2S2O4) An un-stable sulfur oxyacid that is known only insolution. Its salts (dithionites or sulfinates)are powerful reducing agents.
divalent /dÿ-vay-lĕnt/ (bivalent) Having avalence of two.
dl-form See optical activity.
D-L convention See optical activity.
DNA (deoxyribonucleic acid) A nucleicacid, mainly found in the chromosomes,that contains the hereditary information oforganisms. The molecule is made up of twoantiparallel helical polynucleotide chainscoiled around each other to give a doublehelix. It is also known as the Watson–Crickmodel after James Watson and FrancisCrick who first proposed this model in1953. Phosphate molecules alternate withdeoxyribose sugar molecules along bothchains and each sugar molecule is alsojoined to one of four nitrogenous bases –adenine (A), guanine (G), cytosine (C), orthymine (T). The two chains are joined toeach other by hydrogen bonding betweenbases. The two purine bases (adenine andguanine) always bond with the pyrimidinebases (thymine and cytosine), and the pair-ing is quite specific: adenine with thymineand guanine with cytosine. The two chainsare therefore complementary. The se-quence of bases along the chain makes upa code – the genetic code – that determinesthe precise sequence of amino acids in pro-teins.
DNA is the hereditary material of allorganisms with the exception of RNAviruses. See also RNA.
Döbereiner’s triads /doh-ber-ÿ-nerz/ Tri-ads of chemically similar elements in whichthe central member, when placed in orderof increasing relative atomic mass, has arelative atomic mass approximately equalto the average of the outer two. Otherchemical and physical properties of thecentral member also lie between those ofthe first and last members of the triad. TheGerman chemist Johann Wolfgang
distillation
92
93
dubnium
Döbereiner (1780–1849) noted this rela-tionship in 1817; the triads are now recog-nized as consecutive members of a group ofthe periodic table; e.g. Ca, Sr, and Ba andCl, Br, and I.
dodecanoic acid /doh-dek-ă-noh-ik/ (lau-ric acid; CH3(CH2)10COOH) A whitecrystalline carboxylic acid, used as a plasti-cizer and for making detergents and soaps.Its glycerides occur naturally in coconutand palm oils.
dolomite /dol-ŏ-mÿt/ (pearl spar) A min-eral, CaCO3.MgCO3, used as an ore ofmagnesium and for the lining of open-hearth steel furnaces and Bessemer con-verters.
donor 1. The atom, ion, or molecule thatprovides the pair of electrons in forming acovalent bond.2. The impurity atoms used in doping semi-conductors.
doping The incorporation of impuritieswithin the crystal lattice of a solid so as toalter its physical properties. For instance,silicon, when doped with boron, becomessemiconducting.
double bond A covalent bond betweentwo atoms that includes two pairs of elec-trons, one pair being the single bond equiv-alent (the sigma pair) and the otherforming an additional bond, the pi bond (πbond). It is conventionally represented bytwo lines, for example H2C=O. See multi-ple bond; orbital.
double decomposition (metathesis) Achemical technique for making an insolu-ble salt by mixing two soluble salts. Theions from the two reactants ‘change part-ners’ to form a new soluble compound andthe insoluble compound, which is precipi-tated. For example, mixing solutions ofpotassium iodide and lead nitrate forms asolution of potassium nitrate and a yellowprecipitate of lead iodide.
double helix See DNA.
double salt When equivalent quantities ofcertain salts are mixed in aqueous solutionand the solution evaporated, a salt mayform, e.g. FeSO4.(NH4)2SO4.6H2O. Inaqueous solution the salt behaves as a mix-ture of the two individuals. These salts arecalled double salts to distinguish themfrom complex salts, which yield complexions in solution.
Downs process An electrolytic processfor making chlorine from fused sodiumchloride. The anode is a central piece ofgraphite, surrounded by a cylindrical steelcathode. A grid and dome between the twokeeps the products separate.
Dow process /dow/ An industrial methodwhereby magnesium is extracted from sea-water via the precipitation of Mg(OH)2 byCa(OH)2, followed by solvation of the pre-cipitated hydroxide by hydrochloric acid.The process is named for the American in-dustrial chemist Herbert Henry Dow(1866–1930), who founded the DowChemical Company (1897).
dry cell A voltaic cell in which the elec-trolyte is in the form of a jelly or paste. TheLeclanché dry cell is extensively used forflashlights and other portable applications.See also Leclanché cell.
dryers Devices used in chemical processesto remove a liquid from a solid by evapo-ration. Drying equipment is classified bythe method of transferring heat to a wetsolid. This can be by direct contact be-tween hot gases and the solid (direct dry-ers), heat transfer by conduction through aretaining metallic wall (indirect dryers), orinfrared rays (infrared dryers).
dry ice Solid carbon dioxide, used as a re-frigerant.
drying oil A natural oil, such as linseedoil, that hardens in air. Such oils containunsaturated fatty acids, which polymerizeon oxidation.
dubnium /dub-nee-ŭm/ A radioactive syn-thetic element made by bombarding 249Cf
nuclei with 15N atoms. It was first reportedby workers at Dubna, a town nearMoscow. Symbol: Db; p.n. 105; most sta-ble isotope 262Db (half-life 34 s).
Dulong and Petit’s law /doo-long ănd pĕ-teez / The molar thermal capacity of a solidelement is approximately equal to 3R,where R is the gas constant (25 J K–1
mol–1). The law applies only to elementswith simple crystal structures at normaltemperatures. At lower temperatures themolar heat capacity falls with decreasingtemperature (it is proportional to T3).Molar thermal capacity was formerlycalled atomic heat – the product of theatomic weight (relative atomic mass) andthe specific thermal capacity. The law isnamed for the French physicists Pierre-Louis Dulong (1785–1838) and Alexis-Thérèse Petit (1791–1820).
Dumas’ method /doo-mahz/ 1. A methodfor determining the vapor densities ofvolatile liquids. The method utilizes aDumas bulb; i.e. a glass bulb with a narrowentrance tube. The bulb is weighed ‘empty’(i.e. full of air) then the sample is intro-duced and the bulb immersed in a heatingbath so that the sample boils and expels allthe air. When the surplus vapor has beenexpelled, the bulb is sealed off, cooled,dried, and weighed. The tip of the tube isthen broken under water so that the watercompletely fills the tube and the wholeweighed again. This enables the volume ofthe bulb to be calculated from the knowndensity of water, and knowing the densityof air one can compute the vapor density ofthe sample. The method is named for theFrench chemist Jean Baptiste André Dumas(1800–84). See also Hofmann’s method;Victor Meyer’s method.
2. A method of finding the amount of ni-trogen in an organic compound by heatingthe compound with copper oxide to con-vert the nitrogen into nitrogen oxides.These are reduced by passing them overhot copper and the volume of nitrogen col-lected is measured.
Duralumin /dewr-al-yŭ-min/ (Trademark)A strong lightweight aluminum alloy con-taining 3–4% copper, with small amountsof magnesium, manganese, and sometimessilicon. It is widely used in aircraft bodies.
dye A coloring material for fabric, leather,etc. Most dyes are now synthetic organiccompounds (the first such was the dyemauve synthesized from aniline in 1856 byW. H. Perkin). Dyes are often unsaturatedorganic compounds containing conjugateddouble bonds – the bond system responsi-ble for the color is called the chromophore.See also azo compound.
dynamite A high explosive made by ab-sorbing nitroglycerine into an earthy mate-rial such as diatomite (kieselguhr). Solidsticks of dynamite are much safer to handlethan the highly sensitive liquid nitroglycer-ine.
dyne /dÿn/ Symbol: dyn The former unitof force used in the c.g.s. system. It is equalto 10–5 N.
dysprosium /dis-proh-see-ŭm, -shee-ŭm/A soft malleable silvery element of the lan-thanoid series of metals. It occurs in asso-ciation with other lanthanoids. One of itsfew uses is as a neutron absorber in nuclearreactors; it is also a constituent of certainmagnetic alloys.Symbol: Dy; m.p. 1412°C; b.p. 22562°C;r.d. 8.55 (20°C); p.n. 66; r.a.m. 162.50.
Dulong and Petit’s law
94
95
ebulioscopic constant /i-bul-ee-ŏ-skop-ik/ See elevation of boiling point.
ebullition /eb-ŭ-lish-ŏn/ The boiling orbubbling of a liquid.
eclipsed conformation See conforma-tion.
Edison cell See nickel–iron accumulator.
edta /ee-dee-tee-ay/ (ethylenediaminetetraacetic acid) A compound with the for-mula
(HOOCCH2)2N(CH2)2N(CH2COOH)2It is used in forming chelates of transitionmetals. See chelate.
effervescence /ef-er-vess-ĕns/ The evolu-tion of gas in the form of bubbles in a liq-uid.
efficiency Symbol: η A measure used forprocesses of energy transfer; the ratio ofthe useful energy produced by a system ordevice to the energy input. For a reversibleheat engine, the efficiency is given by
η = (T1 – T2)/T1T1 being the source temperature and T2 the‘sink’ temperature.
efflorescence /ef-flŏ-ress-ĕns/ The processin which a crystalline hydrated solid loseswater of crystallization to the air. A pow-dery deposit is gradually formed.
einsteinium /ÿn-stÿ-nee-ŭm/ A radio-active transuranic element of the actinoidseries, not found naturally on Earth. It canbe produced in milligram quantities bybombarding 239Pu with neutrons to give253Es (half-life 20.47 days). Several othershort-lived isotopes have been synthesized.
Symbol: Es; m.p. 860 ± 30°C; p.n. 99;most stable isotope 254Es (half-life 276days). The element is named for the Ger-man–Swiss–American theoretical physicistAlbert Einstein (1879–1955).
elastomer /i-las-tom-ĕ-ter/ An elastic sub-stance, e.g. a natural or synthetic rubber.
electrochemical equivalent /i-lek-trŏ-kem-ă-kăl/ Symbol: z The mass of an el-ement released from a solution of its ionwhen a current of one ampere flows forone second during electrolysis.
electrochemical series (electromotive se-ries) A series giving the activities of metalsfor reactions that involve ions in solution.In decreasing order of activity, the series is
K, Na, Ca, Mg, Al, Zn, FePb, H, Cu, Hg, Ag, Pt, Au
Any member of the series will displace ions of a lower member from solution. Forexample, zinc metal will displace Cu2+
ions:Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Zinc has a greater tendency than copper toform positive ions in solution. Similarly,metals above hydrogen displace hydrogenfrom acids:
Zn + 2HCl → ZnCl2 + H2The series is based on electrode potentials,which measure the tendency to form posi-tive ions. The series is one of increasingelectrode potential for half cells of the typeMn+|M. Thus, copper (EŠ for Cu2+|Cu = +0.34 V) is lower than zinc (EŠ for Zn2+|Zn= –0.76V). The hydrogen half cell has avalue EŠ = 0.
electrochemistry /i-lek-trŏ-kem-iss-tree/The study of the formation and behavior ofions in solutions. It includes electrolysis
E
electrochromatography
96
and the generation of electricity by chemi-cal reactions in cells.
electrochromatography /i-lek-troh-kroh-mă-tog-ră-fee/ See electrophoresis.
electrocyclic reaction /i-lek-troh-sÿ-klik/See pericyclic reaction.
electrode Any part of an electrical deviceor system that emits or collects electrons orother charge carriers. An electrode mayalso be used to deflect charged particles bythe action of the electrostatic field that itproduces. See also half cell.
electrodeposition /i-lek-troh-dep-ŏ-zish-ŏn/ (electroplating) The process of de-positing a layer of solid (metal) on anelectrode by electrolysis. Positive ions insolution gain electrons at the cathode andare deposited as atoms. Copper, for in-stance, can be deposited on a metal cath-ode from an acidified copper sulfatesolution.
electrode potential Symbol: E A meas-ure of the tendency of an element to formions in solution. For example, a metal in asolution containing M+ ions may dissolvein the solution as M+ ions; the metal thenhas an excess of electrons and the solutionan excess of positive ions – thus, the metalbecomes negative with respect to the solu-tion. Alternatively, the positive ions maygain electrons from the metal and be de-posited as metal atoms. In this case, themetal becomes positively charged with re-spect to the solution. In either case, a po-tential difference is developed betweensolid and solution, and an equilibriumstate will be reached at which further reac-tion is prevented. The equilibrium value ofthis potential difference would give an in-dication of the tendency to form aqueousions.
It is not, however, possible to measurethis for an isolated half cell – any measure-ment requires a circuit, which sets up an-other half cell in the solution. Therefore,electrode potentials (or reduction poten-tials) are defined by comparison with a hy-drogen half cell, which is connected to the
half cell under investigation by a saltbridge. The e.m.f. of the full cell can thenbe measured.
In referring to a given half cell the morereduced form is written on the right for ahalf-cell reaction. For the half cell Cu2+|Cu,the half-cell reaction is a reduction:
Cu2+(aq) + 2e → CuThe cell formed in comparison with a hy-drogen electrode is:
Pt(s)H2(g)|H+(aq)|Cu2+(aq)|CuThe e.m.f. of this cell is +0.34 volt meas-ured under standard conditions. Thus, thestandard electrode potential (symbol: EŠ)is +0.34 V for the half cell Cu2+|Cu. Thestandard conditions are 1.0 molar solu-tions of all ionic species, standard pressure,and a temperature of 298 K.
Half cells can also be formed by a solu-tion of two different ions (e.g. Fe2+ andFe3+). In such cases, a platinum electrode isused under standard conditions.
electrodialysis /i-lek-troh-dÿ-al-ă-sis/ (pl.electrodialyses) A method of removingions from water by selective flow throughmembranes under the influence of an elec-tric field. In a simple arrangement a cell isdivided into three compartments by twosemipermeable membranes, one permeableto positive ions and the other permeable tonegative ions. Electrodes are placed in theouter compartments of the cell – the posi-tive electrode next to the membrane thatallows negative ions to pass. In thisarrangement, positive and negative ionspass through the membranes in leavingdeionized water in the center compartmentof the cell. In practice, an array of alternat-ing membranes is used. Electrodialysis isused for making drinking water in areas inwhich brackish ground water is available.
electrolysis /i-lek-trol-ă-sis/ The produc-tion of chemical change by passing electriccharge through certain conducting liquids(electrolytes). The current is conducted bymigration of ions – positive ones (cations)to the cathode (negative electrode), andnegative ones (anions) to the anode (posi-tive electrode). Reactions take place at theelectrodes by transfer of electrons to orfrom them.
In the electrolysis of water (containing asmall amount of acid to make it conductadequately) hydrogen gas is given off at thecathode and oxygen is evolved at theanode. At the cathode the reaction is:
H+ + e– → H2H → H2
At the anode:OH– → e– + OH
2OH → H2O + O2O → O2
In certain cases the electrode materialmay dissolve. For instance, in the electrol-ysis of copper(II) sulfate solution with cop-per electrodes, copper atoms of the anodedissolve as copper ions
Cu → 2e– + Cu2+
See Faraday’s laws.
electrolyte /i-lek-trŏ-lÿt/ A liquid contain-ing positive and negative ions that con-ducts electricity by the flow of thosecharges. Electrolytes can be solutions ofacids or metal salts (‘ionic compounds’),usually in water. Alternatively they may bemolten ionic compounds – again the ionscan move freely through the substance.Liquid metals (in which conduction is byfree electrons rather than ions) are not clas-sified as electrolytes. See also electrolysis.
electrolytic /i-lek-trŏ-lit-ik/ Relating tothe behavior or reactions of ions in solu-tion.
electrolytic cell See cell; electrolysis.
electrolytic corrosion Corrosion by elec-trochemical reaction. See rusting.
electrolytic refining A method of purify-ing metals by electrolysis. Copper is puri-fied by making the impure metal the anodein an electrolytic cell with an acidified cop-per sulfate electrolyte. The cathode is a thinstrip of pure copper. The copper at theanode dissolves (as Cu2+ ions) and purecopper is deposited on the cathode. In thisparticular process, gold and silver are ob-tained as by-products, deposited as ananode sludge on the bottom of the cell.
electromagnetic radiation Energy prop-agated by vibrating electric and magneticfields. Electromagnetic radiation forms awhole electromagnetic spectrum, depend-ing on frequency and ranging from high-frequency radio waves to low-frequencygamma rays. It can be thought of as waves(electromagnetic waves) or as streams ofphotons. The frequency and wavelengthare related by
λv = cwhere c is the speed of light. The energycarried depends on the frequency.
electromotive series See electrochemicalseries.
electron /i-lek-tron/ An elementary parti-cle of negative charge (–1.602 192 ×10–19C) and rest mass 9.109 558 × 10–31
kg. Electrons are present in all atoms.
electron affinity Symbol: A The energyreleased when an atom (or molecule orgroup) gains an electron in the gas phase toform a negative ion. It is thus the energy of:
A + e– → A–
97
electron affinity
ELECTROMAGNETIC SPECTRUM(note: the figures are only approximate)
Radiation Wavelength (m) Frequency (Hz)gamma radiation –10–12 1019–x-rays 10–12–10–9 1017–1020
ultraviolet radiation 10–9 –10–7 1015–1018
visible radiation 10–7 –10–6 1014–1015
infrared radiation 10–6 –10–4 1012–1014
microwaves 10–4 –1 109–1013
radio waves 1 – –109
A positive value of A (often in electron-volts) indicates that heat is given out. Oftenthe molar enthalpy is given for this processof electron attachment (∆H). Here theunits are joules per mole (J mol–1), and, bythe usual convention, a negative value indi-cates that energy is released.
electron-deficient compounds Com-pounds in which the number of electronsavailable for bonding is insufficient for thebonds to consist of conventional two-elec-tron covalent bonds. Diborane, B2H6, is anexample in which each boron atom hastwo terminal hydrogen atoms bound byconventional electron-pair bonds and inaddition the molecule has two hydrogenatoms bridging the boron atoms (B–H–B).In each bridge there are only two electronsfor the bonding orbital. See also multicen-ter bond.
electron diffraction A technique used todetermine the structure of substances, prin-cipally the shapes of molecules in thegaseous phase. A beam of electrons di-rected through a gas at low pressure pro-duces a series of concentric rings on aphotographic plate. The dimensions ofthese rings are related to the interatomicdistances in the molecules. See also x-raydiffraction.
electron donor See reduction.
electronegative /i-lek-troh-neg-ă-tiv/ De-scribing an atom or molecule that attractselectrons, forming negative ions. Examplesof electronegative elements include thehalogens (chlorine etc.), which readilyform negative ions (F–, Cl–, etc.). See alsoelectronegativity.
electronegativity /i-lek-troh-neg-ă-tiv-ă-tee/ A measure of the tendency of an atomin a molecule to attract electrons to itself.Elements to the right-hand side of the peri-odic table are strongly electronegative (val-ues from 2.5 to 4); those on the left-handside have low electronegativities (0.8–1.5)and are sometimes called electropositive el-ements. Different electronegativities ofatoms in the same molecule give rise to
polar bonds and sometimes to polar mol-ecules.
As the concept of electronegativity isnot precisely defined it cannot be preciselymeasured and several electronegativityscales exist. Although the actual values dif-fer the scales are in good relative agree-ment. See also electron affinity; ionizationpotential.
electronic energy levels See atom; en-ergy levels.
electronic transition The demotion orpromotion of an electron between elec-tronic energy levels in an atom or molecule.
electron pair Two electrons in one or-bital with opposing spins (spin paired),such as the electrons in a covalent bond orlone pair.
electron spin See atom.
electron spin resonance (ESR) A similartechnique to nuclear magnetic resonance,but applied to unpaired electrons in a mol-ecule (rather than to the nuclei). It is apowerful method of studying free radicalsand transition-metal complexes.
electron-transfer reaction A chemicalreaction that involves the transfer, addi-tion, or removal of electrons. Many elec-tron-transfer reactions involve complexesof transition metals. The rates of such re-actions vary enormously and can be ex-plained in terms of the way in whichmolecules of the solvent, which start offsolvating the reactants, rearrange so as tosolvate the products.
electronvolt /i-lek-tron-vohlt/ Symbol: eVA unit of energy equal to 1.602 191 7 ×10–19 joule. It is defined as the energy re-quired to move an electron charge across apotential difference of one volt. It is oftenused to measure the kinetic energies of ele-mentary particles or ions, or the ionizationpotentials of molecules.
electrophile /i-lek-trŏ-fÿl, -fil/ An elec-tron-deficient ion or molecule that takes
electron-deficient compounds
98
part in an organic reaction. The elec-trophile can be either a positive ion (H+,NO2
+) or a molecule that can accept anelectron pair (SO3, O3). The electrophileattacks negatively charged areas of mol-ecules, which usually arise from the pres-ence in the molecule of an electronegativeatom or group or of pi-bonds. Comparenucleophile.
electrophilic addition /i-lek-trŏ-fil-ik/ Areaction involving the addition of a smallmolecule to an unsaturated organic com-pound, across the atoms held by a doubleor triple bond. The reaction is initiated bythe attack of an electrophile on the elec-tron-rich area of the molecule. The mecha-nism of electrophilic addition is thought tobe ionic, as in the addition of HBr toethene:
H2C:CH2 + H+Br– →H3CCH2
+ + Br– →H3CCH2Br
In the case of higher alkenes (more thantwo carbon atoms) several isomeric prod-ucts are possible. The particular isomerproduced depends on the stability of the al-ternative intermediates and this is summa-rized empirically by Markovnikoff’s rule.See also addition reaction.
electrophilic substitution A reaction in-volving substitution of an atom or group ofatoms in an organic compound with anelectrophile as the attacking substituent.Electrophilic substitution is very commonin aromatic compounds, in which elec-trophiles are substituted onto the ring. Anexample is the nitration of benzene:
C6H6 + NO2+ → C6H5NO2 + H+
The nitronium ion (NO2+) is formed by
mixing concentrated nitric and sulfuricacids:
HNO3 + H2SO4 → H2NO3+ + HSO4
–
H2NO3+ → NO2
+ + H2OThe accepted mechanism for a simple
electrophilic substitution on benzene in-volves an intermediate of the formC6H5HNO2
+. See also substitution reac-tion.
electrophoresis /i-lek-troh-fŏ-ree-sis/ Theuse of an electric field (between two elec-
trodes) to cause charged particles of a col-loid to move through a solution. The tech-nique is used to separate and identifycolloidal substances such as carbohydrates,proteins, and nucleic acids. Various exper-imental arrangements are used. One simpletechnique uses a strip of adsorbent papersoaked in a buffer solution with electrodesplaced at two points on the paper. Thistechnique is sometimes called electrochro-matography. In gel electrophoresis, used toseparate DNA fragments, the medium is alayer of gel.
electroplating /i-lek-trŏ-play-ting/ Theprocess of coating a solid surface with alayer of metal by means of electrolysis (i.e.by electrodeposition).
electropositive /i-lek-troh-poz-ă-tiv/ De-scribing an atom or molecule that tends tolose electrons, forming positive ions. Ex-amples of electropositive elements includethe alkali metals (lithium, sodium, etc.),which readily form positive ions (Li+, Na+,etc.).
electrovalent bond /i-lek-trŏ-vay-lĕnt/(ionic bond) A binding force between theions in compounds in which the ions areformed by complete transfer of electronsfrom one element to another element orradical. For example, Na + Cl becomes Na+
+ Cl–. The electrovalent bond arises fromthe excess of the net attractive force be-tween the ions of opposite charge over thenet repulsive force between ions of likecharge. The magnitude of electrovalent in-teractions is of the order 102–103 kJ mol–1
and electrovalent compounds are generallysolids with rigid lattices of closely packedions. The strengths of electrovalent bondsvary with the reciprocal of the inter-ionicdistances and are discussed in terms of lat-tice energies.
electrum /i-lek-trŭm/ Originally, a natu-rally occurring alloy of gold and silver (upto 45% silver) that resembles pure gold inappearance. A synthetic alloy of copper,nickel, and zinc is also known as electrum.
99
electrum
element /el-ĕ-mĕnt/ A substance that can-not be chemically decomposed into moresimple substances. The atoms of an el-ement all have the same proton num-ber (and thus the same number ofelectrons, which determines the chemicalactivity).
At present there are 112 reported chem-ical elements, although research is continu-ing all the time to synthesize new ones. Theelements from hydrogen (p.n. 1) to ura-nium (92) all occur naturally, with the ex-ception of technetium (43), which isproduced artificially by particle bombard-ment. Technetium and elements with pro-ton numbers higher than 84 (polonium)are radioactive. Radioactive isotopes alsoexist for other elements, either naturally insmall amounts or synthesized by particlebombardment. The elements with protonnumber higher than 92 are the transuranicelements. Neptunium (93) and plutonium(94) both occur naturally in small quanti-ties in uranium ores, but the transuranicsare all synthesized. Thus, neptunium andplutonium are made by neutron bombard-ment of uranium nuclei. Other transuran-ics are made by high-energy collisionprocesses between nuclei. The higher pro-ton number elements have been detectedonly in very small quantities – in somecases, only a few atoms have been pro-duced.
There has been some controversy aboutthe naming of the higher synthetic elementsbecause of disputes about their discovery.A long-standing problem concerned el-ement-104 (rutherfordium) which was for-merly also known by its Russian name ofkurchatovium. More recent confusion hasbeen caused by differences between namessuggested by the International Union ofPure and Applied Chemistry (IUPAC) andthe names suggested by the AmericanChemical Union (ACU).The original IUPAC names (1994) were:
mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) dubnium (Db, 104)joliotium (Jl, 105) rutherfordium (Rf, 106) bohrium (Bh, 107)
hahnium (Hn, 108) meitnerium (Mt, 109)
The ACU names were: mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) rutherfordium (Rf, 104) hahnium (Ha, 105) seaborgium (Sg, 106) nielsbohrium (Ns, 107) hassium (Hs, 108) meitnerium (Mt, 109)
A compromise list of names was adoptedby IUPAC in 1997:
mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) rutherfordium (Rf, 104) dubnium (Db, 105) seaborgium (Sg, 106) bohrium (Bh, 107) hassium (Hs, 108) meitnerium (Mt, 109)
This is the list used in this dictionary. Dub-nium is named after Dubna in Russia,where much work has been done on syn-thetic elements. Hassium is the Latin namefor the German state of Hesse, where theGerman research group is located
In addition to the above elements, sevenother elements have been reported withproton numbers 110, 111, 112, 113, 114,115, and 116. Element 110 has beennamed darmstadtium for the town in Ger-many where it was first discovered. El-ement 111 has been named roentgenium(for Roentgen). So far the others have notbeen named and have their temporary sys-tematic IUPAC names (see unnil-).
elevation of boiling point A colligativeproperty of solutions in which the boilingpoint of a solution is raised relative to thatof the pure solvent. The elevation is di-rectly proportional to the number of solutemolecules introduced rather than to anyspecific aspect of the solute composition.The proportionality constant, kB, is calledthe boiling-point elevation constant orsometimes the ebulioscopic constant. Therelationship is
∆t = kBCM
element
100
where ∆t is the rise in boiling point and CMis the molal concentration; the units of kBare kelvins kilograms moles–1 (K kg mol–1).The property permits the measurement ofrelative molecular mass of involatilesolutes. An accurately weighed amount ofpure solvent is boiled until the temperatureis steady, a known weight of solute ofknown molecular mass is quickly intro-duced, the boiling continued, and the ele-vation measured using a Beckmannthermometer. The process is repeated sev-eral times and the average value of kB ob-tained by plotting ∆t against CM. Thewhole process is then repeated with the un-known material and its relative molecularmass obtained using the value of kB previ-ously obtained.
There are several disadvantages withthis method and it is therefore used largelyfor demonstration purposes. The mainproblem is that the exact amount of solventremaining in the liquid phase is unknownand varies with the rate of boiling. The the-oretical explanation of the effect is identi-cal to that for the lowering of vaporpressure; the boiling points are those tem-
peratures at which the vapor pressureequals the atmospheric pressure.
elimination reaction A reaction involv-ing the removal of a small molecule, e.g.water or hydrogen chloride, from an or-ganic molecule to give an unsaturated com-pound. An example is the elimination of awater molecule from an alcohol to producean alkene.
An elimination reaction is often in com-petition with a substitution reaction andthe predominant product will depend onthe reaction conditions. The reaction ofbromoethane with sodium hydroxidecould yield either ethene (by elimination ofHBr) or ethanol (by substitution of the Brwith OH). The former product predomi-nates if the reaction is carried out in an al-coholic solution and the latter if thesolution is aqueous.
Elinvar /el-in-var/ (Trademark) An alloyof chromium, iron, and nickel. It is used formaking hairsprings for clocks and watchesbecause its elasticity does not vary withtemperature.
elution /i-loo-shŏn/ The removal of an ad-sorbed substance in a chromatography col-umn or ion-exchange column using asolvent (eluent), giving a solution called theeluate. The chromatography column canselectively adsorb one or more componentsfrom the mixture. To ensure efficient re-covery of these components graded elutionis used. The eluent is changed in a regularmanner starting with a non-polar solventand gradually replacing it by a more polarone. This will wash the strongly polar com-ponents from the column. See columnchromatography.
emery A mineral form of corundum (alu-minum oxide) with some iron oxide andsilica impurities. It is extremely hard and isused as an abrasive and polishing material.
emission spectrum See spectrum.
empirical formula The formula of acompound showing the simplest ratio ofthe atoms present. The empirical formula
101
empirical formula
Beckmannthermometer
tube forintroducing
solute
heater
condenser
Apparatus for measuring elevation of boiling point
is the formula obtained by experimentalanalysis of a compound and it can be re-lated to a molecular formula only if themolecular weight is known. For exampleP2O5 is the empirical formula of phospho-rus(V) oxide although its molecular for-mula is P4O10. Compare molecularformula; structural formula.
emulsion A colloid in which a liquidphase (small droplets with a diameterrange 10–5–10–7 cm) is dispersed or sus-pended in a liquid medium. Emulsions areclassed as lyophobic (solvent-repelling andgenerally unstable) or lyophilic (solvent-at-tracting and generally stable).
enantiomer /en-an-tee-ŏ-mer/ (enan-tiomorph) A compound whose structure isnot superimposable on its mirror image:one of any pair of optical isomers. See alsoisomerism; optical activity.
enantiotropy /en-an-tee-ot-rŏ-pee/ Theexistence of different stable allotropes ofan element at different temperatures; sul-fur, for example, exhibits enantiotropy.The phase diagram for an enantiotropic el-ement has a point at which all the al-lotropes can coexist in a stableequilibrium. At temperatures above orbelow this point, one of the allotropes willbe more stable than the other(s). See alsoallotropy; monotropy.
enclosure compound See clathrate.
endothermic /en-doh-th’er-mik/ Describ-ing a process in which heat is absorbed (i.e.heat flows from outside the system, or thetemperature falls). The dissolving of a saltin water, for instance, is often an endother-mic process. Compare exothermic.
end point See equivalence point; volu-metric analysis.
energy Symbol: W A property of a system;a measure of its capacity to do work. En-ergy and work have the same unit: the joule(J). It is convenient to divide energy into ki-netic energy (energy of motion) and poten-tial energy (‘stored’ energy). Names are
given to many different forms of energy(chemical, electrical, nuclear, etc.); the onlyreal difference lies in the system under dis-cussion. For example, chemical energy isthe kinetic and potential energies of elec-trons in a chemical compound.
energy level One of the discrete energiesthat an atom or molecule, for instance, canhave according to quantum theory. Thus inan atom there are certain definite orbitsthat the electrons can be in, correspondingto definite electronic energy levels of theatom. Similarly, a vibrating or rotatingmolecule can have discrete vibrational androtational energy levels.
energy profile A diagram that traces thechanges in the energy of a system duringthe course of a reaction. Energy profiles areobtained by plotting the potential energyof the reacting particles against the reac-tion coordinate. To obtain the reaction co-ordinate the energy of the total interactingsystem is plotted against position for themolecules. The reaction coordinate is thepathway for which the energy is a mini-mum.
The energy profile for the hydrogena-tion of an alkene with and without a cata-lyst has certain interesting features: theactivation energy without the catalyst isvery much larger than that with the cata-lyst and any point along the horizontal axisrepresents the changes that the collidingparticles experience as atoms are separatedin bond breaking and as atoms are broughttogether in bond formation.
enol /ee-nol, -nohl/ An organic compoundcontaining the C:CH(OH) group; i.e. onein which a hydroxyl group is attached tothe carbon of a double bond. See keto-enoltautomerism.
enthalpy /en-thal-pee, en-thal-/ Symbol: HThe sum of the internal energy (U) and theproduct of pressure (p) and volume (V) ofa system:
H = U + pVIn a chemical reaction carried out at
constant pressure, the change in enthalpy
emulsion
102
measured is the internal energy change plusthe work done by the volume change:
∆H = ∆U + p∆V
entropy /en-trŏ-pee/ (pl. entropies) Sym-bol: S In any system that undergoes a re-versible change, the change of entropy isdefined as the heat absorbed divided by thethermodynamic temperature:
δS = δQ/TA given system is said to have a certain en-tropy, although absolute entropies are sel-dom used: it is change in entropy that isimportant. The entropy of a system meas-ures the availability of energy to do work.
In any real (irreversible) change in aclosed system the entropy always increases.Although the total energy of the system hasnot changed (first law of thermodynamics)the available energy is less – a consequenceof the second law of thermodynamics.
The concept of entropy has beenwidened to take in the general idea of dis-order – the higher the entropy, the moredisordered the system. For instance, achemical reaction involving polymeriza-tion may well have a decrease in entropybecause there is a change to a more orderedsystem. The ‘thermal’ definition of entropyis a special case of this idea of disorder –here the entropy measures how the energytransferred is distributed among the parti-cles of matter. See also Boltzmann formula.
enzyme /en-zÿm/ A protein, with a rela-tive molecular mass between 105 and 107,that catalyzes a specific biochemical reac-tion. An enzyme mediates the conversionof one substance (the substrate) to another(the product) by combining with the sub-strate to form an intermediate complex.The enzyme molecule is very much largerthan the substrate molecule. Enzymes dif-fer from inorganic catalysts in the follow-ing ways:1. they have a high degree of specificity –
enzymes can even distinguish betweenenantiomorphs;
2. an increase in temperature can cause adecrease in the rate of reaction as a re-sult of the enzyme being denatured;
3. they are destroyed by too great a changein pH;
4. they are inactivated by low concentra-tions of heavy-metal ions.
epimerism /i-pim-ĕ-riz-ăm/ A form of iso-merism exhibited by carbohydrates inwhich the isomers (epimers) differ in thepositions of –OH groups. The α- and β-forms of glucose are epimers. See sugar.
epoxide /i-poks-ÿd/ A type of organiccompound containing a three-memberedring in which two carbon atoms are eachbonded to the same oxygen atom.
epoxyethane /i-poks-ee-eth-ayn/ (ethyleneoxide; C2H4O) A colorless gaseous ether.Epoxyethane has a cyclic structure with athree-membered ring. It is made by oxida-tion of ethene over a silver catalyst. Thering is strained and the compound is con-sequently highly reactive. It polymerizes toepoxy polymers (resins). Hydrolysis pro-duces 1,2-ethanediol.
epoxy resin /i-poks-ee/ See epoxyethane.
Epsom salt /ep-sŏm/ See magnesium sul-fate.
equation See chemical equation.
equation of state An equation that inter-relates the pressure, temperature, and vol-ume of a system, such as a gas. The idealgas equation (see gas laws) and the van derWaals equation are examples.
equilibrium /ee-kwă-lib-ree-ŭm, ek-wă-/(pl. equilibriums or equilibria) In a re-versible chemical reaction:
A + B ˆ C + DThe reactants are forming the products:
A + B → C + D
103
equilibrium
CH2CH2
O
Epoxyethane
which also react to give the original reac-tants:
C + D → A + BThe concentrations of A, B, C, and Dchange with time until a state is reached atwhich both reactions are taking place atthe same rate. The concentrations (or pres-sures) of the components are then constant– the system is said to be in a state of chem-ical equilibrium. Note that the equilibriumis a dynamic one; the reactions still takeplace but at equal rates. The relative pro-portions of the components determine the‘position’ of the equilibrium, which may bedisplaced by changing the conditions (e.g.temperature or pressure).
equilibrium constant In a chemical equi-librium of the type
xA + yB ˆ zC + wDThe expression:
[A]x[B]y/[C]z[D]w
where the square brackets indicate concen-trations, is a constant (Kc) when the systemis at equilibrium. Kc is the equilibrium con-stant of the given reaction; its units dependon the stoichiometry of the reaction. Forgas reactions, pressures are often used in-stead of concentration. The equilibriumconstant is then Kp, where Kp = Kc
n. Here nis the number of moles of product minusthe number of moles of reactant; for in-stance, in
3H2 + N2 ˆ 2NH3
n is 2 – (1 + 3) = –2.
equipartition of energy /ee-kwă-par-tish-ŏn, ek-wă-/ The principle that the total en-ergy of a molecule is, on average, equallydistributed among the available DEGREES OF
FREEDOM. It is only approximately true inmost cases.
equivalence point The point in a titra-tion at which the reactants have beenadded in equivalent proportions, so thatthere is no excess of either. It differsslightly from the end point, which is theobserved point of complete reaction, be-cause of the effect of the indicator, errors,etc.
equivalent proportions, law of (law ofreciprocal proportions) When two chemi-cal elements both form compounds with athird element, a compound of the first twoelements contains them in the relative pro-portions they have in compounds with thethird element. For example, the mass ratioof carbon to hydrogen in methane (CH4) is12:4; the ratio of oxygen to hydrogen inwater (H2O) is 16:2. In carbon monoxide(CO), the ratio of carbon to oxygen is12:16.
equivalent weight A measure of ‘com-bining power’ formerly used in calcula-tions for chemical reactions. Theequivalent weight of an element is the num-ber of grams that could combine with ordisplace one gram of hydrogen (or 8 gramsof oxygen or 35.5 grams of chlorine). It isthe relative atomic mass (atomic weight)divided by the valence. For a compoundthe equivalent weight depends on the reac-tion considered. An acid, for instance, inacid–base reactions has an equivalentweight equal to its molecular weight di-vided by the number of acidic hydrogenatoms.
erbium /er-bee-ŭm/ A soft malleable sil-very element of the lanthanoid series ofmetals. It occurs in association with otherlanthanoids. Erbium has uses in the metal-lurgical and nuclear industries and in mak-ing glass for absorbing infrared radiation.
Symbol: Er; m.p. 1529°C; b.p. 2863°C;r.d. 9.066 (25°C); p.n. 68; r.a.m. 167.26.
erg A former unit of energy used in thec.g.s. system. It is equal to 10–7 joule.
Erlenmeyer flask /er-lĕn-mÿ-er/ A coni-cal glass laboratory flask with a narrowneck. It is named for the German chemistRichard August Carl Emil Erlenmeyer(1825–1909).
ESR See electron spin resonance.
essential amino acid See amino acid.
essential oil Any pleasant-smellingvolatile oil obtained from various plants,
equilibrium constant
104
widely used in making flavorings and per-fumes. Most consist of terpenes and theyare obtained by steam distillation or sol-vent extraction.
ester A type of organic compound formedby reaction between an acid and an alco-hol. If the acid is a carboxylic acid, the for-mula is RCOOR′, where R and R′ areorganic groups. Esters of low-molecularweight acids and alcohols are volatile fra-grant compounds. Esters of some long-chain carboxylic acids occur naturally infats and oils. Esters can be prepared by di-rect reaction of acids and alcohols, with adehydrating agent to take out the waterand displace the equilibrium. Reaction ofalcohols with acyl halides also gives esters.See also carboxylic acid; saponification;glyceride.
esterification /es-te-ri-fă-kay-shŏn/ Thereaction of an acid with an alcohol to forman ester and water. The reaction is an equi-librium of the form:
CH3COOH + C2H5OH ˆCH3COOC2H5 + H2O
In the preparation of esters the ester isdistilled off or the water is removed to in-crease the yield. The reverse reaction is hy-drolysis.
ethanal /eth-ă-nal/ (acetaldehyde;CH3CHO) A water-soluble liquid alde-hyde used as a starting material in the man-ufacture of several other compounds.Ethanal can be prepared by the oxidationof ethanol. It is manufactured by the cat-alytic oxidation of ethyne with oxygenusing copper(II) chloride and palladium(II)chloride as catalysts. The mixture of gasesis bubbled through an aqueous solution ofthe catalysts; the reaction involves forma-tion of an intermediate organometalliccomplex with Pd2+ ions. With dilute acidsethanal polymerizes to ethanal trimer(C3O3H3(CH3)3, formerly called paralde-hyde), which is a sleep-inducing drug.Below 0°C ethanal tetramer is formed(C4O4H4(CH3)4, formerly called metalde-hyde), which is used as a slug poison and afuel in small portable stoves.
ethane /eth-ayn/ (C2H6) A gaseous alkaneobtained either from the gaseous fractionof crude oil or by the ‘cracking’ of heavierfractions. Ethane is the second member ofthe homologous series of alkanes.
ethanedioic acid /eth-ayn-dÿ-oh-ik/ (ox-alic acid; (COOH)2) A white crystallineorganic acid that occurs naturally inrhubarb, sorrel, and other plants of thegenus Oxalis. It is slightly soluble in water,highly toxic, and used in dyeing and as achemical reagent.
ethane-1,2-diol (ethylene glycol; glycol;CH2(OH)CH2(OH)) A syrupy organic liq-uid commonly used as antifreeze and as astarting material in the manufacture ofDacron. The compound is manufacturedfrom ethene by oxidation over suitable cat-alysts to form epoxyethane, with subse-quent hydrolysis to the diol.
105
ethane-1,2-diol
CH3
CH
O
CH
CH3O
O
HC
CH3
CH3
CH
O
HC CH3
O
CH
CH3
O
CHH3C
Ethanal trimer
O
Ethanal tetramer
ethanoate /eth-ă-noh-ayt/ (acetate;CH3COO–) A salt or ester of ETHANOIC
ACID (acetic acid).
ethanoic acid /eth-ă-noh-ik/ (acetic acid;CH3COOH) A colorless viscous liquid or-ganic acid with a pungent odor (it is theacid in vinegar). Below 16.7°C it solidifiesto a glassy solid (glacial ethanoic acid). It ismade by the oxidation of ethanol or bu-tane, or by the continued fermentation ofbeer or wine. It is made into ethenylethanoate (vinyl acetate) for making poly-mers. Cellulose ethanoate (acetate) is madefrom ethanoic anhydride. See cellulose ac-etate.
ethanol /eth-ă-nol, -nohl/ (ethyl alcohol;alcohol; C2H5OH) A colorless volatile liq-uid alcohol. Ethanol occurs in intoxicatingdrinks, in which it is produced by fermen-tation of a sugar:
C6H12O6 → 2C2H5OH + 2CO2Yeast is used to cause the reaction. Atabout 15% alcohol concentration (by vol-ume) the reaction stops because the yeast iskilled. Higher concentrations of alcoholare produced by distillation.
Apart from its use in drinks, alcohol isused as a solvent and to form ethanal. For-merly, the main source was by fermenta-tion of molasses, but now catalytichydration of ethene is used to manufactureindustrial ethanol. See also methylatedspirits.
ethanoyl chloride /eth-ă-noh-ăl/ (acetylchloride; CH3COCl) A liquid acyl chlorideused as an acetylating agent.
ethanoyl group (acetyl group) The groupRCO–.
ethene /eth-een/ (ethylene; C2H4) Agaseous alkene. Ethene is not normally pre-sent in the gaseous fraction of crude oil butcan be obtained from heavier fractions bycatalytic cracking. This is the principal in-dustrial source. The compound is impor-tant as a starting material in theorganic-chemicals industry (e.g. in themanufacture of ethanol) and as the startingmaterial for the production of polyethene.
Ethene is the first member of the homolo-gous series of alkenes.
ether /ee-th’er/ A type of organic com-pound containing the group –O–. Simpleethers have the formula R–O–R′, where Rand R′ are alkyl or aryl groups, which mayor may not be the same. They are eithergases or very volatile liquids and are veryflammable. The commonest example isethoxyethane (diethylether, C2H5OC2H5)used formerly as an anesthetic. Ethers nowfind application as solvents. They are pre-pared in the laboratory by the dehydrationof alcohols with concentrated sulfuric acid.An excess of alcohol is used to ensure thatonly one molecule of water is removedfrom each pair of alcohol molecules. Theyare generally unreactive, but the C–O bondcan be cleaved by reaction with HI or PCl5.
ethoxyethane /i-thoks-ee-eth-ayn/ (ether;diethylether; C2H5OC2H5) A colorlessvolatile liquid. Ether is well known for itscharacteristic smell and anesthetic proper-ties, also for its extreme flammability. Itstill finds some application as an anestheticwhen more modern materials are unsuit-able; it is also an excellent solvent. Its man-ufacture is an extension of the laboratorysynthesis: ethanol vapor is passed into amixture of excess ethanol and concen-trated sulfuric acid at 140°C:
C2H5OH + H2SO4 →C2H5.O.SO2.OH + H2O
C2H5O.SO2.OH + C2H5OH →C2H5OC2H5 + H2SO4
ethyl acetate /eth-ăl/ See ethyl ethanoate.
ethyl alcohol See ethanol.
ethylamine /eth-ă-lă-meen, -lam-een/(aminoethane; C2H5NH2) A colorless liq-uid amine. It can be prepared from
ethanoate
106
R R’
O.. ..
Ether
chloroethane heated with concentratedaqueous ammonia:
C2H5Cl + NH3 → C2H5NH2 + HClIt is used in manufacturing certain dyes.
ethyl bromide See bromoethane.
ethyl carbamate See urethane.
ethyl chloride See chloroethane.
ethyl dibromide /dÿ-broh-mÿd/ See di-bromoethane.
ethylene /eth-ă-leen/ See ethene.
ethylenediamine /eth-ă-leen-dÿ-ă-meen/An organic compound, H2NCH2CH2NH2.It is important in inorganic chemistry be-cause it may function as a bidentantate lig-and, coordinating to a metal ion by thelone pairs on the two nitrogen atoms. Inthe names of complexes it is given the ab-breviation en.
ethylenediamine tetraacetic acid /eth-ă-leen-dÿ-ă-meen tet-ră-see-tik/ See edta.
ethylene glycol See ethane-1,2-diol.
ethylene oxide See epoxyethane.
ethylene tetrachloride /tet-ră-klor-ÿd, -id, -kloh-rÿd, -rid/ See tetrachloroethane.
ethyl ethanoate (ethyl acetate;C2H5OOCCH3) An ester formed fromethanol and ethanoic acid. It is a fragrantliquid used as a solvent for plastics and inflavoring and perfumery.
ethyl iodide See iodoethane.
ethyne /eth-ÿn/ (acetylene; C2H2) Agaseous alkyne. Traditionally ethyne hasfound use in oxy-acetylene weldingtorches, since its combustion with oxygenproduces a flame of very high temperature.It is also important in the organic chemi-cals industry for the production ofchloroethene (vinyl chloride), which is thestarting material for the production ofpolyvinyl chloride (PVC), and for the pro-
duction of other vinyl compounds. Untilrecently, ethyne was manufactured by thesynthesis and subsequent hydrolysis of cal-cium dicarbide, a very expensive pro-cedure. Modern methods increasinglyemploy the cracking of alkanes.
ethynide /eth-ă-nÿd, -nid/ See carbide.
eudiometer /yoo-dee-om-ĕ-ter/ An appa-ratus for the volumetric analysis of gases.
europium /yû-roh-pee-ŭm/ A silvery el-ement of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids. Its main use is in a mixture of eu-ropium and yttrium oxides widelyemployed as the red phosphor in televisionscreens. The metal is used in some super-conducting alloys.
Symbol: Eu; m.p. 822°C; b.p. 1597°C;r.d. 5.23 (25°C); p.n. 63; r.a.m. 151.965.
eutectic /yoo-tek-tik/ A mixture of twosubstances in such proportions that noother mixture of the same substances has alower freezing point. If a solution contain-ing the proportions of the eutectic is cooledno solid is deposited until the freezingpoint of the eutectic is reached, when twosolid phases are simultaneously depositedin the same proportions as the mixture;these two solid phases form the eutectic. Ifone of the substances is water the eutecticcan also be referred to as a cryohydrate.
evaporation 1. A change of state fromliquid to gas (or vapor). Evaporation cantake place at any temperature, the rate in-creasing with temperature. Some moleculesin the liquid have enough energy to escapeinto the gas phase (if they are near the sur-face and moving in the right direction). Be-cause these are the molecules with higherkinetic energies, evaporation results in acooling of the liquid.2. A change from solid to vapor, especiallyoccurring at high temperatures close to themelting point of the solid. Thin films ofmetal can be evaporated onto a surface inthis way.
exa- Symbol: E A prefix denoting 1018.
107
exa-
excitation /eks-ÿ-tay-shŏn/ The processof producing an excited state of an atom,molecule, etc.
excitation energy The energy required tochange an atom, molecule, etc. from onequantum state to a state with a higher en-ergy. The excitation energy (sometimescalled excitation potential) is the differencebetween two energy levels of the system.
excited state A state of an atom, mol-ecule, or other system, with an energygreater than that of the ground state. Com-pare ground state.
exclusion principle The principle, enun-ciated by Pauli in 1925, that no two elec-trons in an atom can have an identical setof quantum numbers.
exothermic /eks-oh-th’er-mik/ Denotinga chemical reaction in which heat isevolved (i.e. heat flows from the system orthe temperature rises). Combustion is anexample of an exothermic process. Com-pare endothermic.
explosive A substance or mixture whichcan rapidly decompose upon detonationproducing large amounts of heat and gases.The three most important classes of explo-sives are: 1. Propellants which burn steadily, and are
used as rocket fuels. 2. Initiators which are very sensitive and
are used in small amounts to detonateless sensitive explosives.
3. High explosives which need an initiator,but are very powerful.
E–Z convention A convention for the de-scription of a molecule showing cis–transisomerism. In a molecule ABC=CDE,where A, B, D, and E are different groups,a sequence rule (see CIP system) is appliedto the pair A and B to find which has pri-ority and similarly to the pair C and D. Ifthe two groups of higher priority are on thesame side of the bond then the isomer isdesignated Z (from German zusammen, to-gether). If they are on opposite sides theisomer is designated E (German entgegen,opposite).
excitation
108
109
face-centered cubic crystal (f.c.c.) Acrystal structure in which the unit cell hasatoms, ions, or molecules at each cornerand at the center of each face of a cube(also called cubic close-packed). It has a co-ordination number of 12. The structure isclose-packed and made up of layers ofatoms in which each atom is surrounded bysix others arranged hexagonally. Copperand aluminum have face-centered cubicstructures. The layers are packed one ontop of the other, with the second layer fit-ting into the holes of the first layer. Thethird layer is in a different set of holesformed by the second layer. If the layers aredesignated A, B, C, the packing is arranged
ABCABC in contrast to hexagonal close packing, inwhich the layers are arranged
ABABAB.
Fahrenheit scale A TEMPERATURE SCALE
in which the ice temperature is taken as 32°and the steam temperature is taken as 212°(both at standard pressure). The scale isnot used for scientific purposes. To convertbetween degrees Fahrenheit (F) and de-grees Celsius (C) the formula
C/5 = (F – 32)/9is used. The scale is named for the Germanphysicist Daniel Fahrenheit (1686–1736).
Fajan’s rules /faj-ănz/ Rules that dealwith variations in the degree of covalentcharacter in ionic compounds in terms ofpolarization effects. Decrease in size andincrease in charge for cations is regarded asmaking them more polarizing and for an-ions an increase in both charge and sizemakes them more polarizable. Thus cova-lent character is said to increase with in-creasing polarizing power of the cationand/or increasing polarizability of the
anion (originally rules 1 and 2). Fajan’sthird rule states that covalent character isgreater for cations with a non-rare gas con-figuration than for those with a completeoctet, charge and size being approximatelyequal. For example, Cu+ is more polarizingthan Na+ (approximately the same size) be-cause the d-electrons around Cu+ do notshield the nucleus as effectively as the com-plete octet in Na+. The rules are named forthe Polish–American physical chemistKasimir Fajans (1887–1975).
farad /fa-răd, -rad/ Symbol: F The SI unitof capacitance. When the plates of a capac-itor are charged by one coulomb and thereis a potential difference of one volt betweenthem, then the capacitor has a capacitanceof one farad. One farad = 1 coulomb volt–1
(CV–1). The unit is named for the Britishphysicist and chemist Michael Faraday(1791–1867).
faraday /fa-ră-day/ Symbol: F A unit ofelectric charge equal to the charge requiredto discharge one mole of a singly-chargedion. One faraday is 9.648 670 × 104
coulombs. See Faraday’s laws.
Faraday constant See Faraday’s laws.
Faraday’s laws (of electrolysis) Two lawsresulting from the work of Michael Fara-day on electrolysis:1. The amount of chemical change pro-
duced is proportional to the electriccharge passed.
2. The amount of chemical change pro-duced by a given charge depends on theion concerned.
More strictly it is proportional to the rela-tive ionic mass of the ion, and the chargeon it. Thus the charge Q needed to deposit
F
m grams of an ion of relative ionic mass,M, carrying a charge Z is given by:
Q = FmZ/MF, the Faraday constant, has a value of onefaraday, i.e. 9.648 670 × 104 coulombs.
fat See glyceride.
fatty acid See carboxylic acid.
f-block elements The block of elementsin the periodic table that have two outer s-electrons and an incomplete penultimatesubshell of f-electrons. The f-block consistsof the lanthanoids (cerium to lutetium) andthe actinoids (thorium to lawrencium).
f.c.c. See face-centered cubic crystal.
Fehling’s solution /fay-lingz/ A solutionused to test for the aldehyde group(–CHO). It is a freshly made mixture ofcopper(II) sulfate solution with alkalinepotassium sodium 2,3-dihydroxybutane-dioate (tartrate). The aldehyde, whenheated with the mixture, is oxidized to acarboxylic acid, and a red precipitate ofcopper(I) oxide and copper metal is pro-duced. The tartrate is present to complexwith the original copper(II) ions to preventprecipitation of copper(II) hydroxide. Thesolution is named for the German organicchemist Hermann Christian von Fehling(1812–85).
feldspar (felspar) A member of a largegroup of abundant crystalline igneousrocks, mainly silicates of aluminum withcalcium, potassium, and sodium. The al-kali feldspars have mainly potassium witha little sodium and no calcium; the plagio-clase feldspars vary from sodium-only tocalcium-only, with little or no potassium.Feldspars are used in making ceramics,enamels, and glazes.
femto- Symbol: f A prefix denoting 10–15.For example, 1 femtometer (fm) = 10–15
meter (m).
femtochemistry /fem-toh-kem-iss-tree/The study of atomic and molecularprocesses, particularly chemical reactions,
that take place on timescales of about afemtosecond (10–15s). Femtochemistry be-came possible with the construction oflasers that could be pulsed in femtosec-onds. Femtochemistry can be regarded asvery high speed ‘photography’ of chemicalreactions. Typically, a short pulse of radia-tion can initiate a change, with subsequentpulses at short intervals used to investigatethe intermediates spectroscopically. Thetechnique has enabled entities that existonly for a short time, such as activatedcomplexes, to be studied. The detailedmechanisms of many chemical reactionshave been investigated in this way.
fermentation A chemical reaction pro-duced by microorganisms (molds, bacteria,or yeasts). A common example is the for-mation of ethanol from sugars:
C6H12O6 → 2C2H5OH + 2CO2
fermi /fer-mee/ A unit of length equal to10–15 meter. It was formerly used in atomicand nuclear physics. The unit is named forthe Italian–American physicist EnricoFermi (1901–54).
fermium /fer-mee-ŭm/ A radioactivetransuranic element of the actinoid series,not found naturally on Earth. It is pro-duced in very small quantities by bom-barding 239Pu with neutrons to give 253Fm(half-life 3 days). Several other short-livedisotopes have been synthesized.
Symbol: Fm; p.n. 100; most stable iso-tope 257Fm (half-life 100.5 days).
ferric chloride /fe-rik/ See iron(III) chlo-ride.
ferric oxide See iron(III) oxide.
ferric sulfate See iron(III) sulfate.
ferricyanide See cyanoferrate.
ferrite /fe-rÿt/ A nonconducting magneticceramic material, general formula MO.Fe2O4 (where M is a divalent transitionmetal such as cobalt, nickel, or zinc). Fer-rites are used to make powerful magnets in
fat
110
111
fine structure
high-frequency electronic devices such asradar sets.
ferrocene /fe-roh-seen/ (Fe(C5H5)2) Anorange crystalline solid. It is an example ofa sandwich compound, in which an iron(II)ion is coordinated to two cyclopentadienylions. The bonding involves overlap of d or-bitals on the iron with the pi electrons inthe cyclopentadienyl ring. The compoundmelts at 173°C, is soluble in organic sol-vents, and can undergo substitution reac-tions on the rings. It can be oxidized to theblue cation (C5H5)2Fe+. The systematicname is di-π-cyclopentadienyl iron(II).
ferrocyanide /fe-roh-sÿ-ă-nÿd/ See cyano-ferrate.
ferromagnetism /fe-roh-mag-nĕ-tiz-ăm/See magnetism.
ferrosoferric oxide /fe-ros-oh-fe-rik/ Seetriiron tetroxide.
ferrous chloride /fe-rŭs/ See iron(II) chlo-ride.
ferrous oxide See iron(II) oxide.
ferrous sulfate See iron(II) sulfate.
fertilizer A substance added to soil to in-crease its fertility. Artificial fertilizers gen-erally consist of a mixture of chemicalsdesigned to supply nitrogen (N), as well assome phosphorus (P) and potassium (K),known as PKN fertilizers. Typical chemi-cals include ammonium nitrate, ammo-nium phosphate, ammonium sulfate, andpotassium carbonate.
Fick’s law /fiks/ A law describing the dif-fusion that occurs when solutions of differ-ent concentrations come into contact, withmolecules moving from regions of higherconcentration to regions of lower concen-tration. Fick’s law states that the rate ofdiffusion dn/dt, called the diffusive fluxand denoted J, across an area A is given by:dn/dt = J = –DA∂c/∂x, where D is a con-stant called the diffusion constant, ∂c/∂x isthe concentration gradient of the solute,
and dn/dt is the amount of solute crossingthe area A per unit time. D is constant fora specific solute and solvent at a specifictemperature. Fick’s law was formulated bythe German physiologist Adolf Eugen Fick(1829–1901) in 1855.
filler A solid material used to modify thephysical properties or reduce the cost ofsynthetic compounds, such as rubbers,plastics, paints, and resins. Slate powder,glass fiber, mica, and cotton are all used asfillers.
filter See filtration.
filter pump A type of vacuum pump inwhich a jet of water forced through a noz-zle carries air molecules out of the system.Filter pumps cannot produce pressuresbelow the vapor pressure of water. Theyare used in the laboratory for vacuum fil-tration, distillation, and similar techniquesrequiring a low-grade vacuum.
filtrate See filtration.
filtration The process of removing sus-pended particles from a fluid by passing orforcing the fluid through a porous material(the filter). The fluid that passes throughthe filter is the filtrate. In laboratory filtra-tion, filter paper or sintered glass is com-monly used.
fine organic chemicals Carbon com-pounds, such as pesticides, dyes, anddrugs, produced only in small quantities.Their main requirement is that they musthave a high degree of purity, often higherthan 95%. They are manufactured for spe-cial purposes, e.g. for use in spectroscopy,pharmacology, and electronics.
fine structure Closely spaced lines seen athigh resolution in a spectral line or band.Fine structure may be caused by vibrationof the molecules or by electron spin. Hy-perfine structure, seen at very high resolu-tion, is caused by the atomic nucleusaffecting the possible energy levels of theatom.
fireclay
112
fireclay A refractory clay containing highproportions of alumina (aluminum oxide)and silica (silicon(IV) oxide), used for mak-ing firebricks and furnace linings.
firedamp Methane occurring in coalmines. Explosions can occur if quantitiesof methane accumulate. The air left afteran explosion is called blackdamp. After-damp is the poisonous carbon monoxideformed.
first-order reaction A reaction in whichthe rate of reaction is proportional to theconcentration of one of the reacting sub-stances. The concentration of the reactingsubstance is raised to the power one; i.e.rate = k[A]. For example, the decomposi-tion of hydrogen peroxide is a first-orderreaction,
rate = k[H2O2]Similarly the rate of decay of radioactivematerial is a first-order reaction,
rate = k[radioactive material]For a first-order reaction, the time for adefinite fraction of the reactant to be con-sumed is independent of the original con-centration. The units of k, the rateconstant, are s–1.
Fischer projection /fish-er/ A way of rep-resenting the three-dimensional structureof a molecule in two dimensions. The mol-ecule is drawn using vertical and horizon-tal lines. Horizontal lines represent bondsthat come out of the paper. Vertical linesrepresent bonds that go into the paper (orare in the plane of the paper). Named forEmil Fischer, the convention was formerlyused for representing the absolute configu-ration of sugars.
Fischer–Tropsch process /fish-er tropsh/The catalytic hydrogenation of carbonmonoxide in the ratio of 2:1 hydrogen tocarbon monoxide at 200°C to produce hy-drocarbons, especially motor fuel. The car-bon monoxide is derived from water gas,which uses coal or natural gas as a sourceof carbon, and the catalyst is finely dividednickel. The process was extensively used inGermany during World War II and is stillused in countries in which crude oil is not
an easily available source of motor fuel.The process is named for the Germanchemists Franz Fischer (1877–1948) andHans Tropsch (1889–1935).
Fittig reaction See Wurtz reaction.
fixation of nitrogen See nitrogen fixa-tion.
fixing, photographic A stage in the de-velopment of exposed photographic filmor paper in which unexposed silver halidesin the emulsion are dissolved away. Thefixing bath contains a solution of ammo-nium or sodium thiosulfate(IV), and is em-ployed after the initial development(chemical reduction) of the latent silverimage.
flame-ionization detector See gas chro-matography.
flame test A preliminary test in qualita-tive analysis in which a small sample of achemical is introduced into a nonluminousBunsen-burner flame on a clean platinumwire. The flame vaporizes part of the sam-ple and excites some of the atoms, whichemit light at wavelengths characteristic ofthe metallic elements in the sample. Thusthe observation of certain colors in theflame indicates the presence of these ele-ments. The same principles are applied inthe modern instrumental method of spec-trographic analysis.
flare stack A chimney at the top of whichunwanted gases are burnt in an oil refineryor other chemical plant.
flash photolysis A technique for investi-
FLAME TEST COLORSElement Flame colorbarium greencalcium brick redlithium crimsonpotassium pale lilacsodium yellowstrontium red
gating free radicals in gases. The gas is heldat low pressure in a long glass or quartztube, and an absorption spectrum takenusing a beam of light passing down thetube. The gas can be subjected to a verybrief intense flash of light from a lamp out-side the tube, producing free radicals,which are identified by their spectra. Mea-surements of the intensity of spectral linescan be made with time using an oscillo-scope, and the kinetics of very fast reac-tions can thus be investigated.
flash point The lowest temperature atwhich sufficient vapor is given off by aflammable liquid to ignite in the presenceof a spark.
flocculation /flok-yŭ-lay-shŏn/ (coagula-tion) The combining of the particles of afinely divided precipitate, such as a colloid,into larger particles or clumps that sinkand are easier to filter off.
flocculent /flok-yŭ-lănt/ Describing a pre-cipitate that has aggregated in woolymasses.
flotation See froth flotation.
fluid A state of matter that is not a solid –that is, a liquid or a gas. All fluids can flow,and the resistance to flow is the viscosity.
fluidization /floo-id-ă-zay-shŏn/ The sus-pension of a finely-divided solid in an up-ward-flowing liquid or gas. Thissuspension mimics many properties of liq-uids, such as allowing objects to ‘float’ init. Fluidized beds so constructed are impor-tant industrially.
fluorescein /floo-ŏ-ress-ee-in/ A fluores-cent dye used as an absorption indicator.
fluorescence /floo-ŏ-ress-ĕns/ The ab-sorption of energy by atoms, molecules,etc., followed by immediate emission ofelectromagnetic radiation as the particlesmake transitions to lower energy states.Compare phosphorescence.
fluoridation /floo-ŏ-ră-day-shŏn/ The in-troduction of small quantities of fluoridecompounds into the water supply as a pub-lic-health measure to reduce the incidenceof tooth decay in children.
fluoride /floo-ŏ-rÿd, -rid/ See halide.
fluorination /floo-ŏ-ră-nay-shŏn/ Seehalogenation.
fluorine /floo-ŏ-reen, -rin/ A slightlygreenish-yellow highly reactive gaseous el-ement belonging to the halogens (group 17of the periodic table, formerly VIIA). It oc-curs notably as fluorite (CaF2) and cryolite(Na3AlF3) but traces are also widely dis-tributed with other minerals. It is slightlymore abundant than chlorine, accountingfor about 0.065% of the Earth’s crust. Thehigh reactivity of the element delayed itsisolation. Fluorine is now prepared by elec-trolysis of molten KF/HF electrolytes,using copper or steel apparatus. Its prepa-ration by chemical methods is impossible.
Fluorine compounds are used in thesteel industry, glass and enamels, uraniumprocessing, aluminum production, and in arange of fine organic chemicals. Fluorine isthe most reactive and most electronegativeelement known; in fact it reacts directlywith almost all the elements, including therare gas xenon. As the most powerful oxi-dizing agent known, it has the pronouncedcharacteristic of bringing out the higheroxidation states when combined withother elements. Ionic fluorides contain thesmall F– ion, which is very similar to theO2– ion. With the more electronegative ele-ments, fluorine forms an extensive range ofcompounds, e.g. SF6, NF3, and PF5, inwhich the bonding is essentially covalent.A vast number of organic compounds areknown in which hydrogen may be replacedby fluorine and in which the C–F bond ischaracteristically very stable. Examples areCF2=CF2, C6H4F2, and CF3COOH; thebonding is again essentially covalent.
Fluorine reacts explosively with hydro-gen, even in the dark, to form hydrogenfluoride, HF, which polymerizes as a resultof hydrogen bonding and has a boilingpoint very much higher than that of HCl
113
fluorine
(HF b.p. 19°C; HCl b.p. –85°C). Unlikethe other halogens, fluorine does not formhigher oxides or oxyacids; oxygen difluo-ride in fact reacts with water to give hy-drogen fluoride.
Fluorine is strongly electronegative andexhibits a strong electron withdrawing ef-fect on adjacent bonds, thus CF3COOH isa strong acid (whereas CH3COOH is not).Although the coordination number seldomexceeds one there are cases in which fluo-rine bridging occurs, e.g. (SbF5)n. The ele-ment is sufficiently reactive to combinedirectly with the rare gas xenon at 400°Cto form XeF2 and XeF4. At 600°C and highpressure it will even form XeF6.
Fluorine and hydrogen fluoride are ex-tremely dangerous and should only be usedin purpose-built apparatus; gloves and faceshields should be used when working withhydrofluoric acid and accidental exposureshould be treated as a hospital emergency.
Symbol: F; b.p. –188.14°C; m.p.–219.62°C; d. 1.696 kg m–3 (0°C); p.n. 9;r.a.m. 18.99840.32.
fluorite /floo-ŏ-rÿt/ (fluorspar) A mineralform of calcium fluoride (CaF2), used inmaking some cements and types of glass.See also calcium-fluoride structure.
fluorite structure See calcium-fluoridestructure.
fluorocarbon /floo-ŏ-rŏ-kar-bŏn/ A com-pound derived from a hydrocarbon by re-placing hydrogen atoms with fluorineatoms. Fluorocarbons are unreactive andmost are stable up to high temperatures.They have a variety of uses – in aerosolpropellants, oils and greases, and syntheticpolymers such as PTFE. See also chloroflu-orocarbon.
fluorspar /floo-er-spar/ See fluorite.
flux 1. A substance used to keep metalsurfaces free of oxide in soldering. See sol-der.2. A substance used in smelting metals toreact with silicates and other impuritiesand form a low-melting slag.
fluxional molecule A molecule in whichthe constituent atoms change their relativepositions so quickly at room temperaturethat the normal concept of structure is in-adequate; i.e. no specific structure existsfor longer than about 10–2 second and therelative positions become indistinguish-able. For example ClF3 at –60°C has a dis-tinct ‘T’ shape but at room temperature thefluorine atoms are visualized as movingrapidly over the surface of the chlorineatom in a state of exchange and are effec-tively identical.
foam A dispersion of bubbles of gas in aliquid, usually stabilized by a SURFACTANT.Solid foams, such as expanded polystyreneor foam rubber, are made by allowing liq-uid foams to set.
formaldehyde /for-mal-dĕ-hÿd/ Seemethanal.
formalin /for-mă-lin/ See methanal.
formate /for-mayt/ See methanoate.
formic acid /for-mik/ See methanoic acid.
formula (pl. formulas or formulae) A rep-resentation of a chemical compound usingsymbols for the atoms and subscript num-bers to show the numbers of atoms present.See empirical formula; general formula;molecular formula; structural formula.
formyl group /for-măl/ The groupHCO–.
fossil fuel A mineral fuel that forms un-derground from the remains of living or-ganisms. Fossil fuels include coal, naturalgas, peat, and petroleum.
fraction A mixture of liquids with similarboiling points collected by fractional distil-lation.
fractional crystallization Crystalliza-tion of one component from a mixture insolution. When two or more substances arepresent in a liquid (or in solution), on cool-ing to a lower temperature one substance
fluorite
114
will preferentially form crystals, leavingthe other substance in the liquid (or dis-solved) state. Fractional crystallization canthus be used to purify or separate sub-stances if the correct conditions areknown.
fractional distillation (fractionation) Adistillation carried out with partial reflux,using a long vertical column (fractionatingcolumn). It utilizes the fact that the vaporphase above a liquid mixture is generallyricher in the more volatile component. Ifthe region in which refluxing occurs is suf-ficiently long, fractionation permits thecomplete separation of two or morevolatile liquids. Fractionation is the funda-mental process for producing petroleumfrom crude oil.
Unlike normal reflux, the fractionatingcolumn may be insulated to reduce heatloss, and special designs are used to maxi-mize the liquid-vapour interface.
fractionation /frak-shŏ-nay-shŏn/ Seefractional distillation.
francium /fran-see-ŭm/ A radioactive ele-ment of the alkali-metal group. It is foundon Earth only as a short-lived product ofradioactive decay, occurring in uraniumores in minute quantities. A large numberof radioisotopes of francium are known.
Symbol: Fr; m.p. 27°C; b.p. 677°C; p.n.87; most stable isotope 223Fr (half-life 21.8minutes).
Frasch process /frash/ See sulfur.
free electron An electron that can movefrom one atom or molecule to anotherunder the influence of an applied electricfield. Movement of free electrons in a con-ductor constitutes an electric current. Freeelectrons are also involved in metallicbonds (see metals).
free energy A measure of the ability of asystem to do useful work. See Gibbs func-tion; Helmholtz function.
free radical An atom or group of atomswith a single unpaired electron. Free radi-
cals are produced by breaking a covalentbond; for example:
CH3Cl → CH3• + Cl•They are often formed in light-induced
reactions. Free radicals are extremely reac-tive and can be stabilized and isolated onlyunder special conditions.
free-radical substitution See substitu-tion reaction.
freezing The process by which a liquid isconverted into a solid by cooling; the re-verse of melting.
freezing mixtures Two or more sub-stances mixed together to produce a lowtemperature. A mixture of sodium chlorideand ice in water (–20°C) is a common ex-ample.
freezing point The temperature at whicha liquid is in equilibrium with its solidphase at standard pressure and belowwhich the liquid freezes or solidifies. Thistemperature is always the same for a par-ticular liquid and is numerically equal tothe melting point of the solid.
freezing point constant See depressionof freezing point.
French chalk See talc.
Frenkel defect /frenk-ĕl/ See defect.
Freons /free-onz/ (Trademark) Variouschlorofluorocarbons (CFCs) and fluoro-carbons used as refrigerants. See chloroflu-orocarbon; fluorocarbon.
Friedel–Crafts reaction /free-dĕl kraft/A method for the substitution of an alkylor acyl group onto a benzene ring. The aro-matic hydrocarbon (e.g. benzene) is re-fluxed with a haloalkane or acyl halideusing aluminum chloride catalyst. Theproduct is an alkylarene, e.g.:
C6H6 + CH3I → C6H5CH3 + HIor a ketone, for example:
C6H6 + CH3COCl → C6H5COCH3 +HCl
115
Friedel–Crafts reaction
The aluminum chloride accepts a lonepair of electrons on the chlorine, polarizingthe chloroalkane to produce positivecharge on the carbon. Electrophilic substi-tution then occurs. The reaction is namedfor the French chemist Charles Friedel(1832–99) and the American chemistJames Crafts (1839–1917).
frontier orbital Either of two orbitals ina molecule: the highest occupied molecularorbital (the HOMO) or the lowest unoccu-pied molecular orbital (the LUMO). TheHOMO is the orbital with the highest en-ergy level occupied at absolute zero tem-perature. The LUMO is the lowest-energyunoccupied orbital at absolute zero. For aparticular molecule the nature of these twoorbitals is very important in determiningthe chemical properties. Frontier-orbitaltheory, which considers the symmetry ofthese orbitals, has been very successful inexplaining such reactions as theDIELS–ALDER REACTION. See also Wood-ward–Hoffmann rules.
froth flotation An industrial techniquefor separating the required parts of oresfrom unwanted gangue. The mineral is pul-verized and mixed with water, to which afrothing agent is added. The mixture is aer-
ated and particles of one constituent arecarried to the surface by bubbles of air,which adhere preferentially to one of theconstituents. Various additives are alsoused to modify the surface properties of theparticles (e.g. to increase the adherence ofair bubbles).
fructose /fruk-tohs, frûk-/ (fruit sugar;C6H12O6) A sugar found in fruit juices,honey, and cane sugar. It is a ketohexose,existing in a pyranose form when free. Incombination (e.g. in sucrose) it exists in thefuranose form.
fruit sugar See fructose.
fuel cell A type of cell in which fuel is con-verted directly into electricity. In one form,hydrogen gas and oxygen gas are fed to thesurfaces of two porous nickel electrodesimmersed in potassium hydroxide solu-tion. The oxygen reacts to form hydroxyl(OH–) ions, which it releases into the solu-tion, leaving a positive charge on the elec-trode. The hydrogen reacts with the OH–
ions in the solution to form water, givingup electrons to leave a negative charge onthe other electrode. Large fuel cells cangenerate tens of amperes. Usually the e.m.f.
frontier orbital
116
Friedel-Crafts acetylation
+ CH3CI
+ CH3COCI
benzene chloromethane methylbenzene(toluene)
benzene ethanoyl chloride phenyl methylketone
C
O
CH3
CH3
Friedel–Crafts methylation
Friedel–Crafts reactions
is about 0.9 volt and the efficiency around60%.
fullerene /fûl-ĕ-reen/ See buckminster-fullerene.
fullerite /fûl-ĕ-rÿt/ See buckminster-fullerene.
fuller’s earth A natural clay used as anabsorbent and industrial catalyst.
fumaric acid /fyoo-ma-rik/ See butene-dioic acid.
fuming sulfuric acid See oleum.
functional group A group of atoms in acompound that is responsible for the char-acteristic reactions of the type of com-pound. Examples are:
alcohol –OHaldehyde –CHOamine –NH2ketone =CO.carboxylic acid –CO.OHacyl halide –CO.X (X = halogen)nitro compound –NO2sulfonic acid –SO2.OHnitrile –CNdiazonium salt –N2
+
diazo compound –N=N–
fundamental units The units of length,
mass, and time that form the basis of mostsystems of units. In SI, the fundamentalunits are the meter, the kilogram, and thesecond. See also base unit.
furan /fyoo-ran, fyoo-ran/ (furfuran;C4H4O) A heterocyclic liquid organiccompound. Its five-membered ring con-tains four carbon atoms and one oxygenatom. The structure is characteristic ofsome monosaccharide sugars (furanoses).
furanose /fyoo-ră-nohs/ A SUGAR that hasa five-membered ring (four carbon atomsand one oxygen atom).
furfuran /fer-fŭ-răn, fer-fŭ-ran/ See furan.
fused Describing a solid that has beenmelted and solidified into a single mass.Fused silica, for example, is produced bymelting sand.
fused ring See ring.
fusel oil /fyoo-zĕl, -sĕl/ A mixture of high-molecular-weight alcohols together withsome esters and fatty acids, formed fromalcoholic fermentation and obtained dur-ing distillation. It is used as a source ofhigher alcohols.
fusion Another word for melting.
117
fusion
118
gadolinium /gad-ŏ-lin-ee-ŭm/ A ductilemalleable silvery element of the lanthanoidseries of metals. It occurs in associationwith other lanthanoids. Gadolinium isused in alloys, magnets, and in the elec-tronics industry.
Symbol: Gd; m.p. 1313°C; b.p.3266°C; r.d. 7.9 (25°C); p.n. 64; r.a.m.157.25.
galactose /gă-lak-tohs/ (C6H12O6) ASUGAR found in lactose and many polysac-charides. It is an aldohexose, isomeric withglucose.
galena /gă-lee-nă/ (lead glance) A mineralform of lead(II) sulfide. It is the principalore of lead.
gallium /gal-ee-ŭm/ A soft silvery low-melting metallic element belonging togroup 3 (formerly IIIA) of the periodictable. It is found in minute quantities inseveral ores, including zinc blende (ZnS)and bauxite (Al2O3.H2O). Gallium is usedin low-melting alloys, high-temperaturethermometers, and as a doping impurity insemiconductors. Gallium arsenide is asemiconductor used in light-emittingdiodes and in microwave apparatus.
Symbol: Ga; m.p. 29.78°C; b.p.2403°C; r.d. 5.907 (solid at 20°C), 6.114(liquid); p.n. 31; r.a.m. 69.723.
galvanic cell /gal-van-ik/ See cell.
galvanizing /gal-vă-nÿ-zing/ A process forcoating steel with zinc by dipping in a bathof molten zinc, or by electrodeposition.The zinc protects the steel from corrosion.
gamma radiation A form of electromag-netic radiation emitted by changes in the
nuclei of atoms. Gamma waves have veryhigh frequency (short wavelength).Gamma radiation shows particle proper-ties much more often than wave properties.The energy of a gamma photon, given by
W = hv(where h is the Planck constant), can bevery high. A gamma photon of 1024 Hz hasan energy of 6.6 × 10–10 J. See also electro-magnetic radiation.
gamma rays Streams of gamma radia-tion.
gangue /gang/ The rock or other undesir-able material occurring in an ore.
gas The state of matter in which forces ofattraction between the particles of a sub-stance are small. The particles have free-dom of movement and gases, therefore,have no fixed shape or volume. The atomsand molecules of a gas are in a continualstate of motion and are continually collid-ing with each other and with the walls ofthe containing vessel. These collisions withthe walls create the pressure of a gas.
gas chromatography A technique widelyused for the separation and analysis ofmixtures. Gas chromatography employs acolumn packed with either a solid station-ary phase (gas-solid chromatography orGSC) or a solid coated with a non-volatileliquid (gas-liquid chromatography orGLC). The whole column is placed in athermostatically controlled heating jacket.A volatile sample is introduced into the col-umn using a syringe, and an unreactive car-rier gas, such as nitrogen, passed throughit. The components of the sample will becarried along in this mobile phase. How-ever, some of the components will cling
G
more readily to the stationary phase thanothers, either because they become at-tached to the solid surface or because theydissolve in the liquid. The time taken fordifferent components to pass through thecolumn is characteristic and can be used toidentify them. The emergent sample ispassed through a detector, which registersthe presence of the different components inthe carrier gas.
Two types of detector are in commonuse: the katharometer, which measureschanges in thermal conductivity, and theflame-ionization detector, which turns thevolatile components into ions and registersthe change in electrical conductivity.
gas constant (universal gas constant)Symbol: R The universal constant8.314 34 J mol–1 K–1 appearing in theequation of state for an ideal gas. See gaslaws.
gaseous diffusion separation A tech-nique for the separation of gases that relieson their slightly different atomic masses.For example, the isotopes of uranium (235Uand 238U) can be separated by first prepar-ing gaseous uranium hexafluoride fromuranium ore. If this is then made to passthrough a series of fine pores, the mole-cules containing the lighter isotope of ura-nium will pass through more quickly. Asthe gases pass through successive ‘di-aphragms’, the proportion of lighter mol-ecules increases and a separation of over99% is attainable. This method has alsobeen applied to the separation of the iso-topes of hydrogen.
gas equation See gas laws.
gas laws Laws relating the temperature,pressure, and volume of a fixed mass ofgas. The main gas laws are Boyle’s law andCharles’ law. The laws are not obeyed ex-actly by any real gas, but many commongases obey them under certain conditions,particularly at high temperatures and lowpressures. A gas that would obey the lawsover all pressures and temperatures is aperfect or ideal gas.
Boyle’s and Charles’ laws can be com-bined into an equation of state for idealgases:
pVm = RTwhere Vm is the molar volume and R themolar gas constant. For n moles of gas
pV = nRTAll real gases deviate to some extent
from the gas laws, which are applicableonly to idealized systems of particles ofnegligible volume with no intermolecularforces. There are several modified equa-tions of state that give a better descriptionof the behavior of real gases, the bestknown being the van der Waals equation.
gas–liquid chromatography See gaschromatography.
gasohol /gas-ŏ-hôl/ Alcohol (ethanol) ob-tained by the industrial fermentation ofsugar for use as a motor fuel. It has beenproduced on a large scale in some coun-tries.
gas oil One of the main fractions obtainedfrom PETROLEUM by distillation, used as afuel for diesel engines. See diesel fuel.
gasoline /gas-ŏ-leen, gas-ŏ-leen/ See petro-leum.
gas-solid chromatography See gas chro-matography.
Gatterman–Koch reaction A reactionfor substituting a formyl (methanoyl)group (HCO–) onto a benzene ring of anaromatic hydrocarbon. It is used in the in-dustrial production of benzaldehyde frombenzene:
C6H6 → C6H5CHOThe aromatic hydrocarbon is mixed with aLewis acid, such as aluminum chloride,and a mixture of carbon monoxide and thehydrogen chloride is passed through. Thefirst stage is the production of an H–C≡O+
ion:HCl + CO + AlCl3 → HCO+ + AlCl4–
Copper (I) chloride (CuCl) is also added asa catalyst. The HCO+ ion acts as an elec-trophile in electrophilic substitution on thebenzene ring.
119
Gatterman–Koch reaction
Gatterman reaction
120
A variation of the reaction in which theHCO– group is substituted onto the ben-zene ring of a phenol uses hydrogencyanide rather than carbon monoxide.Typically, a mixture of zinc cyanide andhydrochloric acid is used, to give zinc chlo-ride (which acts as a Lewis acid) and hy-drogen cyanide. The electrophile in thiscase is protonated hydrogen cyanide:
HCN + HCl + ZnCl2 → HCNH+ +ZnCl3–
The phenol is first substituted to give animine:
C6H5OH → HOC6H4CH=NHThis then hydrolyzes to the aromatic alde-hyde:
HOC6H4CH=NH2 → HOC6H4CHOSimilar reactions using alkyl cyanides (ni-triles) rather than hydrogen cyanide givearomatic ketones. This type of reaction, inwhich a cyanide is used to produce an alde-hyde (or ketone) is often called the Gatter-man reaction. The Gatterman–Kochreaction was reported by the Germanchemist Ludwig Gatterman (1860–1920)in 1897 (with J. C. Koch). The use of hy-drogen cyanide was reported by Gatter-man in 1907.
Gatterman reaction 1. See Sandmeyerreaction.2. See Gatterman–Koch reaction.
gauche conformation /gohsh/ See con-formation.
gauss /gows/ Symbol: G The unit of mag-netic flux density in the c.g.s. system. It isequal to 10–4 tesla.
Gay-Lussac’s law 1. Gases react in vol-umes that are in simple ratios to each otherand to the products if they are gases (allvolumes measured at the same temperatureand pressure).2. See Charles’ law.
gel /jel/ A lyophilic colloid that is normallystable but may be induced to coagulatepartially under certain conditions (e.g.lowering the temperature). This produces apseudo-solid or easily deformable jelly-likemass, called a gel, in which intertwining
particles enclose the whole dispersingmedium. Gels may be further subdividedinto elastic gels (e.g. gelatin) and rigid gels(e.g. silica gel).
gelatin /jel-ă-tin/ (gelatine) A pale yellowprotein obtained from the bones, hides,and skins of animals, which forms a col-loidal jelly when dissolved in hot water. Itis used in jellies and other foods, to makecapsules for various medicinal drugs, as anadhesive and sizing medium, and in photo-graphic emulsions.
gel electrophoresis see electrophoresis.
gel filtration See molecular sieve.
gem positions Positions in a molecule onthe same atom. For example, 1,1-dichloroethane (CH3CHCl2) is a gem di-halide.
general formula A representation of thechemical formula common to a group ofcompounds. For example, CnH2n+2 is thegeneral formula for an alkane, whosemembers form a homologous series. Seealso empirical formula; homologous series;molecular formula; structural formula.
geochemistry /jee-oh-kem-iss-tree/ Thestudy of the chemistry of the Earth. Thesubject includes the chemical compositionof the Earth, the abundance of the elementsand their isotopes, the atomic structure ofminerals, and chemical processes thatoccur at the high temperatures and pres-sures inside the Earth.
geometrical isomerism See isomerism.
germanium /jer-may-nee-ŭm/ A hardbrittle gray metalloid element belonging togroup 14 (formerly IVA) of the periodictable. It is found in sulfide ores such as ar-gyrodite (4Ag2S.GeS2) and in zinc ores andcoal. Most germanium is recovered duringzinc or copper refining as a by-product.Germanium was extensively used in earlysemiconductor devices but has now beenlargely superseded by silicon. It is used as
an alloying agent, catalyst, phosphor, andin infrared equipment.
Symbol: Ge; m.p. 937.45°C; b.p.2830°C; r.d. 5.323 (20°C); p.n. 32; r.a.m.72.61.
germanium(IV) oxide (GeO2) A com-pound made by strongly heating germa-nium in air or by hydrolyzinggermanium(IV) chloride. It occurs in twoforms. One is slightly soluble in waterforming an acidic solution; the other is in-soluble in water. Germanium oxide dis-solves in alkalis to form the anion GeO3
2–.
German silver See nickel-silver.
Gibbs function (Gibbs free energy) Sym-bol: G A thermodynamic function definedby
G = H – TSwhere H is the enthalpy, T the thermody-namic temperature, and S the entropy. It isuseful for specifying the conditions ofchemical equilibrium for reactions for con-stant temperature and pressure (G is a min-imum). The function is named for theAmerican mathematician and theoreticalphysicist Josiah Williard Gibbs(1839–1903). See also free energy.
giga- Symbol: G A prefix denoting 109.For example, 1 gigahertz (GHz) = 109 hertz(Hz).
glacial ethanoic (acetic) acid Purewater-free ethanoic acid.
glacial ethanoic acid (glacial acetic acid)Pure water-free ethanoic acid.
glass A hard transparent material madeby heating calcium oxide (lime), sodiumcarbonate, and sand (silicon(IV) oxide).This produces a calcium silicate – the nor-mal type of glass, called soda glass. Specialtypes of glass can be obtained by incorpo-rating boron oxide in the glass (borosili-cate glass, used for laboratory apparatus)or by including metals other than calcium,e.g. lead or barium.
Glass is an amorphous substance, in thesense that there is no long-range ordering
of the atoms on a lattice. It can be regardedas a supercooled liquid, which has not crys-tallized. Solids with similar noncrystallinestructures are also called glasses
Glauber’s salt /glow-ber, glaw-/ Seesodium sulfate.
GLC Gas-liquid chromatography. See gaschromatography.
glove box A sealed box with gloves fittedto ports in one side and having a transpar-ent top, used for safety reasons or to per-form experiments in an inert or sterileatmosphere.
gluconic acid /gloo-kon-ik/ (dextronicacid CH2OH(CHOH)4COOH) A solublecrystalline organic acid made by the oxida-tion of glucose (using specific molds). It isused in paint strippers.
glucosan A POLYSACCHARIDE that isformed of glucose units. Cellulose andstarch are examples.
glucose /gloo-kohs/ (dextrose; grapesugar; C6H12O6) A monosaccharide oc-curring widely in nature as D-glucose. It oc-curs as glucose units in sucrose, starch, andcellulose. It is important to metabolism be-cause it participates in energy-storage andenergy-release systems. See also sugar.
glucoside /gloo-kŏ-sÿd/ See glycoside.
glue An adhesive, of which there are vari-ous types. Aqueous solutions of starch andethyl cellulose are used as pastes for stick-ing paper; traditional wood glue is made byboiling animal bones (see gelatin); quick-drying adhesives are made by dissolvingrubber or a synthetic polymer in a volatilesolvent; and some polymers, such as epoxyresins and polyvinyl acetate (PVA), arethemselves used as glues.
glutamic acid /gloo-tam-ik/ See aminoacids.
glutamine /gloo-tă-meen, -min/ See aminoacids.
121
glutamine
glyceride /gliss-ĕ-rÿd, -rid/ An esterformed between glycerol (propane-1,2,3-triol) and one or more carboxylic acids.Glycerol has three alcohol groups, and ifall three groups have formed esters, thecompound is a triglyceride. Naturally oc-curring fats and oils are triglycerides oflong-chain carboxylic acids (hence thename ‘fatty acid’). The main carboxylicacids occurring in fats and oils are:1. octadecanoic acid (stearic acid), a satu-
rated acid CH3(CH2)16COOH.2. hexadecanoic acid (palmitic acid), a sat-
urated acid CH3(CH2)14COOH.3. cis-9-octadecenoic acid (oleic acid), an
unsaturated acid.CH3(CH2)7CH:CH(CH2)7COOH
glycerine /gliss-ĕ-rin, -reen, gliss-ĕ-reen/(glycerin) See propane-1,2,3-triol.
glycerol /gliss-ĕ-rol, -rohl/ See propane-1,2,3-triol.
glyceryl trinitrate /gliss-ĕ-răl trÿ-nÿ-trayt/See nitroglycerine.
glycine /glÿ-seen, glÿ-seen/ See aminoacids.
glycogen /glÿ-kŏ-jĕn/ (animal starch) Apolysaccharide that is the main carbohy-drate store of animals. It is composed ofmany glucose units linked in a similar wayto starch. Glycogen is readily hydrolyzed ina stepwise manner to glucose itself. It isstored largely in the liver and in muscle butis found widely distributed in the body.
glycol /glÿ-kol. -kohl/ See diol.
glycoside /glÿ-kŏ-sÿd/ A compound madeby replacing a hydroxyl group of a mono-saccharide sugar with an alcoholic, pheno-lic, or other group. The bond is a glycosidiclink. If the sugar is glucose, the compoundis a glucoside. Glycosides occur in plants .
glycosidic link /glÿ-kŏ-sid-ik/ See glyco-side.
gold A transition metal that occurs native.It is unreactive and is very ductile and mal-
leable. Gold is used in jewelry, often al-loyed with copper, and in electronics andcolored glass. Pure gold is 24 carat; 9 caratindicates that 9 parts in 24 consist of gold.
Symbol: Au; m.p. 1064.43°C; b.p.2807°C; r.d. 19.320 (20°C); p.n. 79; r.a.m.196.96654.
gold(III) chloride (gold trichloride; auricchloride; AuCl3) A compound prepared bydissolving gold in aqua regia. The brightyellow crystals (chloroauric acid) producedon evaporation are heated to form dark redcrystals of gold(III) chloride. The chloridedecomposes easily (at 175°C) to givegold(I) chloride and chlorine; at highertemperatures it decomposes to give goldand chlorine. Gold(III) chloride is used inphotography. It exists as a dimer, Au2Cl6.
Goldschmidt process /gohld-shmit/ Aprocess for extracting certain metals fromtheir oxides by reduction with aluminum.It is named for the German chemist Johann(Hans) Wilhelm Goldschmidt (1861–1923). See thermite.
gold trichloride /trÿ-klor-ÿd, -kloh-rÿd/See gold(III) chloride.
graft copolymer See polymerization.
Graham’s law (of diffusion) The princi-ple that gases diffuse at a rate that is in-versely proportional to the square root oftheir density. Light molecules diffuse fasterthan heavy molecules. The principle is usedin the separation of isotopes. The law isnamed for the Scottish chemist ThomasGraham (1805–69).
grain A crystal in a metal that has beenprevented from attaining its regular geo-metrical form.
gram Symbol: g A unit of mass defined as10–3 kilogram.
gram-atom See mole.
gram-equivalent The equivalent weightof a substance in grams.
glyceride
122
gram-molecule See mole.
granulation A process for enlarging par-ticles to improve the flow properties ofsolid reactants and products in industrialchemical processes. The larger a particle,and the freer from fine materials in a solid,the more easily it will flow. Dry granula-tion produces pellets from dry materials,which are crushed into the desired size.Wet granulation involves the addition of aliquid to the material, and the resultingpaste is extruded and dried before cuttinginto the required size.
grape sugar See dextrose.
graphite An allotrope of CARBON.Graphite is a good conductor of heat andelectricity. The atoms are arranged in lay-ers which cleave easily and graphite is usedas a solid lubricant.
gravimetric analysis /grav-ă-met-rik/ Amethod of quantitative analysis in whichthe final analytical measurement is madeby weighing. There are many variations inthe method but in essence they all consistof:1. taking an accurately weighed sample
into solution;2. precipitation as a known compound by
a quantitative reaction;3. digestion and coagulation procedures.4. filtration and washing;5. drying and weighing as a pure com-
pound.Filtration is a key element in the
method and a variety of special filter pa-pers and sinter-glass filters are available.
gray Symbol: Gy The SI unit of absorbedenergy dose per unit mass resulting fromthe passage of ionizing radiation throughliving tissue. One gray is an energy absorp-tion of one joule per kilogram of mass. Theunit is named for the British physicianHarold Gray (1905–65).
Grignard reagent /gree-nyar/ A type oforganometallic compound with the generalformula RMgX, where R is an alkyl or aryl
group and X is a halogen (e.g. CH3MgCl).Grignard reagents are prepared by reactingthe haloalkane or haloaryl compound withmagnesium in dry ether:
CH3Cl + Mg → CH3MgClGrignard reagents probably have the
form R2Mg.MgCl2. They are used exten-sively in organic chemistry. With methanala primary alcohol is produced:
RMgX + HCHO → RCH2OH +Mg(OH)X
Other aldehydes give secondary alcohols:RMgX + R′CHO → RR′CHOH +
Mg(OH)XAlcohols and carboxylic acids give hydro-carbons:
RMgX + R′OH → RR′ + Mg(OH)XWater also gives a hydrocarbon:
RMgX + H2O → RH + Mg(OH)XSolid carbon dioxide in acid solution givesa carboxylic acid:
RMgX + CO2 + H2O → RCOOH +Mg(OH)X
ground state The lowest energy state ofan atom, molecule, or other system. Com-pare excited state.
group 1. In the PERIODIC TABLE, a series ofchemically similar elements that have simi-lar electronic configurations. A group isthus a column of the periodic table. For ex-ample, the alkali metals, all of which haveouter s1 configurations, belong to group 1.2. (FUNCTIONAL GROUP) In organic chem-istry, an arrangement of atoms that be-stows a particular type of property on amolecule and enables it to be placed in aparticular class, e.g. the aldehyde group–CHO.
group 0 elements See rare gases.
group 1 elements See alkali metals.
group 2 elements See alkaline-earth met-als.
group 3-12 elements See transition ele-ments.
group 13 elements A group of elementsin the periodic table consisting of the ele-
123
group 13 elements
ments boron (B), aluminum (Al), gallium(Ga), indium (In), and thallium (T1). Theseelements were formerly classified as groupIII elements and belonged to the IIIA sub-group. The IIIB subgroup consisted of theelements scandium (Sc), yttrium (Y), lan-thanum (La), and actinium (Ac). Thegroup 13 elements all have three outer elec-trons (s2p1) with no partly filled innershells. The group is the first group of ele-ments in the p-block. As with all the groupsthere is an increase in metallic character asthe group is descended.
The boron atom is both small and has ahigh ionization potential, so bonds toboron are largely covalent with only smalldegrees of polarization. In fact boron isclassed as a metalloid. It has volatile hy-drides and a weakly acidic oxide. As thegroup is descended the ionization poten-tials decrease and the atomic radii increase,leading to increasingly polar interactionsand the formation of distinct M3+ ions. Theincrease in metallic character is clearly il-lustrated by the hydroxides: boric acidB(OH)3 is acidic; aluminum and galliumhydroxides are amphoteric, dissolving inacids to give Al3+ and Ga3+ and in bases togive aluminates and gallates; the hydrox-ides of indium and thallium are distinctlybasic. As the elements get heavier the bondenergies with other elements become gen-erally smaller. The monovalent state (re-moval of the p electron only) becomesprogressively stable. For example gal-lium(I) is known in the gas phase and incomplex systems sometimes known as‘GaCl2’. Monovalent indium halides areknown, for example InX, and thallium(I) isdistinctly more stable than thallium(III).
group 14 elements A group of elementsin the periodic table consisting of the ele-ments carbon (C), silicon (Si), germanium(Ge), tin (Sn), and lead (Pb). These ele-ments were formerly classified as group IVelements and belonged to the IVA sub-group. The IVB subgroup consisted of theelements titanium (Ti), zirconium (Zr), andhafnium (Hf), which are transition metals.The group 14 elements all have electronicstructures with four outer electrons (s2p2)and no partly filled inner shells. The group
shows the common trend towards metalliccharacter with the heavier elements; thuscarbon is a typical non-metal, silicon andgermanium are metalloids, and tin andlead are characteristically metallic. As ob-served with other groups the first memberof the group is quite different from the rest.The carbon atom is smaller and has ahigher ionization potential, both favouringpredominance of covalence, but additionalfactors are:1. The widespread nature of extensive
catenation in carbon compounds.2. The possibility of strong overlap of p or-
bitals or double bonding.3. The unavailability of d orbitals in car-
bon compounds.Significant differences are:1. Both oxides of carbon are gaseous, CO
and CO2; the other elements form solidoxides, e.g. SiO2, Pb3O4.
2. The heavier elements have no com-pounds analogous to aromatic com-pounds.
3. The heavier elements have no com-pounds analogous to ethene or ketones.
4. The heavier elements readily expandtheir coordination number, e.g. SiF6
2–
and SnCl62–.
group 15 elements A group of elementsin the periodic table consisting of the ele-ments nitrogen (N), phosphorus (P), ar-senic (As), antimony (Sb), and bismuth(Bi). These elements were formerly classi-fied as group V elements and belonged tothe VA subgroup. The VB subgroup con-sisted of the elements vanadium (V), nio-bium (Nb), and tantalum (Ta), which aretransition elements.
The main-group elements all have elec-tronic configurations corresponding toouter s2p3 with no vacancies in inner levels.The ionization potentials are high and thelighter members of the group, N and P, aredistinctly electronegative and non-metallicin character. Arsenic and antimony aremetalloids and bismuth is weakly me-tallic (Bi2O3 dissolves in acids to giveBi(OH)3).
Nitrogen is dissimilar from the othermembers of the group in that it formsstable multiple bonds, it is limited to coor-
group 14 elements
124
dination number 4, e.g. NH4+, it is suffi-
ciently electronegative to form hydrogenbonds and the nitride ion N3–, and its ox-ides are irregular with the rest of the group.
The group displays a remarkable num-ber of allotropes of its members showingthe trend from non-metallic forms throughto metallic forms. Thus nitrogen has onlythe diatomic form; phosphorus has ahighly reactive form P1 (brown), and atetrahedral form P4 (white), forms basedon broken tetrahedra Pn (red and violet),and a hexagonal layer-type lattice (black).Arsenic and antimony have the As4 and Sb4forms, which are less stable than the layertype in this case; bismuth has the layer-lat-tice form only.
group 16 elements A group of elementsof the periodic table sometimes called thechalcogens, consisting of the elements oxy-gen (O), sulfur (S), selenium (Se), tellurium(Te), and polonium (Po). These elementswere formerly classified as group VI ele-ments and belonged to the VIA subgroup.The VIB subgroup consisted of the ele-ments chromium (Cr), molybdenum (Mo),and tungsten (W), which are transition ele-ments. The electronic configurations of thegroup 6 elements are all s2p4 with no va-cant inner orbitals. These configurationsare all just two electrons short of a rare gasstructure, consequently they have highelectron affinities and are almost entirelynon-metallic in character.
The group shows the normal propertyof a trend towards metallic character as itis descended. Selenium, tellurium and polo-nium have ‘metallic’ allotropes and po-lonium has generally metalloid-typeproperties where they have been studied(Po is very rare). All the elements combinewith a large number of other elements,both metallic and non-metallic, but in con-trast to compounds of the halogens theyare more generally insoluble in water, andeven where soluble they do not ionize read-ily.
Oxygen behaves like the first membersof other groups in having great differencesfrom the rest of the group. For example,oxygen forms hydrogen bonds whereassulfur and the others do not; oxygen can-
not expand its outer shell and positive oxi-dation states are uncommon for oxygen(OF2, O2F2, O2
+PtF6–). Oxygen is para-
magnetic. Excluding oxygen, group 16shows similar trends with increasing size tothose in group 15. For example decreasingstability of H2X, greater tendency to formcomplexes such as SeBr6
2–, decreasing sta-bility of high formal positive oxidationstates, and marginal metallic properties(e.g. Po forms a hydroxide).
group 17 elements A group of elementsin the periodic table consisting of the ele-ments fluorine (F), chlorine (Cl), bromine(Br), iodine (I), and astatine (At). Formerlythey belonged to group VII and were clas-sified in the VIIA subgroup. The VIIB sub-group consisted of the elements manganese(Mn), technetium (Te), and rhenium (Re),which are transition elements. See halo-gens.
group 18 elements See rare gases.
group theory A branch of mathematicsused to analyze symmetry in a systematicway. A group consists of a set of elementsA, B, C, etc. and a rule for forming theproduct of these elements, such that certainconditions are satisfied:(1) Every product AB of two elements Aand B is also an element of the set.(2) Any three elements A, B, C, must satifyA(BC) = (AB)C.(3) There is an element of the set, denotedI, which is the identity element, i.e. for allA in the set AI = IA =A.(4) For each element A of the set there is aninverse, denoted A–1, which also belongs tothe set which satisfies the relation AA–1 =A–1 = A = I.
In the case of symmetry operations theproduct is the successive performance ofthese operations. If a group has a finitenumber of elements it is said to be a dis-crete group. The point groups, which arisefrom the rotations and reflections of bodiessuch as isolated molecules are discretegroups.
GSC Gas–solid chromatography. See gaschromatography.
125
GSC
guanidine /gwan-ă-deen, -din, gwah-nă-/(iminourea; HN:C(NH2)2) A stronglybasic crystalline organic compound whichcan be nitrated to make a powerful explo-sive. It is also used in making dyestuffs,medicines and polymer resins.
guanine /gwah-neen, -nin/ A nitrogenousbase found in DNA and RNA. Guanine hasa purine ring structure.
guanosine /gwah-nŏ-seen, -sin/ (guaninenucleoside) A NUCLEOSIDE present in DNAand RNA and consisting of guanine linkedto D-ribose via a β-glycosidic bond.
gun cotton See cellulose trinitrate.
gunmetal See bronze.
gunpowder A powdered mixture of sul-fur, charcoal, and potassium nitrate, usedas an explosive.
gypsum /jip-sŭm/ See calcium sulfate.
gyromagnetic ratio /jÿ-roh-mag-net-ik/The ratio of the magnetic moment of a sys-tem to the angular momentum of that sys-tem. The gyromagnetic ratio is a usefulquantity in the theory of electrons andatomic nuclei.
guanidine
126
127
Haber process /hay-ber/ An importantindustrial process for the manufacture ofammonia, which is used for fertilizers andfor making nitric acid. The reaction is theequilibrium:
N2 + 3H2 ˆ 3NH3The nitrogen used is obtained by fraction-ation of liquid air and the hydrogen by theoxidation of hydrocarbons (from naturalgas). The nitrogen and hydrogen are puri-fied and mixed in the correct proportions.The equilibrium amount of ammonia isfavoured by low temperatures, but in prac-tice the reaction would never reach equilib-rium at normal temperatures. An optimumtemperature of about 450°C is thereforeused. High pressure also favors the reac-tion and a pressure of about 250 atmos-pheres is used. The catalyst is iron withsmall amounts of potassium and aluminumoxides present. The yield is about 15%.Ammonia is condensed out of the gas mix-ture at –50°C. The process is named for theGerman physical chemist Fritz Haber(1868–1934).
habit See crystal habit.
hafnium /haf-nee-ŭm/ A transition metalfound in zirconium ores. Hafnium is diffi-cult to work and can burn in air. It is usedin control rods for nuclear reactors and incertain specialized alloys and ceramics.
Symbol: Hf; m.p. 2230°C; b.p. 5197°C;r.d. 13.31 (20°C); p.n. 72; r.a.m. 178.49.
hahnium /hah-nee-ŭm/ Symbol: Ha or HnA name formerly suggested for element-105 (dubnium) and also for element-108(hassium). See element.
half cell An electrode in contact with a so-lution of ions. In general there will be an
e.m.f. set up between electrode and solu-tion by transfer of electrons to or from theelectrode. The e.m.f. of a half cell cannotbe measured directly since setting up a cir-cuit results in the formation of another halfcell. See electrode potential.
half-life (half-life period; Symbol: t½) Thetime taken for half the nuclei of a sample ofa radioactive nuclide to decay. The half-lifeof a nuclide is a measure of its stability.(Stable nuclei can be thought of as havinginfinitely long half-lives.) If N0 is the origi-nal number of nuclei, the number remain-ing at the end of one half-life is N0/2, at theend of two half-lives is N0/4, etc.
half-sandwich compound See sandwichcompound.
halide /hal-ÿd, hay-lÿd/ A compound con-taining a halogen. The inorganic halides ofelectropositive elements contain ions of thetype Na+Cl– (sodium chloride) or K+Br–
(potassium bromide). Transition metalhalides often have some covalent bonding.Non-metal halides are covalent com-pounds, which are usually volatile. Exam-ples are tetrachloromethane (CCl4) andsilicon tetrachloride (SiCl4). Halides arenamed as bromides, chlorides, fluorides, oriodides.
halite /hal-ÿt, hay-lÿt/ (rock salt) A natu-rally occurring mineral form of sodiumchloride, NaCl. It forms colorless or whitecrystals when pure, but is often colored byimpurities.
haloalkane /hal-oh-al-kayn/ (alkyl halide)A type of organic compound in which oneor more hydrogen atoms of an alkane havebeen replaced by halogen atoms. Haloalka-
H
nes can be made by direct reaction of thealkane with a halogen. Other methods are:1. Reaction of an alcohol with the halogen
acid (e.g. from NaBr + H2SO4) or withphosphorus halides (red phosphorusand iodine can be used):
ROH + HBr → RBr + H2OROH + PCl5 → RCl + POCl3 + HCl
2. Addition of an acid to an alkene:RCH:CH2 + HBr → RCH2CH2Br
The haloalkanes are much more reac-tive than the alkanes, and are useful start-ing compounds for preparing a wide rangeof organic chemicals. In particular, theyundergo nucleophilic substitutions inwhich the halogen atom is replaced bysome other group (iodine compounds arethe most reactive). Some reactions ofhaloalkanes are:1. Refluxing with aqueous potassium hy-
droxide to give an alcohol:RI + OH– → ROH + I–
2. Refluxing with potassium cyanide in al-coholic solution to give a nitrile:
RI + CN– → RCN + I–
3. Refluxing with an alkoxide to give anether:
RI + –OR′ → ROR′ + I–
4. Reaction with alcoholic ammonia solu-tion (100°C in a sealed tube) to give anamine:
RI + NH3 → RNH2 + HI5. Boiling with alcoholic potassium hy-
droxide, to eliminate an acid and pro-duce an alkene:
RCH2CH2I + KOH → KI + H2O +RCH:CH2
See also Fittig reaction; Grignardreagent; Wurtz reaction.
halobutyl /hal -oh-byoo-t’l/ See butyl rub-ber.
halocarbons /hal-oh-kar-bŏnz/ Chemicalcompounds that contain carbon atomsbound to halogen atoms and (sometimes)hydrogen atoms. The halocarbons includehaloalkanes such as tetrachloromethane(CCl4) and the haloforms (CHCl3, CHBr3,etc.). There are various types of halocar-bon that are useful but are also significantpollutants. For example, the chlorofluoro-carbons (CFCs) contain carbon, fluorine,
and chlorine. They are useful as refriger-ants, aerosol propellants, and in makingrigid plastic foams. However, they are alsothought to damage the ozone layer and aninternational agreement exists to phase outtheir use. Similar compounds are the hy-drochlorofluorocarbons (HCFCs), whichcontain hydrogen as well as chlorine andfluorine, and the hydrofluorocarbons(HFCs), which contain hydrogen and fluo-rine.
The halons are a class of halocarbonsthat contain bromine as well as hydrogenand other halogens. Their main use is infire extinguishers. They are, however, sig-nificantly more active than CFCs in theireffect on the ozone layer. The halocarbonsare also thought to contribute to globalwarming.
haloform /hal-ŏ-form/ Any of the fourcompounds CHX3, where X is a halogenatom (F, fluoroform; Cl, chloroform; Br,bromoform; I, iodoform). The systematicnames are fluoromethane, chloromethane,bromomethane, and iodomethane. See alsohaloform reaction.
haloform reaction A reaction of amethyl ketone with NaOX, where X is Cl,Br, or I, to give a haloform. With sodiumchlorate(I), for example:
RCOCH3 + 3NaOCl → RCOCCl3 +3NaOH
RCOCl3 + NaOH → NaOCOR +CHCl3
The reaction can be used to make car-boxylic acids (from the NaOCOR), and isespecially useful when R is an aromaticgroup because the starting ketone,RCOCH3, can be produced by Friedel–Crafts acetylation. See also triiodo-methane.
halogenating agent /hal-lŏ-jĕ-nayt-ing/ Acompound used to introduce halogenatoms into a molecule. Examples are phos-phorus trichloride (PCl3) and aluminumtrichloride (AlCl3).
halogenation /hal-ŏ-jĕ-nay-shŏn/ A reac-tion in which a halogen atom is introducedinto a molecule. Halogenations are speci-
halobutyl
128
129
hardening
fied as chlorinations, brominations, fluori-nations, etc., according to the element in-volved. There are several methods.1. Direct reaction with the element using
high temperature or ultraviolet radia-tion:
CH4 + Cl2 → CH3Cl + HCl2. Addition to a double bond:
H2C:CH2 + HCl → C2H5Cl3. Reaction of a hydroxyl group with a
halogenating agent, such as PCl3:C2H5OH → C2H5Cl + OH–
4. In aromatic compounds direct substitu-tion can occur using aluminum chlorideas a catalyst:
2C6H6 + Cl2 → 2C6H5Cl5. Alternatively in aromatic compounds,
the chlorine can be introduced by react-ing the diazonium ion with copper(I)chloride:
C6H5N2+ + Cl– → C6H5Cl + N2
halogens /hal-ŏ-jĕnz/ A group of elements(group 17 of the periodic table) consistingof fluorine, chlorine, bromine, iodine, andthe short-lived element astatine. The halo-gens all have outer valence shells that areone electron short of a rare-gas configura-tion. Because of this, the halogens are char-acterized by high electron affinities andhigh electronegativities, fluorine being themost electronegative element known. Thehigh electronegativities of the halogensfavor the formation of both uninegativeions, X–, particularly combined with elec-tropositive elements (e.g. NaCl, KBr, CsI,CaF2), and the single covalent bond –Xwith elements of moderate to high elec-tronegativity (e.g. HCl, SiF4, SF4, BrCH3,Cl2O). The halogens all form diatomicmolecules, X2, which are characterized bytheir high reactivities with a wide range ofother elements and compounds. As agroup, the elements increase in both sizeand polarizability as the proton number in-creases and there is an attendant decreasein electronegativity and reactivity. Thismeans that there is no chemistry of positivespecies apart from a few cationic iodinecompounds. The heavier halogens Cl, Br,and I, all form oxo-species with formalpositive oxidation numbers +1 and +5,
chlorine and iodine also forming +3 and +7species (e.g. HOCl, HBrO3, I2O5, HIO4).
The elements decrease in oxidizingpower in the order F2>Cl2>Br2>I2 and theions X– may be arranged in order of in-creasing reducing power F–<Cl–<Br–<I–.Thus any halogen will displace the ele-ments below it from their salts in solution,for example
Cl2 + 2Br– → Br2 + 2Cl–
A wide range of organic halides isformed in which the C–F bond is charac-teristically resistant to chemical attack; theC–Cl bond is also fairly stable, particularlyin aryl compounds but the alkyl halogencompounds become increasingly suscepti-ble to nucleophilic attack and generallymore reactive.
halon /hal-on/ See halocarbon.
halothane /hal-ŏ-thayn/ (CHBrClCF3) Acolorless nonflammable liquid halocarbonused as a general anesthetic. The system-atic name is 1-chloro-1-bromo-2,2,2-triflu-oroethane.
hammer mill A device used in the chemi-cal industry for crushing and grinding solidmaterials at high speeds to a specified size.The impact between the particles, grindingplates, and grinding hammers pulverizesthe particles. Hammer mills can be used fora greater variety of soft material than othertypes of grinding equipment. Compare ballmill.
hardening (of oils) The conversion of liq-uid plant oils into a more solid form by hy-drogenation using a nickel catalyst.Hardening of liquid oils is used in produc-ing margerine. In vegetable oils the fattyacids present (as glycerides) contain doublebonds (i.e. they are unsaturated). The hy-drogenation process increases the amountof unsaturated material, increasing themelting point. The process still leaves someunsaturated fatty acids and, for this rea-son, it is claimed that margarines arehealthier than animal fats (e.g. butter),which contain a much higher proportion ofsaturated fats. This is because the unsatu-rated fats are less likely to lead to choles-
terol build-up in the body, and consequentrisk of coronary heart disease. The hydro-genation process may, however, also affectthe nature of the double bonds. Naturalunsaturated fatty acids mostly have a cisconfiguration about the double bonds. Inthe hydrogenation process, a proportion ofthese are converted into fatty acids with atrans configuration. Glycerides of these areknown as trans-fats. It has been claimedthat there is also a link between trans-fatsand coronary heart disease. See alsoSabatier–Senderens process.
hardness (of water) Hard water is waterthat will not readily form a lather withSOAP owing to the presence of dissolvedcalcium, iron, and magnesium compounds.Such compounds can react with soap toproduce insoluble salts, which collect assolid scum. The effectiveness of the cleans-ing solution is thus reduced. Hardness ofwater is of two types, TEMPORARY HARD-NESS and PERMANENT HARDNESS. Only tem-porary hardness can be removed by boilingthe water.
Hardness of water is usually expressedby assuming that all the hardness is due todissolved calcium carbonate, which is pre-sent as ions. It can be estimated by titrationwith a standard soap solution or with edta.See also water softening.
hard vacuum See vacuum.
hard water See hardness.
hassium /hass-ee-ŭm/ A transactinide ele-ment that is formed artificially.
Symbol: Hs; p.n. 108; most stable iso-tope 265Hs (half-life 2 × 10–3s).
HCFC Hydrochlorofluorocarbon. Seehalocarbon.
heat Energy transferred as a result of atemperature difference. The term is oftenloosely used to mean internal energy (i.e.the total kinetic and potential energy of theparticles). It is common in chemistry to de-fine such quantities as heat of combustion,heat of neutralization, etc. These are in factmolar enthalpies for the change, given the
symbol ∆HMŠ. The superscript symbol de-notes standard conditions, while the sub-script M indicates that the enthalpy changeis for one mole. The unit is usually the kilo-joule per mole (kJ mol–1). By convention,∆H is negative for an exothermic reaction.Molar enthalpy changes stated for chemi-cal reactions are changes for standard con-ditions, which are defined as 298 K (25°C)and 101 325 Pa (1 atmosphere). Thus, thestandard molar enthalpy of reaction is theenthalpy change for reaction of substancesunder these conditions producing reactantsunder the same conditions. The substancesinvolved must be in their normal equilib-rium physical states under these conditions(e.g. carbon as graphite, water as the liq-uid, etc.). Note that the measured enthalpychange will not usually be the standardchange. In addition, it is common to spec-ify the entity involved. For instance∆HfŠ(H2O) is the standard molar en-thalpy of formation for one mole of H2Ospecies.
heat engine (thermodynamic engine) Adevice for converting heat energy intowork. Heat engines operate by transferringenergy from a high-temperature source to alow-temperature sink. The theoretical op-eration of heat engines is useful in the the-ory of thermodynamics. See Carnot cycle.
heat exchangers Devices that enable theheat from a hot fluid to be transferred to acool fluid without allowing them to comeinto contact. The normal arrangement isfor one of the fluids to flow in a coiled tubethrough a jacket containing the secondfluid. Both the cooling and heating effectmay be of benefit in conserving the energyused in a chemical plant and in controllingthe process.
heat of atomization The energy requiredin dissociating one mole of a substance intoatoms. See heat.
heat of combustion The energy liberatedwhen one mole of a substance burns in ex-cess oxygen. See heat.
hardness
130
heat of crystallization The energy liber-ated when one mole of a substance crystal-lizes from a saturated solution of thissubstance.
heat of dissociation The energy requiredto dissociate one mole of a substance intoits constituent elements.
heat of formation The energy changewhen one mole of a substance is formedfrom its elements. See heat.
heat of neutralization The energy liber-ated when one mole of an acid or base isneutralized.
heat of reaction The energy change whenmolar amounts of given substances reactcompletely. See heat.
heat of solution The energy change whenone mole of a substance is dissolved in agiven solvent to infinite dilution (in prac-tice, to form a dilute solution).
heavy hydrogen See deuterium.
heavy-metal pollution See pollution.
heavy water Deuterium oxide, D2O.
hecto- Symbol: h A prefix denoting 102.For example, 1 hectometer (hm) = 102 me-ters (m).
Heisenberg’s uncertainty principle /hÿ-zĕn-bergz/ The impossibility of making si-multaneous measurements of both theposition and the momentum of a sub-atomic particle (e.g. an electron) with un-limited accuracy. The uncertainty arisesbecause, in order to detect the particle, ra-diation has to be ‘bounced’ off it, and thisprocess itself disrupts the particle’s posi-tion. Heisenberg’s uncertainty principle isnot a consequence of ‘experimental error’.It represents a fundamental limit to objec-tive scientific observation, and arises fromthe wave–particle duality of particles andradiation. In one direction, the uncertaintyin position ∆x and momentum ∆p are re-lated by ∆x∆p∼h/4π, where h is the Planck
constant. The uncertainty principle isnamed for the German physicist WernerKarl Heisenberg (1901–76).
helicate /hel-ă-kayt/ See supramolecularchemistry.
helium /hee-lee-ŭm/ A colorlessmonatomic gas; the first member of therare gases (group 18 of the periodic table).Helium has the electronic configuration1s2 and consists of a nucleus of two pro-tons and two neutrons (equivalent to an α-particle) with two extra-nuclear electrons.It has an extremely high ionization poten-tial and is completely resistant to chemicalattack of any sort. The gas accounts foronly 5.2 × 10–4% of the atmosphere; up to7% occurs in some natural gas deposits.Helium is the second most abundant ele-ment in the universe, the primary processon the Sun being nuclear fusion of hydro-gen to give helium.
Helium is recovered commercially fromnatural gas in both the USA and countriesof the former USSR and it also forms partof ammonia plant tail gas if natural gas isused as a feedstock. Its applications are infields in which inertness is required andwhere the cheaper alternatives, such asnitrogen, are too reactive; for example,high-temperature metallurgy, powder tech-nology, and as a coolant in nuclear reac-tors. Helium is also favoured over nitrogenfor diluting oxygen for deep-sea diving(lower solubility in blood) and as a pres-surizer for liquefied gas fuels in rockets(total inertness). It is also used as an idealgas for balloons (no fire risk) and for low-temperature physics research.
Helium is unusual in that it is the onlyknown substance for which there is notriple point (i.e., no combination of pres-sure and temperature at which all threephases can co-exist). This is because the in-teratomic forces, which normally partici-pate in the formation of solids, are so weakthat they are of the same order as the zero-point energy. At 2.2 K helium undergoes atransition from liquid helium I to liquidhelium II, the latter being a true liquid butexhibiting superconductivity and an im-measurably low viscosity (superfluidity).
131
helium
The low viscosity allows the liquid tospread in layers a few atoms thick, de-scribed by some as ‘flowing uphill’.
Helium also has an isotope. 3He isformed in nuclear reactions and by decayof tritium. This also undergoes a phasechange at temperatures close to absolutezero.
Symbol: He; m.p. 0.95 K (pressure);b.p. 4.216 K; d. 0.1785 kg m–3 (0°C); p.n.2; r.a.m. 4.002602.
Hell–Volhard–Zelinsky reaction /hellvoh-lard zem-lin-skee/ A method for thepreparation of halogenated carboxylicacids using free halogen in the presence ofa phosphorus halide. The halogenation oc-curs at the carbon atom adjacent to the–COOH group. With Br2 and PBr3:
RCH2COOH → RCHBrCOOH →RCBr2COOH
The reaction is named for the Germanchemists Carl Magnus von Hell (1849–1926) and Jacob Volhard (1834–1910)and for the Russian chemist Nicolai (Niko-lay Dmitrievich) Zelinsky (1861–1953).
Helmholtz function (Helmholtz free en-ergy) Symbol: F A thermodynamic func-tion defined by
F = U – TSwhere U is the internal energy, T the ther-modynamic temperature, and S the en-tropy. It is a measure of the ability of asystem to do useful work in an isothermalprocess. The function is named for the Ger-man physiologist and theoretical physicistHerman Ludwig Ferdinand von Helmholtz(1821–94). See also free energy.
hematite /hem-ă-tÿt, hee-mă-/ A mineralform of iron(III) oxide. It is the principalore of iron.
heme /heem/ (haeme) An iron-containingporphyrin that is the prosthetic group inhemoglobin, myoglobin, and some cyto-chromes.
hemiacetal /hem-ee-ass-ĕ-tal/ See acetal.
hemihydrate /hem-ee-hÿ-drayt/ A crys-talline compound with one molecule of
water of crystallization per two moleculesof compound (e.g. 2CaSO4.H2O).
hemiketal /hem-ee-kee-t’l/ See ketal.
hemimorphite /hem-ee-mor-fÿt/ Seecalamine.
hemoglobin /hee-mŏ-gloh-bin, hem-ŏ-,hee-mŏ-gloh-bin/ The pigment of the redblood cells that is responsible for the trans-port of oxygen from the lungs to the tis-sues. It consists of a basic protein, globin,linked with four heme groups. Heme is acomplex compound containing an ironatom.
The most important property of hemo-globin is its ability to combine reversiblywith one molecule of oxygen per iron atomto form oxyhemoglobin, which has abright red color. The iron is present in thedivalent state (iron(II)) and this remainsunchanged with the binding of oxygen.There are variations in the polypeptidechains, giving rise to different types of he-moglobins in different species. The bindingof oxygen depends on the oxygen partialpressure; high pressure favors formation ofoxyhemoglobin and low pressure favorsrelease of oxygen.
henry /hen-ree/ Symbol: H The SI unit ofinductance, equal to the inductance of aclosed circuit that has a magnetic flux ofone weber per ampere of current in the cir-cuit. 1 H = 1 Wb A–1. The unit is named forthe American physicist Joseph Henry(1797–1878).
Henry’s law The concentration (C) of agas in solution is proportional to the par-tial pressure (p) of that gas in equilibriumwith the solution, i.e. p = kC, where k is aproportionality constant.
The relationship is similar in form tothat for Raoult’s law, which deals withideal solutions.
A consequence of Henry’s law is thatthe ‘volume solubility’ of a gas is indepen-dent of pressure.
heparin /hep-ă-rin/ A POLYSACCHARIDE
that inhibits the formation of thrombin
Hell–Volhard–Zelinsky reaction
132
from prothrombin and thereby preventsthe clotting of blood. It is used in medicineas an anticoagulant.
heptahydrate /hep-ta-hÿ-drayt/ A crys-talline hydrated compound containing onemolecule of compound per seven moleculesof water of crystallization.
heptane /hep-tayn/ (C7H16) A colorlessliquid alkane obtained from petroleum re-fining. It is used as a solvent.
heptavalent /hep-tă-vay-lĕnt, hep-tav-ă-/(septavalent) Having a valence of seven.
hertz /herts/ Symbol: Hz The SI unit of fre-quency, defined as one cycle per second(s–1). Note that the hertz is used for regu-larly repeated processes, such as vibrationor wave motion. The unit is named for theGerman physicist Heinrich Rudolph Hertz(1857–94).
Hess’s law A derivative of the first law ofthermodynamics. It states that the totalheat change for a given chemical reactioninvolving alternative series of steps is inde-pendent of the route taken and is some-times known as the law of constant heatsummation. The law is named for theSwiss–Russian chemist Germain HenriHess (1802–50).
hetero atom /het-ĕ-roh/ See heterocycliccompound.
heterocyclic compound /het-ĕ-rŏ-sÿ-klik, -sik-lik/ A compound that has a ringcontaining more than one type of atom.Commonly, heterocyclic compounds areorganic compounds with at least one atomin the ring that is not a carbon atom. Pyri-dine and glucose are examples. The non-carbon atom is called a hetero atom.Compare homocyclic compound.
heterogeneous /het-ĕ-rŏ-jee-nee-ŭs/ Re-lating to more than one phase. A heteroge-neous mixture, for instance, contains twoor more distinct phases.
heterolytic fission /het-ĕ-rŏ-lit-ik/ (het-erolysis) The breaking of a covalent bondso that both electrons of the bond remainwith one fragment. A positive ion and anegative ion are produced:
RX → R+ + X–
Compare homolytic fission.
heteropolymer See polymerization.
hexacyanoferrate /heks-ă-sÿ-an-oh-fe-rayt/ See cyanoferrate.
hexadecanoate /heks-ă-dek-ă-noh-ayt/(palmitate) A salt or ester of hexadecanoicacid.
hexadecanoic acid /heks-ă-dek-ă-noh-ik/(palmitic acid) A crystalline carboxylicacid:
CH3(CH2)14COOHIt is present as GLYCERIDES in fats and
oils.
hexagonal close-packed crystal /heks-ag-ŏ-năl/ A crystal structure in which lay-ers of close-packed atoms are stacked in anABABAB arrangement. The second layer Bfits into the holes of the first layer A, withthe third layer over the first. The coordina-tion number is 12. Zinc and magnesiumhave hexagonal close-packed structures.
The unit cell is based on a hexagon withatoms occupying corner and mid positions.See face-centered cubic crystal.
hexagonal crystal See crystal system.
hexamethylene diamine /heks-ă-meth-ă-leen dÿ-ă-meen/ See 1,6-diaminohexane.
hexamethylenetetramine /heks-ă-meth-ă-leen-tet-ră-meen, -min/ See hexamine.
hexamine /heks-ă-meen, -min/ (hexameth-ylenetetramine; C6H12N4) A white crys-talline organic compound made bycondensing methanal with ammonia. It isused as a fuel for camping stoves, in vul-canizing rubber, and as a urinary disinfec-tant. Hexamine can be nitrated to make thehigh explosive cyclonite.
133
hexamine
hexane /heks-ayn/ (C6H14) A liquidalkane obtained from the light fraction ofcrude oil. The principal use of hexane is ingasoline and as a solvent.
hexanedioic acid /heks-ayn-dÿ-oh-ik/(adipic acid; HOOC(CH2)4COOH) A col-orless crystalline organic dicarboxylic acidthat occurs in rosin. It is used in the manu-facture of NYLON.
hexanoate /heks-ă-noh-ayt/ A salt or esterof hexanoic acid.
hexanoic acid /heks-ă-noh-ik/ (caproicacid, CH3(CH2)4COOH) An oily car-boxylic acid found (as glycerides) in cow’smilk and some vegetable oils.
hexose /heks-ohs/ A SUGAR that has sixcarbon atoms in its molecules.
hexyl group /heks-ăl/ The groupC5H11CH2–, having a straight chain of car-bon atoms.
HFC Hydrofluorocarbon. See halocar-bon.
histidine /his-tă-deen, -din/ See aminoacids.
Hofmann degradation /hoff-măn/ Amethod of preparing primary amines fromacid amides. The amide is refluxed withaqueous sodium hydroxide and bromine:
RCONH2 + NaOH + Br2 →RCONHBr + NaBr + H2O
RCONHBr + OH– → RCON–Br + H2ORCON–Br → R–N = C = O + Br–
RNCO + 2OH– → RNH2 + CO22–
The reaction is a ‘degradation’ in that acarbon atom is removed from the amidechain. The degradation is named for theGerman organic chemist August Wilhelmvon Hofman (1818–92).
Hofmann’s method A rather outmodedmethod for determining the vapor densityof volatile liquids. A known weight of sam-ple is introduced into a mercury barometertube, which is surrounded by a heatingjacket. The volume of vapor can thus be
read off directly, the temperature isknown, and the pressure is obtained bytaking the atmospheric pressure minus themercury height in the barometer (with cor-rections for the density of mercury athigher temperatures). The method’s onlyadvantage is that it may be used for sam-ples that decompose at their normal boil-ing point. See also Dumas’ method; VictorMeyer’s method.
holmium /hohl-mee-ŭm/ A soft malleablesilvery element of the lanthanoid series ofmetals. It occurs in association with otherlanthanoids. It has few applications.
Symbol: Ho; m.p. 1474°C; b.p.2695°C; r.d. 8.795 (25°C); p.n. 67; r.a.m.164.93032.
HOMO /hoh-moh/ See frontier orbital.
homocyclic compound /hoh-mŏ-sÿ-klik,hom-ŏ-/ A compound containing a ringmade up of the same atoms. Benzene is anexample of a homocyclic compound. Com-pare heterocyclic compound.
homogeneous /hoh-mŏ-jee-nee-ŭs, hom-ŏ-/ Relating to a single phase. A homoge-neous mixture, for instance, consists ofonly one phase.
homologous series A group of organiccompounds possessing the same functionalgroup and having a regular structural pat-tern so that each member of the series dif-fers from the next one by a fixed number ofatoms. The members of a homologous se-ries can be represented by a general for-mula. For example, the homologous seriesof alkane alcohols CH3OH, C2H5OH,C3H7OH, …, has a general formulaCnH2n+1OH. Each member differs by CH2from the next. Any two successive mem-bers of a series are called homologs.
homologs /hom-ŏ-lôgz/ See homologousseries.
homolytic fission /hoh-mŏ-lit-ik, hom-ŏ-/ (homolysis) The breaking of a covalentbond so that one electron from the bond is
hexane
134
135
hydride
left on each fragment. Two free radicals re-sult:
RR′ → R• + R′•Compare heterolytic fission.
homopolymer /hoh-mŏ-pol-i-mer, hom-ŏ-/ See polymerization.
hornblende /horn-blend/ A dark-coloredrock-forming mineral consisting of silicatesof calcium, iron, and magnesium. It is amajor component of granite and other ig-neous and metamorphic rocks.
host–guest chemistry A branch ofSUPRAMOLECULAR CHEMISTRY in which amolecular structure acts as a ‘host’ to holdan ion or molecule (the ‘guest’). The guestmay be coordinated to the host or may betrapped by its structure. For example, cal-ixarenes are compounds with cup-shapedmolecules that may accept guest molecules.See also crown ethers.
HPLC High-performance liquid chro-matography; a sensitive analytical tech-nique, that is similar to gas–liquidchromatography but using a liquid carrier.The carrier is specifically choosen for theparticular substance to be detected.
Hückel rule /hyoo-kĕl/ See aromatic com-pound.
humectant /hyoo-mek-tănt/ A hygro-scopic substance used to maintain moisturelevels. Glycerol, mannitol, and sorbitol arecommonly used in foodstuffs, tobacco, etc.
Humphreys series /hum-freez/ See hydro-gen atom spectrum.
Hund’s rule /hûnts/ A rule that states thatthe electronic configuration in degenerateorbitals will have the minimum number ofpaired electrons. The rule is named for theGerman physicist Friedrich Hund(1896–1997).
hybrid orbital See orbital.
hydrate /hÿ-drayt/ A compound coordi-nated with water molecules. When water is
bound up in a compound it is known as thewater of crystallization.
hydration /hÿ-dray-shŏn/ The solvationof such species as ions in water.
hydraulic cement See cement.
hydrazine /hÿ-dră-zeen/ (N2H4) A color-less liquid that can be prepared by the oxi-dation of ammonia with sodiumchlorate(I) or by the gas phase reaction ofammonia with chlorine. Hydrazine is aweak base, forming salts (e.g. N2H4.HCl)with strong acids. It is a powerful reducingagent, reducing salts of the noble metals tothe metal. Anhydrous hydrazine ignitesspontaneously in oxygen and reacts vio-lently with oxidizing agents. The aqueoussolution, hydrazine hydrate, has been usedas a fuel for jet engines and for rockets. Seealso hydrazone.
hydrazoic acid /hÿ-dră-zoh-ik/ (hydrogenazide; azoimide; HN3) A colorless liquidwith a nauseating smell. It is highly poiso-nous and explodes in the presence of oxy-gen and oxidizing agents. It can be made bydistilling a mixture of sodium azide(NaN3) and a dilute acid. It is usually usedas an aqueous solution. The salts of hydra-zoic acid (azides), especially lead azide(Pb(N3)2), are used in detonators becauseof their ability to explode when given a me-chanical shock.
hydrazone /hÿ-dră-zohn/ A type of or-ganic compound containing the C:NNH2group, formed by the reaction between analdehyde or ketone and hydrazine (N2H4).Derivatives of hydrazine are often used toproduce crystalline products, which havesharp melting points that can be used tocharacterize the original aldehyde or ke-tone. Phenylhydrazine (C6H5NH.NH2),for instance, produces phenylhydrazones.
hydride /hÿ-drÿd/ A compound of hydro-gen. Ionic hydrides are formed with highlyelectropositive elements and contain the H–
ion (hydride ion). Non-metals form cova-lent hydrides, as in methane (CH4) orsilane (SiH4). The boron hydrides are elec-
tron-deficient covalent compounds. Manytransition metals absorb hydrogen to forminterstitial hydrides.
hydrobromic acid /hÿ-droh-broh-mik/(HBr) A colorless liquid produced byadding hydrogen bromide to water. Itshows the typical properties of a strongacid and it is a strong reducing agent. Aconvenient way of producing hydrobromicacid is to bubble hydrogen sulfide throughbromine water. Although it is not as strongas hydrochloric acid it dissociates exten-sively in water and is a good proton donor.
hydrocarbon /hÿ-droh-kar-bŏn/ Anycompound containing only the elementscarbon and hydrogen. Examples are thealkanes, alkenes, alkynes, and aromaticssuch as benzene and naphthalene.
hydrochloric acid /hÿ-droh-klor-ik, -kloh-rik/ (HCl) A colorless fuming liquidmade by adding hydrogen chloride towater:
HCl(g) + H2O1. → H3O+(aq) + Cl–(aq)Dissociation into ions is extensive and
hydrochloric acid shows the typical prop-erties of a strong acid. It reacts with car-bonates to give carbon dioxide and yieldshydrogen when reacted with all but themost unreactive metals. Hydrochloric acidis used in the manufacture of dyes, drugs,and photographic materials. It is also usedto pickle metals, i.e. clean the surface priorto electroplating. Hydrochloric acid do-nates protons with ease and is the strongestof the hydrohalic acids. The concentratedacid is oxidized to chlorine by such agentsas potassium manganate(VII) and man-ganese(IV) oxide.
hydrochlorofluorocarbon /hÿ-droh-klor-ŏ-floo-ŏ-rŏ-kar-bŏn, -kloh-roh-/ Seehalocarbon.
hydrocyanic acid /hÿ-droh-sÿ-an-ik/(prussic acid; HCN) A highly poisonousweak acid formed when hydrogen cyanidegas dissolves in water. Its salts arecyanides. Hydrogen cyanide is used inmaking acrylic plastics.
hydrofluoric acid /hÿ-droh-floo-or-ik, -oh-rik/ (HF) A colorless liquid producedby dissolving hydrogen fluoride in water. Itis a weak acid, but will dissolve most sili-cates and hence can be used to etch glass.As the interatomic distance in HF is rela-tively small, the H–F bond energy is veryhigh and hydrogen fluoride is not a goodproton donor. It does, however, form hy-drogen bonds. See hydrogen fluoride. – un-able to donate protons easily –hydrofluoric acid is a weak acid. See bondenergy.
hydrofluorocarbon /hÿ-droh-floo-ŏ-rŏ-kar-bŏn/ See halocarbon.
hydrogen /hÿ-drŏ-jĕn/ A colorless gaseouselement. Hydrogen has some similarities toboth the alkali metals (group 1) and thehalogens (group 17), but is not normallyclassified in any particular group of the pe-riodic table. It is the most abundant ele-ment in the Universe and the ninth mostabundant element in the Earth’s crust andatmosphere (by mass). It occurs principallyin the form of water and petroleum prod-ucts; traces of molecular hydrogen arefound in some natural gases and in theupper atmosphere.
The gas may be prepared in the labora-tory by the reaction of dilute hydrochloricacid with a metal that lies above hydrogenin the electromotive series, magnesium andzinc being commonly used:
Zn + 2HCl → H2 + ZnCl2Reactions of the amphoteric metals zinc
and aluminum with dilute aqueous alkali,to form zincates or aluminates, are also aconvenient source of hydrogen. Electroly-sis of dilute mineral acids may be used toobtain hydrogen but care must be taken toavoid mixing with the oxygen released atthe anode as this leads to explosive mix-tures. Industrially, hydrogen is obtained asa by-product of the electrolytic cells used inthe production of sodium hydroxide (reac-tion of the Na/Hg amalgam with water), orby the water-gas route in which steam isdecomposed by hot coke.
The main use of hydrogen is as a chem-ical feedstock for the manufacture of am-monia and of a range of organic
hydrobromic acid
136
compounds. Small-scale uses include re-ducing atmospheres for metallurgy, hard-ening edible oils, and pharmaceuticalmanufacture. Hydrogen also has a poten-tial future use as a fuel.
Hydrogen occupies a unique positionamong the elements as hydrogen atoms arethe simplest of all atoms. The hydrogenatom consists of a proton (positive charge)with one extranuclear electron (1s1). Thechemistry of hydrogen depends on one ofthree processes:1. Loss of the electron to form H+.2. Gain of an electron to form H–.3. Sharing of electrons by covalent bond
formation as in H2 or HCl.The hydrogen atom with the 1s electron
removed would have an extremely smallionic radius and the positive hydrogen ionoccurs only in association with otherspecies as in H+NH3 or H+FH. The ioncommonly written H+ in solution is in factthe solvated proton (hydroxonium or hy-dronium) H3O+, which is formed by ion-ization of acids:
H2O + HCl → H3O+ + Cl–
It is believed that the lifetime of any oneH3O+ ion is extremely short as the protonsappear to undergo very rapid exchange be-tween water molecules.
Hydrogen also forms a number of com-pounds in which it is regarded as gainingan electron and becoming H– (the hydrideion). It can form ionic hydrides only withthe most electropositive elements (groups 1and 2). The ionic nature of these com-pounds is indicated by the fact that themelts are good conductors and that elec-trolysis liberates hydrogen at the anode.These hydrides are prepared by heating theelement in a stream of hydrogen:
H2 + 2M → 2MHExamples are CH4, NH3, and HCl.
These compounds are generally low-boil-ing. General methods of preparation are:1. The hydrolysis of the appropriate ‘-ide’
compound; e.g. silicides give silane, ni-trides give ammonia, sulfides give H2S.
2. Reduction of a chloride; for example:SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3A third class of hydride is that of metal-
lic hydrides formed between hydrogen andmany transition metals. Palladium in par-
ticular is renowned for its ability to absorbhydrogen as PdHx, where x takes values upto 1.8. Titanium and zirconium behavesimilarly but the exact nature of the com-pounds formed and of the bond type re-mains uncertain. There are changes in themagnetic properties of the metal (indicat-ing some electron interaction) and in thelattice dimensions but discrete phases ofthe type MH or MH2 are not isolated. Theclass is sometimes referred to as the inter-stitial hydrides.
Atomic nuclei possess the property of‘spin’ and for diatomic molecules there ex-ists the possibility of having the spins of ad-jacent nuclei aligned (ortho) or opposed(para). Because of the small mass of hydro-gen, these forms are more important in hy-drogen molecules than in other diatomicmolecules. The two forms are in equilib-rium with parahydrogen dominant at lowtemperatures, rising to 75% orthohydro-gen at room temperatures. Although chem-ically identical the melting point andboiling point of the para form are bothabout 0.1° lower than the 3:1 equilibriummixture.
Natural hydrogen in molecular or com-bined forms contains about one part in2000 of deuterium, D, an isotope of hy-drogen that contains one proton and oneneutron in its nucleus. Although the effectof isotopes on chemical properties is nor-mally small, in the case of hydrogen the dif-ference in mass number leads to a loweringof some reaction rates known as the ‘deu-terium isotope effect’. Hydrogen also ex-hibits two less common forms of bonding.Boron hydrides form a wide variety ofcompounds in which the hydrogen acts asa bridging species involving ‘three-centretwo-electron’ bonds. Such species are saidto be ‘electron deficient’ as they do nothave sufficient electrons for conventionaltwo-electron covalent bonds. The second,less common, form is that of the coordi-nated hydrides in which the H– ion acts asa ligand bound to a transition metal atom.
Symbol: H; m.p. 14.01 K; b.p. 20.28 K;d. 0.089 88 kg m–3 (0°C); p.n. 1; r.a.m.1.0079.
137
hydrogen
hydrogenation
138
hydrogenation /hÿ-droj-ĕ-nay-shŏn/ Thereaction of a compound with hydrogen. Anexample is the hydrogenation of nitrogento form ammonia in the Haber process. Inorganic chemistry, hydrogenation refers tothe addition of hydrogen to multiplebonds, usually with the aid of a catalyst.Unsaturated natural liquid vegetable oilscan be hydrogenated to form saturatedsemisolid fats – a reaction used in makingtypes of margarine. See Bergius process;hardening.
hydrogen atom spectrum The spectrumof the hydrogen atom is characterized byseveral series of sharp spectral lines de-scribed by simple laws. The general law forthese series of lines is:
1/λ = R (1/n21 – 1/n2
2),where λ is the wavelength associated witha spectral line, R is the RYDBERG CONSTANT
and n1 and n2 are integers, with n2≥n1.The first of these series to be discovered
was the BALMER SERIES in which n1 = 2, n2 =3,4,5,… This series is in the visible regionand was discovered by the Swiss mathe-matician and physicist Johann JakobBalmer (1825–1898) in 1885. The series inwhich n1 = 1 is the LYMAN SERIES which liesin the ultraviolet region. This series wasdiscovered by the American physicistTheodore Lyman (1874–1954). TheLyman series is a conspicuous feature ofthe spectrum of the Sun.
In the PASCHEN SERIES (n1 = 3), theBrackett series (n1 = 4), the Pfund series (n1= 5) and the Humphreys series (n1 = 6) thespectral lines occur in the infrared region.
The explanation for these regular serieslies in the existence of discrete, quantizedenergy levels. In 1913 Niels Bohr was ableto derive the formula for these series interms of the ad hoc quantum assumptionsof the BOHR THEORY. In the mid-1920s theformula was derived in a deductive wayfrom quantum mechanics.
hydrogen azide See hydrazoic acid.
hydrogen bond (H-bonding) An inter-molecular bond between molecules inwhich hydrogen is bound to a stronglyelectronegative element. Bond polarization
by the electronegative element X leads to apositive charge on hydrogen Xδ––Hδ+; thishydrogen can then interact directly withelectronegative elements of adjacent mole-cules. The hydrogen bond is represented asa dotted line:
Xδ– – Hδ+ ....... Xδ– – Hδ+ …The length of a hydrogen bond is charac-teristically 0.15–0.2 nm. Hydrogen bond-ing may lead to the formation of dimers(for example, in carboxylic acids) and isused to explain the anomalously high boil-ing points of H2O and HF.
hydrogen bromide (HBr) A colorlesssharp-smelling gas that is very soluble inwater. It is produced by direct combinationof hydrogen and bromine in the presence ofa platinum catalyst or by the reaction ofphosphorus tribromide with water. It dis-solves in water to give hydrobromic acid.Hydrogen bromide is rather inactive chem-ically. It will not conduct electricity in theliquid state, indicating that it is a molecu-lar compound.
hydrogencarbonate /hÿ-drŏ-jĕn-kar-bŏ-nayt/ (bicarbonate) A salt containing theion –HCO3.
hydrogen chloride (HCl) A colorless gasthat has a strong irritating odor and fumesstrongly in moist air. It is prepared by theaction of concentrated sulfuric acid onsodium chloride. The gas is made industri-ally by burning a stream of hydrogen inchlorine. It is not particularly reactive butwill form dense white clouds of ammo-nium chloride when mixed with ammonia.It is very soluble in water and ionizes al-most completely to give HYDROCHLORIC
ACID. It will also dissolve in many non-aqueous solvents, including toluene, butwill not ionize and the resultant solutionshows no acidic properties. Hydrogenchloride is used in the manufacture of or-ganic chlorine compounds, such aspolyvinyl chloride (PVC).
Unlike the other hydrogen halides hy-drogen chloride will not dissociate on heat-ing, indicating a strong H–Cl bond.
hydrogen cyanide See hydrocyanic acid.
hydrogen electrode A type of half cellbased on hydrogen, and assigned zeroELECTRODE POTENTIAL, so that other ele-ments may be compared with it. It is alsocalled the standard hydrogen half cell. Thehydrogen is bubbled over a platinum elec-trode, coated in ‘platinum black’, in a 1 Macid solution. Hydrogen is adsorbed on theplatinum black, which has a high surfacearea, enabling the equilibrium
H(g) ˆ H+(aq) + e–
to be set up. The platinum is inert and hasno tendency to form platinum ions in solu-tion.
hydrogen fluoride (HF) A colorless li-quid produced by the reaction of concen-trated sulfuric acid on calcium fluoride:
CaF2(s) + H2SO4(aq) → CaSO4(aq) +2HF(l)
It produces toxic corrosive fumes and dis-solves readily in water to give hydrofluoricacid.
Hydrogen fluoride is atypical of the hy-drogen halides as the individual H–F unitsare associated into much larger units,forming zigzag chains and rings. This iscaused by hydrogen bonds that form be-tween the hydrogen and the highly elec-tronegative fluoride ions. Hydrogenfluoride is used extensively as a catalyst inthe petroleum industry. See hydrofluoricacid.
hydrogen ion A positively charged hy-drogen atom, H+, i.e. a proton. Hydrogenions are produced by all ACIDS in water, inwhich they are hydrated to hydroxonium(hydronium) ions, H3O+. See pH.
hydrogen molecule ion The simplesttype of molecule. It consists of two hydro-gen nuclei and one electron. If the nucleiare regarded as being fixed theSCHRÖDINGER EQUATION for the hydrogenmolecule ion can be solved exactly. Thisenables theories and approximation tech-niques concerned with chemical bondingto be tested quantitatively.
hydrogen peroxide (H2O2) A colorlesssyrupy liquid, usually used in solution inwater. Although it is stable when pure, on
contact with bases such as manganese(IV)oxide it gives off oxygen, themanganese(IV) oxide acting as a catalyst:
2H2O2 → 2H2O + O2
Hydrogen peroxide can act as an oxi-dizing agent, converting iron(II) ions toiron(III) ions, or as a reducing agent withpotassium manganate(VII). It is used as ableach and in rocket fuel. The strength ofsolutions is usually given as volumestrength – the volume of oxygen (dm3) atSTP given by decomposition of 1 dm3 ofthe solution.
hydrogensulfate /hÿ-drŏ-jĕn-sul-fayt/(bisulfate; HSO4
–) An acidic salt or ester ofsulfuric acid (H2SO4), in which only one ofthe acid’s hydrogen atoms has been re-placed by a metal or organic radical. Anexample is sodium hydrogensulfate,NaHSO4.
hydrogen sulfide (sulfuretted hydrogen;H2S) A colorless very poisonous gas withan odor of bad eggs. Hydrogen sulfide isprepared by reacting hydrochloric acidwith iron(II) sulfide. It is tested for by mix-ing with lead nitrate, with which it gives ablack precipitate. Its aqueous solution isweakly acidic. Hydrogen sulfide reducesiron(III) chloride to iron(II) chloride, form-ing hydrochloric acid and a yellow precip-itate of sulfur. Hydrogen sulfideprecipitates insoluble sulfides, and is usedin qualitative analysis. It burns with a blueflame in oxygen to form sulfur(IV) oxideand water. Natural gas contains some hy-drogen sulfide, which is removed beforesupply to the consumer.
hydrogensulfite /hÿ-drŏ-jĕn-sul-fÿt/(bisulfite; HSO3
–) An acidic salt or ester ofsulfurous acid (H2SO3), in which only oneof the acid’s hydrogen atoms has been re-placed by a metal or organic radical. Anexample is sodium hydrogensulfite,NaHSO3.
hydrohalic /hÿ-droh-hal-ik/ Describingacids formed by hydrogen halides, e.g. HF,HCI, HBr, when dissolved in water.
139
hydrohalic
hydroiodic acid /hÿ-droh-ÿ-od-ik/ See io-dine.
hydrolysis /hÿ-drol-ă-sis/ A reaction be-tween a compound and water. Some exam-ples are:Salts of weak acids
Na2CO3 + 2H2O → 2NaOH + H2CO3Esters
CH3COOC2H5 + H2O ˆ CH3COOH+ C2H5OH
Certain inorganic halidesSiCl4 + 4H2O → Si(OH)4 + 4HCl
hydron /hÿ-dron/ The positive ion H+.The name is used when the isotope is notrelevant, i.e. a hydron could be a proton,deuteron, or triton.
hydronium ion /hÿ-droh-nee-ŭm/ Seeacid; hydrogen ion.
hydrophilic /hÿ-drŏ-fil-ik/ Water attract-ing. See lyophilic.
hydrophobic /hÿ-drŏ-foh-bik/ Water re-pelling. See lyophobic.
hydroquinone /hÿ-droh-kwi-nohn, hÿ-droh-kwin-ohn/ See benzene-1,4-diol.
hydrosol /hÿ-drŏ-sol, -sohl/ A colloid inaqueous solution.
hydroxide /hÿ-droks-ÿd, -id/ A com-pound containing the ion OH– or thegroup –OH.
hydroxonium ion /hÿ-droks-oh-nee-ŭm/See hydrogen ion.
hydroxybenzene /hÿ-droks-ee-ben-zeen, -ben-zeen/ See phenol.
hydroxybenzoate /hÿ-droks-ee-ben-zoh-ayt/ See salicylate.
hydroxybenzoic acid /hÿ-droks-ee-ben-zoh-ik/ See salicylic acid.
hydroxyl group /hÿ-drox-ăl/ A group(–OH) containing hydrogen and oxygen,characteristic of alcohols and phenols, and
some hydroxides. It should not be con-fused with the hydroxide ion (OH–).
2-hydroxypropanoic acid /hÿ-droks-ee-proh-pă-noh-ok/ (lactic acid) A colorlessliquid carboxylic acid:
CH3CH(OH)COOHSee optical activity.
hygroscopic /hÿ-grŏ-skop-ik/ Describinga substance that absorbs moisture from theatmosphere. See also deliquescent.
hyperconjugation /hÿ-per-kon-jŭ-gay-shŏn/ The interaction of sigma bonds withpi bonds in a compound. It is sometimesdescribed in terms of resonance structuresof the type:
C6H5CH3 → C6H5CH2–H+
to explain the interaction of the methylgroup with the pi electrons of the benzenering in methylbenzene (toluene).
hyperfine structure /hÿ-per-fÿn/ See finestructure.
hypertonic solution A solution that hasa higher osmotic pressure than some othersolution. Compare hypotonic solution.
hypo /hÿ-poh/ An old name for SODIUM
THIOSULFATE(IV), especially when used inphotography. It was formerly known assodium hyposulfite.
hypobromous acid /hÿ-pŏ-broh-mŭs/ Seebromic(I) acid.
hypochlorous acid /hÿ-pŏ-klor-ŭs, -kloh-rŭs/ See chloric(I) acid.
hypophosphorous acid /hÿ-pŏ-fos-fŏ-rŭs/ See phosphinic acid.
hyposulfuric acid /hÿ-pŏ-sul-fyoor-ik/See dithionic acid.
hyposulfurous acid /hÿ-per-sul-fyoor-ŭs/See dithionous acid.
hypotonic solution A solution that has alower osmotic pressure than some othersolution. Compare hypertonic solution.
hydroiodic acid
140
141
Iceland spar A pure transparent mineralform of calcite (calcium carbonate,CaCO3), noted for its property of birefrin-gence (double refraction).
ideal gas (perfect gas) See gas laws; ki-netic theory.
ideal-gas scale See absolute temperature.
ideal solution A hypothetical solutionthat obeys RAOULT’S LAW.
ignis fatuus /ig-nis fach-û-ŭs/ (will-o’-the-wisp) A light sometimes seen over marshyground. It is caused by methane producedby rotting vegetation, which is ignited bythe presence of small amounts of sponta-neously flammable phosphine (PH3).
ignition temperature The temperatureto which a substance must be heated beforeit will continue to burn (usually in air).
imide /im-ÿd, -id/ An organic compoundcontaining the group –CO.NH.CO–, i.e. a–NH group attached to two carbonylgroups. Simple imides have the general for-mula R1.CO.NH.CO.R2, where R1 and R2
are alkyl or aryl groups. The group isknown as the imido group, and it can formpart of a ring in cyclic imides.
imido group /i-mee-doh, im-i-doh/ Seeimide.
imine /i-meen, im-in/ An organic com-pound containing the group C=N–, inwhich there is a double bond between thecarbon and the nitrogen. A general for-mula for imines is R1R2C=N–R3, whereR1, R2, and R3 are hydrocarbon groups orhydrogen. They can be made by the reac-
tion of aldehydes and ketones with pri-mary amines. For example, propanone(acetone; CH3COCH3) with ethylamine(C2H6NH2):
CH3COCH3 + C2H6NH2 →(CH3)2C=N–C2H6 + H2O
The reaction is acid-catalyzed. Whenammonia is used the imine contains theC=N–H group:
CH3COCH3 + NH3 → (CH3)2C=N–H +H2O
Most imines are unstable and can be de-tected only in solution unless R1, R2, or R3
are aryl groups. Intermediates in which animine adds a proton to form a positive ion(e.g. R1R2C=NR3H+) are known asiminium ions.
iminium ion See imine.
imino group /i-mee-noh, im-i-noh/ Seeimine.
iminourea /i-mee-noh-yû-ree-ă, im-ă-noh- / See guanidine.
immiscible /i-miss-ă-băl/ Describing twoor more liquids that will not mix, such asoil and water. After being shaken togetherthey form separate layers.
indene /in-deen/ (C9H8) A colorless flam-mable hydrocarbon. It has a benzene ringfused to a five-membered ring.
indicator A compound that reversiblychanges color depending on the pH of thesolution in which it is dissolved. The visualobservation of this change is therefore aguide to the pH of the solution and it fol-lows that careful choice of indicators per-mits a wide range of end points to bedetected in acid–base titrations.
I
Redox titrations require either specificindicators, which detect one of the compo-nents of the reaction (e.g. starch for iodine,potassium thiocyanate for Fe3+) or trueredox indicators in which the transitionpotential of the indicator between oxidizedand reduced forms is important. The tran-sition potential of a redox indicator is anal-ogous to the transition pH in acid–basesystems.
Complexometric titrations require indi-cators that complex with metal ions andchange color between the free state and thecomplex state. See also absorption indica-tor.
indigo (C16H10N2O2) A blue organic dyethat occurs (as a glucoside) in plants of thegenus Indigofera. It is a derivative of in-dole, and is now made synthetically.
indium /in-dee-ŭm/ A soft silvery metallicelement belonging to group 13 of the peri-odic table. It is found in minute quantities,primarily in zinc ores and is used in alloys,in several electronic devices, and in electro-plating.
Symbol: In; m.p. 155.17°C; b.p.2080°C; r.d. 7.31 (25°C); p.n. 49; r.a.m.114.818.
indole /in-dohl, -dol/ (benzpyrrole;C8H7N) A colorless solid organic com-pound, whose molecules consist of a ben-zene ring fused to a pyrrole ring. It occursin coal tar and various plants, and is thebasis of indigo and of several plant hor-mones.
inductive effect /in-duk-tiv/ The effect inwhich substituent atoms or groups in anorganic compound can attract (–I) or pushaway electrons (+I), forming polar bonds.
inert gases See rare gases.
infrared /in-fră-red/ (IR) ELECTROMAG-NETIC RADIATION with longer wavelengthsthan visible radiation. The wavelengthrange is approximately 0.7 µm to 1 mm.Many materials transparent to visible lightare opaque to infrared, including glass.Rock salt, quartz, germanium, or poly-
ethene prisms and lenses are suitable foruse with infrared. Infrared radiation is pro-duced by movement of charges on the mol-ecular scale; i.e. by vibrational orrotational motion of molecules. Of partic-ular importance in chemistry is the absorp-tion spectrum of compounds in theinfrared region. Certain bonds betweenpairs of atoms (C–C, C=C, C=O, etc.) havecharacteristic vibrational frequencies,which correspond to bands in the infraredspectrum. Infrared spectra are thus used infinding the structures of new organic com-pounds. They are also used to ‘fingerprint’and thus identify known compounds. Atshorter wavelengths, infrared absorptioncorresponds to transitions between rota-tional energy levels, and can be used to findthe dimensions of molecules (by their mo-ment of inertia).
inhibitor A substance that slows downthe rate of reaction. Hydrogen sulfide, hy-drogen cyanide, mercury salts, and arseniccompounds readily inhibit heterogeneouscatalysts by adsorption. For example, ar-senic compounds inhibit platinum catalystsin the oxidation of sulfur(IV) oxide to sul-fur(VI) oxide. Inhibitors must not be con-fused with negative catalysts; inhibitors donot change the pathway of a reaction.
inner Describing a ring compound that isformed, or could be regarded as formed, byone part of a molecule reacting with an-other. For example, a LACTAM is an inneramide and a LACTONE is an inner ester.
inorganic chemistry The branch ofchemistry concerned with elements otherthan carbon and with the preparation,properties, and reactions of their com-pounds. Certain simple carbon compoundsare treated in inorganic chemistry, includ-ing the oxides, carbon disulfide, thehalides, hydrogen cyanide, and salts, suchas the cyanides, cyanates, carbonates, andhydrogencarbonates.
insertion reaction A reaction in which anatom or group is inserted between twoother groups. See carbene.
indigo
142
143
invert sugar
insoluble /in-sol-yŭ-băl/ Describing acompound that has a very low solubility (ina specified solvent).
instrumentation /in-strŭ-men-tay-shŏn/The measurement of the conditions and thecontrol of processes within a chemicalplant. The instruments can be classifiedinto three groups: those for current infor-mation using mercury thermometers,weighing scales, and pressure gauges; thosefor recording viscosity, fluid flow, pres-sure, and temperature; and those instru-ments that control and maintain thedesired conditions including pH and theflow of materials.
intercallation compound /in-terk-ă-lay-shŏn/ A compound that has a structurebased on layers and in which there are lay-ers of a different character interleaved inthe basic structural units. For example, themicas phlogopite (KMg3(OH)2Si3AlO10)and muscovite (KAl2(OH)2Si3AlO10) areformed by interleaving K+ ions replacing aquarter of the silicon layers in talc and py-rophyllite respectively. See also lamellarcompound.
intermediate 1. A compound that re-quires further chemical treatment to pro-duce a finished industrial product such as adye or pharmaceutical chemical.2. A transient chemical entity in a complexreaction. See also precursor.
intermediate bond A form of covalentbond that also has an ionic or electrovalentcharacter. See polar bond.
intermediate bonding A form of cova-lent bond that also has an ionic or elec-trovalent character. See polar bond.
intermolecular forces /in-ter-mŏ-lek-yŭ-ler/ Forces of attraction between moleculesrather than forces within the molecule(chemical bonding). If these intermolecularforces are weak the material will begaseous and as their strength progressivelyincreases materials become progressivelyliquids and solids. The intermolecularforces are divided into H-bonding forces
and VAN DER WAALS FORCES, and the majorcomponent is the ELECTROSTATIC INTERAC-TION OF DIPOLES. See hydrogen bond.
internal energy Symbol: U The energy ofa system that is the total of the kinetic andpotential energies of its constituent parti-cles (e.g. atoms and molecules). If the tem-perature of a substance is raised, bytransferring energy to it, the internal en-ergy increases (the particles move faster).Similarly, work done on or by a system re-sults in an increase or decrease in the inter-nal energy. The relationship between heat,work, and internal energy is given by thefirst law of thermodynamics. Sometimesthe internal energy of a system is looselyspoken of as ‘heat’ or ‘heat energy’.Strictly, this is incorrect; heat is the trans-fer of energy as a result of a temperaturedifference.
internal resistance Resistance of a sourceof electricity. In the case of a cell, when acurrent is supplied, the potential differencebetween the terminals is lower than thee.m.f. The difference (i.e. e.m.f. – p.d.) isproportional to the current supplied. Theinternal resistance (r) is given by:
r = (E – V)/Iwhere E is the e.m.f., V the potential dif-ference between the terminals, and I thecurrent.
interstitial /in-ter-stish-ăl/ See defect.
interstitial compound A crystallinecompound in which atoms of a non-metal(e.g. carbon, hydrogen, or boron) occupyinterstitial positions in the crystal lattice ofa metal (usually a transition metal). Inter-stitial compounds are often non-stoichio-metric. Their physical properties are oftensimilar to those of metals; e.g. they have ametallic luster and are electrical conduc-tors.
inversion /in-ver-shŏn/ A change fromone optical isomer to the other. See Waldeninversion.
invert sugar See sucrose.
iodic acid /ÿ-od-ik/ See iodic(V) acid.
iodic(V) acid (iodic acid; HIO3) A color-less deliquescent crystalline solid producedby the reaction of concentrated nitric acidwith iodine. Iodic(V) acid is a strong oxi-dizing agent. It will liberate iodine from so-lutions containing iodide ions and it reactsvigorously with organic materials, oftenproducing flames. It dissociates extensivelyin water and hence is a strong acid.
iodic(VII) acid (periodic acid; H5IO6) Awhite crystalline solid made by low-tem-perature electrolysis of concentratediodic(V) acid. It exists in a number offorms, the most common of which isparaiodic(VII) acid. Iodic(VII) acid is apowerful oxidizing agent. It is a weak acid,which – by cautious heating under vacuum– can be converted to dimesoiodic(VII)acid (H4I2O9), and metaiodic(VII) acid(HIO4). All three will oxidize manganeseto manganate(VII) and this reaction can beused to determine small amounts of man-ganese in steel.
iodide /ÿ-ŏ-dÿd, -did/ See halide.
iodine /ÿ-ŏ-dÿn, -deen/ A dark-violetvolatile solid element belonging to thehalogens (group 17 of the periodic table). Itoccurs in seawater and is concentrated byvarious marine organisms in the form of io-dides. Significant deposits also occur in theform of iodates. The element is conve-niently prepared by the oxidation of io-dides in acid solution (using MnO2).Industrial methods similarly use oxidationof iodides or reduction of iodates to iodidesby sulfur(IV) oxide (sulfur dioxide) fol-lowed by oxidation, depending on thesource of the raw materials. Iodine and itscompounds are used in chemical synthesis,photography, pharmaceuticals, and dye-stuffs manufacture.
Iodine has the lowest electronegativityof the stable halogens and consequently isthe least reactive. It combines only slowlywith hydrogen to form hydroiodic acid,HI. Iodine also combines directly withmany electropositive elements, but does somuch more slowly than does bromine or
chlorine. Because of the larger size of theiodine ion and the consequent low latticeenergies, the iodides are generally moresoluble than related bromides or chlorides.As with the other halides, iodides of Ag(I),Cu(I), Hg(I), and Pb(II) are insoluble unlesscomplexing ions are present.
Iodine also forms a range of covalentiodides with the metalloids and non-metal-lic elements (this includes a vast range oforganic iodides) but these are generally lessthermodynamically stable and are morereadily hydrolyzed than chlorine orbromine analogs.
Four oxides are known of which io-dine(V) oxide, I2O5, is the most important.The other oxides, I2O4, I4O9, and I2O7 aremuch less stable and of uncertain structure.Like chlorine (but not bromine) iodineforms oxo-species based on IO–, IO2
–,IO3
–, and IO4–. The chemistry of the other
oxo-species in solution is complex. Ele-mental iodine reacts with alkalis in a simi-lar way to bromine and chlorine.
The ionization potential of iodine is suf-ficiently low for it to form a number ofcompounds in which it is electropositive. Itforms I+ cations, for example, by reactionof solid silver nitrate with iodine solution,and such cations are sufficiently elec-trophilic to substitute aromatic com-pounds such as phenol. Iodine also exhibitsthe interesting property of forming solu-tions that are violet colored in non-donortype solvents, such as tetrachloromethane,but in donor solvents, such as ethanol ordioxan, there is a strong iodine–solvent in-teraction, which gives the solution a deepbrown color. Even though iodine solutionswere commonly used as antiseptic agents,the element is classified as toxic, and careshould be taken to avoid eye intrusions orexcessive skin contact.
Symbol: I; m.p. 113.5°C; b.p. 184°C;r.d. 4.93 (20°C); p.n. 53; r.a.m.126.90447.
iodine monochloride /mon-ŏ-klor-ÿd, -id, -kloh-rÿd, -rid/ (ICl) A dark red liquidmade by passing chlorine over iodine. Ithas properties similar to those of its con-stituent halogens. Iodine monochloride is
iodic acid
144
145
ionic radius
used as a nonaqueous solvent and as an io-dating agent in organic reactions.
iodine number (iodine value) A numberthat gives a measure of the number of un-saturated bonds in an organic compoundsuch as a fat or oil. It is found by measur-ing the amount of iodine in grams taken upby 100 grams of the compound.
iodine(V) oxide (iodine pentoxide; I2O5)A white crystalline solid made by heatingiodic(V) acid to a temperature of 200°C. Itis a very strong oxidizing agent.
Iodine oxide is the acid anhydride ofiodic(V) acid, which is reformed whenwater is added to the oxide. Its main use isin titration work, measuring traces of car-bon monoxide in the air.
iodine pentoxide /pen-toks-ÿd/ See io-dine(V) oxide.
iodine trichloride /trÿ-klor-ÿd, -kloh-rÿd/See diiodine hexachloride.
iodoethane /ÿ-od-oh-eth-ayn/ (ethyl io-dide; C2H5I) A colorless liquid alkyl halidemade by reaction of ethanol with iodine inthe presence of red phosphorus.
iodoform /ÿ-od-ŏ-form/ See triiodo-methane.
iodoform reaction See triiodomethane.
iodomethane /ÿ-od-oh-meth-ayn/ (methyliodide; CH3I) A liquid alkyl halide madeby reaction of methanol with iodine in thepresence of red phosphorus.
ion /ÿ-on, -ŏn/ An atom or molecule thathas a negative or positive charge as a resultof losing or gaining one or more electrons.See electrolysis; ionization.
ion exchange A process that takes placein certain insoluble materials, which con-tain ions capable of exchanging with ionsin the surrounding medium. Zeolites, thefirst ion exchange materials, were used forwater softening. These have largely beenreplaced by synthetic resins made of an
inert backbone material, such aspolyphenylethene, to which ionic groupsare weakly attached. If the ions exchangedare positive, the resin is a cationic resin. Ananionic resin exchanges negative ions.When all available ions have been ex-changed (e.g. sodium ions replacing cal-cium ions) the material can be regeneratedby passing concentrated solutions (e.g.sodium chloride) through it. The calciumions are then replaced by sodium ions. Ion-exchange techniques are used for a vastrange of purification and analytical pur-poses.
For example, solutions with ions thatcan be exchanged for OH– and H+ can beestimated by titrating the resulting solutionwith an acid or a base.
ionic bond /ÿ-on-ik/ See electrovalentbond.
ionic crystal A crystal composed of ionsof two or more elements. The positive andnegative ions are arranged in definite pat-terns and are held together by electrostaticattraction. Common examples are sodiumchloride and cesium chloride.
ionic product The product of concentra-tions:
KW = [H+][OH–]in water as a result of a small amount ofself-ionization.
ionic radius A measure of the effective ra-dius of an ion in a compound. For an iso-lated ion, the concept is not verymeaningful, since the ion is a nucleus sur-rounded by an ‘electron cloud’. Values ofionic radii can be assigned, however, basedon the distances between ions in crystals.
Different methods exist for determiningionic radii and often different values arequoted for the same ion. The two mainmethods are those of Goldschmidt and ofPauling. The Goldschmidt radii are deter-mined by substituting data from one com-pound to another, to produce a set of ionicradii for different ions. The Pauling radiiare assigned by a more theoretical treat-ment for apportioning the distance be-tween an anion and a cation.
ionic strength For an ionic solution aquantity can be introduced which empha-sizes the charges of the ions present:
I = ½Σimiz2i
where m is the molality and z the ioniccharge. The summation is continued overall the different ions in the solution, i.
ionization /ÿ-ŏ-ni-zay-shŏn/ The processof producing ions. There are several waysin which ions may be formed from atomsor molecules. In certain chemical reactionsionization occurs by transfer of electrons;for example, sodium atoms and chlorineatoms react to form sodium chloride,which consists of sodium ions (Na+) andchloride ions (Cl–). Certain molecules canionize in solution; acids, for example, formhydrogen ions as in the reaction
H2SO4 → 2H+ + SO42–
The ‘driving force’ for ionization in asolution is solvation of the ions by mol-ecules of the solvent. H+, for example, issolvated as a hydroxonium (hydronium)ion, H3O+.
Ions can also be produced by ionizingradiation; i.e. by the impact of particles orphotons with sufficient energy to break upmolecules or detach electrons from atoms:A → A+ + e–. Negative ions can be formedby capture of electrons by atoms or mol-ecules: A + e– → A–.
ionization energy See ionization poten-tial.
ionization potential (IP; Symbol: I) Theenergy required to remove an electronfrom an atom (or molecule or group) in thegas phase, i.e. the energy required for theprocess:
M → M+ + e–
It gives a measure of the ability of metals toform positive ions. The second ionizationpotential is the energy required to removetwo electrons and form a doubly chargedion:
M → M2+ + e–
Ionization potentials stated in this wayare positive; often they are given in elec-tronvolts. Ionization energy is the energyrequired to ionize one mole of the sub-
stance, and is usually stated in kilojoulesper mole (kJ mol–1).
In chemistry, the terms ‘second’, ‘third’,etc., ionization potentials are usually usedfor the formation of doubly, triply, etc.,charged ions. However, in spectroscopyand physics, they are often used with a dif-ferent meaning. The second ionization po-tential is the energy to remove the secondleast strongly bound electron in forming asingly charge ion. For lithium (ls22s1) itwould refer to removal of a 1s electron toproduce an excited ion with the configura-tion 1s12s1. Note also that ionization po-tentials are now stated as energies.Originally they were the potential throughwhich an electron had to be accelerated tocause ionization by electron impact:
M + e– → M2+ + 2e–
They were thus stated in volts.
ionizing radiation Radiation of suffi-ciently high energy to cause IONIZATION. Itmay be short-wavelength electromagneticradiation (ultraviolet, x-rays, or gammarays) or streams of particles.
ion pair A positive ion and a negative ionin close proximity in solution, held by theattractive force between their charges. SeeDebye–Hückel theory; electrolysis.
IP See ionization potential.
IR See infrared.
iridium /i-rid-ee-ŭm/ A white transitionmetal that is highly resistant to corrosion.It is used in electrical contacts, in sparkplugs, and in jewelry.
Symbol: Ir; m.p. 2410°C; b.p. 4130°C;r.d. 22.56 (17°C); p.n. 77; r.a.m. 192.217.
iron /ÿ-ern/ A transition element occur-ring in many ores, especially the oxides(haematite and magnetite) and carbonate.It is extracted in a blast furnace using coke,limestone, and hot air. The coke and airform carbon monoxide, which then re-duces the iron ore to iron. The limestoneremoves acidic impurities and forms alayer of slag above the molten iron at thebase of the furnace. Steel is formed by re-
ionic strength
146
ducing the carbon content to between 0.1and 1.5%. Iron is used as a catalyst in theHaber process for ammonia production. Itcorrodes in air and moisture to hydratediron(III) oxide (rust).
The most stable oxidation state is +3,which is yellow, but +2 (green) and +6 (eas-ily reduced) also exist. Solutions of iron(II)ions give a green precipitate with sodiumhydroxide solution, whereas iron(III) ionsgive a brown precipitate. The concentra-tion of iron(II) ions can be estimated inacid solution by titration with standardpotassium manganate(VII) solution.Iron(III) ions must first be reduced toiron(II) ions with sulfur(IV) oxide (sulfurdioxide).
Symbol: Fe; m.p. 1535°C; b.p. 2750°C;r.d. 7.874 (20°C); p.n. 26; r.a.m. 55.845.
iron(II) chloride (ferrous chloride; FeCl2)A compound prepared by passing dry hy-drogen chloride gas over heated iron.White feathery anhydrous crystals are pro-duced. Hydrated iron(II) chloride(FeCl2.6H2O) is prepared by reacting ex-cess iron with dilute or concentrated hy-drochloric acid. Green crystals of thehexahydrate are obtained on crystalliza-tion from solution. Iron(II) chloride isreadily soluble in water, producing anacidic solution due to salt hydrolysis.
iron(III) chloride (ferric chloride; FeCl3)A compound prepared in the anhydrousstate as dark red crystals by passing drychlorine over heated iron. The productsublimes and is collected in a cooled re-ceiver. The hydrated salt (FeCl3.6H2O) isprepared by adding excess iron(III) oxideto concentrated hydrochloric acid. Oncrystallization yellow-brown crystals of thehexahydrate are formed. Iron(III) chlorideis very soluble in water and undergoes salthydrolysis. At temperatures below 400°Cthe anhydrous salt exists as a dimer,Fe2Cl6.
iron(II) oxide (ferrous oxide; FeO) Ablack powder formed by the careful reduc-tion of iron(III) oxide using either carbonmonoxide or hydrogen. It can also be pre-pared by heating iron(II) oxalate in the ab-
sence of air. It is only really stable at hightemperatures and disproportionates slowlyon cooling to give iron(III) oxide and iron.Iron(II) oxide can be reduced by heating ina stream of hydrogen. When exposed toair, it is oxidized to iron(III) oxide. It is abasic oxide, dissolving readily in diluteacids to form iron(II) salt solutions. Ifheated to a high temperature in an inert at-mosphere, iron(II) oxide disproportionatesto give iron and triiron tetroxide.
iron(III) oxide (ferric oxide; Fe2O3) Arusty-brown solid prepared by the actionof heat on iron(III) hydroxide or iron(II)sulfate. It occurs in nature as the mineralhematite. Industrially it is obtained byroasting iron pyrites. Iron(III) oxide dis-solves in dilute acids to produce solutionsof iron(III) salts. It is stable at red heat, de-composes around 1300°C to give triirontetroxide, and can be reduced to iron byhydrogen at 1000°C. Iron(III) oxide is notionic in character but has a structure simi-lar to that of aluminum(III) oxide.
iron(II) sulfate (ferrous sulfate; green vit-riol; FeSO4.7H2O) A compound that oc-curs in nature as the mineral melanterite(or copperas). It is made industrially fromiron pyrites. In the laboratory iron(II) sul-fate is prepared by dissolving excess iron indilute sulfuric acid. On crystallization,green crystals of the heptahydrate are ob-tained. Careful heating of the hydrated saltyields anhydrous iron(II) sulfate; on fur-ther heating the sulfate decomposes to giveiron(III) oxide, sulfur(IV) oxide, and sul-fur(VI) oxide. The hydrated crystals oxi-dize easily on exposure to air owing to theformation of basic iron(III) sulfate. Afreshly prepared solution of iron(II) sulfateabsorbs nitrogen(II) oxide (brown-ringtest).
Iron(II) sulfate crystals are isomor-phous with the sulfates of zinc, magne-sium, nickel, and cobalt.
iron(III) sulfate (ferric sulfate; Fe2(SO4)3)A compound prepared in the hydratedstate by the oxidation of iron(II) sulfatedissolved in dilute sulfuric acid, using anoxidizing agent, such as hydrogen peroxide
147
iron(III) sulfate
or concentrated nitric acid. On crystalliza-tion, the solution deposits a white mass ofthe nonahydrate (Fe2(SO4)3.9H2O). Theanhydrous salt can be prepared by gentlyheating the hydrated salt. Iron(III) sulfatedecomposes on heating to give iron(III)oxide and sulfur(VI) oxide. It forms alumswith the sulfates of the alkali metals.
irreversible change /i-ri-ver-să-băl/ Seereversible change.
irreversible reaction A reaction in whichconversion to products is complete; i.e.there is little or no back reaction.
isobars /ÿ-sŏ-barz/ 1. Two or more nu-clides that have the same nucleon numbersbut different proton numbers.2. Lines joining points of equal pressure.
isocyanide /ÿ-sŏ-sÿ-ă-nÿd/ See isonitrile.
isocyanide test (carbylamine reaction) A
test for the primary amine group. The sus-pected primary amine is warmed withtrichloromethane in an alcoholic solutionof potassium hydroxide. The resulting iso-cyanide (RNC) has a characteristic smell ofbad onions (and is very toxic):
CHCl3 + 3KOH + RNH2 → RNC +3KCl + 3H2O
isoelectronic /ÿ-soh-i-lek-tron-ik/ De-scribing compounds that have the samenumber of electrons. For example, carbonmonoxide (CO) and nitrogen (N2) are iso-electronic.
isoleucine /ÿ-sŏ-loo-seen, -loo-sin/ Seeamino acids.
isomer /ÿ-sŏ-mer/ See isomerism.
isomerism /ÿ-som-ĕ-riz-ăm/ The existenceof two or more chemical compounds withthe same molecular formulae but differentstructural formulae or different spatial
irreversible change
148
H2CH3C
H2
CCH3
C
CH3
CH3 H3
CH
butane(n-butane)
methylpropane(isobutane)
Isomer: isomers differing in carbon skeleton
H3CCH3
C
CH3 OH
dimethyl ether ethanol
Isomer: isomers differing in the nature of the functional group
H2OHC
H2
CCH3
C
OH
CH3 H3
CH
propan-1-ol(n-propanol)
propan-2-ol(isopropanol)
Isomer: isomers differing in the position of a functional group
arrangements of atoms. The differentforms are known as isomers. For example,the compound C4H10 may be butane(CH3CH2CH2CH3, with a straight chainof carbon atoms) or 2-methyl propane(CH3CH(CH3)CH3, with a branchedchain).
Structural isomerism is the type of iso-merism in which the structural formulae ofthe compounds differ. There are two mainforms. In one the isomers are differenttypes of compound. An example of this isthe compounds ethanol (C2H5OH) andmethoxymethane (dimethyl ether, CH3O-CH3), both having the molecular formulaC2H6O but quite different functionalgroups. In the other type of structural iso-
merism, the isomers differ because of theposition of a functional group in the mole-cule. For example, the primary alcoholpropan-1-ol (CH3CH2CH2OH) and thesecondary alcohol propan-2-ol(CH3CH(OH)CH3) are isomers; both havethe molecular formula C3H7OH.
Structural isomerism also occurs in in-organic chemistry. A particular case isfound in complexes in which a ligand maycoordinate to a metal ion in two ways. Forexample, NO2 can coordinate through N(the nitro ligand) or through O (the nitridoligand). Such ligands are said to be am-bidentate. Complexes that differ only inthe way in which the ligand coordinates aresaid to show linkage isomerism.
149
isomerism
propan-1-yne propan-2-yne
CCH3C CH3CH2CCH
CH3
Isomer: isomers differing in the position of a multiple bond
cis-diethyl epoxide trans-diethyl epoxide
CC
H H
C2H5 C2H5
O
C2H5
CC
H C2H5
H
O
Isomer: cis–trans isomerism in a ring compound
E-methylethylketone oxime
NC
CH3 OH
C2H6
Z-methylethylketone oxime
NC
CH3
OHC2H6
Isomer: E–Z isomerism in an oxime
cis-dichloroethene
CC
Cl Cl
HH
trans-dichloroethene
CC
Cl H
ClH
Isomer: cis-trans isomerism in an alkene
isomorphism
150
Stereoisomerism occurs when two com-pounds with the same molecular formulaeand the same groups differ only in thearrangement of the groups in space. Thereare two types of stereoisomerism.
Cis–trans (or syn–anti) isomerism oc-curs when there is restricted rotation abouta bond between two atoms (e.g. a doublebond or a bond in a ring). Groups attached
to each atom may be on the same side ofthe bond (the cis- or syn-isomer) or oppo-site sides (the trans- or anti-isomer). Cis-trans isomerism also occurs insquare-planar complexes of the typeMX2Y2, where M is a metal ion and X andY are different ligands. If the X ligands areadjacent the isomer is a cis-isomer; if theyare opposite, it is a trans-isomer. This type
cis-isomer trans-isomer
M
Y
Y
X
X M
X
Y
X
Y
Isomerism
trans-isomer cis-isomer
X
Y
YM
Y
Y
X
mer-isomerfac-isomer
Y
X
Y
XM
Y
Y
X
X
Y
YM
Y
X
X
X
Y
YM
X
Y
Isomerism
151
of isomerism can also occur in octahedralcomplexes of the type MX2Y4. Cis–transisomerism was formerly called geometricalisomerism.
Octahedral complexes of the typeMX3Y3 show a different type of stereoiso-merism. If the three X ligands are in a planethat includes the M ion (with the three Yligands in a plane at right angles), then thestructure is called the mer-isomer (‘mer’stands for meridional). If the three X (andY) ligands are all on a face of the octahe-dron, the structure is the fac-isomer (‘fac’stands for facial). See also E–Z convention
Optical isomerism occurs when thecompound has no plane of symmetry andcan exist in left- and right-handed formsthat are mirror images of each other. Suchmolecules always contain a CHIRAL ELE-MENT. Molecules that are mirror images ofeach other are more properly called enan-tiomers. Stereoisomers that are not mirrorimages are called diastereoisomers. For ex-ample, ANOMERS are diastereoisomers. Seealso optical activity.
isomorphism /ÿ-sŏ-mor-fiz-ăm/ The exis-tence of compounds with the same crystalstructure.
isomorphism, law of See Mitscherlich’slaw.
isonitrile /ÿ-sŏ-nÿ-trăl, -tril, -trÿl/ (iso-cyanide) An organic compound of the for-mula R–NC.
isoprene /ÿ-sŏ-preen/ See methylbuta-1,3-diene.
isopropanol /ÿ-sŏ-proh-pă-nol, -nohl/Propan-2-ol. See propanol.
isotactic polymer See polymerization.
isotherm /ÿ-sŏ-therm/ A line on a chart orgraph joining points of equal temperature.See also isothermal change.
isothermal change /ÿ-sŏ-therm-ăl/ Aprocess that takes place at a constant tem-perature. Throughout an isothermalprocess, the system is in thermal equilib-
rium with its surroundings. For example, acylinder of gas in contact with a constant-temperature box may be compressedslowly by a piston. The work done appearsas energy, which flows into the reservoir tokeep the gas at the same temperature.Isothermal changes are contrasted withadiabatic changes, in which no energy en-ters or leaves the system, and the tempera-ture changes. In practice no process isperfectly isothermal and none is perfectlyadiabatic, although some can approximatein behavior to one of these ideals.
isotones /ÿ-sŏ-tohnz/ Two or more nu-clides that have the same neutron numbersbut different proton numbers.
isotonic /ÿ-sŏ-tonn-ik/ Describing solu-tions that have the same osmotic pressure.
isotopes /ÿ-sŏ-tohps/ Two or more speciesof the same element differing in their massnumbers because of differing numbers ofneutrons in their nuclei. The nuclei musthave the same number of protons (an ele-ment is characterized by its proton num-ber). Isotopes of the same element havevery similar properties because they havethe same electron configuration, but differslightly in their physical properties. An un-stable isotope is termed a radioactive iso-tope or radioisotope
Isotopes of elements are useful in chem-istry for studies of the mechanisms ofchemical reactions. A standard technique isto label one of the atoms in a molecule byusing an isotope of the element. It is thenpossible to trace the way in which thisatom behaves throughout the course of thereaction. For example, in the esterificationreaction:
ROH + R′COOH ˆ H2O + R′COORit is possible to find which bonds are bro-ken by using a labeled oxygen atom. If thereaction is performed using 18O in the al-cohol it is found that this nuclide appearsin the ester, showing that the C–OH bondof the acid is broken in the reaction. In la-beling, radioisotopes are detected by coun-ters; stable isotopes can also be used, anddetected by a mass spectrum.
isotopes
Isotopes are also used in kinetic studies.For example, if the bond between twoatoms X–Y is broken in the rate-determin-ing step, and Y is replaced by a heavier iso-tope of the element, Y*, then the reactionrate will be slightly lower with the Y* pre-sent. This kinetic isotope effect is particu-larly noticeable with hydrogen anddeuterium compounds, because of thelarge relative difference in mass.
isotope separation /ÿ-sŏ-tohp/ The sepa-ration of different isotopes of a chemical el-ement, making use of differences in thephysical properties of these isotopes. Forsmall-scale isotope separation a MASS SPEC-TROMETER is frequently used. For large-scale isotope separation the methods usedinclude diffusion in the gas phase (used for
uranium using the gas uranium hexafluo-ride), distillation (which was used to pro-duce heavy water), electrolysis (also usedfor heavy water), and the use of cen-trifuges. Laser separation may also be em-ployed, with excitation of one isotopeoccurring, followed by its separation usingan electromagnetic field.
isotopic mass /ÿ-sŏ-top-ik/ (isotopicweight) The mass number of a given iso-tope of an element.
isotopic number The difference betweenthe number of neutrons in an atom and thenumber of protons.
isotopic weight See isotopic mass.
isotope separation
152
153
jeweler’s rouge Iron(III) oxide (Fe2O3),used as a mild abrasive.
joliotium /zhoh-lee-oh-shee-ŭm/ Symbol:Jl A name formerly suggested for element-105 (dubnium). See element.
joule /jool/ Symbol: J The SI unit of energy
and work, equal to the work done whenthe point of application of a force of onenewton moves one meter in the direction ofaction of the force. 1 J = 1 N m. The jouleis the unit of all forms of energy. The unitis named for the British physicist JamesPrescott Joule (1818–89).
J
154
kainite /kay-ă-nÿt, kÿ-nÿt/ A mineral formof hydrated crystalline magnesium sulfate(MgSO4) containing also some potassiumchloride. It is used as a fertilizer and sourceof potassium compounds.
kaolin /kay-ŏ-lin/ See china clay.
kaolinite /kay-ŏ-li-nÿt/ A hydrated alumi-nosilicate mineral, Al2(OH)4Si2O5, themajor constituent of kaolin. It is formed bythe weathering of FELDSPARS from granites.
katal /kay-t’l/ Symbol: kat The SI derivedunit of catalytic activity, equal to theamount of substance in moles that can becatalyzed per second. 1 kat = l mol s–1.
katharometer /kath-ă-rom-ĕ-ter/ See gaschromatography.
Kekulé structure /kay-koo-lay/ A struc-ture of BENZENE in which there is a hexag-onal ring with alternate double and singlebonds. Two possible Kekulé structurescontribute to the RESONANCE hybrid of ben-zene. The structure is named for the Ger-man chemist Friedrich Augus Kekulé vonStradonitz (1829–96).
kelvin Symbol: K The SI base unit of ther-modynamic temperature. It is defined asthe fraction 1/273.16 of the thermody-namic temperature of the triple point ofwater. Zero kelvin (0 K) is absolute zero.One kelvin is the same as one degree on theCelsius scale of temperature. The unit isnamed for the British theoretical and ex-perimental physicist William Thompson,Baron Kelvin (1824–1907).
keratin /ke-ră-tin/ Any of a class of fi-brous proteins found in hair, feathers,
horn, and hooves. The keratins containpolypeptide chains linked by disulfidebonds between cystein amino acids.
kerosene /ke-rŏ-seen, ke-rŏ-seen/ See pe-troleum.
ketal /kee-t’l/ A type of organic compoundformed by addition of an alcohol to a ke-tone. Addition of one molecule gives ahemiketal; two molecules give a full ketal.See acetal.
ketene /kee-teen/ (keten) A member of agroup of organic compounds, general for-mula R2C:CO, where R is hydrogen or anorganic radical. The simplest member isthe colorless gas ketene, CH2:CO; theother ketenes are colored because of thepresence of the double bonds. Ketenes areunstable and react with other unsaturatedcompounds to give cyclic compounds.
keto–enol tautomerism /kee-toh ee-nol/An equilibrium between two isomers inwhich one isomer is a ketone (the ketoform) and the other an enol. The equilib-rium is by transfer of a hydrogen atom be-tween the oxygen atom of the carbonylgroup and an adjacent carbon atom.
keto form /kee-toh/ See keto–enol tau-tomerism.
ketohexose /kee-toh-heks-ohs/ A ketoseSUGAR with six carbon atoms.
ketone /kee-tohn/ A type of organic com-pound with the general formula RCOR,having two alkyl or aryl groups bound to acarbonyl group. They are made by oxidiz-ing secondary alcohols (just as ALDEHYDES
are made from primary alcohols). Simple
K
examples are propanone (acetone,CH3COCH3) and butanone (methyl ethylketone, CH3COC2H5).
The chemical reactions of ketones aresimilar in many ways to those of aldehydes.The carbonyl group is polarized, with pos-itive charge on the carbon and negativecharge on the oxygen. Thus nucleophilicaddition can occur at the carbonyl group.Ketones thus:1. Undergo addition reactions with hydro-
gen cyanide and hydrogensulfite ions.2. Undergo condensation reactions with
hydroxylamine, hydrazine, and their de-rivatives.
3. Are reduced to (secondary) alcohols.They are not, however, easily oxidized.Strong oxidizing agents give a mixtureof carboxylic acids. They do not reactwith Fehling’s solution or Tollen’sreagent, and do not easily polymerize.
ketopentose /kee-toh-pen-tohs/ A ketoseSUGAR with five carbon atoms.
ketose /kee-tohs/ A SUGAR containing aketo- (=CO) or potential keto- group.
kieselguhr /kee-zĕl-goor/ See diatomite.
killed spirits Zinc(II) chloride solutionused as a flux for solder, so called becauseit is made by adding zinc to hydrochloricacid (‘spirits of salt’).
kilo- Symbol: k A prefix denoting 103. Forexample, 1 kilometer (km) = 103 meters(m).
kilocalorie /kil-ŏ-kal-ŏ-ree/ See calorie.
kilogram /kil-ŏ-gram/ (kilogramme; Sym-bol: kg) The SI base unit of mass, equal tothe mass of the international prototype ofthe kilogram, which is a piece of plat-inum–iridium kept at Sèvres in France.
kilogramme /kil-ŏ-gram/ An alternativespelling of kilogram.
kilowatt-hour /kil-ŏ-wot/ Symbol: kWhA unit of energy, usually electrical, equal tothe energy transferred by one kilowatt ofpower in one hour. It has a value of 3.6 ×106 joules.
kinetic energy /ki-net-ik/ See energy.
kinetic isotope effect See isotopes.
kinetics /ki-net-iks/ A branch of physicalchemistry concerned with the study ofrates of chemical reactions and the effect ofphysical conditions that influence the rateof reaction, e.g. temperature, light, concen-tration, etc. The measurement of theserates under different conditions gives infor-mation on the mechanism of the reaction;i.e. on the sequence of processes by whichreactants are converted into products.
kinetic theory A theory explaining phys-ical properties in terms of the motion ofparticles. The kinetic theory of gases as-sumes that the molecules or atoms of a gasare in continuous random motion and thepressure (p) exerted on the walls of a con-
155
kinetic theory
C C
H
OH
C C
H
OH
ˆenol form keto form
Keto-enol tautomerism
RC O
R
Ketone
taining vessel arises from the bombard-ment by these fast moving particles. Whenthe temperature is raised the speeds in-crease; so consequently does the pressure.If more particles are introduced or the vol-ume is reduced there are more particles tobombard unit area of the walls and thepressure also increases. When a particlecollides with the wall it experiences a rateof change of momentum, which is propor-tional to the force exerted. For a largenumber of particles this provides a steadypressure on the wall.
Additional assumptions are:1. The particles behave as if they are hard
smooth perfectly elastic points.2. They do not exert any appreciable force
on each other except during collisions.3. The volume occupied by the particles
themselves is a negligible fraction of thevolume of the gas.
4. The duration of each collision is negligi-ble compared with the time between col-lisions.By considering the change in momen-
tum on impact with the walls it can beshown that
p = ρc2/3
where ρ is the density of the gas and c is theroot-mean-square speed of the molecules.The mean-square speed of the molecules isproportional to the absolute temperature:
Nmc2 = RTSee also degrees of freedom.
Kipps apparatus /kips/ An apparatus forthe production of a gas from the reactionof a liquid on a solid. It consists of threeglobes, the upper globe being connectedvia a wide tube to the lower globe. Theupper globe is the liquid reservoir. Themiddle globe contains the solid and alsohas a tap at which the gas may be drawnoff. When the gas is drawn off the liquidrises from the lower globe to enter the mid-dle globe and reacts with the solid, therebyreleasing more gas. When turned off thegas released forces the liquid back downinto the lower globe and up into the reser-voir, thus stopping the reaction. The appa-ratus is named for the Dutch chemistPetrus Jacobus Kipps (1808–64).
Kjeldahl’s method /kyel-dahlz/ Amethod used for the determination of ni-trogen in organic compounds. The nitroge-nous substance is converted to ammoniumsulfate by boiling with concentrated sulfu-ric acid (often with a catalyst such asCuSO4) in a specially designed long-neckedKjeldahl flask. The mixture is then madealkaline and the ammonia distilled off intostandard acid for measurement by titra-tion. The method is named for the Danishchemist Johan Gustav Christoffer Thor-sager Kjeldahl (1849–1900).
Kolbe electrolysis /kol-bee/ The electrol-ysis of sodium salts of carboxylic acids toprepare alkanes. The alkane is produced atthe anode after discharge of the carboxy-late anion and decomposition of the radi-cal:
RCOO– → RCOO• + e–
RCOO• → R• + CO2R• + RCOO• → R – R + CO2
and2R• → R – R
As the reaction is a coupling reaction,only alkanes with an even number of car-bon atoms in the chain can be prepared inthis way. The process is named for the Ger-man chemist Adolph Wilhelm HermannKolbe (1818–84).
Kroll process /krol/ A method of obtain-ing certain metals by reducing the metalchloride with magnesium. Titanium can beobtained in this way:
TiCl4 + 2Mg → 2MgCl2 + TiThe process is named for the Luxem-
bourg metallurgist William Justin Kroll(1889–1873).
krypton /krip-ton/ A colorless odorlessmonatomic element of the rare-gas group,known to form unstable compounds withfluorine. It occurs in minute quantities(0.001% by volume) in air. Krypton is usedin fluorescent lights. Symbol: Kr; m.p.–156.55°C; b.p. –152.3°C; d. 3.749 (0°C)kg m–3; p.n. 36; r.a.m. 83.80.
kurchatovium /ker-chă-toh-vee-ŭm/ Sym-bol: Ku A former name for element-104(rutherfordium). See element.
Kipps apparatus
156
157
label A stable or radioactive nuclide usedto investigate some process, such as achemical reaction. See isotopes.
labile /lay-băl/ Describing a complex withligands that can easily be replaced by morestrongly bonded ligands.
lactam /lak-tam/ A type of organic com-pound containing the –NH.CO– group aspart of a ring in the molecule. Lactams canbe regarded as formed from a straight-chain compound that has an amine group(–NH2) at one end of the molecule and acarboxylic acid group (–COOH) at theother; i.e. from an amino acid. The reac-tion of the amine group with the carboxylicacid group, with elimination of water,leads to the cyclic lactam, which is thus aninternal amide. Lactams may exist in an al-ternate tautomeric form in which the hy-drogen atom has migrated from the N ontothe O of the carbonyl group. This, the lac-tim form, contains the group –N=C(OH)–.
lactate /lak-tayt/ A salt or ester of lacticacid.
lactic acid /lak-tik/ See 2-hydroxy-propanoic acid.
lactim /lak-tim/ See lactam.
lactone /lak-tohn/ A type of organic com-pound containing the group –O.CO– aspart of a ring in the molecule. A lactonecan be regarded as formed from a com-pound with an alcohol (–OH) group onone end of the chain and a carboxylic acid(–COOH) group on the other. The lactonethen results from reaction of the –OHgroup with the –COOH group; i.e. it is aninternal ester.
lactose /lak-tohs/ (milk sugar; C12H22O11)A sugar found in milk. It is a disaccharidecomposed of glucose and galactose units.
Ladenburg benzene /lan-dĕn-berg/ Astructure once suggested for BENZENE inwhich the six carbon atoms are at the cor-ners of a triangular prism with each carbonatom bonded to a hydrogen atom. It isnamed for the German chemist AlbertLadenburg (1842–1911). The actual com-pound with this structure, known as pris-mane, was synthesized in 1973.
lake A pigment made by combining an or-ganic dyestuff with an inorganic com-pound (e.g. aluminum oxide).
lamellar compound /lă-mel-er/ A com-pound with a crystal structure composed
L
C ON
C
H
C
Lactam
C OO
C
C
Lactone
of thin plates or layers. Silicates form manycompounds with distinct layers. Typicalexamples are talc (Mg3(OH)2Si4O10) andpyrophyllite (Al2(OH)2Si4O10). See also in-tercallation compound.
lamp black A black pigment; a finely di-vided form of carbon formed by incom-plete combustion of an organic compound.
Langmuir isotherm /lang-myoor/ Anequation used to describe the adsorption ofa gas onto a plane surface at a fixed tem-perature. It can be written in the form:
θ = bp/ (1 + bp),where θ is the fraction of the surface cov-ered by the gas, p is the gas pressure, and bis a constant known as the adsorption co-efficient, which is the equilibrium constantfor the adsorption process. The equationwas derived by the American chemist Irv-ing Langmuir (1881–1957) in 1916 usingthe kinetic theory of gases.
lanolin /lan-ŏ-lin/ A yellowish viscoussubstance obtained from wool fat. It con-tains cholesterol and terpene compounds,and is used in cosmetics, in ointments, andin treating leather.
lanthanides /lan-thă-nÿdz, -nidz/ See lan-thanoids.
lanthanoids /lan-thă-noidz/ (lanthanides;lanthanons) A group of elements whoseelectronic configurations display back fill-ing of the 4f-level. There is a maximum of14 electrons in an f-orbital. The elementlanthanum itself has no f-electrons (La[Xe]5d16s2) and is thus strictly not a lan-thanoid, but it is included by convention,thus giving a closely related series of 15 el-ements. Excluding lanthanum, the ele-ments all have 4fx6s2 configurations butgadolinium and lutecium have an addi-tional 5d1 electron.
The characteristic oxidation state isM3+ and the great similarity in the size ofthe ions leads to a very close similarity ofchemical properties and hence to great dif-ficulties of separation using conventionalmethods. Chromatographic and solvent-
extraction methods have been specially de-veloped for the lanthanoids.
lanthanons /lan-thă-nŭs/ See lanthanoids.
lanthanum /lan-thă-nŭm/ A soft ductilemalleable silvery metallic element that isthe first member of the lanthanoid series. Itis found associated with other lanthanoidsin many minerals, including monazite andbastnaesite. Lanthanum is used in severalalloys (especially for lighter flints), as a cat-alyst, and in making optical glass.
Symbol: La; m.p. 921°C; b.p. 3457°C;r.d. 6.145 (25°C); p.n. 57; r.a.m.138.9055.
lapis lazuli /lap-is laz-yŭ-lÿ, -lee/ A deepblue mineral consisting of sodium alu-minum sulfate with some sulfur, widelyused for ornaments and as a semipreciousgemstone. It was the original source of thepigment ultramarine.
laser /lay-zer/ An acronym for Light Am-plification by Stimulated Emission of Radi-ation. A laser device produces high-intensity, monochromatic, coherent beamsof light. In the laser process the moleculesof a sample (such as ruby doped with Cr3+
ions) are promoted to an excited state. Asthe sample is in a cavity between two re-flective surfaces, when a molecule emitsspontaneously, the photon so generatedricochets backwards and forwards. In thisway other molecules are stimulated to emitphotons of the same energy. If one of thereflective surfaces is partially transmittingthis radiation can be tapped.
laser spectroscopy Spectroscopy thatuses lasers as a radiation source. Laserspectroscopy is particularly importantwhen an intense beam of monochromaticradiation is needed, as in the RAMAN EF-FECT. Pulsed-laser techniques are also used(see femtochemistry).
latent heat /lay-tĕnt/ The heat evolved orabsorbed when a substance changes itsphysical state, e.g. the latent heat of fusionis the heat absorbed when a substancechanges from a solid to a liquid.
lamp black
158
159
lead
laterite /lat-ĕ-rÿt/ A red fine-grained typeof clay formed in tropical climates by theweathering of igneous rocks. Its colorcomes from the presence of iron(III) hy-droxide.
lattice A regular three-dimensionalarrangement of points. A lattice is used todescribe the positions of the particles(atoms, ions, or molecules) in a crystallinesolid. The lattice structure can be exam-ined by x-ray diffraction techniques.
lattice energy The energy released whenions of opposite charge are brought to-gether from infinity to form one mole of agiven crystal. The lattice energy is a mea-sure of the stability of a solid ionic sub-stance, with respect to ions in the gas. Seealso Born–Haber cycle.
laughing gas See dinitrogen oxide.
lauric acid /lô-rik/ See dodecanoic acid.
law of conservation of energy See con-servation of energy; law of.
law of conservation of mass See con-servation of mass; law of.
law of constant composition See con-stant proportions; law of.
law of constant heat summation SeeHess’s law.
law of constant proportions See con-stant proportions; law of.
law of definite proportions See definiteproportions; law of.
law of equivalent proportions Seeequivalent proportions, law of.
law of isomorphism See Mitscherlich’slaw.
law of mass action See mass action; lawof.
law of octaves See Newlands’ law.
law of reciprocal proportions Seeequivalent proportions, law of.
lawrencium /lor-en-see-ŭm/ A radioactivetransuranic element of the actinoid series,not found naturally on Earth. Several veryshort-lived isotopes have been synthesizedby bombarding 252Cf with boron nuclei or249Bk with 18O nuclei.
Symbol: Lr; p.n. 103; most stable iso-tope 262Lr (half-life 261 minutes).
laws of chemical combination Seechemical combination; laws of.
lazurite /laz-yŭ-rÿt/ An uncommon typi-cally azure blue mineral consisting of a sil-icate of sodium, calcium, and aluminumwith some sulfur. It and its parent rock,lapis lazuli, are widely used for ornamentsand as semiprecious gemstones. Lazuritewas the original source of the pigment ul-tramarine.
leaching The washing out of a solublematerial from an insoluble solid using asolvent. This is often carried out in batchtanks or by dispersing the crushed solid ina liquid.
lead /led/ A dense, dull, gray, soft metallicelement; the fifth member of group 14 (for-merly VA) of the periodic table and the endproduct of radioactive decay series. It oc-curs in small quantities in a wide variety ofminerals but only a few are economicallyimportant. The most important one isgalena (PbS), found in Australia, Mexico,the USA, and Canada. Other minerals areanglesite (PbSO4), litharge (PbO), andcerussite (PbCO3).
Galena is often associated with zincores and the smelting operations of bothlead and zinc industries are closely inte-grated. The ore is concentrated by frothflotation and the concentrate roasted thenreduced,
2PbS + 3O2 → 2PbO + 2SO2PbS + 2O2 → PbSO4
2PbO + 2C → 2Pb + 2CO2PbO + PbS → 3Pb + SO2
PbSO4 + 2C → Pb + 2CO + SO2
Silver is often recovered in economic quan-tities from crude lead.
The outer s2p2 configurations of tin andlead give rise to similar properties for thetwo metals. There is however a muchgreater predominance of the divalent statefor lead. Both oxides are amphoteric,lead(II) oxide (PbO) leading to plumbitesand lead(IV) oxide (PbO2) to plumbates ondissolution in alkalis. Lead also formsmixed oxides Pb2O3 (a yellow solid, betterwritten as Pb(II)Pb(IV)O3) and Pb3O4 (redlead, Pb2(II)Pb(IV)O4). Both metals havelow melting points and neither is attackedby dilute acids; they differ however in theirreaction with concentrated nitric acid
3Pb + 8HNO3 → 3Pb(NO3)2 + 2NO +4H2O
Tin gives hydrated Sn(IV) oxide. Concen-trated hydrochloric acid will not attacklead.
Lead, like tin, forms halides, PbX2, withall halogens. PbCl4 is the only knownlead(IV) halide (all halides of SnX4 areknown). The great stability of the Pb(II)state leads to Pb(IV) compounds beingpowerful oxidizing agents.
Lead is used in accumulators(lead–acid), alloys, radiation shielding, andwater and sound proofing. It is also used inthe petrochemical, paint, and glass indus-tries.
Symbol: Pb; m.p. 327.5°C; b.p.1830°C; r.d. 11.35 (20°C); p.n. 82; r.a.m.207.2.
lead(II) acetate See lead(II) ethanoate.
lead–acid accumulator A type of electri-cal accumulator used in vehicle batteries. Ithas two sets of plates: spongy lead platesconnected in series to the negative terminaland lead(IV) oxide plates connected to thepositive terminal. The material of the elec-trodes is held in a hard lead-alloy grid. Theplates are interleaved. The electrolyte is di-lute sulfuric acid.
The e.m.f. when fully charged is about2.2 V. This falls to a steady 2 V when cur-rent is drawn. As the accumulator begins torun down, the e.m.f. falls further. Duringdischarge the electrolyte becomes more di-lute and its relative density falls. To
recharge the accumulator, charge is passedthrough it in the opposite direction to thedirection of current supply. This reversesthe cell reactions and increases the relativedensity of the electrolyte (c. 1.25 for a fullycharged accumulator).
The electrolyte contains hydrogen ions(H+) and sulfate ions (SO4
2–). During dis-charge, H+ ions react with the lead(IV)oxide to give lead(II) oxide and water
PbO2 + 2H+ + 2e– → PbO + H2OThis reaction takes electrons from theplate, causing the positive charge. There isa further reaction to yield soft lead sulfate:
PbO + SO42– + 2H+ → PbSO4 + H2O +
2e–
Electrons are released to the electrode, pro-ducing the negative charge. During charg-ing the reactions are reversed:
PbSO4 + 2e– → Pb + SO42–
PbSO4 + 2H2O → PbO2 + 4H+ + SO42–
+ 2e–
lead(II) carbonate (PbCO3) A white poi-sonous powder that occurs naturally as themineral cerussite. It forms rhombic crystalsand can be precipitated by reacting a coldaqueous solution of a soluble lead salt (e.g.lead(II) nitrate) with ammonium carbon-ate.
lead(II) carbonate hydroxide (whitelead; 2PbCO3.Pb(OH)2) The most impor-tant basic lead carbonate. It can be manu-factured electrolytically. It has been used asa pigment in white and colored paints butit has the considerable drawback that it ispoisonous.
lead-chamber process A former processfor the manufacture of sulfuric acid by ox-idizing sulfur(IV) oxide with nitrogenmonoxide. The reaction, which was car-ried out in large lead chambers, has nowbeen replaced by the contact process.
lead dioxide /dÿ-oks-ÿd, -id/ See lead(IV)oxide.
lead(II) ethanoate (lead(II) acetate;Pb(CH3CO2)2) A compound usually oc-curring as the hydrate Pb(CH3CO2)2.3H2O, forming monoclinic crystals. At
lead(II) acetate
160
100°C it loses ethanoic acid and water,forming a basic lead(II) ethanoate. Its solu-bility in water is 50 g per 100 g of water at25°C. Lead(II) ethanoate can be obtainedas an anhydrous salt. Its chief asset is thatit is one of the few common lead(II) saltsthat are soluble in water.
lead-free fuel Vehicle fuel that containsnone of the anti-knock agent leadtetraethyl. See lead tetraethyl.
lead(IV) hydride See plumbane.
lead monoxide /mon-oks-ÿd, -id/ Seelead(II) oxide.
lead(II) oxide (lead monoxide; PbO) Ayellow crystalline powder formed by roast-ing molten lead in air. Litharge, the mostcommon form, is obtained when lead(II)oxide is heated above its melting point. Ifprepared below its melting point, anotherform, called massicot, is obtained. Lithargeis used in the rubber industry, in the man-ufacture of paints and varnishes, and in themanufacture of lead glazes for pottery.
lead(IV) oxide (lead dioxide; PbO2) Adark-brown solid forming hexagonal crys-tals. On heating to 310°C it decomposesinto lead(II) oxide and oxygen. It can beprepared electrolytically or by reactinglead(II) oxide with potassium chlorate.Lead(IV) oxide has been used in the manu-facture of matches.
lead(II) sulfate (PbSO4) A white crys-talline solid that occurs naturally as themineral anglesite. It is almost insoluble inwater and can be precipitated by reactingan aqueous solution containing sulfate ionswith a solution of a soluble lead(II) salt(e.g. lead(II) ethanoate). It forms basiclead(II) sulfates when shaken together withlead(II) hydroxide and water. Basic lead(II)sulfates are useful as pigments.
lead(II) sulfide (PbS) A black solid thatoccurs naturally as the mineral galena. Itcan be prepared as a precipitate by reactinga solution of a soluble lead(II) salt with hy-drogen sulfide or with a solution of a solu-
ble sulfide. Lead(II) sulfide is useful as arectifier in electrical components.
lead tetraethyl /tet-ră-eth-ăl/ (tetraethyllead; Pb(CH2CH3)4) A poisonous liquidthat is insoluble in water but soluble in or-ganic solvents. It is manufactured by the re-action of an alloy of sodium and lead with1-chloroethane. The product is obtainedby steam distillation. Lead tetraethyl isused as an additive in internal-combustionengine fuel to increase its octane numberand thus prevent preignition (knocking).
leaving group The group leaving a mol-ecule in a substitution or elimination reac-tion. The nature of the leaving group isoften an important factor in the progress ofthe reaction.
Leblanc process /lă-blahnk/ An obsoleteprocess for the manufacture of sodium car-bonate. Sodium chloride is converted tosodium sulfate by heating with sulfuricacid. This sulfate is then roasted in a rotaryfurnace where it is first reduced to the sul-fide using carbon, and then immediatelyconverted to the carbonate by the action oflimestone. Sodium carbonate solution isobtained by leaching with water and this issubsequently dried and calcined to obtainthe solid. The process is named for theFrench physician Nicolas Leblanc(1742–1806).
Le Chatelier’s principle /lă-sha-tel-yayz/If a system is at equilibrium and a change ismade in the conditions, the equilibrium ad-justs so as to oppose the change. The prin-ciple can be applied to the effect oftemperature and pressure on chemical re-actions. A good example is the Haberprocess for synthesis of ammonia:
N2 + 3H2 ˆ 2NH3The ‘forward’ reaction
N2 + 3H2 → 2NH3is exothermic. Thus, reducing the tempera-ture displaces the equilibrium towards pro-duction of NH3 (as this tends to increasetemperature). Increasing the pressure alsofavors formation of NH3, because thisleads to a reduction in the total number ofmolecules (and hence pressure). The
161
Le Chatelier’s principle
Leclanché cell
162
process is named for the French chemistHenri Louis Le Chatelier (1850–1936).
Leclanché cell /lă-klahn-shay/ A primaryvoltaic cell consisting, in its ‘wet’ form, ofa carbon-rod anode and a zinc cathode,with a 10–20% solution of ammoniumchloride as electrolyte. Manganese(IV)oxide mixed with crushed carbon in aporous bag or pot surrounding the anodeacts as a depolarizing agent. The dry form(dry cell) is widely used for flashlight bat-teries, transistor radios, etc. It has a mix-ture of ammonium chloride, zinc chloride,flour, and gum forming an electrolytepaste. Sometimes the dry cell is arranged inlayers to form a rectangular battery, whichhas a longer life than the cylinder type. Thecell is named for the French engineer andinventor Georges Leclanché (1839–82).
LEED (low-energy electron diffraction) Amethod of electron diffraction that is usedto investigate surfaces. A surface is bom-barded with a beam of low-energy elec-trons, with the diffracted electrons hittinga fluorescent screen or other detector. Theelectron beam has a low energy so that itinteracts with the surface rather than thebulk of the material. The technique givesuseful information about the structure ofsurfaces and processes occurring at sur-faces.
leucine /loo-seen/ See amino acids.
levo-form /lee-vo-form/ See optical activ-ity.
levorotatory /lee-voh-roh-tă-tor-ee. -toh-ree/ See optical activity.
Lewis acid /loo-is/ A substance that canaccept an electron pair to form a coordi-nate bond; a Lewis base is an electron-pairdonor. In this model, the neutralization re-action is seen as the acquisition of a stableoctet by the acid, for example:
Cl3B + :NH3 → Cl3B:NH3Metal ions in coordination compounds
are also electron-pair acceptors and there-fore Lewis acids. The definition includesthe ‘traditional’ Brønsted acids since H+ is
an electron acceptor, but in common usagethe terms Lewis acid and Lewis base are re-served for systems without acidic hydrogenatoms. The acid is named for the Americanphysical chemist Gilbert Newton Lewis(1875–1946).
Lewis base See Lewis acid.
Lewis octet theory See octet.
Lewis structure (Lewis formula) A two-dimensional depiction, by means of chemi-cal element and electron dot symbols, ofthe possible structure of a molecule or ion,showing each atom in relation to its neigh-bors, the bonds that hold the atoms to-gether, and the lone pairs in each atom’souter shell.
l-form See optical activity.
L-form See optical activity.
Liebig condenser /lee-big/ A simple typeof laboratory condenser. It consists of astraight glass tube, in which the vapor iscondensed, with a surrounding glass jacketthrough which cooling water flows. Thecondenser is named for the Germanchemist Justus von Liebig (1803–73).
ligand /lig-ănd/ A molecule or ion thatforms a coordinate bond to a metal atomor ion in a COMPLEX. See also chelate.
ligand-field theory See crystal-field the-ory.
light (visible radiation) A form of electro-magnetic radiation able to be detected bythe human eye. Its wavelength range is be-tween about 400 nm (far red) and about700 nm (far violet). The boundaries are notprecise as individuals vary in their ability todetect extreme wavelengths; this abilityalso declines with age.
lignite /lig-nÿt/ (brown coal) The poorestgrade of coal, containing up to 45% car-bon and with a high moisture content.
ligroin /lig-roh-in/ A mixture of hydrocar-
bons obtained from PETROLEUM, used as ageneral solvent. It has a boiling range of 80to 120°C.
lime See calcium oxide; calcium hydrox-ide.
limestone A natural form of calcium car-bonate. It is used in making calcium com-pounds, carbon dioxide, and cement.
lime water A solution of calcium hydrox-ide in water. If carbon dioxide is bubbledthrough lime water a milky precipitate ofcalcium carbonate forms. Prolonged bub-bling of carbon dioxide turns the solutionclear again as a result of the formation ofsoluble calcium hydrogencarbonate(Ca(HCO3)2).
limonite /lÿ-mŏ-nÿt/ (bog iron ore) Adark-colored mineral that consists mainlyof the hydroxide and oxides of iron, amajor iron ore. It is also used as a pigment.
Linde process /lin-dĕ/ A method of lique-fying gases by compression followed by ex-pansion through a nozzle. The process isused for producing liquid oxygen and ni-trogen by liquefying air and then fraction-ating it. It is named for the Germanengineer Karle von Linde (1842–1934).
line defect See defect.
line spectrum A SPECTRUM composed of anumber of discrete lines corresponding tosingle wavelengths of emitted or absorbedradiation. Line spectra are produced byatoms or simple (monatomic) ions in gases.Each line corresponds to a change in elec-tron orbit, with emission or absorption ofradiation.
linkage isomerism See isomerism.
linoleic acid /lin-ŏ-lee-ik, lă-noh-lee-ik/(C17H31COOH) An unsaturated car-boxylic acid that occurs in LINSEED OIL andother plant oils. It is used in making paintsand varnishes.
linolenic acid /lin-ŏ-lee-nik, -len-ik/(C17H29COOH) A liquid unsaturated car-boxylic acid that occurs in LINSEED OIL andother plant oils. It contains three doublebonds.
linseed oil An oil extracted from the seedsof flax (linseed). It hardens on exposure toair (it is a drying oil) because it containsLINOLEIC ACID and LINOLENIC ACID, and isused in enamels, paints, putty, and var-nishes.
lipid /lip-id/ A collective term used to de-scribe a group of substances in cells char-acterized by their solubility in organicsolvents such as ether and benzene, andtheir low solubility in water. The group israther heterogeneous in terms of bothfunction and structure. The simple lipidsinclude neutral lipids or glycerides, whichare esters of glycerol and fatty acids, andthe waxes, which are esters of long-chainmonohydric alcohols and fatty acids.
liquefied natural gas /lik-wĕ-fÿd/ (LNG)Liquid METHANE, obtained from NATURAL
GAS and used as a fuel.
liquefied petroleum gas (LPG) A mix-ture of liquefied hydrocarbon gases(mainly PROPANE) extracted from PETRO-LEUM, used as a fuel for internal combus-tion engines and for heating.
liquid The state of matter in which theparticles of a substance are loosely boundby intermolecular forces. The weakness ofthese forces permits movement of the par-ticles and consequently liquids can changetheir shape within a fixed volume. The liq-uid state lacks the order of the solid state.Thus, amorphous materials, such as glass,in which the particles are disordered andcan move relative to each other, can beclassed as liquids.
liquid air A pale blue liquid. It is a mix-ture of liquid oxygen (boiling at –183°C)and liquid nitrogen (boiling at –196°C).
liquid crystal A substance that can flowlike a viscous liquid but has a considerable
163
liquid crystal
amount of molecular order. There arethree types of liquid crystal. In a smecticcrystal there are layers of aligned mole-cules, with the long axes of the moleculesbeing perpendicular to the layers. In a cho-lesteric crystal there are also layers ofaligned molecules, but with the axes of themolecules being parallel to the planes ofthe layers. In a nematic crystal the mol-ecules are all aligned in the same directionbut not arranged in layers.
liter /lee-ter/ Symbol: l A unit of volumenow defined as 10–3 meter3; i.e. 1000 cm3.The milliliter (ml) is thus the same as thecubic centimeter (cm3). However, the nameis not recommended for precise measure-ments since the liter was formerly definedas the volume of one kilogram of purewater at 4°C and standard pressure. Onthis definition, one liter is the same as1000.028 cubic centimeters.
litharge /lith-arj/ See lead(II) oxide.
lithia /lith-ee-ă/ See lithium oxide.
lithia water See lithium hydrogencarbon-ate.
lithium /lith-ee-ŭm/ A light silvery moder-ately reactive metal; the first member of thealkali metals (group 1 of the periodictable). It occurs in a number of complex sil-icates, such as spodumene, lepidolite, andpetalite, and a mixed phosphate, tryphilite.It is a rare element accounting for0.0065% of the Earth’s crust. Lithium oresare treated with concentrated sulfuric acidand lithium sulfate subsequently separatedby crystallization. The element can be ob-tained by conversion to the chloride andelectrolysis of the fused chloride or of solu-tions of LiCl in pyridine.
Lithium has the electronic configura-tion 1s22s1 and is entirely monovalent in itschemistry. The lithium ion is, however,much smaller than the ions of the other al-kali metals; consequently it is polarizingand a certain degree of covalence occurs inits bonds. Lithium also has the highest ion-ization potential of the alkali metals.
The element reacts with hydrogen toform lithium hydride, LiH, a colorlesshigh-melting solid, which releases hydro-gen at the anode during electrolysis (con-firming the ionic nature Li+H–). Thecompound reacts with water to release hy-drogen and is also frequently used as a re-ducing agent in organometallic synthesis.Lithium reacts with oxygen to give Li2O(sodium gives the peroxide) and with ni-trogen to form Li3N on fairly gentle warm-ing. The metal itself reacts only slowly withwater, giving the hydroxide (LiOH) butlithium oxide reacts much more vigorouslyto give again the hydroxide; the nitride ishydrolyzed to ammonia. The metal reactswith halogens to form halides (LiX).
Apart from the fluoride the halides arereadily soluble both in water and in oxy-gen-containing organic solvents. In thisproperty lithium partly resembles magne-sium which has a similar charge/size ratio.Compared to the other carbonates ofgroup 1, lithium carbonate is thermally un-stable decomposing to Li2O and CO2. Thisis because the small Li+ ion leads to partic-ularly high lattice energies favouring theformation of Li2O.
Lithium also forms a wide range ofalkyl- and aryl-compounds with organiccompounds, which are particularly usefulin organic synthesis. Lithium compoundsimpart a characteristic purple color toflames.
Symbol: Li; m.p. 180.54°C; b.p.1347°C; r.d. 0.534 (20°C); p.n. 3; r.a.m.6.941.
lithium aluminum hydride See lithiumtetrahydridoaluminate(III).
lithium carbonate (Li2CO3) A whitesolid obtained by the addition of excesssodium carbonate solution to a solution ofa lithium salt. It is soluble in the presenceof excess carbon dioxide, forming lithiumhydrogencarbonate. If heated (to 780°C) ina stream of hydrogen it undergoes thermaldecomposition to give lithium oxide andcarbon dioxide. Lithium carbonate formsmonoclinic crystals and differs from theother group 1 carbonates in being onlysparingly soluble in water. It has a close re-
liter
164
165
localized bond
semblance to the carbonates of magnesiumand calcium – an example of a diagonal re-lationship in the periodic table.
lithium chloride (LiCl) A white solidproduced by dissolving lithium carbonateor oxide in dilute hydrochloric acid andcrystallizing the product. Below 19°C thedihydrate (LiCl.2H2O) is obtained; at19°C the chloride loses one molecule ofwater and becomes anhydrous at 93.5°C.Lithium chloride is used as a flux in weld-ing aluminum. The compound forms cubiccrystals; the anhydrous salt is isomorphouswith sodium chloride. It dissolves in suchorganic solvents as alcohols, ketones, andesters. Lithium chloride is probably themost deliquescent substance known andforms hydrates with 1, 2, and 3 moleculesof water.
lithium hydride (LiH) A white crystallinesolid produced by direct combination ofthe elements at a temperature above500°C. Electrolysis of the fused salt yieldshydrogen at the anode. Lithium hydride re-acts violently with water to give lithiumhydroxide and hydrogen; the reaction ishighly exothermic. The compound is usedas a reducing agent and in the preparationof other hydrides. Lithium hydride has acubic structure and is more stable than thegroup 1 hydrides.
lithium hydrogencarbonate (LiHCO3)A compound known only in solution, pro-duced by the action of carbon dioxide,water, and lithium carbonate. When a so-lution of the hydrogencarbonate is heated,carbon dioxide and lithium carbonate areproduced. Solutions of the hydrogencar-bonate are sold and used in medicine underthe name of lithia water.
lithium hydroxide (LiOH) A white solidmade industrially as the monohydrate(LiOH.H2O) by reacting lime with alithium ore or with a salt made from theore. Lithium hydroxide has a closer resem-blance to the group 2 hydroxides than tothe group 1 hydroxides.
lithium oxide (lithia; Li2O) A white solidproduced by burning metallic lithium in airabove its melting point. It can also be pro-duced by the thermal decomposition of thecarbonate or hydroxide. Lithium oxide re-acts slowly with water forming a solutionof lithium hydroxide; the reaction isexothermic. The compound has a calcium-fluoride structure.
lithium sulfate (Li2SO4) A white solidprepared by the addition of excess lithiumoxide or carbonate to a solution of sulfuricacid. It is readily soluble in water fromwhich it crystallizes as the monohydrate. Incontrast to the other group 1 sulfates itdoes not form alums and it is not isomor-phous with these other sulfates.
lithium tetrahydridoaluminate(III) /tet-ră-hÿ-dri-doh-ă-loo-mă-nit, -nayt/ (lithiumaluminum hydride; LiAlH4) A white solidproduced by action of lithium hydride onaluminum chloride, the hydride being inexcess. Lithium tetrahydridoaluminate re-acts violently with water. It is a powerfulreducing agent, reducing ketones and car-boxylic acids to their corresponding alco-hols. In inorganic chemistry it is used in thepreparation of hydrides.
litmus /lit-mŭs/ A natural pigment thatchanges color when in contact with acidsand alkalis; above a pH of 8.3 it is blue andbelow a pH of 4.5 it is red. Thus it gives arough indication of the acidity or basicityof a solution; because of its rather broadrange of color change it is not used for pre-cise work. Litmus is used both in solutionand as litmus paper.
lixiviation /lik-siv-ee-ay-shŏn/ The pro-cess of separating soluble componentsfrom a mixture by washing them out withwater.
LNG See liquefied natural gas.
localized bond A bond in which the elec-trons contributing to the bond remain be-tween the two atoms concerned, i.e. thebonding orbital is localized. The majority
of bonds are of this type. Compare delo-calized bond.
lodestone /lohd-stohn/ A naturally occur-ring magnetic oxide of iron, a form of MAG-NETITE (Fe3O4). A piece of lodestone is anatural magnet and lodestones were usedas magnetic compasses in ancient times.
lone pair A pair of valence electrons hav-ing opposite spin that are located togetheron one atom, i.e. are not shared as in a co-valent bond. Lone pairs occupy similar po-sitions in space to bond pairs and accountfor the shapes of molecules. A moleculewith a lone pair can donate this to an elec-tron acceptor, such as H+ or a metal ion, toform coordinate bonds. See complex;Lewis base.
long period See period.
lowering of vapor pressure A colliga-tive property of solutions in which thevapor pressure of a solvent is lowered as asolute is introduced. When both solventand solute are volatile the effect of increas-ing the solute concentration is to lower thepartial vapor pressure of each component.When the solute is a solid of negligiblevapor pressure the lowering of the vaporpressure of the solution is directly propor-tional to the number of species introducedrather than to their nature and the propor-tionality constant is regarded as a generalsolvent property. Thus the introduction ofthe same number of moles of any solutecauses the same lowering of vapor pres-sure, if dissociation does not occur. If thesolute dissociates into two species on dis-solution the effect is doubled. The kineticmodel for the lowering of vapor pressuretreats the solute molecules as occupyingpart of the surface of the liquid phase andthereby restricting the escape of solventmolecules. The effect can be used in themeasurement of relative molecular masses,particularly for large molecules, such aspolymers. See also Raoult’s law.
Lowry–Brønsted theory /low-ree bron-sted/ See acid.
LPG See liquefied petroleum gas.
LSD See lysergic acid diethylamide.
lumen /loo-mĕn/ (pl. lumens or lumina)Symbol: lm The SI unit of luminous flux,equal to the luminous flux emitted by apoint source of one candela in a solid angleof one steradian. 1 lm = 1 cd sr.
luminescence /loo-mă-ness-ĕns/ Theemission of radiation from a substance inwhich the particles have absorbed energyand gone into excited states. They then re-turn to lower energy states with the emis-sion of electromagnetic radiation. If theluminescence persists after the source ofexcitation is removed the process is calledphosphorescence: if not, it is called fluores-cence.
lutetium /loo-tee-shee-ŭm/ A silvery ele-ment of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids. Lutetium is a very rare lan-thanoid and has few uses.
Symbol: Lu; m.p. 1663°C; b.p. 3395°C;r.d. 9.84 (25°C); p.n. 71; r.a.m. 174.967.
lux /luks/ (pl. lux) Symbol: lx The SI unitof illumination, equal to the illuminationproduced by a luminous flux of one lumenfalling on a surface of one square meter. 1lx = 1 lm m–2.
Lyman series /lÿ-măn/ See hydrogenatom spectrum. It is named for the Ameri-can physicist Theodore Lyman (1874–1954).
lyophilic /lÿ-ŏ-fil-ik/ Solvent attracting.When the solvent is water, the word hy-drophilic is often used. The terms are ap-plied to: 1. Ions or groups on a molecule. In aque-
ous or other polar solutions ions orpolar groups are lyophilic. For example,the –COO– group on a soap is thelyophilic (hydrophilic) part of the mole-cule.
2. The disperse phase in colloids. Inlyophilic colloids the dispersed particleshave an affinity for the solvent, and the
lodestone
166
colloids are generally stable. Comparelyophobic.
lyophobic /lÿ-ŏ-foh-bik/ Solvent re-pelling. When the solvent is water, theword hydrophobic is used. The terms areapplied to:1. Ions or groups on a molecule. In aque-
ous or other polar solvents, the lyopho-bic group will be non-polar. Forexample, the hydrocarbon group on asoap molecule is the lyophobic (hy-drophobic) part.
2. The disperse phase in colloids. Inlyophobic colloids the dispersed parti-cles are not solvated and the colloid is
easily solvated. Gold and sulfur sols areexamples. Compare lyophilic.
lysergic acid diethylamide /lÿ-ser-jik; dÿ-eth-ăl-am-ÿd/ (LSD) A synthetic organiccompound that has physiological effectssimilar to those produced by alkaloids incertain fungi. Even small quantities, if in-gested, produce hallucinations and ex-treme mental disturbances. The initialsLSD come from the German form of thechemical’s name, Lysergic-Saure-Diathy-lamide.
lysine /lÿ-seen, -sin/ See amino acids.
167
lysine
168
macromolecular crystal /mak-roh-mŏ-lek-yŭ-ler/ A crystal composed of atomsjoined together by covalent bonds, whichform giant three-dimensional or two-dimensional networks. Diamond and sili-cates are examples of macromolecularcrystals.
macromolecule /mak-roh-mol-ĕ-kyool/ Alarge molecule; e.g. a natural or syntheticpolymer.
magic acid See superacid.
magnesia /mag-nee-zhă, -shă/ See magne-sium oxide.
magnesite /mag-nĕ-sÿt/ A mineral form ofmagnesium carbonate, MgCO3.
magnesium /mag-nee-zee-ŭm, -seee-ŭm/A light metallic element; the second mem-ber of group 2 of the periodic table (thealkaline earths). It has the electronic con-figuration of neon with two additionalouter 3s electrons. The element accounts for2.09% of the Earth’s crust and is eighth inorder of abundance. It occurs in a wide va-riety of minerals such as brucite (Mg(OH)2),carnallite (MgCl2.KCl.6H2O), epsomite(MgSO4. 7H2O), magnesite (MgCO3), anddolomite (MgCO3.CaCO3); and also asMgCl2 in sea water. The metal is obtainedby several routes, depending on the mineralused, but all leading to the chloride. This isfollowed by electrolysis of the fused chlo-ride.
The element has a fairly low ionizationpotential and is an electropositive element.It reacts directly with oxygen, nitrogen,and sulfur on heating, to form the oxide,MgO, the nitride, Mg3N2, and the sulfideMgS.
Magnesium halides are also formed bydirect reaction with the halogen. Theyshow a slight deviation in the general be-havior of halides by dissolving normally inwater and only undergoing a significantamount of hydrolysis when solutions areevaporated:
MgX2 + 2H2O → Mg(OH)2 + 2HXMagnesium hydroxide is considerably
less soluble than the hydroxides of theheavier elements in group 2 and is a weakerbase. It forms a soluble sulfate, chlorate,and nitrate. Like the heavier alkalineearths, magnesium forms an insoluble car-bonate, but MgCO3 is the least thermallystable of the group. Unlike calcium, it doesnot form a carbide.
Magnesium metal is industrially impor-tant as a major component in lightweightalloys (with aluminum and zinc). Themetal surfaces develop an imperviousoxide film which protects them from pro-gressive deterioration.
Because of its ionic nature magnesiumforms very few coordination compounds,Donor–acceptor species in aqueous solu-tions are short-lived, but magnesium bro-mide and iodide have sufficient acceptorproperties to dissolve in donor solventssuch as alcohols and ketones. Magnesiumappears to be an essential element for life,occurring in chlorophyll.
Certain specialized organometallic com-pounds of magnesium are important in or-ganic chemistry as synthetic reagents.Magnesium forms Grignard reagents,RMgX, and related di-alkyls, R2Mg, by re-action of the metal directly with the alkylbromide or iodide.
The element crystallizes with the close-packed hexagonal structure.
Symbol: Mg; m.p. 649°C; b.p. 1090°C;r.d. 1.738 (20°C); p.n. 12; r.a.m. 24.3050.
M
magnesium bicarbonate See magnesiumhydrogencarbonate.
magnesium carbonate (MgCO3) Awhite solid that occurs naturally as themineral magnesite and in association withcalcium carbonate in dolomite. Magne-sium carbonate is sparingly soluble inwater but reacts with dilute acids to givethe salt, carbon dioxide, and water. Be-cause of this it is used in medicine as a mildantacid. Magnesium carbonate also de-composes easily to give magnesium oxide,which is an important refractory material.
magnesium chloride (MgCl2) A solidthat exists in a variety of hydrated forms,most commonly as the hexahydrate,MgCl2.6H2O. When heated, this hydratedform is hydrolyzed by its water of crystal-lization, with the evolution of hydrogenchloride and the formation of the oxide:
MgCl2 + H2O → MgO + 2HClThe anhydrous chloride must therefore beprepared by evaporating an aqueous solu-tion in an atmosphere of hydrogen chlo-ride. It is used in the preparation of cottonfabrics.
magnesium hydrogencarbonate (mag-nesium bicarbonate; Mg(HCO3)2) Thesolid hydrogencarbonate is unknown atroom temperature. The compound is pro-duced in solution when water containingcarbon dioxide dissolves magnesium car-bonate:
MgCO3 + H2O + CO2 → Mg(HCO3)2It is a cause of temporary hardness inwater.
magnesium hydroxide (Mg(OH)2) Awhite solid that dissolves sparingly inwater to give an alkaline solution. It can beprepared by adding an alkali to a solublemagnesium compound. Magnesium hy-droxide is used in medicine as an antacid.
magnesium oxide (magnesia; MgO) Awhite solid that occurs naturally as themineral periclase. It can be prepared byheating magnesium in oxygen or by de-composition of magnesium hydroxide, car-bonate, or nitrate. Magnesium oxide is
weakly basic because of the attraction ofthe oxide ions for protons from water mol-ecules:
O2– + H2O → 2OH–
Magnesium oxide melts at 2800°C and iswidely used as a refractory lining for fur-naces.
magnesium peroxide (MgO2) A whiteinsoluble solid prepared by reactingsodium or barium peroxide with a concen-trated solution of a magnesium salt. Mag-nesium peroxide is used as a bleach fordyestuffs and silks.
magnesium sulfate (MgSO4) A solid thatoccurs naturally in many salt deposits incombination with other minerals. One hy-drated form, MgSO4.7H2O, is known asEpsom salt or epsomite. Epsom salt ismade on a commercial scale by reactingmagnesium carbonate with sulfuric acid.Magnesium sulfate is used as a purgativedrug and as an antidote for barium andbarbiturate poisoning. It is also used in thedyeing and sizing of textiles, in syntheticfiber production, and as a source of mag-nesium in fertilizers.
magnetic constant See magnetic perme-ability.
magnetic moment The ratio of thetorque exerted on a magnet or a loop thatcarries an electrical current or on a movingelectrical charge in a magnetic field to thefield strength of that magnetic field. Anelectron has an orbital magnetic momentbecause of the magnetic field generated byits orbital motion round the nucleus and aspin magnetic moment because of its quan-tum mechanical spin. Atomic nuclei canalso have magnetic moments because oftheir spin. The existence of electron andnuclear magnetic moments is made use ofin various branches of spectroscopy and inmagnetochemistry.
magnetic permeability symbol µ. Aquantity that characterizes the response ofa medium to a magnetic field. The perme-ability of free space, which is denoted µ0and is also called the magnetic constant has
169
magnetic permeability
the value 4π × 10–7 Hm–1 in SI units. Themagnetic permeability of a medium relatesthe magnetic flux density B to the magneticfield strength H by the equation: B = µH.The relative magnetic permeability of amedium is defined by µ = µrµ0.
magnetic quantum number See atom.
magnetic separation A method of sepa-rating crushed mineral mixtures using themagnetic properties that a component maypossess, such as ferromagnetism or dia-magnetism.
magnetic susceptibility Symbol χ. A di-mensionless quantity, related to the MAG-NETIC PERMEABILITY of a medium, thatcharacterizes the magnetic nature of thatmedium. The relation between the mag-netic susceptibility and the relative mag-netic permeability µr is given by χ = µr
–1. Ifthe material is diamagnetic χ has a smallnegative value. If the material is paramag-netic χ has a small positive value. If the ma-terial is ferromagnetic χ has a large positivevalue.
magnetism /mag-nĕ-tiz-ăm/ The study ofthe nature and cause of magnetic forcefields, and how different substances are af-fected by them. Magnetic fields are pro-duced by moving charge – on a large scale(as with a current in a coil, forming an elec-tromagnet), or on the small scale of themoving charges in the atoms. It is generallyassumed that the Earth’s magnetism andthat of other planets, stars, and galaxieshave the same cause.
Substances may be classified on thebasis of how samples interact with fields.Different types of magnetic behavior resultfrom the type of atom. Diamagnetism,which is common to all substances, is dueto the orbital motion of electrons. Para-magnetism is due to electron spin, and aproperty of materials containing unpairedelectrons. It is particularly important intransition-metal chemistry, in which thecomplexes often contain unpaired elec-trons. Magnetic measurements can give in-formation about the bonding in thesecomplexes. Ferromagnetism, the strongest
effect, also involves electron spin and thealignment of magnetic moments in do-mains.
magnetite /mag-nĕ-tÿt/ A black mineralform of iron(II)–iron(III) oxide, Fe2O3, amajor type of iron ore. It is also used as aflux in making ceramics. See also lode-stone.
magnetochemistry /mag-nee-toh-kem-iss-tree/ The use of magnetic measurementsto give information about bonding.
magneton /mag-nĕ-ton/ Symbol µ. A unitfor measuring magnetic moments at theatomic and subatomic scales. The Bohrmagneton µB is given by µB = eh/4πme,where e and m are the charge and mass ofan electron respectively, and h is the Planckconstant. The Bohr magneton is the small-est unit the orbital magnetic moment of anelectron in an atom can have. The nuclearmagneton µN is given by replacing the massof the electron by the mass of the protonand is therefore about 1840 times smallerthan the Bohr magneton.
malachite /mal-ă-kÿt/ A green mineralconsisting of copper(II) carbonate and hy-droxide (CuCO3.Cu(OH)2). It is used as anore and a pigment.
maleic acid /mă-lee-ik/ See butenedioicacid.
malic acid /mal-ik/ (2-hydroxybutane-dioic acid; HCOOCH2CH(OH)COOH) Acolorless crystalline carboxylic acid that isfound in unripe fruits. It tastes of applesand is used in food flavorings.
malonic acid /mă-lon-ik/ See propane-dioic acid.
maltose /môl-tohs/ (C12H22O11) A sugarfound in germinating cereal seeds. It is adisaccharide composed of two glucoseunits. Maltose is an important intermedi-ate in the enzyme hydrolysis of starch. It isfurther hydrolyzed to glucose.
magnetic quantum number
170
171
mannose
manganate(VI) /mang-gă-nayt/ A saltcontaining the ion MnO4
2–.
manganate(VII) (permanganate) A saltcontaining the ion MnO4
–.Manganate(VII) salts are purple, andstrong oxidizing agents. In basic solutionsthe dark green manganate(VI) ion isformed.
manganese /mang-gă-neez, -nees/ A tran-sition metal occurring naturally as oxides,e.g. pyrolusite (MnO2). Nodules found onthe ocean floor are about 25% manganese.Its main use is in alloy steels made byadding pyrolusite to iron ore in an electricfurnace. Manganese decomposes coldwater and dilute acids to give hydrogenand reacts with oxygen and nitrogen whenheated. The oxidation states are +7, +6, +4,and (the most stable) +2. Manganese(II)salts are pale pink and with alkali the solu-tions precipitate manganese(II) hydroxide,which rapidly oxidizes in air to brownmanganese(III) oxide.
Symbol: Mn; m.p. 1244°C; b.p.1962°C; r.d. 7.44 (20%C); p.n. 25; r.a.m.54.93805.
manganese dioxide /dÿ-oks-ÿd, -id/ Seemanganese(IV) oxide.
manganese(II) oxide (manganous oxide;MnO) A green powder prepared by heat-ing manganese(II) carbonate or oxalate inthe absence of air. Alternatively it may beprepared by heating the higher manganeseoxides in a stream of hydrogen. Man-ganese(II) oxide is a basic oxide and is al-most insoluble in water. At hightemperatures it is reduced by hydrogen tomanganese. On exposure to air, man-ganese(II) oxide rapidly oxidizes. The com-pound has a crystal lattice similar to that ofsodium(I) chloride.
manganese(III) oxide (manganic oxide;manganese sesquioxide; Mn2O3) A blackpowder obtained by ignitingmanganese(IV) oxide or a manganese(II)salt in air at 800°C. It reacts slowly withcold dilute acids to form manganese(III)salts. Manganese(III) oxide occurs in na-
ture as braunite (3Mn2O3.MnSiO3) andthe monohydrate (Mn2O3.H2O) occurs inthe mineral manganite. The manganese(III)oxide crystal lattice contains Mn3+ and O2–
ions. With concentrated alkalis, it under-goes disproportionation to give man-ganese(II) and manganese(IV) ions.
manganese(IV) oxide (manganese diox-ide; MnO2) A black powder prepared bythe action of heat on manganese(II) nitrate.A hydrated form occurs naturally as themineral pyrolusite. Manganese(IV) oxide isinsoluble in water. It is a powerful oxidiz-ing agent: it reacts with hot concentratedhydrochloric acid to produce chlorine andwith warm sulfuric acid to give oxygen.The dihydrate (MnO2.2H2O) is formedwhen potassium manganate(VII) is re-duced in alkaline solution. It is used as acatalyst in the laboratory preparation ofchlorine, as a depolarizer in electric drycells, and in the glass industry. At500–600°C, manganese(IV) oxide decom-poses to give manganese(III) oxide and tri-manganese tetroxide. It has a goodelectrical conductivity.
manganese sesquioxide /ses-kwee-oks-ÿd, -id/ See manganese(III) oxide.
manganic oxide /man-gan-ik/ See man-ganese(III) oxide.
manganin /mang-gă-nin/ An alloy of cop-per, manganese, and nickel. It has a highelectrical resistivity and is used to make re-sistors.
manganous oxide /mang-gă-nŭs/ Seemanganese(II) oxide.
mannitol /man-ă-tol, -tohl/ (HOCH2-(CHOH)4CH2OH) A soluble hexahydricalcohol that occurs in many plants andfungi. It is used in medicines and as asweetener (particularly in foods for diabet-ics). It is an isomer of sorbitol.
mannose /man-ohs/ (C6H12O6) A simplesugar found in many polysaccharides. It isan aldohexose, isomeric with glucose.
manometer /mă-nom-ĕ-ter/ A device formeasuring pressure. A simple type is a U-shaped glass tube containing mercury orother liquid. The pressure difference be-tween the arms of the tube is indicated bythe difference in heights of the liquid.
marble A dense mineral form of calciumcarbonate (CaCO3), formed from lime-stone.
Markovnikoff’s rule /mar-koff-ni-koffs/A rule that predicts the quantities of theproducts formed when an acid (HA) addsto the double bond in an alkene. If thealkene is not symmetrical two productsmay result; for instance (CH3)2C:CH2 canyield either (CH3)2HCCH2A or(CH3)2ACCH3. The rule states that themajor product will be the one in which thehydrogen atom attaches itself to the carbonatom with the larger number of hydrogenatoms. In the example above, therefore, themajor product is (CH3)2ACCH3.
The Markovnikoff rule is explainable ifthe mechanism is ionic. The first step is ad-dition of H+ to one side of the double bond,forming a carbonium ion. The more stableform of carbonium ion will be the form inwhich the positive charge appears on thecarbon atom with the largest number ofalkyl groups – thus the hydrogen tends toattach itself to the other carbon. The posi-tive charge is partially stabilized by theelectron-releasing (inductive) effect of thealkyl groups.
Additions of this type do not alwaysfollow the Markovnikoff rule. Under cer-tain conditions the reaction may involvethe free radicals H• and A•, in which casethe opposite (anti-Markovnikoff) effect oc-curs. The rule is named for the Russianchemist Vladimir Vasilyevich Markov-nikoff (1838–1904).
marsh gas Methane that is produced inmarshes by decomposing vegetation.
Marsh’s test See arsine.
mass Symbol: m A measure of the quan-tity of matter in an object. Mass is deter-mined in two ways: the inertial mass of a
body determines its tendency to resistchange in motion; the gravitational massdetermines its gravitational attraction forother masses. The SI unit of mass is thekilogram.
mass action, law of At constant temper-ature, the rate of a chemical reaction is di-rectly proportional to the active mass ofthe reactants, the active mass being takenas the concentration. In general, the rate ofreaction decreases steadily as the reactionproceeds; a measure of the concentrationof any of the reactants will give a measureof the rate of reaction.
For the reaction A + B → products, thelaw of mass action states that
rate = k[A][B]where [A] represents the concentration ofA in mol dm–3 and k is a constant depen-dent on the reaction. The term active masscan equal the concentration in mol dm–3
only if there is no interaction or interfer-ence between the reacting molecules. Manysystems exhibit interactions and interfer-ence and consequently the concentrationhas to be multiplied by an activity coeffi-cient in order to obtain the effective activemass. See activity coefficient.
mass-energy equation The equation E =mc2, where E is the total energy (rest-massenergy + kinetic energy + potential energy)of a mass m, c being the speed of light. Theequation is a consequence of Einstein’sSpecial theory of relativity; mass is a formof energy and energy also has mass. Con-version of rest-mass energy into kinetic en-ergy is the source of power in radioactivesubstances and the basis of nuclear-powergeneration.
massicot /mass-ă-kot/ See lead(II) oxide.
mass number See nucleon number.
mass spectrometer An instrument forproducing ions in a gas and analyzing themaccording to their charge/mass ratio. Theearliest experiments by J. J. Thomson useda stream of positive ions from a dischargetube, which were deflected by parallel elec-tric and magnetic fields at right angles to
manometer
172
the beam. Each type of ion formed a para-bolic trace on a photographic plate (a massspectrograph).
In modern instruments, the ions areproduced by ionizing the gas with elec-trons. The positive ions are accelerated outof this ion source into a high-vacuum re-gion. Here, the stream of ions is deflectedand focused by a combination of electricand magnetic fields, which can be varied sothat different types of ion fall on a detector.In this way, the ions can be analyzed ac-cording to their mass, giving a mass spec-trum of the material. Mass spectrometersare used for accurate measurements of rel-ative atomic mass and for analysis of iso-tope abundance. They can also be used toidentify compounds and analyze mixtures.An organic compound bombarded withelectrons forms a number of fragment ions.(Ethane (C2H6), for instance, might formCH+, C2H5
+, CH2+, etc.) The relative pro-
portions of different types of ions is used tofind the structure of new compounds. Thecharacteristic spectrum can also identifycompounds by comparison with standardspectra.
mass spectrum See mass spectrometer.
matrix /may-triks, mat-riks/ (pl. matricesor matrixes) A continuous solid phase inwhich particles of a different solid phaseare embedded.
matte /mat/ A mixture of iron and coppersulfides obtained at an intermediate stagein smelting copper ores.
maxwell /maks-wĕl/ Symbol: Mx A unitof magnetic flux used in the c.g.s. system. Itis equal to 10–8 Wb. The unit is named forthe British physicist James Clerk Maxwell(1831–79).
mechanism A step-by-step description ofthe events taking place in a chemical reac-tion. It is a theoretical framework account-ing for the fate of bonding electrons andillustrates which bonds are broken andwhich are formed. For example, in thechlorination of methane to givechloromethane:
step 1Cl:Cl → 2Cl•
step 2Cl• + CH4 → HCl + CH3•
step 3CH3• + Cl:Cl → CH3Cl + Cl•
mega- Symbol: M A prefix denoting 106.For example, 1 megahertz (MHz) = 106
hertz (Hz).
meitnerium /mÿt-neer-ee-ŭm/ A radioac-tive metallic element not found naturallyon Earth. Only a few atoms of the elementhave ever been detected; it can be made bybombarding a bismuth target with iron nu-clei. The isotope 266Mt has a half-life ofabout 3.4 × 10–3s.
Symbol: Mt; p.n. 109.
melamine /mel-ă-mÿn/ (triaminotriazine;C3N3(NH2)3) A white solid organic com-pound whose molecules consist of a six-membered heterocyclic ring of alternatecarbon and nitrogen atoms with threeamino groups attached to the carbons.Condensation polymerization withmethanal or other aldehydes producesmelamine resins, which are important ther-mosetting plastics.
melting (fusion) The process by which asolid is converted into a liquid by heat orpressure.
melting point The temperature at whicha solid is in equilibrium with its liquidphase at standard pressure and abovewhich the solid melts. This temperature isalways the same for a particular solid. Ion-ically bonded solids generally have muchhigher melting points than those in whichthe forces are covalent or intermolecular.
membrane A thin pliable sheet of tissueor other material acting as a boundary. Themembrane may be either natural (as incells, skin, etc.) or synthetic modificationsof natural materials (cellulose derivativesor rubbers). In many physicochemicalstudies membranes are supported onporous materials, such as porcelain, to pro-
173
membrane
vide mechanical strength. Membranes aregenerally permeable to some degree.
Membranes can be prepared to permitthe passage of other molecules and micro-molecular material. Because of permeabil-ity effects, concentration diffferences at amembrane give rise to a whole range ofmembrane-equilibrium studies, of whichosmosis, dialysis, and ultrafiltration are ex-amples. See also semipermeable mem-brane.
mendelevium /men-dĕ-lee-vee-ŭm/ A ra-dioactive transuranic element of the acti-noid series, not found naturally on Earth.Several short-lived isotopes have been syn-thesized.
Symbol: Md; p.n. 101; most stable iso-tope 258Md (half-life 57 minutes).
Mendius reaction /men-dee-ŭs/ The re-duction of the cyanide group to a primaryamine group using sodium in alcohol:
RCN + 2H2 → RCH2NH2It is a method of increasing the chain lengthof compounds in ascending a homologousseries.
mer See polymer.
mercaptan /mer-kap-tăn/ See thiol.
mercuric chloride /mer-kyoo-rik/ Seemercury(II) chloride.
mercuric oxide See mercury(II) oxide.
mercuric sulfide See mercury(II) sulfide.
mercurous chloride /mer-kyoo-rŭs/ Seemercury(I) chloride.
mercurous oxide See mercury(I) oxide.
mercurous sulfide See mercury(I) sul-fide.
mercury /mer-kyŭ-ree/ A transition metalthat occurs naturally as cinnabar (mer-cury(II) sulfide); small drops of metallicmercury also occur in cinnabar and insome volcanic rocks. The vapor is very poi-sonous. Mercury is used in thermometers,
special amalgams for dentistry, scientificapparatus, and in mercury cells. Mercurycompounds are used as fungicides, timberpreservatives, and detonators.
Symbol: Hg; m.p. –38.87°C; b.p.356.58°C; r.d. 13.546 (20°C); p.n. 80;r.a.m. 200.59. See amalgam; zinc group.
mercury cell A voltaic or electrolytic cellin which one or both of the electrodes con-sists of mercury or an amalgam. Amalgamelectrodes are used in the Daniell cell andthe Weston cadmium cell. Flowing mer-cury electrodes are also used in electrolyticcells (see polarography). In the productionof chlorine, sodium chloride solution iselectrolyzed in a cell with carbon anodesand a flowing mercury cathode. At thecathode, sodium metal is formed, whichforms an amalgam with the mercury.
mercury(I) chloride (mercurous chloride;calomel; Hg2Cl2) A white precipitate pre-pared by adding dilute hydrochloric acid toa mercury(I) salt solution or by sublimingmercury(II) chloride with mercury. Mer-cury(I) chloride is sparingly soluble inwater and is blackened by both ammoniagas, which it absorbs, and by alkalis. It wasformerly used as a purgative.
mercury(II) chloride (mercuric chloride;corrosive sublimate; HgCl2) A colorlesscrystalline compound prepared by directcombination of mercury with cold drychlorine. Mercury(II) chloride is soluble inwater; it also dissolves in concentrated hy-drochloric acid because of the formation ofcomplex ions, HgCl42– and HgCl3–. Onheating, mercury(II) chloride sublimesforming a white translucent mass. It is ex-tremely poisonous, but in dilute solution(1:1000) it is used as an antiseptic.
mercury(I) oxide (mercurous oxide;Hg2O) The black precipitate formed onaddition of sodium hydroxide solution to asolution of mercury(I) nitrate is thought bysome to be mercury(I) oxide; others, whodoubt its existence, think that the black-ness of the precipitate is due to some freemercury. X-ray examination of this black
mendelevium
174
compound has shown it to be an intimatemixture of mercury(II) oxide and mercury.
mercury(II) oxide (mercuric oxide; HgO)A poisonous compound formed as a yellowpowder by the addition of sodium(I) hy-droxide to a solution of mercury(II) nitrateor as a red solid by heating mercury at350°C for a long time. The difference incolor is simply one of particle size. If theoxide is strongly heated it decomposes togive mercury and oxygen.
mercury(I) sulfide (mercurous sulfide;Hg2S) The brownish black precipitateformed when mercury is treated with coldconcentrated sulfuric acid for a long time isthought to be mercury(I) sulfide. Alterna-tively it may be prepared by the action ofhydrogen sulfide or an alkaline sulfide on amercury(I) salt solution. As soon as themercury(I) sulfide has been formed it dis-proportionates to give mercury(II) sulfideand mercury.
mercury(II) sulfide (mercuric sulfide;vermilion; HgS) A compound that occursin nature as the minerals cinnabar (a redsolid) and metacinnabar (a black solid). Itis prepared as a black precipitate by the ac-tion of hydrogen sulfide on a soluble mer-cury(II) salt solution. On heating,mercury(II) sulfide sublimes and becomesred. It is insoluble in dilute hydrochloricand nitric acids but will dissolve in concen-trated nitric acid and aqua regia. Mer-cury(II) sulfide is used as the pigmentvermilion.
meso-form /mess-oh-form/ See optical ac-tivity.
mesomerism /mes-som-ĕ-riz-ăm/ See res-onance.
meta- 1. Designating a benzene com-pound with substituents in the 1,3 posi-tions. The position on a benzene ring thatis two carbon atoms away from a sub-stituent is the meta position. This was usedin the systematic naming of benzene deriv-atives. For example, meta-dinitrobenzene
(or m-dinitrobenzene) is 1,3-dinitroben-zene.2. Certain acids regarded as formed froman anhydride and water are named metaacids to distinguish them from the morehydrated ortho acids. For example, H2SiO3(SiO2 + H2O) is metasilicic acid; H4SiO4(SiO2 + 2H2O) is orthosilicic acid.
See also ortho-; para-.
metabolism /mĕ-tab-ŏ-liz-ăm/ The bio-chemical reactions that take place in cells.The molecules taking part in these reac-tions are termed metabolites. Metabolic re-actions characteristically occur in smallsteps that together make up a metabolicpathway. They involve the breaking downof molecules to provide energy (catabo-lism) and the building up of more complexmolecules and structures from simplermolecules (anabolism).
metabolite /mĕ-tab-ŏ-lÿt/ A substancethat takes part in a metabolic reaction, ei-ther as reactant or product. Some are syn-thesized within the organism itself,whereas others have to be taken in as food.
metaiodic(VII) acid /met-ă-ÿ-od-ik/ Seeiodic(VII) acid.
metal See metals.
metal carbonyl A coordination com-pound formed between a metal and car-bonyl groups. Transition metals formmany such compounds.
metaldehyde /mĕ-tal-dĕ-hÿd/ See ethanal.
metallic bond A bond formed betweenatoms of a metallic element in its zero oxi-dation state and in an array of similaratoms. The outer electrons of each atomare regarded as contributing to an ‘electrongas’, which occupies the whole crystal ofthe metal. It is the attraction of the positiveatomic cores for the negative electron gasthat provides the strength of the metallicbond.
Quantum mechanical treatment of theelectron gas restricts its energy to a series of‘bands’. It is the behavior of electrons in
175
metallic bond
these bands that gives rise to semiconduc-tors.
metallic crystal A crystal formed bymetal atoms in the solid state. Each atomcontributes its valence (outer) electrons toa ‘sea’ of electrons, which are free to mi-grate through the solid, and the ions re-maining are arranged in a lattice. Theability of electrons to move through the lat-tice accounts for the electrical and thermalconductivity of metals.
metallocene /mĕ-tal-ŏ-seen/ A sandwichcompound in which a metal atom or ion iscoordinated to two cyclopentadienyl ions.Ferrocene (Fe(C5H5)2) is the commonestexample.
metalloid /met-ă-loid/ Any of a class ofchemical elements that are intermediate inproperties between metals and non-metals.Examples are germanium, arsenic, and tel-lurium.
There is, in fact, no clear-cut distinctionbetween metals and non-metals. In the pe-riodic table, there is a change from metallicto non-metallic properties across the table,and an increase in metallic propertiesdown a group. Consequently there is a di-agonal around the center of the table (B, Si,As, Te) in which there is a borderline be-tween metals and non-metals, and the met-alloids are the borderline cases. Elementssuch as arsenic, germanium, and telluriumare semiconductors, but other elements areoften said to be metalloids according totheir chemical properties. Tin, for instance,forms salts with acids but also forms stan-nates with alkalis. Its oxide is amphoteric.Note also that tin has metallic (white tin)and non-metallic (gray tin) allotropes.
metalloporphyrin /mĕ-tal-oh-por-fă-rin/See porphyrins.
metallurgy /met-ă-ler-jee/ The study ofmetals, especially methods of extractingmetals from their ores and the formationand properties of alloys.
metals Any of a class of chemical elementswith certain characteristic properties. In
everyday usage, metals are elements (andalloys) such as iron, aluminum, and cop-per, which are lustrous malleable solids –usually good conductors of heat and elec-tricity. Note that this is not a strict defini-tion – some metals are poor conductors,mercury is a liquid, etc.
In chemistry, metals are distinguishedby their chemical properties, and there aretwo main groups. Reactive metals, such asthe alkali metals and alkaline-earth metals,are electropositive elements. They are highin the electromotive series and tend to formcompounds by losing electrons to give pos-itive ions. They have basic oxides and hy-droxides. This typical metallic behaviordecreases across the periodic table and in-creases down a group in the table.
The other type of metals are the transi-tion elements, which are less reactive, havevariable valences, and tend to form com-plexes. In the solid and liquid states metalshave metallic bonds, formed by positiveions with free electrons. See also metalloid;non-metal.
metastable species /met-ă-stay-băl/ Anexcited state of an atom, ion, or molecule,that has a relatively long lifetime before re-verting to the ground state. Metastablespecies are intermediates in some reactions.
metastable state A condition of a systemor body in which it appears to be in stableequilibrium but, if disturbed, can settleinto a lower energy state. For example, su-percooled water is liquid below 0°C (atstandard pressure). When a small crystal ofice or dust (for example) is introduced,rapid freezing occurs.
metathesis /mĕ-tath-ĕ-sis/ See double de-composition.
meter /mee-ter/ Symbol: m The SI baseunit of length, defined as the distance trav-eled by light in vacuum in 1/(2.99 792 458× 108) second. This definition was adoptedin 1983 to replace the 1967 definition of alength equal to 1 650 763.73 wavelengthsin vacuum corresponding to the transitionbetween the levels 2p10 and 5d5 of the 86Kratom.
metallic crystal
176
methanal /meth-ă-nal/ (formaldehyde;HCOH) A colorless gaseous aldehyde. It ismanufactured by the oxidation ofmethanol (500°C and a silver catalyst):
2CH3OH + O2 → 2HCOH + 2H2OThe compound is used in the manufactureof urea–formaldehyde resins. A solution ofmethanal (40%) in water is called forma-lin. It is extensively used as a preservativefor biological specimens.
If an aqueous solution of methanal isevaporated a polymer – polymethanal – isformed:
–O–CH2–O–CH2–O–CH2–This was formerly called paraformalde-hyde. If methanal is distilled from acidicsolutions a cyclic methanal trimer(C3O3H6) is produced.
methane /meth-ayn/ (CH4) A gaseousalkane. Natural gas is about 99% methaneand this provides an important startingmaterial for the organic-chemicals indus-try. Methane can be chlorinated directly toproduce the more reactive chloromethanes,or it can be ‘reformed’ by partial oxidationor using steam to give mixtures of carbonoxides and hydrogen. Methane is the firstmember of the homologous series of alka-nes.
methanide /meth-ă-nÿd/ See carbide.
methanoate /meth-ă-noh-ayt/ (formate) Asalt or ester of methanoic acid.
methanoic acid /meth-ă-noh-ik/ (formicacid; HCOOH) A liquid carboxylic acidmade by the action of sulfuric acid on
sodium methanoate (NaOOCH). It is astrong reducing agent. Methanoic acid oc-curs naturally in ants and nettles.
methanol /meth-ă-nol, -nohl/ (methyl al-cohol; wood alcohol; CH3OH) A colorlessliquid alcohol, which is used as a solventand in the manufacture of methanal(formaldehyde) for the plastics and drugsindustries. Methanol was originally pro-duced from the distillation of wood. Nowit is manufactured by the catalytic oxida-tion of methane from natural gas.
methionine /meth-ÿ-ŏ-neen, -nin/ Seeamino acids.
methoxy group /meth-oks-ee/ The groupCH3)–.
methyl acetate /meth-ăl/ See methylethanoate.
methyl alcohol See methanol.
methylamine /meth-ăl-ă-meen, -am-een/(CH3NH2) A colorless flammable gas thatsmells like ammonia. It is the simplest pri-mary amine, used for making herbicidesand other organic chemicals.
methylaniline /meth-ăl-an-ă-lin, -lÿn/ Seetoluidine.
methylated spirits /meth-ă-lay-tid/Ethanol to which is added methanol (about9.5%), pyridine (about 0.5%), and a bluedye. The ethanol is denatured in this wayso that it can be sold without excise dutyfor use as a fuel and solvent.
methylation /meth-ă-lay-shŏn/ A reac-tion in which a methyl group (CH3–) is in-troduced in a compound. Friedel–Craftsreactions involving halo-methanes are ex-amples of methylations.
methylbenzene /meth-ăl-ben-zeen, -been-zeen/ (toluene; C6H5CH3) A colorless liq-uid hydrocarbon, similar to benzene bothin structure and properties. As methylben-zene is much less toxic than benzene it ismore widely used, especially as a solvent.
177
methylbenzene
CH2
O
CH2O
H2C
O
Methanal trimer
Large quantities are required for the man-ufacture of TNT (trinitrotoluene).
Methylbenzene is itself produced eitherfrom the fractional distillation of coal taror from methylcyclohexane (a constituentof some crude oils):
C6H11CH3 → C6H5CH3 + 3H2A catalyst of aluminum and molybdenumoxides is employed at high temperaturesand pressures.
methyl bromide See bromomethane.
2-methylbuta-1,3-diene (isoprene;CH2:CH(CH3)CH:CH2) A colorless un-saturated liquid hydrocarbon, which oc-curs in terpenes and natural rubber. It isused to make synthetic rubber.
methyl chloride See chloromethane.
methyl cyanide (acetonitrile; CH3CN) Apleasant-smelling poisonous colorless li-quid organic nitrile. Methyl cyanide is apolar solvent, and is widely used for dis-solving both inorganic and organic com-pounds.
methylene /meth-ă-leen/ See carbene.
methylene group The group :CH2.
methyl ethanoate (methyl acetate;CH3COOCH3) A colorless liquid esterwith a fragrant odor. It is commonly usedas a solvent.
methyl ethyl ketone See butanone.
methyl group The group CH3–.
methyl iodide See iodomethane.
methyl methacrylate /meth-ak-ră-layt/(CH2:CCH3COOCH3) The methyl esterof methacrylic acid. The compound is usedin the manufacture of a number of acrylicpolymers, such as Plexiglas (polymethyl-methacrylate).
methyl orange An acid–base indicatorthat is red in solutions below a pH of 3 andyellow above a pH of 4.4. As the transition
range is clearly on the acid side, methyl or-ange is suitable for the titration of an acidwith a moderately weak base, such assodium carbonate.
methylphenols /meth-ăl-fee-nol, -nohl/(cresols; HOC6H4CH3) Compounds withboth methyl and hydroxyl groups substi-tuted onto the benzene ring. There arethree isomers, with the methyl group in the2–, 3–, and 4– positions, respectively. Amixture of the isomers can be obtainedfrom coal tar. It is used as a germicide(known as Lysol).
methyl red An acid–base indicator that isred in solutions below a pH of 4.2 and yel-low above a pH of 6.3. It is often used forthe same types of titration as methyl or-ange but the transition range of methyl redis nearer neutral (pH7) than that of methylorange. The two molecules are structurallysimilar.
methyl salicylate (oil of wintergreen;methyl 2-hydroxybenzoate; C8H8O3) Themethyl ester of SALICYLIC ACID, which oc-curs in certain plants. It is absorbedthrough the skin and used medicinally torelieve rheumatic symptoms. It is also usedin perfumes and as a flavoring agent in var-ious foods.
metric system A system of units based onthe meter and the kilogram and using mul-tiples and submultiples of 10. SI units,c.g.s. units, and m.k.s. units are all scien-tific metric systems of units.
metric ton See tonne.
mho /moh/ See siemens.
mica /mÿ-kă/ A member of an importantgroup of aluminosilicate minerals thathave a characteristic layered structure. Thethree main types are biotite, lepidolite, andmuscovite, which differ in their content ofother elements (such as potassium, magne-sium, and iron). Mica flakes are used aselectrical insulators, dielectrics, and smallheat-proof windows.
methyl bromide
178
micelle /mi-sell/ An aggregate of mol-ecules in a COLLOID.
micro- Symbol: µ A prefix denoting 10–6.For example, 1 micrometer (µm) = 10–6
meter (m).
micron /mÿ-kron/ Symbol: µ A unit oflength equal to 10–6 meter.
microwaves /mÿ-kroh-wayvz/ A form ofELECTROMAGNETIC RADIATION, ranging inwavelength from about 1 mm (where itmerges with infrared) to about 120 mm(bordering on radio waves). Microwavesare produced by various electronic devicesincluding the klyston; they are often car-ried over short distances in tubes of rectan-gular section called waveguides.
Spectra in the microwave region cangive information on the rotational energylevels of certain molecules.
migration 1. The movement of an atom,group, or double bond from one positionto another in a molecule.2. The movement of ions in an electricfield.
milk of lime See calcium hydroxide.
milk sugar See lactose.
milli- Symbol: m A prefix denoting 10–3.For example, 1 millimeter (mm) = 10–3
meter (m).
millimeter of mercury See mmHg.
mineral A naturally occurring inorganiccompound, usually crystalline and of defi-nite chemical composition; a mineral’sphysical properties are also more or lessconstant. All rocks are composed of mix-tures of individual minerals. See rock.
mineral acid An inorganic acid, espe-cially an acid used commercially in largequantities. Examples are hydrochloric, ni-tric, and sulfuric acids.
mirror image A shape that is identical toanother except that its structure is reversed
as if viewed in a mirror. If an object is notsymmetrical it cannot be superimposed onits mirror image. For example, the lefthand is the mirror image of the right hand.
misch metal /mish/ A pyrophoric alloy ofcerium and other lanthanoids, used inlighter flints.
miscible /miss-ă-băl/ Denoting combina-tions of substances that, when mixed, giverise to only one phase; i.e. substances thatdissolve in each other. See solid solution;solution.
Mitscherlich’s law /mich-er-lik/ (law ofisomorphism) The law stating that sub-stances that crystallize in isomorphousforms (i.e. have identical crystalline formsand form mixed crystals) have similarchemical compositions. The law can beused to indicate the formulae of com-pounds. For instance, the fact thatchromium(III) oxide is isomorphous withFe2O3 and Al2O3 implies that its formula isCr2O3. The law is named for the Germanchemist Eilhard Mitscherlich (1794–1863).
mixed indicator A mixture of two ormore indicators so as to decrease the pHrange or heighten the color change, etc.
mixture Two or more substances forminga system in which there is no chemicalbonding between the two. In homogeneousmixtures (e.g. solutions or mixtures ofgases) the molecules of the substances aremixed, and there is only one phase. In het-erogeneous mixtures (e.g. gunpowder orcertain alloys) different phases can be dis-tinguished. Mixtures differ from chemicalcompounds in that:1. The chemical properties of the compo-
nents of a mixture are the same as thoseof the pure substances.
2. The mixture can be separated by physi-cal means (e.g. distillation or crystalliza-tion) or mechanically.
3. The proportions of the components canvary. Some mixtures (e.g. certain solu-tions) can only vary in proportions be-tween definite limits.
179
mixture
m.k.s. system A system of units based onthe meter, the kilogram, and the second. Itformed the basis for SI units.
mmHg (millimeter of mercury) A formerunit of pressure defined as the pressure thatwill support a column of mercury one mil-limeter high under specified conditions. Itis equal to 133.322 4 Pa, and is almostidentical to the torr.
mobile phase See chromatography.
moiety /moi-ĕ-tee/ A distinct part of amolecule; for example, the sugar moiety ina nucleoside.
molal concentration /moh-lăl/ See con-centration.
molar /moh-ler/ 1. Denoting a physicalquantity divided by the amount of sub-stance. In almost all cases the amount ofsubstance will be in moles. For example,volume (V) divided by the number of moles(n) is molar volume Vm = v/n.2. A molar solution contains one mole ofsolute per cubic decimeter of solvent.
molarity /moh-la-ră-tee/ A measure of theconcentration of solutions based upon thenumber of molecules or ions present,rather than on the mass of solute, in anyparticular volume of solution. The molar-ity (M) is the number of moles of solute inone cubic decimeter (litre). Thus a 0.5Msolution of hydrochloric acid contains 0.5× (1 + 35.5)g HCl per dm3 of solution.
mole Symbol: mol The SI base unit ofamount of substance, defined as theamount of substance that contains as manyelementary entities as there are atoms in0.012 kilogram of 12C. The elementary en-tities may be atoms, molecules, ions, elec-trons, photons, etc., and they must bespecified. The amount of substance is pro-portional to the number of entities, theconstant of proportionality being the Avo-gadro number. One mole contains6.022 045 × 1023 entities. One mole of anelement with relative atomic mass A has a
mass of A grams (this mass was formerlycalled one gram-atom of the element).
molecular beam /mŏ-lek-yŭ-ler/ A beamof molecules (or atoms or ions) at low pres-sure such that the entities in the beam aremoving in the same direction with few in-termolecular collisions. Molecular beamsare used in spectroscopy and in the study ofintermolecular forces, chemical reactions,and surfaces.
molecular crystal /mŏ-lek-yŭ-ler/ A crys-tal in which molecules, as opposed toatoms, occupy lattice points. Examples in-clude iodine and solid carbon dioxide (dryice). Because the forces holding the mole-cules together are weak, molecular crystalshave low melting points. When the mole-cules are small, the crystal structure ap-proximates to a close-packed arrangement.See close packing.
molecular formula The formula of acompound showing the number and typesof the atoms present in a molecule, but notthe arrangement of the atoms. For exam-ple, C2H6O represents the molecular for-mula both of ethanol (C2H5OH) andmethoxymethane (CH3OCH3). The molec-ular formula can be determined only if themolecular weight is known. Compare em-pirical formula; general formula; structuralformula.
molecularity /mŏ-lek-yŭ-la-ră-tee/ Thetotal number of reacting molecules in theindividual steps of a chemical reaction.Thus, a unimolecular step has molecularity1, a bimolecular step 2, etc. Molecularity isalways an integer, whereas the order of areaction need not necessarily be so. Themolecularity of a reaction gives no infor-mation about the mechanism by which ittakes place.
molecular orbital See orbital.
molecular sieve A substance throughwhich molecules of a limited range of sizescan pass, enabling volatile mixtures to beseparated. Zeolites and other metal alu-minum silicates can be manufactured with
m.k.s. system
180
pores of constant dimensions in their mol-ecular structure. When a sample is passedthrough a column packed with granules ofthis material, some of the molecules enterthese pores and become trapped. The re-mainder of the mixture passes through theinterstices in the column. The trapped mol-ecules can be recovered by heating. Molec-ular-sieve chromatography is widely usedin chemistry and biochemistry laborato-ries. A modified form of molecular sieve isused in gel filtration. The sieve is a contin-uous gel made from a polysaccharide. Inthis case, molecules larger than the largestpore size are totally excluded from the col-umn.
molecular spectrum The absorption oremission spectrum that is characteristic ofa molecule. Molecular spectra are usuallyband spectra.
molecular weight See relative molecularmass.
molecule /mol-ĕ-kyool/ A particle formedby the combination of atoms in a whole-number ratio. A molecule of an element(combining atoms are the same, e.g. O2) orof a compound (different combiningatoms, e.g. HCl) retains the properties ofthat element or compound. Thus, anyquantity of a compound is a collection ofmany identical molecules. Molecular sizesare characteristically 10–10 to 10–9 m.
Many molecules of natural products areso large that they are regarded as giantmolecules (macromolecules); they maycontain thousands of atoms and have com-plex structural formulae that require veryadvanced techniques to identify. See alsoformula; relative molecular mass.
mole fraction The number of moles of agiven component in a mixture divided bythe total number of moles present of all thecomponents. The mole fraction of compo-nent A is
nA/(nA + nB + nC + …)where nA is the number of moles of A, etc.
molybdenum /mŏ-lib-dĕ-nŭm/ A transi-tion element that occurs naturally in
molybdenite (MoS2) and wulfenite(PbMoO4). It is used in alloy steels, lampbulbs, and catalysts. The compound am-monium molybdate, dissolved in nitricacid, is used as a test for phosphates(V).Molybdenum sulfide (MoS2) is used in lu-bricants to enhance viscosity.
Symbol: Mo; m.p. 2620°C; b.p.4610°C; r.d. 10.22 (20°C); p.n. 42; r.a.m.95.94.
monatomic /mon-ă-tom-ik/ Denoting amolecule, radical, or ion consisting of onlyone atom. For example, helium is amonatomic gas and H• is a monatomicradical.
Mond process /mohnt/ See nickel.
monobasic acid /mon-oh-bay-sik/ Anacid that has only one active proton, suchas hydrochloric acid. See also dibasic acid.
monochlorobenzene /mon-ŏ-klor-oh-ben-zeen, -klor-oh-ben-zeen, -kloh-roh-/See chlorobenzene.
monoclinic crystal /mon-ŏ-klin-ik/ Seecrystal system.
monohydrate /mon-ŏ-hÿ-drayt/ A saltthat has a single molecule of WATER OF
CRYSTALLIZATION, such as sodium carbon-ate monohydrate, Na2CO3.H2O.
monohydric alcohol /mon-ŏ-hÿ-drik/See alcohol.
monomer /mon-ŏ-mer/ The molecule,group, (or compound) from which a dimer,trimer, or POLYMER is formed.
monosaccharide /mon-ŏ-sak-ă-rÿd, -rid/A sugar that cannot be hydrolyzed to sim-pler carbohydrates of smaller carbon con-tent. Glucose and fructose are examples.
monosodium glutamate /mon-ŏ-soh-dee-ŭm gloo-tă-mayt/ (MSG) A white crys-talline solid compound, made fromsoya-bean protein. It is a sodium salt ofglutamic acid (see amino acid) used as aflavor enhancer, particularly in Chinese
181
monosodium glutamate
cuisine. Monosodium glutamate can causean allergic reaction in people who are ul-trasensitive to it.
monoterpene See terpene.
monotropy /mŏ-not-rŏ-pee/ The exis-tence of a single allotrope of an elementthat is always more stable than the other(s)regardless of temperature; phosphorus, forexample, exhibits monotropy. The phasediagram for a monotropic element showsthat one allotrope always has a lowervapor pressure than the other(s) at all tem-peratures; this is the stable allotrope.
monovalent /mon-ŏ-vay-lĕnt, mŏ-nov-ă-/(univalent) Having a valence of one.
mordant /mor-dănt/ An inorganic com-pound used to fix dye in cloth. The mor-dant (e.g. aluminum hydroxide orchromium salts) is precipitated in the fibersof the cloth, and the dye then absorbs inthe particles.
Moseley’s law Lines in the x-ray spectraof elements have frequencies that dependon the proton number of the element. Fora set of elements, a graph of the square rootof the frequency of x-ray emission againstproton number is a straight line (for spec-tral lines corresponding to the same transi-tion). The law is named for the Britishphysicist Henry Gwyn Jeffreys Moseley(1887–1915).
mother liquor The solution remainingafter the formation of crystals.
MSG See monosodium glutamate.
multicenter bond /mul-ti-sen-trik/ Atwo-electron bond formed by the overlapof orbitals from more than two atoms (usu-ally 3). The bridging in diborane is believedto take place by overlap of an sp3 hybridorbital from each boron atom with the 1sorbital on the hydrogen atom. This multi-center bond is called a two-electron three-center bond. The molecule iselectron-deficient. See also electron-defi-cient compounds.
multidecker sandwich compound Seesandwich compound.
multidentate ligand /mul-ti-den-tayt/ Aligand that possesses at least two sites atwhich it can coordinate.
multiple bond A bond between twoatoms involving more than one pair of elec-trons, e.g. a double bond or a triple bond.This additional bonding arises from over-lap of atomic orbitals that are perpendicu-lar to the inter-nuclear axis and gives riseto an increase in electron density above andbelow the inter-nuclear axis. Such bondsare called pi bonds. If the sigma bond axisis taken as the z-axis (i.e. the inter-nuclearaxis) then overlap of orbitals along the x-axis gives rise to a pi bond in the xz plane.Similarly orbitals on the y-axis form a pibond in the yz plane.
multiple proportions, law of Proposedby Dalton in 1804, the principle that whentwo elements A and B combine to formmore than one compound, the weights of Bthat combine with a fixed weight of A arein small whole-number ratios. For exam-ple, in dinitrogen oxide, N2O, nitrogenmonoxide, NO, and dinitrogen tetroxide,N2O4, the amounts of nitrogen combinedwith a fixed weight of oxygen are in theratio 4:2:1.
multiple-range indicator See universalindicator.
Mumetal /myoo-met-ăl/ (Trademark) Amagnetic alloy containing about 75%nickel, the remainder being iron, copper,and chromium and sometimes molybde-num. It is easily magnetized and demagne-tized, has a high permeability, and is usedin transformer cores and electromechanicalequipment.
muriate /myoor-ee-ayt/ An obsolete namefor a chloride; e.g. muriate of potash (KCl).
mustard gas (2,2′′-dichlorodiethyl sulfide;((CH2ClCH2)2S) A poisonous vesicant gasused as a war gas.
monoterpene
182
mutarotation /myoo-tă-roh-tay-shŏn/ Achange in the optical rotation of a solutionwith time, as when one optical isomer dis-appears and another is formed.
myoglobin /mÿ-oh-gloh-bin/ A globularprotein formed of a heme group and a sin-gle polypeptide chain. It occurs in muscletissue where it acts as an oxygen store.
183
myoglobin
184
nano- Symbol: n A prefix denoting 10–9.For example, 1 nanometer (nm) = 10–9
meter (m).
nanotechnology /nan-oh-tek-nol-ŏ-jee/The technology of devices at the nanome-ter scale. In such small devices the quantummechanical nature of electrons has to betaken into account fully. Instruments suchas the ATOMIC FORCE MICROSCOPE whichcan identify and manipulate individualatoms are very useful in nanotechnology.
nanotubes /nan-ŏ-tewbz/ Tubular struc-tures with diameters of a few nanometers(1nm = 10–9 m). Examples of nanotubesare the carbon structures known as ‘buckytubes’, which have a structure similar tothat of BUCKMINSTERFULLERENE. Interesthas been shown in nanotubes as possiblemicroscopic probes in experiments, assemiconductor materials, and as a compo-nent of composite materials. Nanotubescan also be produced by joining aminoacids to give tubular polypeptide struc-tures.
naphtha /naf-thă, nap-thă/ (solvent naph-tha) An imprecise term for a mixture ofhydrocarbons obtained from coal and pe-troleum. It has a boiling range of70–160°C and is used as a solvent and as araw material for making various other or-ganic chemicals.
naphthalene /naf-thă-leen, nap-thă-leen/(C10H8) A white crystalline solid with adistinctive smell of mothballs. Naphtha-lene is found in both the middle- andheavy-oil fractions of crude oil and is ob-tained by fractional crystallization. It isused in the manufacture of benzene-1,2-di-carboxylic anhydride (phthalic anhydride)
and thence in the production of plasticsand dyes.
The structure of naphthalene is ‘ben-zene-like’, having two six-membered ringsfused together. The reactions are charac-teristic of AROMATIC COMPOUNDS.
nascent hydrogen /nay-sĕnt/ A particu-larly reactive form of hydrogen, which isbelieved to exist briefly between its genera-tion (e.g. by the action of dilute acid onmagnesium) and its appearance as bubblesof normal hydrogen gas. It is thought thatpart of the free energy of the production re-action remains with the hydrogen mol-ecules for a short time. Nascent hydrogenmay be used to produce the hydrides ofphosphorus, arsenic, and antimony, whichare not readily formed from ordinary hy-drogen.
natron /nay-tron/ A naturally occurringform of hydrated SODIUM CARBONATE
N
Representations of naphthalene
(Na2CO3.10H2O), found on the beds ofdried-out soda lakes.
Natta process /nat-ă/ A method for themanufacture of isotactic polypropeneusing Ziegler catalysts. The process isnamed for the Italian chemist Giulio Natta(1903–79). See also addition polymeriza-tion; Ziegler process.
natural abundance See abundance.
natural gas Gas obtained from under-ground deposits and often associated withsources of petroleum. It contains a highproportion of methane and other volatilehydrocarbons.
neighboring-group participation Aneffect in an organic reaction in whichgroups close to the point at which reactionoccurs affect the rate of reaction or stereo-chemistry of the products in some way.
nematic crystal /ni-mat-ik/ See liquidcrystal.
neodymium /nee-ŏ-dim-ee-ŭm/ A toxicsilvery element belonging to the lanthanoidseries of metals. It occurs in associationwith other lanthanoids. Neodymium isused in various alloys, as a catalyst, in com-pound form in carbon-arc searchlights,etc., and in the glass industry.
Symbol: Nd; m.p. 1021°C; b.p.3068°C; r.d. 7.0 (20°C); p.n. 60; r.a.m.144.24.
neon /nee-on/ An inert colorless odorlessmonatomic element of the rare-gas group.Neon forms no compounds. It occurs inminute quantities (0.0018% by volume) inair and is obtained from liquid air. It isused in neon signs and lights, electricalequipment, and gas lasers.
Symbol: Ne; m.p. –248.67°C; b.p.–246.05°C; d. 0.9 kg m–3 (0°C); p.n. 10;r.a.m. 20.18.
neoprene /nee-ŏ-preen/ A type of syn-thetic rubber made by polymerization of 2-chlorobuta-1,2,-diene (H2C:CHCCl:CH2).
It is more resistant to oil, solvents, andtemperature than natural rubbers.
neptunium /nep-tew-nee-ŭm/ A toxic ra-dioactive silvery element of the actinoid se-ries of metals that was the first transuranicelement to be synthesized (1940). Foundon Earth only in minute quantities in ura-nium ores, it is obtained as a by-productfrom uranium fuel elements.
Symbol: Np; m.p. 640°C; b.p. 3902°C;r.d. 20.25 (20°C); p.n. 93; most stable iso-tope 237Np (half-life 2.14 × 106 years).
neutralization The stoichiometric reac-tion of an acid and a base in volumetricanalysis. The neutralization point or endpoint is detected with indicators.
neutral oxide An oxide that forms nei-ther an acid nor a base on reaction withwater. Examples include carbon monoxide(CO), dinitrogen oxide (N2O), and water(H2O). See acidic oxide; amphoteric; basicoxide.
neutron diffraction A method of struc-ture determination that makes use of thequantum mechanical wave nature of neu-trons. Thermal neutrons with average ki-netic energies of about 0.025 eV have awavelength of about 0.1 nanometer, mak-ing them suitable for investigating thestructure of matter at the atomic level.Neutron diffraction is particularly usefulfor identifying the positions of hydrogenatoms. These are difficult to establish usingx-rays since x-rays interact with electrons,and hence are scattered weakly by hydro-gen atoms, which have only one electron.Protons scatter neutrons strongly, meaningthat the positions of protons can readily bedetermined using neutron scattering. Neu-tron diffraction has also been used in in-vestigations of the magnetic ordering insolids because there is an interaction be-tween the magnetic moments of the neu-trons and the magnetic moments of atomsin the sample.
neutron number Symbol: N The numberof neutrons in the nucleus of an atom; i.e.
185
neutron number
the nucleon number (A) minus the protonnumber (Z).
Newlands’ law (law of octaves) The ob-servation that, when the elements arearranged in order of increasing relativeatomic mass (atomic weight), there is asimilarity between members that are eightelements apart (inclusive). For example,lithium has similarities to sodium, beryl-lium to magnesium, and so on. Newlandsdiscovered this relationship in 1863 beforethe announcement of Mendeléev’s famousperiodic law (1869). It is now recognizedthat Newlands’ law arises from there beingeight members in each short period. Thelaw is named for the British chemist JohnAlexander Reina Newlands (1837–98).
newton Symbol: N The SI unit of force,equal to the force needed to accelerate onekilogram by one meter second–2. 1 N = 1 kgm s–2. The unit is named for the Englishphysicist and mathematician Sir IsaacNewton (1642–1727).
Nichrome /nÿ-krohm/ (Trademark) Anyof a group of nickel–chromium–iron al-loys, containing 60–80% nickel and about16% chromium; small amounts of other el-ements, such as carbon or silicon, may beadded. They can withstand very high tem-peratures and their high electrical resistiv-ity makes them suitable for use in heatingelements.
nickel /nik-ăl/ A transition metal that oc-curs naturally as the sulfide and silicate. Itis extracted by the Mond process, which in-volves reduction of nickel oxide using car-bon monoxide followed by the formationand subsequent decomposition of volatilenickel carbonyl. Nickel is used as a catalystin the hydrogenation of alkenes, e.g. mar-garine manufacture, and in coinage alloys.Its main oxidation state is +2 and thesecompounds are usually green.
Symbol: Ni; m.p. 1453°C; b.p. 2732°C;r.d. 8.902 (25°C); p.n. 28; r.a.m. 58.6934.
nickel carbonyl (tetracarbonyl nickel(0);Ni(CO)4) A colorless liquid with a highlytoxic vapor. It is prepared by passing car-
bon monoxide over finely divided nickel ata temperature of approximately 60°C. Theproduct is purified by fractional distilla-tion. Nickel carbonyl decomposes on heat-ing to give pure nickel. It is used as acatalyst and in the preparation of nickel(Mond process).
nickelic oxide See nickel(III) oxide.
nickel–iron accumulator (Edison cell;Nife or NIFE cell) A type of secondary cellin which the electrodes are formed by steelgrids. The positive electrode is impreg-nated with a nickel–nickel hydride mixtureand the negative electrode is impregnatedwith iron oxide. Potassium hydroxide solu-tion forms the electrolyte. The nickel–ironcell is lighter and more durable than thelead accumulator, and it can work withhigher currents. Its e.m.f. is approximately1.3 volts. The cell is also named for theAmerican physicist and inventor ThomasAlva Edison (1847–1931).
nickelous oxide See nickel(II) oxide.
nickel(II) oxide (nickelous oxide; NiO)A pale green powder formed by heatingnickel(II) hydroxide, nitrate, or carbonatein the absence of air. It is a basic oxide dis-solving in dilute acids to give green solu-tions of nickel(II) salts. Nickel(II) oxidemay be reduced by carbon, carbon monox-ide, or hydrogen.
nickel(III) oxide (nickelic oxide; Ni2O3)A black powder formed by the action ofheat on nickel(II) oxide in air. It can also beprepared by igniting nickel(II) nitrate orcarbonate. It exists as the dihydrate,Ni2O3.2H2O.
nickel-silver (German silver) Any of agroup of alloys containing nickel, copper,and zinc in various proportions (but no sil-ver). They are white or silvery in color, canbe highly polished, and have good corro-sion-resistance. They are used, for exam-ple, in silver plating, chromium plating,and enameling.
nielsbohrium /neels-bor-ee-ŭm/ Symbol:
Newlands’ law
186
187
nitro compound
Ns A name formerly suggested for ele-ment-107 (bohrium). See element.
NIFE cell See nickel–iron accumulator.
ninhydrin /nin-hÿ-drin/ A colorless or-ganic compound that gives a blue col-oration with amino acids. It is used as a testfor amino acids, in particular to show thepositions of spots of amino acids in paperchromatography.
niobium /nÿ-oh-bee-ŭm/ A soft silverytransition element used in welding, specialsteels, and nuclear reactor work.
Symbol: Nb; m.p. 2468°C; b.p.4742°C; r.d. 8.570 (20°C); p.n. 41; r.a.m.92.90638.
niter /nÿ-ter/ See potassium nitrate.
nitrate /nÿ-trayt/ A salt or ester of nitricacid.
nitration /nÿ-tray-shŏn/ A reaction intro-ducing the nitro (–NO2) group into an or-ganic compound. Nitration of aromaticcompounds is usually carried out using amixture of concentrated nitric and sulfuricacids, although the precise conditions dif-fer from compound to compound. The at-tacking species is NO2
+ (the nitryl cation),and the reaction is an example of elec-trophilic substitution.
nitric acid /nÿ-trik/ (HNO3) A colorlessfuming corrosive liquid that is a strongacid. Nitric acid can be made in a labora-tory by the distillation of a mixture of analkali metal nitrate and concentrated sulfu-ric acid. Commercially it is prepared by thecatalytic oxidation of ammonia and is sup-plied as concentrated nitric acid, whichcontains 68% of the acid and is often col-ored yellow by dissolved oxides of nitro-gen.
Nitric acid is a strong oxidizing agent.Most metals are converted to their nitrateswith the evolution of oxides of nitrogen(the composition of the mixture of the ox-ides depends on the temperature and on theconcentration of the nitric acid used).Some non-metals (e.g. sulfur and phospho-
rus) react to produce oxyacids. Organicsubstances (e.g. sawdust and ethanol) reactviolently, but the more stable aromaticcompounds, such as benzene and toluene,can be converted to nitro compounds incontrollable reactions.
nitric oxide See nitrogen monoxide.
nitride /nÿ-trÿd/ A compound of nitrogenwith a more electropositive element such asmagnesium or calcium.
nitriding /nÿ-trÿ-ding/ See case hardening.
nitrification /ny-tră-fă-kay-shŏn/ The ac-tion of nitrifying bacteria in convertingammonia and nitrites to nitrates. It is animportant stage in the NITROGEN CYCLE. Seealso nitrogen fixation.
nitrile /nÿ-trăl, -tril, -trÿl/ (cyanide) A typeof organic compound containing the –CNgroup. Nitriles are colorless liquids withpleasant smells. They can be prepared byeither refluxing an organic halogen com-pound with an alcoholic solution of potas-sium cyanide, or by dehydration of anamide with phosphorus(V) oxide. Charac-teristic reactions are hydrolysis (to the car-boxylic acid and ammonia) and reductionwith hydrogen gas (to an amine).
nitrile rubber A copolymer of butadieneand propenonitrile (acrylonitrile)(CH2=CHCN). It is an important rubberdue to its resistance to oil and solvents.
nitrite /nÿ-trÿt/ A salt or ester of nitrousacid.
nitrobenzene /nÿ-troh-ben-zeen, -ben-zeen/ (C6H5NO2) A yellow organic oil ob-tained by refluxing benzene with a mixtureof concentrated nitric and sulfuric acids.The reaction is a typical electrophilic sub-stitution on the benzene ring by the nitrylcation (NO2
+).
nitrocellulose /nÿ-troh-sell-yŭ-lohs/ Seecellulose trinitrate.
nitro compound /nÿ-troh/ A type of or-
ganic compound containing the nitro(–NO2) group attached to an aromaticring. Nitro compounds can be prepared bynitration using a mixture of concentratednitric and sulfuric acids. They can be re-duced to aromatic amines:
RNO2 + 3H2 → RNH2 + 2H2OThey can also undergo further substitutionon the benzene ring. The nitro group di-rects substituents into the 3 position.
nitrogen /nÿ-trŏ-jĕn/ The first element ofgroup 15 of the periodic table; a very elec-tronegative element existing in the uncom-bined state as gaseous diatomic N2molecules. The nitrogen atom has the elec-tronic configuration [He]2s22p3. It is typi-cally non-metallic and its bonding isprimarily by polarized covalent bonds.With electropositive elements the nitrideion N3– may be formed.
Nitrogen accounts for about 78% ofthe atmosphere (by volume) and it also oc-curs as sodium nitrate in various mineraldeposits. It is separated for industrial useby the fractionation of liquid air. Pure ni-trogen is prepared by thermal decomposi-tion of azides;
2NaN3 → 2Na + 3N2The ‘active nitrogen’ gas obtained by
passing an electric discharge through nitro-gen is a mixture of ordinary nitrogen mol-ecules and excited single nitrogen atoms.
Nitrogen is a diatomic molecule, whichis effectively triple-bonded and has a highdissociation energy (940 kJ mol–1). It istherefore inert and it only reacts readilywith lithium and other highly electroposi-tive elements. The direct combination ofnitrogen and hydrogen occurs at elevatedtemperatures and pressures (400–600°C,100 atmospheres) and is the basis of the in-dustrially important Haber process for themanufacture of ammonia.
Nitrogen forms a number of oxides:1. Dinitrogen monoxide (nitrous oxide,
N2O) – a neutral oxide.2. Nitrogen monoxide (nitric oxide, NO) –
a neutral oxide.3. Nitrogen(III) oxide (nitrogen sesquiox-
ide, N2O3) – an unstable oxide that isthe anhydride of nitrous acid.
4. dioxide (NO2), which gives a mixture ofnitrous and nitric acids in water.
5. Dinitrogen tetroxide (N2O4), which is inequilibrium with nitrogen dioxide.
6. Dinitrogen pentoxide (N2O5) – the an-hydride of nitric acid.
7. Nitrogen trioxide (NO3).Nitrogen also forms a number of binary
halides such as NF3, NCl3, and halogenazides such as ClN3. Except for NF3 theseare all very explosive, and even NF3, whichis reputedly stable, has been known to ex-plode.
Transition metals form nitrides that aredifferent from the ionic or covalent ni-trides, the stoichiometries are, for example,ZrN, W2N, Mn4N. They are often called‘interstitial’ nitrides. They are very hardand their formation is the basis of surfacehardening by exposing hot metal to ammo-nia gas (see case hardening).
Nitrogen has two isotopes; 14N, thecommon isotope, and 15N (natural abun-dance 0.366%), which is used as a markerin mass spectrometric studies.
Symbol: N; m.p. –209.86°C; b.p.–195.8°C; d. 1.2506 kg m–3 (0°C); p.n. 7;r.a.m. 14.
nitrogen cycle The circulation of nitro-gen and its compounds in the environment,one of the major natural cycles of an ele-ment. Nitrogen occurs mainly as a gas inthe atmosphere and as nitrates in the soil.Denitrification of these nitrates convertsthem to nitrogen. Plants also take up ni-trates and are eaten by animals; the plantsand animals convert them to proteins,which after the death of the organisms ap-pears as ammonia, which nitrification con-verts back to nitrates. Fixation ofatmospheric nitrogen by bacteria or light-ning also gives rise to nitrates. See also ni-trification; nitrogen fixation.
nitrogen
188
O
O
N
Nitro compound
nitrogen dioxide /dÿ-oks-ÿd, -id/ (NO2)A brown gas produced by the dissociationof dinitrogen tetroxide (with which it is inequilibrium), the dissociation being com-plete at 140°C. Further heating causes dis-sociation to colorless nitrogen monoxideand oxygen:
2NO2(g) = 2NO(g) + O2(g)Nitrogen dioxide can also be made by
the action of heat on metal nitrates (not thenitrates of the alkali metals or some of thealkaline-earth metals).
nitrogen fixation A reaction in which at-mospheric nitrogen is converted into nitro-gen compounds. Nitrogen fixation occursnaturally by certain bacteria (in the soiland in the roots of leguminous plants).Small amounts of nitrogen(II) oxide (NO)are also formed in thunderstorms by directcombination of the elements in the electri-cal discharge. Fixation of nitrogen is im-portant because nitrogen is an essentialelement for plant growth, and vastamounts of nitrogenous fertilizers are alsorequired for agriculture. Former processesfor fixing nitrogen were the Birkeland-Eyde process (reacting N2 and O2 in anelectric arc – the equivalent of the thunder-storm in nature), and the cyanamideprocess (heating calcium dicarbide with ni-trogen to give CaCN2). Now the majormethod is the Haber process for makingammonia.
nitrogen monoxide /mon-oks-ÿd, -id/ (ni-tric oxide; NO) A colorless gas that is in-soluble in water but dissolves in a solutioncontaining iron(II) ions owing to the for-mation of the complex ion (FeNO)2+: thenitrogen monoxide can be released byheating. Nitrogen monoxide is prepared bythe action of nitric acid on copper turnings;the impure product can be purified byusing a solution of iron(II) ions to absorbthe product. Commercially, nitrogenmonoxide is prepared by the catalytic oxi-dation of ammonia or by the direct unionof nitrogen and oxygen in an electric arc.Nitrogen monoxide is the most heat-stableof the oxides of nitrogen, only decompos-ing above 1000°C. At ordinary tempera-
tures it combines immediately with oxygento give nitrogen dioxide:
2NO(g) + O2(g) → 2NO2(g)
nitrogenous base /nÿ-troj-ĕ-nŭs/ See qua-ternary ammonium compound.
nitrogen oxides Any of the various com-pounds formed between nitrogen and oxy-gen. See dinitrogen oxide; nitrogen;nitrogen dioxide; nitrogen monoxide.
nitroglycerine /nÿ-troh-gliss-ĕ-rin, -reen/(glyceryl trinitrate) A highly explosive sub-stance used in dynamite. It is obtained bytreating glycerol (1,2,3-trihydroxy-propane) with a mixture of concentratednitric and sulfuric acids. It is not a nitrocompound, but a nitrate ester
CH2(NO3)CH(NO3)CH2(NO3)
nitro group The group –NO2; the func-tional group of nitro compounds.
nitro ligand See isomerism.
nitronium ion /nÿ-troh-nee-ŭm/ See nitrylion.
nitrophenols /nÿ-troh-fee-nolz, -nohlz/(C6H4(OH)NO2) Organic compoundsformed directly or indirectly by the nitra-tion of phenol. Three isomeric forms arepossible. The 2 and 4 isomers are producedby the direct nitration of phenol and can beseparated by steam distillation, the 2 iso-mer being steam volatile. The 3 isomer isproduced from nitrobenzene by formationof 1,3-dinitrobenzene, conversion to 3-ni-trophenylamine, and thence by diazotiza-tion to 3-nitrophenol.
nitroso ligand See isomerism.
nitrous acid /nÿ-trŭs/ (HNO2) A weakacid known only in solution, obtained byacidifying a solution of a nitrite. It readilydecomposes on warming or shaking to ni-trogen monoxide and nitric acid. The useof nitrous acid is very important in thedyestuffs industry in the diazo reaction: ni-trous acid is liberated by acidifying a solu-tion of a nitrite (usually sodium nitrite) in
189
nitrous acid
the presence of the compound to be diazo-tized. Nitrous acid and the nitrites are nor-mally reducing agents but in certaincircumstances they can behave as oxidizingagents, e.g. with sulfur dioxide and hydro-gen sulfide.
nitrous oxide See dinitrogen oxide.
nitryl ion /nÿ-trăl/ (nitronium ion) Thepositive ion NO2
+. See nitration.
NMR See nuclear magnetic resonance.
nobelium /noh-bel-ee-ŭm/ A radioactivetransuranic element of the actinoid series,not found naturally on Earth. Several veryshort-lived isotopes have been synthesized.
Symbol: No; p.n. 102; most stable iso-tope 259No (half-life 58 minutes).
noble gases See rare gases.
nonbenzenoid aromatic /non-ben-zĕ-noid/ See aromatic compound.
nonessential amino acid See aminoacid.
nonionic detergent /non-ÿ-on-ik/ See de-tergents.
nonlocalized bond See delocalized bond.
nonmetal Any of a class of chemical ele-ments. Non-metals lie in the top right-handregion of the periodic table. They are elec-tronegative elements with a tendency toform covalent compounds or negative ions.They have acidic oxides and hydroxides. Inthe solid state, nonmetals are either cova-lent volatile crystals or macromolecular(giant-covalent) crystals. See also metal;metalloid.
nonpolar compound A compound thathas molecules with no permanent dipolemoment. Examples of non-polar com-pounds are hydrogen, tetrachloromethane,and carbon dioxide.
nonpolar solvent See solvent.
nonstoichiometric compound A chem-ical compound whose molecules containfractional numbers of atoms. For example,titanium(IV) oxide in the form of the min-eral rutile has the chemical formula TiO1.8.
normality The number of gram equiva-lents per cubic decimeter of a given solu-tion.
normal solution A solution that containsone gram equivalent weight per liter of so-lution. Values are designated by the sym-bol N, e.g. 0.2N, N/10, etc. Because thereis not a clear definition of equivalentweight suitable for all reactions, a solutionmay have one value of normality for onereaction and another value in a different re-action. Because of this many workers pre-fer the molar solution notation.
NTP See STP.
nuclear magnetic resonance (NMR) Amethod of investigating nuclear spin. In anexternal magnetic field the nucleus canhave certain quantized energy states, corre-sponding to certain orientations of the spinmagnetic moment. Hydrogen nuclei, forinstance, can have two energy states, andtransitions between the two occur by ab-sorption of radiofrequency radiation. Inchemistry, this is the basis of a spectro-scopic technique for investigating thestructure of molecules. Radiofrequency ra-diation is fed to a sample and the magneticfield is changed slowly. Absorption of theradiation is detected when the differencebetween the nuclear levels corresponds toabsorption of a quantum of radiation. Thisdifference depends slightly on the electronsaround the nucleus – i.e. the position of theatom in the molecule. Thus a different ab-sorption frequency is seen for each type ofhydrogen atom (this is an example of achemical shift). In ethanol, for example,there are three frequencies, correspondingto hydrogen atoms on the CH3, the CH2,and the OH. The intensity of absorptionalso depends on the number of hydrogenatoms (3:2:1). NMR spectroscopy is apowerful method of finding the structuresof organic compounds.
nitrous oxide
190
nuclear magneton See magneton.
nucleic acids Organic acids whose mol-ecules consist of chains of alternating sugarand phosphate units, with nitrogenousbases attached to the sugar units. Theyoccur in the cells of all organisms. In DNAthe sugar is deoxyribose; in RNA it is ri-bose. See DNA; RNA.
nucleon number (mass number) Symbol:A The number of nucleons (protons plusneutrons) in an atomic nucleus.
nucleophile /new-klee-ŏ-fÿl/ An electron-rich ion or molecule that takes part in anorganic reaction. The nucleophile can be anegative ion (Br–, CN–) or a molecule witha lone pair of electrons (NH3, H2O). Thenucleophile attacks positively chargedparts of molecules, which usually arisefrom the presence of an electronegativeatom elsewhere in the molecule. Compareelectrophile.
nucleophilic addition /new-klee-ŏ-fil-ik/A class of reaction involving the additionof a small molecule to the double bond inan unsaturated organic compound. Sincethese reactions are initiated by nucle-ophiles, the unsaturated bond must con-tain an electronegative atom, which createsan electron-deficient area in the molecule.Nucleophilic addition is a characteristic re-action of aldehydes and ketones and isoften followed by the subsequent elimina-tion of a different small molecule, particu-
larly water. The addition of hydrogencyanide to propanone is an example:
CH3COCH3 + HCN →CH3C(CN)(OH)CH3
See also condensation reaction.
nucleophilic reagent A compound thatcontains electron-rich groups of atoms thatcan donate electrons during a chemical re-action. They are generally oxidizingagents. See oxidation.
nucleophilic substitution A reaction in-volving the substitution of an atom orgroup of atoms in an organic compoundwith a nucleophile as the attacking sub-stituent. Since nucleophiles are electron-rich species, nucleophilic substitutionoccurs in compounds in which a stronglyelectronegative atom or group leads to adipolar bond. The electron-deficient centercan then be attacked by the electron-richnucleophile causing the electronegativeatom or group to be displaced. In generalterms:
R–Le + Nu– → R–Nu + Le–
where Nu– represents the incoming nucle-ophile and Le– represents the leavinggroup.
There are two possible mechanisms fornucleophilic substitution. In a SN1 reactionthe molecule first forms a carbonium ion;for example:
RCH2Cl → RCH2+ + Cl–
The nucleophile then attaches itself to thiscarbonium ion:
RCH2+ + OH– → RCH2OH
191
nucleophilic substitution
C Cl
X
Y
Z Y Z
C OH
X
Y
Z
X
Y
Z
C
X
+ Cl–+
HO C
OH– OH–
Nucleophilic substitution: the SN1 reaction
In a SN2 reaction the nucleophile ap-proaches as the other group leaves, form-ing a transition state in which the carbonhas five attached groups.
The preferred mechanism depends onseveral factors:1. The stability of the intermediate in the
SN1 mechanism.2. Steric factors affecting the formation of
the transition state in the SN2 mecha-nism.
3. The solvent in which the reaction oc-curs: polar solvents will stabilize polarintermediates and so favor the SN1mechanism.
In practice, however, both routes or someintermediate mechanism is thought to op-erate in many cases.
See also substitution reaction.
nucleoside /new-klee-ŏ-sÿd/ A moleculeconsisting of a purine or pyrimidine baselinked to a sugar, either ribose or deoxyri-bose. ADENOSINE, CYTIDINE, GUANOSINE,THYMIDINE, and URIDINE are common nu-cleosides.
nucleotide /new-klee-ŏ-tÿd/ The com-pound formed by condensation of a ni-trogenous base (a purine, pyrimidine, orpyridine) with a sugar (ribose or deoxyri-bose) and phosphoric acid. The nucleicacids are polynucleotides (consisting ofchains of many linked nucleotides).
nucleus /new-klee-ŭs/ (pl. nuclei or nucle-
uses) The compact positively charged cen-ter of an atom made up of one or more nu-cleons (protons and neutrons) aroundwhich is a cloud of electrons. The densityof nuclei is about 1015 kg m–3. The numberof protons in the nucleus defines the ele-ment, being its proton number (or atomicnumber). The nucleon number, or atomicmass number, is the sum of the protons andneutrons. The simplest nucleus is that of ahydrogen atom, 1H, being simply one pro-ton (mass 1.67 × 10–27 kg). The most mas-sive naturally occurring nucleus is 238U of92 protons and 146 protons (mass 4 ×10–25 kg, radius 9.54 × 10–15 m). Only cer-tain combinations of protons and neutronsform stable nuclei. Others undergo sponta-neous decay.
A nucleus is depicted by a symbol indi-cating nucleon number (mass number),proton number (atomic number), and ele-ment name. For example, 121
3Na represents anucleus of sodium having 11 protons andmass 23, hence there are (23 – 11) = 12neutrons.
nuclide /new-klÿd/ A nuclear species thatis characterized by a given number of pro-tons and neutrons; for example, 23Na,24Na, and 24Mg are all different nuclides.Thus:
12
13Na has 11 protons and 12 neutrons
12
14Na has 11 protons and 13 neutrons
12
24Mg has 12 protons and 12 neutrons
The term is applied to the nucleus andoften also to the atom.
nucleoside
192
C Cl
X
Y
Z Y Z
X
Y
Z
X
HO C
OH–
+ Cl–
... Cl...HO C–
Nucleophilic substitution: the SN2 reaction
193
nylon
nylon A type of synthetic polymer linkedby amide groups –NH.CO–. Nylon poly-mers can be made by copolymerization of
a molecule containing two amine groupswith one containing two carboxylic acidgroups.
HN
HN
H
H(CH2)6
O
HO(CH2)4C C
OH
O
HN (CH2)6 NH C
O
C
O
(CH2)4 NH (CH2)6
1,6-diaminohexane hexanedioic acid
Formation of nylon
194
occlusion /ŏ-kloo-zhŏn/ 1. The process inwhich small amounts of one substance aretrapped in the crystals of another; for ex-ample, pockets of liquid occluded duringcrystallization from a solution.2. Absorption of a gas by a solid; for ex-ample, the occlusion of hydrogen by palla-dium.
ocher /oh-ker/ A clay mineral containingiron(III) oxide (Fe2O3), used as a yellow,orange, or brown pigment.
octadecanoic acid /ok-tă-dek-ă-noh-ik/(stearic acid; CH3(CH2)16COOH) A solidcarboxylic acid present in fats and oils asthe glyceride.
octadecenoic acid /ok-tă-dek-ă-noh-ik/cis-9-octadecenoic acid (or oleic acid) is anaturally occurring carboxylic acid present(as glycerides) in fats and oils:
CH3(CH2)7CH:CH(CH2)7COOH
octahedral complex /ok-tă-hee-drăl/ Seecomplex.
octahydrate /ok-tă-hÿ-drayt/ A crys-talline compound containing eight mol-ecules of water of crystallization permolecule of compound.
octane /ok-tayn/ (C8H18) A liquid alkaneobtained from the light fraction of crudeoil. Octane and its isomers are the principalconstituents of gasoline, which is obtainedas the refined light fraction from crude oil.See also octane rating.
octane rating (octane number) A ratingfor the performance of gasoline in internal-combustion engines. The octane ratingmeasures the freedom from ‘knocking’ –
i.e. preignition of the fuel in the engine.This depends on the relative proportions ofbranched-chain and straight-chain hydro-carbons present. High proportions ofbranched-chain alkanes are better in high-performance engines. In rating fuels, 2,2,4-trimethylpentane (isooctane) is given avalue of 100 and heptane is given a value 0.The performance of a fuel is comparedwith a mixture of these hydrocarbons.
octavalent /ok-tă-vay-lĕnt/ Having a va-lence of eight.
octaves, law of See Newlands’ law.
octet /ok-tet/ A stable shell of eight elec-trons in an atom. The completion of theoctet gives rise to particular stability andthis is the basis of the Lewis octet theory(named for the American physical chemistGilbert Newton Lewis (1875–1946)), thus:1. The rare gases have complete octets and
are chemically inert2. The bonding in small covalent molecules
is frequently achieved by the centralatom completing its octet by sharingelectrons with other atoms, e.g. CH4.
3. The ions formed by electropositive andelectronegative elements are generallythose with a complete octet, e.g. Na+,Ca2+, O2–, Cl–.
oersted Symbol: Oe A unit of magneticfield strength in the c.g.s. system. It is equalto 103/4π A m–1. The unit is named for theDanish physicist Hans Christian Oersted(1777–1851).
ohm /ohm/ Symbol: Ω The SI unit of elec-trical resistance, equal to a resistance thatpasses a current of one ampere when thereis an electric potential difference of one
O
volt across it. 1 Ω = 1 V A–1. Formerly, itwas defined in terms of the resistance of acolumn of mercury under specified condi-tions. The unit is named for the Germanphysicist Georg Simon Ohm (1787–1854).
oil Any of various viscous liquids. Seeglyceride; petroleum.
oil of vitriol /vit-ree-ŏl/ Sulfuric(VI) acid.
oil of wintergreen See methyl salicylate.
oil shale A sedimentary rock that includesin its structure 30–60% of organic matter,mainly in the form of bitumen. Heated inthe absence of air it produces an oily sub-stance resembling petroleum, which is richin nitrogen and sulfur compounds.
oleate /oh-lee-ayt/ A salt or ester of oleicacid; i.e. an octadecenoate.
olefin /oh-lĕ-fin/ See alkene.
oleic acid /oh-lee-ik/ See octadecenoicacid.
oleum /oh-lee-ŭm/ (pyrosulfuric acid;H2S2O7) A colorless fuming liquid formedby dissolving sulfur(VI) oxide in concen-trated sulfuric acid. It gives sulfuric acid ondiluting with water. See contact process.
oligomer /ŏ-lig-ŏ-mer/ A POLYMER formedfrom a relatively few monomer molecules.
oligosaccharide /ol-ă-goh-sakă-rÿd, -rid/A carbohydrate formed of a small numberof monosaccharide units (up to around20).
olivine /ol-ă-veen, ol-ă-veen/ A greenishrock-forming silicate mineral containingiron and magnesium. Its clear green form isthe gemstone peridot.
one-pot synthesis A synthesis of a chem-ical compound in which the reactants formthe required product in a single reactionmixture.
onium ion /oh-nee-ŭm/ An ion formed byaddition of a proton (H+) to a molecule.The hydronium ion (H3O+) and ammo-nium ion (NH4
+) are examples.
oolite /oh-ŏ-lÿt/ A sedimentary rock, atype of limestone, that contains smallspherical masses of calcium carbonate(CaCO3).
opal /oh-păl/ A naturally occurring hy-drated and noncrystalline form of silica(SiO2), valued as a gemstone. Various lay-ers within a gemstone opal can reflect andrefract light to give a play of spectral col-ors. There are also milky white and blackvarieties.
open-hearth process A method of mak-ing steel by heating pig iron in an open-hearth furnace by burning a mixture of gasand air. Oxygen is injected through pipesto oxidize impurities and burn off carbon,and lime is used to form a slag.
optical activity /op-tă-kăl/ The ability ofcertain compounds to rotate the plane ofpolarization of plane-polarized light whenthe light is passed through them. Opticalactivity can be observed in crystals, gases,liquids, and solutions. The amount of rota-tion depends on the concentration of theactive compound.
Optical activity is caused by the interac-tion of the varying electric field of the lightwith the electrons in the molecule. It occurswhen the molecules are asymmetric – i.e.they have no plane of symmetry. Such mol-ecules have a mirror image that cannot besuperimposed on the original molecule. Inorganic compounds this usually means thatthey contain a carbon atom attached tofour different groups, forming a chiral cen-ter. The two mirror-image forms of anasymmetric molecule are optical isomers.One isomer will rotate the polarized lightin one sense and the other by the sameamount in the opposite sense. Such isomersare described as dextrorotatory or levoro-tatory, according to whether they rotatethe plane to the ‘right’ or ‘left’ respectively(rotation to the left is clockwise to an ob-server viewing the light coming toward the
195
optical activity
optical isomerism
196
observer). Dextrorotatory compounds aregiven the symbol d or (+) and levorotatorycompounds l or (–). A mixture of the twoisomers in equal amounts does not showoptical activity. Such a mixture is some-times called the (±) or dl-form, a racemate,or a racemic mixture
Optical isomers have identical physicalproperties (apart from optical activity) andcannot be separated by fractional crystal-lization or distillation. Their general chem-ical behavior is also the same, althoughthey do differ in reactions involving otheroptical isomers. Many naturally occurringsubstances are optically active (only oneoptical isomer exists naturally) and bio-chemical reactions occur only with the nat-ural isomer. For instance, the natural formof glucose is d-glucose and living organ-isms cannot metabolize the l-form.
The terms ‘dextrorotatory’ and ‘levoro-tatory’ refer to the effect on polarized light.A more common method of distinguishingtwo optical isomers is by their D-form orL-form. This convention (the D–L conven-tion) refers to the absolute structure of the
isomer according to specific rules (i.e. theabsolute configuration of the molecule).Sugars are related to a particular configu-ration of glyceraldehyde (2,3-dihydrox-ypropanal). For alpha amino acids the cornrule is used: the structure of the acidRC(NH2)(COOH) H is drawn with H atthe top; viewed from the top the groupsspell CORN in a clockwise direction for allD-amino acids (i.e. the clockwise order is–COOH,R,NH2). The opposite is true forL-amino acids. Note that this conventiondoes not refer to optical activity: L-alanineis dextrorotatory by D-cystine is levorota-tory.
An alternative is the R–S convention forshowing configuration. There is an orderof priority of attached groups based on theproton number of the attached atom:I, Br, Cl, SO3H, OCOCH3, OCH3, OH,NO2, NH2, COOCH3, CONH2, COCH3,CHO, CH2OH, C6H5, C2H5, CH3, H
Hydrogen has the lowest priority. Thechiral carbon is viewed such that the groupof lowest priority is hidden behind it. If theother three groups are in descending prior-
(S)-lactic acid(R)-lactic acid
COOH
COH
H
CH3
COOH
CH
OH
CH3
Optical isomers of lactic acid
C
HCOOH
OH
H
COOHOH
C
C
HOCOOH
H
C
HOCOOH
H
OH
C
HCOOH
H
COOHOH
C
Isomers of tartaric acid
Optical isomers
ity in a clockwise direction, the compoundis R-. If descending priority is anticlockwiseit is S-.
The existence of a carbon atom boundto four different groups is not the strictcondition for optical activity. The essentialpoint is that the molecule should be asym-metric. Inorganic octahedral complexes,for example, can show optical isomerism.It is also possible for a molecule to containasymmetric carbon atoms and still have a plane of symmetry. One structure oftartaric acid has two parts of the mol-ecule that are mirror images, thus having aplane of symmetry. This (called the meso-form) is not optically active. See also reso-lution).
optical isomerism See isomerism; opticalactivity.
optical rotary dispersion (ORD) Thephenomenon in which the amount of rota-
tion of plane-polarized light by an opticallyactive substance depends on the wave-length of the light. Plots of rotation againstwavelength can be used to give informa-tion about the molecular structure of opti-cally active compounds.
optical rotation Rotation of the plane ofpolarization of plane-polarized light by anoptically active substance.
orbit The path of an electron as it movesaround the nucleus in an atom.
orbital A region around an atomic nu-cleus in which there is a high probability offinding an electron. The modern picture ofthe atom according to quantum mechanicsdoes not have electrons moving in fixed el-liptical orbits. Instead, there is a finiteprobability that the electron will be foundin any small region at all possible distancesfrom the nucleus. In the hydrogen atom the
197
orbital
The corn rule for absolute configuration of alpha amino acids
R–configuration S–configuration
The R/S system
H
C
R
COOH
H2N
1
C
3 2
1
C
2 3
Absolute configuration
probability is low near the nucleus, in-creases to a maximum, and falls off to in-finity. It is useful to think of a region inspace around the nucleus – in the case ofhydrogen the region within a sphere –within which there is a high chance of find-ing the electron. Each of these, called anatomic orbital, corresponds to a sub-shelland can ‘contain’ a single electron or twoelectrons with opposite spins. Another wayof visualizing an orbital is as a cloud ofelectron charge (the average distributionwith time).
Similarly, in molecules the electronsmove in the combined field of the nucleiand can be assigned to molecular orbitals.In considering bonding between atoms it isuseful to treat molecular orbitals as formedby overlap of atomic orbitals.
It is possible to calculate the shapes andenergies of atomic and molecular orbitalsby quantum theory. The shapes of atomicorbitals depend on the orbital angular mo-mentum (the sub-shell). For each shellthere is one s orbital, three p orbitals, fived orbitals, etc. The s orbitals are spherical,the p orbitals each have two lobes; d or-
bitals have more complex shapes, typicallywith four lobes.
Molecular orbitals are formed by over-lap of atomic orbitals, and again there aredifferent types. If the orbital is completelysymmetrical about an axis between the nu-clei, it is a sigma orbital. This can occur,for instance, by overlap of two s orbitals,as in the hydrogen atom, or two p orbitalswith their lobes along the axis. However,two p orbitals overlapping at right anglesto the axis form a different type of molecu-lar orbital – a pi orbital – with regionsabove and below the axis. Pi orbitals arealso formed by overlap of d orbitals. Eachmolecular orbital can contain a pair ofelectrons, forming a sigma bond or pibond. A double bond, for example thebond in ethene, is a combination of a sigmabond and a pi bond.
Hybrid orbitals are atomic orbitalsformed by combinations of s, p, and datomic orbitals, and are useful in describ-ing the bonding in compounds. There arevarious types. In carbon, for instance, theelectron configuration is 1s22s22p2. Car-bon, in its outer (valence) shell, has one
orbital
198
s orbitaldxy orbital
px orbital py orbital pz orbital
z
y
x
z
y
x
z
y
x
z
y
x
z
y
x
z
y
x
dz2 orbital
Orbital: atomic orbitals
199
z
A
B
C
x x
x x
x x
s orbitals sigma
px orbitals
pz orbitals
sigma
pi
z
Orbital: molecular orbitals
one s
one s
one s
+
+
+
ATOMIC ORBITALS HYBRID ORBITALS SHAPE
one p two sp3 linear
two p three sp2 planar
sp
three p tetrahedralfour sp3
sp3
sp2
Orbital: hybrid orbitals
filled s orbital, two filled p orbitals, andone ‘empty’ p orbital. These four orbitalsmay hybridize (sp3 hybridization) to act asfour equal orbitals arranged tetrahedrally,each with one electron. In methane, eachhybrid orbital overlaps with a hydrogen sorbital to form a sigma bond. Alterna-tively, the s and two of the p orbitals mayhybridize (sp2 hybridization) and act asthree orbitals in a plane at 120°. The re-maining p orbital is at right angles to theplane, and can form pi bonds. Finally, sphybridization may occur, giving two or-bitals in a line. More complex types of hy-bridization, involving d orbitals, explainthe geometries of inorganic complexes.
The combination of two atomic orbitalsin fact produces two molecular orbitals.One – the bonding orbital – has a concen-tration of electron density between the nu-clei, and thus tends to hold the atomstogether. The other – the antibonding or-bital – has slightly higher energy and tendsto repel the atoms. If both atomic orbitalsare filled, the two molecular orbitals arealso filled and cancel each other out – thereis no net bonding effect. If each atomic or-bital has one electron, the pair occupies thelower energy bonding orbital, producing anet attraction.
ORD See optical rotary dispersion.
order The sum of the indices of the con-centration terms in the expression that de-termines the rate of a chemical reaction.For example, in the expression
rate = k[A]x[B]y
x is called the order with respect to A, y theorder with respect to B, and (x + y) theorder overall. The values of x and y are notnecessarily equal to the coefficients of Aand B in the molecular equation. Order isan experimentally determined quantity de-rived without reference to any equation ormechanism. Fractional orders do occur.For example, in the reaction
CH3CHO → CH4 + COthe rate is proportional to [CH3CHO]1.5
i.e. it is of order 1.5.
ore A mineral source of a chemical ele-ment.
ore dressing See beneficiation.
organic acid /or-gan-ik/ An organic com-pound that can release hydrogen ions (H+)to a base, such as a CARBOXYLIC ACID or aPHENOL. See acid.
organic base An organic compound thatcan function as a base. Organic bases aretypically amines that gain H+ ions. Seeamine; base.
organic chemistry The chemistry ofcompounds of carbon. Originally the termorganic chemical referred to chemical com-pounds present in living matter, but now itcovers any carbon compound with the ex-ception of certain simple ones, such as thecarbon oxides, carbonates, cyanides, andcyanates. These are generally studied in in-organic chemistry. The vast numbers ofsynthetic and natural organic compoundsexist because of the ability of carbon toform chains of atoms (catenation). Otherelements are involved in organic com-pounds; principally hydrogen and oxygenbut also nitrogen, halogens, sulfur, andphosphorus.
organometallic compound /or-gă-noh-mĕ-tal-ik/ An organic compound contain-ing a carbon–metal bond. Tetraethyl lead,(C2H5)4Pb, is a typical example of anorganometallic compound, used as an ad-ditive in petrol.
ortho- 1. Designating the form of a di-atomic molecule in which both nuclei havethe same spin direction; e.g. orthohydro-gen, orthodeuterium.2. Designating a benzene compound withsubstituents in the 1,2 positions. The posi-tion next to a substituent is the ortho posi-tion on the benzene ring. This was used inthe systematic naming of benzene deriva-tives. For example, orthodinitrobenzene(or o-dinitrobenzene) is 1,2-dinitroben-zene.3. Certain acids, regarded as formed froman anhydride and water, were named orthoacids to distinguish them from the less hy-drated meta acids. For example, H4SiO4(from SiO2 + 2H2O) is orthosilicic acid;
ORD
200
H2SiO3 (SiO2 + H2O) is metasilicic acid.See also meta-; para-.
orthoboric acid /or-thŏ-bô-rik/ See boricacid.
orthohydrogen /or-thoh-hÿ-drŏ-jĕn/ Seehydrogen.
orthophosphoric acid /or-thoh-fos-fô-rik/ See phosphoric(V) acid.
orthophosphorous acid /or-thoh-fos-fŏ-rŭs/ See phosphonic acid.
orthorhombic crystal /or-thŏ-rom-bik/See crystal system.
osmiridium /oz-mă-rid-ee-ŭm/ A natu-rally occurring alloy of osmium and irid-ium, used to make the tips of pen nibs. Seeiridium; osmium.
osmium /oz-mee-ŭm/ A transition metalthat is found associated with platinum. Os-mium is the most dense of all metals. It hasa characteristic smell resulting from theproduction of osmium(VIII) oxide (os-mium tetroxide, OsO4). The metal is usedin catalysts and in alloys for pen nibs, piv-ots, and electrical contacts.
Symbol: Os; m.p. 3054°C; b.p. 5027°C;r.d. 22.59 (20°C); p.n. 76; r.a.m. 190.23.
osmium(IV) oxide (osmium tetroxide;OsO4) A volatile yellow crystalline solidwith a penetrating odor, used as an oxidiz-ing agent and, in aqueous solution, as acatalyst for organic reactions.
osmium tetroxide /te-troks-ÿd/ See os-mium(IV) oxide.
osmosis /oz-moh-sis/ Systems in which asolvent is separated from a solution by asemipermeable membrane approach equi-librium by solvent molecules on the solventside of the membrane migrating through itto the solution side; this process is calledosmosis and always leads to dilution of thesolution. The phenomenon is quantified bymeasurement of the osmotic pressure. Theprocess of osmosis is of fundamental im-
portance in transport and control mecha-nisms in biological systems; for example,plant growth and general cell function.
Osmosis is a colligative property and itstheoretical treatment is similar to that forthe lowering of vapor pressure. The mem-brane can be regarded as equivalent to theliquid-vapour interface, i.e. one that per-mits free movement of solvent moleculesbut restricts the movement of solute mol-ecules. The solute molecules occupy a cer-tain area at the interface and thereforeinhibit solvent egress from the solution.Just as the development of a vapor pressurein a closed system is necessary forliquid–vapour equilibrium, the develop-ment of an OSMOTIC PRESSURE on the solu-tion side is necessary for equilibrium at themembrane.
osmotic pressure Symbol: π The pressurethat must be exerted on a solution to pre-vent the passage of solvent molecules intoit when the solvent and solution are sepa-rated by a semipermeable membrane. Theosmotic pressure is therefore the pressurerequired to maintain equilibrium betweenthe passage of solvent molecules throughthe membrane in either direction and thusprevent the process of osmosis proceeding.The osmotic pressure can be measured byplacing the solution, contained in a smallperforated thimble covered by a semiper-meable membrane and fitted with a lengthof glass tubing, in a beaker of the pure sol-vent. Solvent molecules pass through themembrane, diluting the solution andthereby increasing the volume on the solu-tion side and forcing the solution to rise upthe glass tubing. The process continuesuntil the pressure exerted by the solventmolecules on the membrane is balanced bythe hydrostatic pressure of the solution inthe tubing. A sample of the solution is thenremoved and its concentration determined.Osmosis is a colligative property; thereforethe method can be applied to the determi-nation of relative molecular masses, partic-ularly for large molecules, such as proteins,but it is restricted by the difficulty ofpreparing good semipermeable mem-branes.
201
osmotic pressure
As the osmotic pressure is a colligativeproperty it is directly proportional to themolar concentration of the solute if thetemperature remains constant; thus π isproportional to the concentration n/V,where n is the number of moles of solute,and V the solvent volume. The osmoticpressure is also proportional to the ab-solute temperature. Combining these twoproportionalities gives πV = nCT, whichhas the same form as the gas equation, PV= nRT, and experimental values of C aresimilar to those for R, the universal gasconstant. This gives considerable supportto the kinetic theory of colligative proper-ties.
Ostwald’s dilution law /ost-valts/ Seedissociation constant.
oxalate /oks-ă-layt/ A salt or ester ofethanedioic acid (oxalic acid). See ethane-dioic acid.
oxalic acid /oks-al-ik/ See ethanedioicacid.
oxidant /oks-ă-dănt/ An oxidizing agent.In rocket fuels, the oxidant is the substancethat provides the oxygen for combustion(e.g. liquid oxygen or hydrogen peroxide).
oxidation /oks-ă-day-shŏn/ An atom, anion, or a molecule is said to undergo oxi-dation or to be oxidized when it loses elec-trons. The process may be effectedchemically, i.e. by reaction with an oxidiz-ing agent, or electrically, in which case ox-idation occurs at the anode. For example,
2Na + Cl2 → 2Na+ + 2Cl–
where chlorine is the oxidizing agent andsodium is oxidized, and
4CN– + 2Cu2+ → C2N2 + 2CuCNwhere Cu2+ is the oxidizing agent and CN–
is oxidized.The oxidation state of an atom is indi-
cated by the number of electrons lost or ef-fectively lost by the neutral atom, i.e. theoxidation number. The oxidation numberof a negative ion is negative. The process ofoxidation is the converse of reduction. Seealso redox.
oxidation–reduction See redox.
oxide /oks-ÿd/ A compound of oxygenand another element. Oxides can be madeby direct combination of the elements or byheating a hydroxide, carbonate, or othersalt. See also acidic oxide; basic oxide; neu-tral oxide; peroxide.
oxidizing acid /oks-ă-dÿ-zing/ An acidthat acts as an oxidizing agent, e.g. sulfu-ric(VI) acid or nitric(V) acid. Because it isoxidizing, nitric(V) acid can dissolve met-als below hydrogen in the electromotive se-ries:
2HNO3 + M → MO + 2NO2 + H2OMO + 2HNO3 → H2O + M(NO3)2
oxidizing agent See oxidation.
oxime /oks-eem, -im/ A type of organiccompound containing the C:NOH group,formed by reaction of an aldehyde or ke-tone with hydroxylamine (NH2OH).
oxo process A method of manufacturingaldehydes by passing a mixture of carbonmonoxide, hydrogen, and alkanes over acobalt catalyst at high pressure (100 at-mospheres and 150°C). The aldehydes cansubsequently be reduced to alcohols, mak-ing the process a useful source of alcoholsof high molecular weight.
2-oxopropanoic acid /oks-oh-proh-pă-noh-ik/ (pyruvic acid; CH3COCOOH) Acolorless liquid carboxylic acid containinga keto (–CO) group. It is important inmetabolic reactions in the body.
oxyacid /oks-ee-ass-id/ An ACID in whichthe replaceable hydrogen atom is part of ahydroxyl group, including carboxylicacids, phenols and inorganic acids such asphosphoric(V) acid and sulfuric(VI) acid.
oxygen /oks-ă-jĕn/ A colorless odorless di-atomic gas; the first member of group 16 ofthe periodic table. It has the electronic con-figuration [He]2s22p4 and its chemistry in-volves the acquisition of electrons to formeither the di-negative ion, O2– or two cova-lent bonds. In each case the oxygen atom
Ostwald’s dilution law
202
attains the configuration of the rare gasneon.
Oxygen is the most plentiful element inthe Earth’s crust accounting for over 40%by weight. It is present in the atmosphere(20%) and is a constituent of the majorityof minerals and rocks (e.g. sandstones,SiO2, carbonates, CaCO3, aluminosili-cates) as well as the major constituent ofthe sea. Oxygen is an essential element foralmost all living things. Elemental oxygenhas two forms: the diatomic molecule O2and the less stable molecule trioxygen(ozone), O3, which is formed by passing anelectric discharge through oxygen gas.There are several convenient laboratoryroutes to oxygen:1. Heat on unstable oxides.
2HgO → 2Hg + O22. Decomposition of peroxides.
2H2O2 → 2H2O + O23. Heat on ‘-ate’ compounds such as chlo-rates or manganate(VII).
2KClO3 → KCl + 3O22KMnO4 → K2MnO4 + MnO2 + O2
Industrially oxygen is obtained by thefractionation of liquid air.
Oxygen reacts with elements from allgroups of the periodic table except group 0and even this unreactive group forms sev-eral compounds containing oxygen (XeO3,XeO4, XeOF2, XeO2
2–).The nature of the oxides has long been
a convenient guide to metallic and non-metallic character. Thus, metals typicallycombine with oxygen to form oxides,which if soluble react with water to formbases (via OH– ion); the more electroposi-tive the metal the more vigorous the reac-tion. Most of the group 1 and group 2metals behave like this forming oxides,O2–, (e.g., Na2O, CaO), peroxides, O2
2–,(e.g., Na2O2, BaO2), and superoxides, O2
–,(e.g., RbO2, CsO2). Oxides of metallic ele-ments are solids and x-ray studies showthat discrete O2– ions do exist in the solidstate even though they do not exist in solu-tion:
O2– + H2O → 2OH–
The non-metals typically form molecu-lar oxides such as SO2, ClO2, and NO2.When soluble in water they generally dis-solve to form acids. The less electronega-
tive elements among the non-metals gener-ally form weaker acids, e.g. B(OH)3 andH2CO3. The more electronegative elementsform stronger acids. The higher the oxida-tion state of the central atom the strongerthe acid (HOCl < HClO2 < HClO4). Non-metal oxides may be gaseous (SO2, CO2),liquid (N2O4), or solid (P2O5). The weaklynon-metallic elements such as silicon mayrequire bases for dissolution and in somecases the oxides have extended polymericstructures, e.g., SiO2. In between the acidicand basic oxides are the amphoteric ox-ides, which behave acidically to strongbases and basically towards strong acids.This behavior is associated with elementsthat are physically metallic but are chemi-cally only weakly cationic, e.g., ZnO givesZn2– with acids and the zincates,Zn(OH)4
2–, with bases. There is also asmall class of mixed oxides that can be re-garded as salts between a ‘basic’ and an‘acidic’ oxide of the same metal: for exam-ple,
FeO + Fe2O3 → Fe(FeO2)2 = Fe3O42PbO + PbO2 → Pb3O4
The neutral oxides are essentially insol-uble non-metal oxides such as N2O, NO,CO, F2O, although extreme conditions canlead to some degree of acidic behavior inthe case of CO, for example,
CO + OH– → HCOO–
Oxygen occurs in three natural isotopicforms, 16O (99.76%), 17O (0.0374%), 18O(0.2039%); the rarer isotopes are used indetailed studies of the behavior of oxygen-containing groups during reactions (tracerstudies).
Oxygen is very limited in its catenationbut an interesting range of O–O species isformed by the following: O2
2– peroxide,O2
– superoxide, O2 oxygen gas, O2+
oxygenyl cation. The O–O bond gets bothprogressively shorter and stronger in theseries. The peroxides are formed with Na,Ca, Sr, and Ba and are formally related tohydrogen peroxide (prepared by the actionof acids on ionic peroxides). The superox-ides of K, Rb, and Cs are prepared by burn-ing the metal in air (the less electropositivemetals, Na, Mg, and Zn form less stablesuperoxides). Oxygenyl cation, O2
+, isformed by reaction of PtF6 (which has a
203
oxygen
high electron affinity) with oxygen at roomtemperature. Other oxygenyl cationspecies can be made with group 15 fluo-rides, e.g. O2
+AsF6–.
Symbol: O; m.p. –218.4°C; b.p.–182.962°C; d. 1.429 kg m–3 (0°C); p.n. 8;r.a.m. 15.9994.
ozone /oh-zohn/ (trioxygen; O3) A poiso-nous, blue-colored allotrope of oxygenmade by passing oxygen through a silentelectric discharge. Ozone is unstable anddecomposes to oxygen on warming. It ispresent in the upper layers of the atmos-phere, where it screens the Earth fromharmful short-wave ultraviolet radiation.There is concern that the ozone layer ispossibly being depleted by the use of fluo-rocarbons and other compounds producedby industry.
Ozone is a strong oxidizing agent. Itsconcentration can be determined by react-ing it with iodide ions and titrating the io-dine formed with standard sodiumthiosulfate(VI). It forms ozonides withalkenes. See ozonolysis.
ozonide /oh-zŏ-nÿd/ See ozonolysis.
ozonolysis /oh-zŏ-nol-ă-sis/ The additionof ozone (O3) to alkenes and the subse-quent hydrolysis of the ozonide into hy-drogen peroxide and a mixture of carbonylcompounds. The carbonyl compounds canbe separated and identified, which in turn,identifies the groups and locates the posi-tion of the double bond in the originalalkene. Ozonolysis was formerly an impor-tant analytical technique.
ozone
204
RC O
RC C
R’
OO
O
RC C
R’C
OO
CR O
H+
H2O
R’CO
H2O2
R’ O3
alkene primary ozonide
secondary ozonide
Ozonolysis of an alkene to form a mixture of carbonyl compounds
205
PAH See particulate matter.
paint A suspension of powdered pigmentin a liquid vehicle, used for decorative andprotective surface coatings. In oil-basedpaints, the vehicle contains solvents and adrying oil or synthetic resin. In water-based or emulsion paints, the vehicle ismostly water with an emulsified resin.
palladium /pă-lay-dee-ŭm/ A silverywhite ductile transition metal occurring inplatinum ores in Canada and as the nativemetal in Brazil. Most palladium is obtainedas a by-product in the extraction of copperand zinc. It is used in electrical relays andas a catalyst in hydrogenation processes.Hydrogen will diffuse through a hot palla-dium barrier.
Symbol: Pd; m.p. 1552°C; b.p. 3140°C;r.d. 12.02 (20°C); p.n. 46; r.a.m. 106.42.
palmitic acid /pal-mit-ik/ See hexade-canoic acid.
paper chromatography A technique
widely used for the analysis of mixtures.Paper chromatography usually employs aspecially produced paper as the stationaryphase. A base line is marked in pencil nearthe bottom of the paper and a small sampleof the mixture is spotted onto it using acapillary tube. The paper is then placedvertically in a suitable solvent, which risesup to the base line and beyond by capillaryaction. The components within the samplemixture dissolve in this mobile phase andare carried up the paper. However, thepaper holds a quantity of moisture andsome components will have a greater ten-dency than others to dissolve in this mois-ture than in the mobile phase. In addition,some components will cling to the surfaceof the paper. Therefore, as the solventmoves through the paper, certain compo-nents will be left behind and separatedfrom each other.
When the solvent has almost reachedthe top of the paper, the paper is removedand quickly dried. The paper is developedto locate the positions of colorless fractionsby spraying with a suitable chemical, e.g.ninhydrin, or by exposure to ultraviolet ra-diation. The components are identified bycomparing the distance they have traveledup the paper with standard solutions thathave been run simultaneously, or by com-puting an RF value. A simplified version ofpaper chromatography uses a piece of filterpaper. The sample is spotted at the centerof the paper and solvent passed through it.Separation of the components of the mix-ture again takes place as the mobile phasespreads out on the paper.
Paper chromatography is an applica-tion of the partition law.
para- 1. Designating the form of a di-atomic molecule in which both nuclei have
P
developingchamber
paper strip
mobile phase
spotted samples
Apparatus for paper chromatography
opposite spin directions; e.g. parahydro-gen, paradeuterium.2. Designating a benzene compound withsubstituents in the 1,4 positions. The posi-tion on a benzene ring directly opposite asubstituent is the para position. This wasused in the systematic naming of benzenecompounds. For example, para-dinitro-benzene (or p-dinitrobenzene) is 1,4-dini-trobenzene.
See also meta-; ortho-.
paraffin /pa-ră-fin/ 1. See petroleum.2. See alkane.
paraffin oil See kerosene. See also petro-leum.
paraffin wax A solid mixture of hydro-carbons, obtained from petroleum.
paraformaldehyde /pa-ră-for-mal-dĕ-hÿd/See methanal.
parahydrogen /pa-ră-hÿ-drŏ-jĕn/ See hy-drogen.
paraiodic (VII) acid /pa-ră-ÿ-od-ik/ Seeiodic (VII) acid.
paraldehyde /pă-ral-dĕ-hÿd/ See ethanal.
paramagnetism /pa-ră-mag-nĕ-tiz-ăm/See magnetism.
partial ionic character The electrons ofa covalent bond between atoms or groupswith different electronegativities will bepolarized towards the more electronegativeconstituent; the magnitude of this effectcan be measured by the ionic character ofthe bond. When the effect is small the bondis referred to simply as a polar bond and isadequately treated using dipole moments;as the effect grows the theoretical treat-ment requires other contributions to ioniccharacter. Examples of partial ionic char-acter (from nuclear quadrupole resonance)are H–I, 21%; H–Cl, 40%; Li–Br, 90%;.
partial pressure In a mixture of gases, thecontribution that one component makes tothe total pressure. It is the pressure that the
gas would have if it alone were present inthe same volume. See Dalton’s law.
particulate matter (PM) An airbornepollutant consisting of small particles ofsilicate, carbon, or large polyaromatic hy-drocarbons (PAHs). These pollutants areoften referred to as particulates. They areclassified according to size; e.g. PM10 isparticulate matter formed of particles lessthan 10 µm diameter.
particulates See particulate matter.
partition coefficient If a solute dissolvesin two non-miscible liquids, the partitioncoefficient is the ratio of the concentrationin one liquid to the concentration in theother liquid.
pascal /pas-kăl/ Symbol: Pa The SI unit ofpressure, equal to a pressure of one newtonper square meter (1 Pa = 1 N m–2). The unitis named for the French mathematician,physicist, and religious philosopher BlaisePascal (1623–62).
Paschen series /pash-ĕn/ A series of linesin the infrared spectrum emitted by excitedhydrogen atoms. The lines correspond tothe atomic electrons falling into the thirdlowest energy level and emitting energy asradiation. The wavelength (λ) of the radia-tion in the Paschen series is given by 1/λ =R(1/32 – 1/n2) where n is an integer and Ris the Rydberg constant. The series isnamed for the German physicist LouisPaschen (1865–1947). See also spectral se-ries.
passive Describing a metal that is unreac-tive because of the formation of a thin pro-tective layer. Iron, for example, does notdissolve in concentrated nitric(V) acid be-cause of the formation of a thin oxidelayer.
Pauli exclusion principle /pow-lee/ Seeexclusion principle.
p-block elements The elements of themain groups 13 (B to Tl), 14 (C to Pb), 15(N to Bi), 16 (O to Po), 17 (F to At), and 18
paraffin
206
207
pericyclic reaction
(He to Rn). They are so called because theyhave outer electronic configurations thathave occupied p levels.
PCB See polychlorinated biphenyl.
pearl A lustrous accretion of calcium car-bonate (nacre, CaCO3) that builds up on aparticle of foreign matter inside the shellsof oysters and some other mollusks.
pearl spar See dolomite.
peat A brown or black material formed bythe partial decomposition of plant remainsin marshy ground, the first stage in the for-mation of coal. It is used for making char-coal and compost, and, when dried, can beburned as a fuel.
PEC See photoelectrochemical cell.
pentahydrate /pen-tă-hÿ-drayt/ A crys-talline compound that has five moleculesof water of crystallization per molecule ofcompound.
pentane /pen-tayn/ (C5H12) A straight-chain alkane obtained by distillation ofcrude oil.
pentanoic acid /pen-tă-noh-ik/ (valericacid; CH3(CH2)3COOH) A colorless li-quid carboxylic acid, used in making per-fumes.
pentavalent /pen-tă-vay-lĕnt, pen-tav-ă-/(quinquevalent) Having a valence of five.
pentose /pen-tohs/ A SUGAR that has fivecarbon atoms in its molecules.
pentyl group /pen-tăl/ (amyl group) Thegroup
CH3CH2CH2CH2CH2–.
peptide /pep-tÿd/ A compound formed bylinkage of two or more amino-acid groupstogether.
percentage composition A way of ex-pressing the composition of a chemicalcompound in terms of the percentage (by
mass) of each of the elements that make itup. It is calculated by dividing the mass ofeach element (taking into account the num-ber of atoms present) by the relative mole-cular mass of the whole molecule. Forexample, methane (CH4) has a relativemolecular mass of 16 and its percentagecomposition is 12/16 = 75% carbon and (4× 1)/16 = 25% hydrogen.
perchloric acid /per-klor-ik, -kloh-rik/See chloric(VII) acid.
perfect gas (ideal gas) See gas laws; ki-netic theory.
pericyclic reaction /pe-ră-sÿ-klik/ A typeof CONCERTED REACTION in which the tran-sition state is cyclic and can be regarded asformed by movement of electrons in a cir-cle, from one bond to an adjacent bond. Itis usual to distinguish three types of peri-cyclic reaction:Cycloadditions. In these reactions a conju-gated diene adds to a double bond to forma ring. This involves the formation of twonew sigma bonds. The DIELS–ALDER REAC-TION is a cycloaddition. The reverse reac-tion, in which a ring containing a doublebond breaks to a diene and a compoundcontaining a double bond, is also a peri-cyclic reaction.Sigmatropic rearrangements. In these reac-tions a sigma bond breaks and another isformed. The COPE REARRANGEMENT is anexample.Electrocyclic reactions. In these reactions aring is formed across the ends of a conju-gated system of double bonds. The reversereaction, in which a ring containing twodouble bonds breaks by movement of elec-
R
C
H
O
C
H
N
R
C
H
Peptide linkage
trons in a cycle, is also an electrocyclic re-action.Pericyclic reactions are often treated usingFRONTIER-ORBITAL theory or the WOOD-WARD–HOFFMANN RULES.
peridot /pe-ră-dot/ See olivine.
period One of the horizontal rows in theconventional periodic table. Each periodrepresents the elements arising from pro-gressive filling of the outer shell (i.e. the ad-dition of one extra electron for each newelement), the elements being arranged inorder of ascending proton number. In astrict sense hydrogen and helium representone period but convention refers to the el-ements lithium to neon (8 elements) as thefirst short period (n = 2), and the elementssodium to argon (8 elements) as the secondshort period (n = 3). With entry to the n =4 level there is filling of the 4s, then backfilling of the 3d, before the 4p are filled.Thus this set contains a total of 18 elec-trons (potassium to krypton) and is calleda long period. The next set, rubidium toxenon, is similarly a long period. See alsoAufbau principle.
periodic acid See iodic(VII) acid.
periodic law The law upon which themodern periodic table is based. Enunciatedin 1869 by Mendeléev, this law stated thatthe properties of the elements are a peri-odic function of their atomic weights: ifarranged in order of increasing atomicweight then elements having similar prop-erties occur at fixed intervals. Certain ex-ceptions or gaps in the table lead to theview that the nuclear charge is a more char-acteristic function, thus the modern state-ment of the periodic law is that the physicaland chemical properties of elements are aperiodic function of their proton number.
periodic table A table of the elementsarranged in order of increasing protonnumber to show similarities in chemicalbehavior between elements. Horizontalrows of elements are called PERIODS. Acrossa period there is a general trend frommetallic to non-metallic behavior. Vertical
columns of related elements are calledGROUPS. Down a group there is an increasein atomic size and in electropositive (metal-lic) behavior.
Originally the periodic table wasarranged in eight groups with the alkalimetals as group I, the halogens as groupVII, and the rare gases as group 0. Thetransition elements were placed in a blockin the middle of the table. Groups weresplit into two sub-groups. For example,group I contained the main-group ele-ments, Li, Na, K, Rb, Cs, in subgroup IAand the subgroup IB elements, Cu, Ag, Au.The system was confusing because therewas a difference in usage for subgroupsand the current form of the table has 18groups (see Appendix).
permanent gas A gas that cannot be liq-uefied by pressure alone – that is, a gasabove its critical temperature. See criticaltemperature.
permanent hardness A type of waterhardness caused by the presence of dis-solved calcium, iron, and magnesium sul-fates or chlorides. This form of hardnesscannot be removed by boiling. Comparetemporary hardness. See also water soften-ing.
permanganate /per-mang-gă-nayt, -nit/See manganate(VII).
Permutit (Trademark) A substance usedto remove unwanted chemicals that havedissolved in water. It is a zeolite consistingof a complex chemical compound, sodiumaluminum silicate. When hard water ispassed over this material, calcium andmagnesium ions exchange with sodiumions in the Permutit. This is a good exam-ple of ion exchange. Once all the availablesodium ions have been used up, the Permu-tit can be regenerated by washing it with asaturated solution of sodium chloride. Theexcess of sodium ions now exchange withthe calcium and magnesium ions in the Per-mutit.
peroxide /pĕ-roks-ÿd/ 1. An oxide con-taining the –O–O– ion.
peridot
208
2. A compound containing the –O–O–group.
peroxosulfuric(VI) acid /pĕ-roks-oh-sul-fyoor-ik/ (Caro’s acid; H2SO5) A whitecrystalline solid formed by reacting hydro-gen peroxide with sulfuric acid. It is a pow-erful oxidizing agent. The acid is alsonamed for the German chemist HeinrichCaro (1834–1910).
perturbation theory An approximationtechnique in which a system is divided intotwo parts, with one part that can be solvedexactly and a second part that makes thesystem too complicated to be solved ex-actly. For perturbation theory to be applic-able it is necessary that the second part is asmall correction to the first part. The ef-fects of the small corrections are expressedas an infinite series, called a perturbationseries, which modifies the results thatcould be calculated exactly. A classic ex-ample of perturbation theory is the modifi-cation of the orbits of planets round theSun because of their gravitational interac-tions with other planets. In quantum me-chanics an application of perturbationtheory is the calculation of the effects ofsmall electric or magnetic fields on atomicenergy levels.
peta- Symbol: P A prefix denoting 1015.
petrochemicals /pet-roh-kem-ă-kălz/Chemicals that are obtained from crude oilor from natural gas.
petrol See petroleum.
petroleum A mixture of hydrocarbonsformed originally from marine animals andplants, found beneath the ground trappedbetween layers of rock. It is obtained bydrilling (also called crude oil). Differentoilfields produce petroleum with differingcompositions. The mixture is separatedinto fractions by fractional distillation in avertical column. The main fractions are:Diesel oil (gas oil) in the range 220–350°C,consisting mainly of C13–C25 hydrocar-bons. It is used in diesel engines.
Kerosene (paraffin) in the range160–250°C, consisting mainly of C11 andC12 hydrocarbons. It is a fuel both for do-mestic heating and jet engines.Gasoline (petrol) in the range 40–180°C,consisting mainly of C5–C10 hydrocarbons.It is used as motor fuel and as a raw ma-terial for making other chemicals.Refinery gas, consisting of C1–C4 gaseoushydrocarbons.
In addition lubricating oils and paraffinwax are obtained from the residue. Theblack material left is bitumen tar.
petroleum ether A flammable mixture ofhydrocarbons, mainly pentane and hexane,used as a solvent. Note that it is not anether.
pewter A tarnish-resistant alloy of tin andlead, usually containing 80–90% tin. Mod-ern pewter is hardened by antimony andcopper, which in the best grades replace thelead content. It is lighter than old pewterand having a bright or a soft satiny finish isoften used for decorative and ceremonialpurposes.
pH The logarithm to base 10 of the recip-rocal of the hydrogen-ion concentration ofa solution. In pure water at 25°C the con-centration of hydrogen ions is 1.00 × 10–7
mol l–1, thus the pH equals 7 at neutrality.An increase in acidity increases the value of[H+], decreasing the value of the pH below7. An increase in the concentration of hy-droxide ion [OH–] proportionately de-creases [H+], therefore increasing the valueof the pH above 7 in basic solutions. pHvalues can be obtained approximately byusing indicators. More precise measure-ments use electrode systems.
phase /fayz/ One of the physically separa-ble parts of a chemical system. For exam-ple, a mixture of ice (solid phase) andwater (liquid phase) consists of two phases.A system consisting of only one phase issaid to be homogeneous. A system consist-ing of more than one phase is said to beheterogeneous.
209
phase
phase diagram A graphical representa-tion of the state in which a substance willoccur at a given pressure and temperature.The lines show the conditions under whichmore than one phase can coexist at equi-librium. For one-component systems (e.g.water) the point at which all three phasescan coexist at equilibrium is called thetriple point and is the point on the graph atwhich the pressure–temperature curves in-tersect.
phase equilibrium A state in which theproportions of the various phases in achemical system are fixed. When two ormore phases are present at fixed tempera-ture and pressure a dynamic condition willbe established in which individual particleswill leave one phase and enter another; atequilibrium this will be balanced by anequal number of particles making the re-verse change, leaving the total compositionunchanged.
phase rule In a system at equilibrium thenumber of phases (P), number of compo-nents (C), and number of degrees of free-dom (F) are related by the formula:
P + F = C + 2
phenol /fee-nol, -nohl/ 1. (carbolic acid,hydroxybenzene, C6H5OH) A white crys-talline solid used to make a variety of otherorganic compounds.2. A type of organic compound in which atleast one hydroxyl group is bound directlyto one of the carbon atoms of an aromaticring. Phenols do not show the behaviortypical of alcohols. In particular they aremore acidic because of the electron-with-drawing effect of the aromatic ring. Thepreparation of phenol itself is by fusing thesodium salt of the sulfonic acid withsodium hydroxide:
C6H5SO2.ONa + 2NaOH → C6H5ONa+ Na2SO3 + H2O
The phenol is then liberated by sulfuricacid:
2C6H5ONa + H2SO4 → 2C6H5OH +Na2SO4
Reactions of phenol include:
1. Replacement of the hydroxyl group witha chlorine atom using phosphorus(V)chloride.
2. Reaction with acyl halides to form estersof carboxylic acids.
3. Reaction with haloalkanes under alka-line conditions to give mixed alkyl–arylethers.
In addition phenol can undergo furthersubstitution on the benzene ring. The hy-droxyl group directs other substituentsinto the 2- and 4-positions.
phenolphthalein /fee-nol-thal-een, -nohl-,-fthal-/ An acid–base indicator that is col-orless in acid solutions and becomes red ifthe pH rises above the transition range of8–9.6. It is used as the indicator in titra-tions for which the end point lies clearly onthe basic side (pH > 7), e.g. oxalic acid orpotassium hydrogentartrate against causticsoda.
phenoxy resin /fi-noks-ee/ A type of ther-moplastic resin made by condensation ofphenols.
phenylalanine /fen-ăl-al-ă-neen, fee-năl-/See amino acids.
phenylamine /fen-al-am-een, fee-năl-/ Seeaniline.
phenylethene /fen-ăl-eth-een, fee-năl-/(styrene; C6H5CHCH2) A liquid hydrocar-bon used as the starting material for theproduction of polystyrene. The manufac-ture of phenylethene is by dehydrogenationof ethyl benzene:
C6H5C2H5 → C6H5CH:CH2 + H2
phenyl group /fen-ăl, fee-năl/ The groupC6H5–, derived from benzene.
phenylhydrazone /fen-ăl-hÿ-dră-zohn,fee-năl-/ See hydrazone.
phenylmethanol /fen-ăl-meth-ă-nol, fee-năl-, -nohl/ (benzyl alcohol; C6H5CH2OH)An aromatic primary alcohol. Phenyl-methanol is synthesized by Cannizzaro’sreaction, which involves the simultaneousoxidation and reduction of benzenecar-
phase diagram
210
baldehyde (benzaldehyde) by refluxing inan aqueous solution of sodium hydroxide:
2C6H5CHO → C6H5CH2OH +C6H5COOH
Phenylmethanol undergoes the reac-tions characteristic of alcohols, especiallythose in which the formation of a stablecarbonium ion as an intermediate(C6H5CH2
+) enhances the reaction. Substi-tution onto the benzene ring is also possi-ble; the –CH2OH group directs into the 2-or 4-position by the donation of electronsto the ring.
phenyl methyl ketone (acetophenone;C6H5COCH3) A colorless sweet-smellingorganic liquid, which solidifies below20°C. It is used as a solvent for methyl andethyl cellulose plastics.
3-phenylpropenoic acid /fen-ăl-proh-pĕ-noh-ik, fee-năl-/ (cinnamic acid;C6H5CH:CHCOOH) A white pleasant-smelling crystalline carboxylic acid. It oc-curs in amber but can be synthesized and isused in perfumes and flavorings.
Phillips process A method for the manu-facture of high-density polyethene using acatalyst of chromium(III) oxide on a pro-moter of silica and alumina. The reactionconditions are 150°C and 30 atm pressure.See also Ziegler process.
Philosopher’s stone See alchemy.
phlogiston theory /floh-jis-tŏn/ A formertheory of combustion (disproved in theeighteenth century by Lavoisier). Flamma-ble compounds were supposed to contain asubstance (phlogiston), which was releasedwhen they burned, leaving ash.
phosgene /fos-jeen/ See carbonyl chloride.
phosphates /fos-fayts/ See phosphoric(V)acid.
phosphide /fos-fÿd, -fid/ A compound ofphosphorus with a more electropositive el-ement.
phosphine /fos-feen, -fin/ (phosphorus(III)
hydride; PH3) A colorless gas that isslightly soluble in water. It has a character-istic fishy smell. It can be made by reactingwater and calcium phosphide or by the ac-tion of yellow phosphorus on a concen-trated alkali. Phosphine usually ignitesspontaneously in air because of contamina-tion with diphosphine. It decomposes intoits elements if heated to 450°C in the ab-sence of oxygen and it burns in oxygen orair to yield phosphorus oxides. It reactswith solutions of metal salts to precipitatephosphides. Like its nitrogen analog am-monia it forms salts, called phosphoniumsalts. It also forms complex addition com-pounds with metal ions. As in ammonia,one or more of the hydrogen atoms can bereplaced by alkyl groups.
phosphinic acid /fos-fin-ik/ (hypophos-phorous acid; H3PO2) A white crystallinesolid. It is a monobasic acid forming theanion H2PO2
– in water. The sodium salt,and hence the acid, can be prepared byheating yellow phosphorus with sodiumhydroxide solution. The free acid and itssalts are powerful reducing agents.
phosphonic acid /fos-fon-ik/ (phospho-rous acid; orthophosphorous acid; H3PO3)A colorless deliquescent solid that can beprepared by the action of water on phos-phorus(III) oxide or phosphorus(III) chlo-ride. It is a dibasic acid producing theanions H2PO3
– and HPO32– in water. The
acid and its salts are slow reducing agents.On warming, phosphonic acid decomposesto phosphine and phosphoric(V) acid.
phosphonium ion /fos-foh-nee-ŭm/ Theion PH4
+ derived from phosphine.
phosphonium salt See phosphine.
phosphor /fos-fer/ A substance thatshows luminescence or phosphorescence.
phosphorescence /fos-fŏ-ress-ĕns/ 1. Theabsorption of energy by atoms followed byemission of electromagnetic radiation.Phosphorescence is a type of luminescence,and is distinguished from fluorescence bythe fact that the emitted radiation contin-
211
phosphorescence
ues for some time after the source of exci-tation has been removed. In phosphores-cence the excited atoms have relativelylong lifetimes before they make transitionsto lower energy states. However, there isno defined time distinguishing phosphores-cence from fluorescence.2. In general usage the term is applied tothe emission of ‘cold light’ – light producedwithout a high temperature. The namecomes from the fact that white phosphorusglows slightly in the dark as a result of achemical reaction with oxygen. The lightcomes from excited atoms produced di-rectly in the reaction – not from the heatproduced. It is thus an example of chemi-luminescence. There are also a number ofbiochemical examples termed biolumines-cence; for example, phosphorescence issometimes seen in the sea from marine or-ganisms, or on rotting wood from certainfungi (known as ‘fox fire’).
phosphoric(V) acid /fos-for-ik, -foh-rik/(orthophosphoric acid; H3PO4) A whitesolid that can be made by reacting phos-phorus(V) oxide with water or by heatingyellow phosphorus with nitric acid. Thenaturally occurring phosphates (or-thophosphates, M3PO4) are salts of phos-phoric(V) acid. An aqueous solution ofphosphoric(V) acid has a sharp taste andhas been used in the manufacture of softdrinks. At about 220°C phosphoric(V)acid is converted to pyrophosphoric acid(H4P2O7). Pure pyrophosphoric acid in theform of colorless crystals is made by warm-ing solid phosphoric(V) acid with phos-phorus(III) chloride oxide. Metaphos-phoric acid is obtained as a polymer,(HPO3)n, by heating phosphoric(V) acid to320°C. Both meta- and pyrophosphoricacids change to phosphoric(V) acid inaqueous solution; the change will takeplace faster at higher temperatures. Phos-phoric(V) acid is tri-basic, forming threeseries of salts with the anions H2PO4
–,HPO4
2–, and PO43–. These series of salts
are acidic, neutral, and alkaline respec-tively. The alkali metal phosphates (exceptlithium phosphate) are soluble in water butother metal phosphates are insoluble.
Phosphates are useful in water softeningand as fertilizers.
phosphorous acid /fos-fŏ-rŭs, fos-for-ŭs,-foh-rŭs/ See phosphonic acid.
phosphorus /fos-fŏ-rŭs/ A reactive solidnon-metallic element; the second elementin group 15 of the periodic table. It has theelectronic configuration [Ne]3s23p3 and istherefore formally similar to nitrogen. It ishowever very much more reactive than ni-trogen and is never found in nature in theuncombined state. Phosphorus is wide-spread throughout the world; economicsources are phosphate rock (Ca3(PO4)2)and the apatites, variously occurring asboth fluoroapatite (3Ca3(PO4)2.CaF2) andas chloroapatite (3Ca3(PO4)2CaCl2).Guano formed from the skeletal phosphateof fish in sea-bird droppings is also an im-portant source of phosphorus. The largestamounts of phosphorus compounds pro-duced are used as fertilizers, with the de-tergents industry producing increasinglylarge tonnages of phosphates. Phosphorusis an essential constituent of living tissueand bones, and it plays a very importantpart in metabolic processes and muscle ac-tion.
The element is obtained industrially bythe reduction of phosphate rock using sand(SiO2) and coke (C) in an electric furnace.The phosphorus is distilled off as P4 mol-ecules and collected under water as whitephosphorus.
2Ca3(PO4)2 + 6SiO2 + 10C → P4 +6CaSiO3 + 10CO
There are three common allotropes ofphosphorus and several other modifica-tions of these, some of which have indefi-nite structures. The common forms are:1. white (or yellow) phosphorus (distilla-
tion of phosphorus; soluble in some or-ganic solvents)
2. red phosphorus (heat on white phospho-rus, insoluble in organic solvents)
3. black phosphorus (heat on white phos-phorus under high pressures).Phosphorus, being in group 15 is non-
metallic in character and has a sufficientlyhigh ionization potential for the chemistryof bonds to phosphorus to be almost en-
phosphoric(V) acid
212
tirely covalent. Phosphorus compounds areformally P(III) and P(V) compounds. Theelement forms a hydride, PH3 (phosphine),which is analogous to ammonia, but it isnot formed by direct combination. Phos-phorus(V) hydrides are not known.
When phosphorus is burned in air theproduct is the highly hygroscopic whitesolid P4O10 (commonly called phosphoruspentoxide, P2O5); this is the P(V) oxide. Ina limited supply of air the P(III) oxide,P4O6 (called phosphorus trioxide) contam-inated with unreacted phosphorus is ob-tained. The oxides both react with water toform acids;
P4O6 + 6H2O →4H3PO3 phosphorous acid
P4O10 + 2H2O →4HPO3 metaphosphoric acid
P4O10 + 6H2O →4H3PO4 orthophosphoric acid.
There are also many poly-phosphoricacids and salts with different ring sizes anddifferent chain lengths. Pyrophosphoricacid is obtained by melting phosphoricacid, further dehydration above 200°Cleads to metaphosphoric acids,
2H3PO4 → H4P2O7 + H2OH4P2O7 → 2HPO3 + H2O
Polyphosphates with molecular weightsfrom 1000 to 10 000 are industrially im-portant in detergent formulations and thelower members are used to complex metalions (sequestration).
Phosphorus forms the P(III) halides,PX3, with all the halogens, and P(V)halides, PX5, with X = F, Cl, Br. The P(III)halides are formed by direct combinationbut because of hazards in the reaction ofphosphorus and fluorine, PF3 is preparedfrom PCl3
The P(III) halide molecules are all pyra-midal (as is PH3) and are readily hy-drolyzed. The P(V) halides are also readilyhydrolyzed by water in two stages givingfirst the oxyhalide, then phosphoric acid,
PX5 + H2O → POX3 + 2HXPOX3 + 3H2O → H3PO4 + 3HX
In the gas phase the trihalides are pyra-midal and the pentahalides are trigonalbipyramids. However, in tetrachloro-methane solution phosphorus pentachlo-ride is a dimer with two chlorine bridges
exhibiting the ready expansion of the coor-dination number to six. In the solid state,(PCl5) is [PCl4]+[PCl6]– (the first tetrahe-dral, the second octahedral) obtained bytransfer of a chloride ion.
Phosphorus interacts directly with sev-eral metals and metalloids to give:1. Ionic phosphides such as Na3P, Ca3P2,
Sr3P2.2. Molecular phosphides such as P4S3 and
P4S5.3. Metallic phosphides such as Fe2P.
Symbol: P; m.p. 44.1°C (white) 410°C(red under pressure); b.p. 280.5°C; r.d.1.82 (white) 2.2 (red) 2.69 (black) (all at20°C); p.n. 15; r.a.m. 30.973762.
phosphorus(III) bromide (phosphorustribromide; PBr3) A colorless liquid madeby reacting phosphorus with bromine. It isreadily hydrolyzed by water to phosphonicacid and hydrogen bromide. Phos-phorus(III) bromide is important in or-ganic chemistry, being used to replace ahydroxyl group with a bromine atom.
phosphorus(V) bromide (phosphoruspentabromide; PBr5) A yellow crystallinesolid that sublimes easily. It can be madeby the reaction of bromine and phospho-rus(III) bromide. Phosphorus(V) bromideis readily hydrolyzed by water to phos-phoric(V) acid and hydrogen bromide. Itsmain use is in organic chemistry to replacea hydroxyl group with a bromine atom.
phosphorus(III) chloride (phosphorustrichloride; PCl3) A colorless liquidformed from the reaction of phosphoruswith chlorine. It is rapidly hydrolyzed bywater to phosphonic acid and hydrogenchloride. Phosphorus(III) chloride is usedin organic chemistry to replace a hydroxylgroup with a chlorine atom.
phosphorus(V) chloride (phosphoruspentachloride; PCl5) A white easily sub-limed solid formed by the action of chlo-rine on phosphorus(III) chloride. It ishydrolyzed by water to phosphoric(V) acidand hydrogen chloride. Its main use is as achlorinating agent in organic chemistry to
213
phosphorus(V) chloride
replace a hydroxyl group with a chlorineatom.
phosphorus(III) chloride oxide (phos-phorus trichloride oxide; phosphorus oxy-chloride; phosphoryl chloride, POCl3) Acolorless liquid that can be obtained by re-acting phosphorus(III) chloride with oxy-gen or by distilling phosphorus(III)chloride with potassium chlorate. The re-actions of phosphorus(III) chloride oxideare similar to those of phosphorus(III)chloride. The chlorine atoms can be re-placed by alkyl groups using Grignardreagents or by alkoxo groups using alco-hols. Water hydrolysis yields phos-phoric(V) acid. Phosphorus(III) chlorideoxide forms complexes with many metalions.
phosphorus(III) hydride See phosphine.
phosphorus(III) oxide (phosphorus tri-oxide; P2O3) A white waxy solid with acharacteristic smell of garlic; it usually ex-ists as P4O6 molecules. It is soluble in or-ganic solvents, such as benzene and carbondisulfide, and is made by burning phos-phorus in a limited supply of air. Phospho-rus(III) oxide oxidizes in air tophosphorus(V) oxide at room temperatureand inflames above 70°C. It dissolvesslowly in cold water or dilute alkalis to givephosphonic acid or one of its salts. Phos-phorus(III) oxide reacts violently with hotwater to form phosphine and phos-phoric(V) acid.
phosphorus(V) oxide (phosphorus pen-toxide; P2O5) A white powder that is solu-ble in organic solvents. It usually exists asP4O10 molecules. Phosphorus(V) oxide canbe prepared by burning phosphorus in aplentiful supply of oxygen. It readily com-bines with water to form phosphoric(V)acid and is therefore used as a drying agentfor gases. It is a useful dehydrating agentbecause it is able to remove the elements ofwater from certain oxyacids and othercompounds containing oxygen and hydro-gen; for example, it reacts with nitric acid(HNO3) to give nitrogen pentoxide (N2O5)on heating.
phosphorus oxychloride /oks-ă-klor-ÿd,-kloh-rÿd/ See phosphorus(III) chlorideoxide.
phosphorus pentabromide /pen-tă-broh-mÿd/ See phosphorus(V) bromide.
phosphorus pentachloride /pen-tă-klor-ÿd, -kloh-rÿd/ See phosphorus(V) chloride.
phosphorus pentoxide /pen-toks-ÿd/ Seephosphorus(V) oxide.
phosphorus tribromide /trÿ-broh-mÿd/See phosphorus(III) bromide.
phosphorus trichloride /trÿ-klor-ÿd, -kloh-rÿd/ See phosphorus(III) chloride.
phosphorus trichloride oxide /trÿ-klor-ÿd, -kloh-rÿd/ See phosphorus(III) chlorideoxide.
phosphorus trioxide /trÿ-oks-ÿd/ Seephosphorus(III) oxide.
phosphoryl chloride /fos-fŏ-răl, fos-for-ăl, -foh-răl/ See phosphorus(III) chlorideoxide.
phot /foht, fot/ A unit of illumination inthe c.g.s. system, equal to an illuminationof one lumen per square centimeter. It isequal to 104 lx.
photochemical reaction /foh-toh-kem-ă-kăl/ A reaction brought about by light; ex-amples include the bleaching of coloredmaterial, the reduction of silver halides,and the photosynthesis of carbohydrates.Chemical changes only occur when the re-acting atoms or molecules absorb photonsof the appropriate energy. The amount ofsubstance that reacts is proportional to thequantity of energy absorbed. For example,in the reaction between hydrogen and chlo-rine, it is not the concentrations of hydro-gen or chlorine that dictate the rate ofreaction but the intensity of the radiation.
photochemistry /foh-toh-kem-iss-tree/The branch of chemistry dealing with reac-
phosphorus(III) chloride oxide
214
tions induced by light or ultraviolet radia-tion.
photoelectric effect The emission ofelectrons from a solid (or liquid) surfacewhen it is irradiated with electromagneticradiation. For most materials the photo-electric effect occurs with ultraviolet radia-tion or radiation of shorter wavelength;some show the effect with visible radiation.
In the photoelectric effect, the numberof electrons emitted depends on the inten-sity of the radiation and not on its fre-quency. The kinetic energy of the electronsthat are ejected depends on the frequencyof the radiation. This was explained, byEinstein, by the idea that electromagneticradiation consists of streams of photons.The photon energy is hv, where h is thePlanck constant and v the frequency of theradiation. To remove an electron from thesolid a certain minimum energy must besupplied, known as the work function, φ.Thus, there is a certain minimum thresholdfrequency v0 for radiation to eject elec-trons: hv0 = θ. If the frequency is higherthan this threshold the electrons areejected. The maximum kinetic energy (W)of the electrons is given by Einstein’s equa-tion:
W = hv – φThe photoelectric effect also occurs
with gases. See photoionization.
photoelectrochemical cell /foh-toh-i-lek-troh-kem-ă-kăl/ (PEC) A type ofvoltaic cell in which one of the electrodes isa light-sensitive semiconductor (such asgallium arsenide). A current is generatedby light falling on the semiconductor, caus-ing electrons to pass between the electrodeand the electrolyte and produce chemicalreactions in the cell.
photoelectron /foh-toh-i-lek-tron/ Anelectron ejected from a solid, liquid, or gasby the photoelectric effect or by photoion-ization.
photoelectron spectroscopy See pho-toionization.
photoemission /foh-toh-i-mish-ŏn/ Theemission of photoelectrons by the photo-electric effect or by photoionization.
photoionization /foh-toh-ÿ-ŏ-ni-zay-shŏn/ The ionization of atoms or mole-cules by electromagnetic radiation.Photons absorbed by an atom may havesufficient photon energy to free an electronfrom its attraction by the nucleus. Theprocess is
M + hv → M+ + e–
As in the photoelectric effect, the radia-tion must have a certain minimum thresh-old frequency. The energy of thephotoelectrons ejected is given by W = hv –I, where I is the ionization potential of theatom or molecule. Analysis of the energiesof the emitted electrons gives informationon the ionization potentials of the sub-stance – a technique known as photoelec-tron spectroscopy.
photolysis /fŏ-tol-ă-sis/ A chemical reac-tion that is produced by electromagneticradiation (light or ultraviolet radiation).Many photolytic reactions involve the for-mation of free radicals.
photosynthesis /foh-tŏ-sin-th’ĕ-sis/ Thephotochemical process green plants use tosynthesize organic compounds from car-bon dioxide and water.
phthalic acid /thal-ik, fthal-ik/ See ben-zene-1,2-dicarboxylic acid.
phthalic anhydride See benzene-1,2-di-carboxylic acid.
physical change A change to a substancethat does not alter its chemical properties.Physical changes (e.g. melting, boiling, anddissolving) are comparatively easy to re-verse.
physical chemistry The branch of chem-istry concerned with physical properties ofcompounds and how these depend on thechemical bonding.
physisorption /fiz-ă-sorp-shŏn, -zorp-/See adsorption.
215
physisorption
piano-stool compound See sandwichcompound.
pi bond See orbital.
pico- /pÿ-koh/ Symbol: p A prefix denot-ing 10–12. For example, 1 picofarad (pF) =10–12 farad (F).
pi complex A complex in which the metalion is bound to a pi-electron system, as inFERROCENE and ZEISE’S SALT.
picrate /pik-rayt/ A salt of PICRIC ACID;metal picrates are explosive.
picric acid /pik-rik/ (2,4,6-trinitrophenol;C6H2(NO3)3OH) A yellow crystallinesolid made by nitrating phenolsulfonicacid. It is used as a dye and as an explosive.With aromatic hydrocarbons picric acidforms characteristic charge-transfer com-plexes (misleadingly called picrates), usedin analysis for identifying the hydrocarbon.
pig iron The crude iron produced from ablast furnace, containing carbon, silicon,and other impurities. The molten iron isrun out of the furnace into channels (called‘sows’), which branch out into a number ofoffshoots (called ‘pigs’) in which the metalis allowed to cool.
pigment An insoluble, particulate color-ing material.
pine-cone oil See turpentine.
pi orbital See orbital.
pipette /pi-pet/ A device used to transfer aknown volume of solution from one con-tainer to another; in general, several sam-ples of equal volume are transferred forindividual analysis from one stock solu-tion. Pipettes are of two types, bulbpipettes, which transfer a known and fixedvolume, and graduated pipettes, which cantransfer variable volumes. Pipettes were atone time universally mouth-operated butsafety pipettes using a plunger or rubberbulb are now preferred.
pitchblende /pich-blend/ (uraninite) Ablack mineral consisting mostly of ura-nium(VI) oxide, UO3. It is the chief ore ofuranium and radium.
pK The logarithm to the base 10 of thereciprocal of an acid’s dissociation con-stant:
log10(1/Ka)
Planck constant /plank/ Symbol: h Afundamental constant; the ratio of the en-ergy (W) carried by a photon to its fre-quency (v). A basic relationship in thequantum theory of radiation is W = hv.The value of h is 6.626 196 × 10–34 J s. ThePlanck constant appears in many relation-ships in which some observable measure-ment is quantized (i.e. can take onlyspecific discrete values rather than any of arange of values). The constant is named forthe German physicist Max Planck(1858–1947).
plane polarization A type of polariza-tion of electromagnetic radiation in whichthe vibrations take place entirely in oneplane.
plasma /plaz-mă/ A mixture of ions andelectrons, as in an electrical discharge.Sometimes, a plasma is described as afourth state of matter.
plaster of Paris See calcium sulfate.
plasticizer /plas-tă-sÿ-zer/ A substanceadded to a synthetic resin to make it moreflexible.
plastics A common term for syntheticpolymers, especially when mixed withother substances such as plasticizers,fillers, preservatives, and colorants.
platinic chloride /plă-tin-ik/ See chloro-platinic acid.
platinum /plat-ă-nŭm/ A silvery-whitemalleable ductile transition metal. It occursnaturally in Australia and Canada, eitherfree or in association with other platinummetals. It is resistant to oxidation and is
piano-stool compound
216
not attacked by acids (except aqua regia)or alkalis. Platinum is used as a catalyst forammonia oxidation (to make nitric acid)and in catalytic converters. It is also used injewelry.
Symbol: Pt; m.p. 1772°C; b.p. 3830 ±100°C; r.d. 21.45 (20°C); p.n. 78; r.a.m.195.08.
platinum black A finely divided blackform of platinum produced, as a coating,by evaporating platinum onto a surface inan inert atmosphere. Platinum-black coat-ings are used as pure absorbent electrodecoatings in experiments on electric cells.They are also used, like carbon-black coat-ings, to improve the ability of a surface toabsorb radiation.
platinum(II) chloride (PtCl2) A gray-brown powder prepared by the partial de-composition of platinum(IV) chloride. Itmay also be prepared by passing chlorineover heated platinum. Platinum(II) chlo-ride is insoluble in water but dissolves inconcentrated hydrochloric acid to form acomplex acid.
platinum(IV) chloride (PtCl4) A reddishhygroscopic solid prepared by the action ofheat on chloroplatinic acid. Crystals of thepentahydrate, PtCl4.5H2O, are formed; an-hydrous platinum(IV) chloride is preparedby treating the hydrated crystals with con-centrated sulfuric acid. The chloride dis-solves in water to produce a strongly acidicsolution. Platinum(IV) chloride forms a se-ries of hydrates having 1, 2, 4, and 5 mol-ecules of water; the tetrahydrate is the moststable. The strongly acidic solution pro-duced on dissolving in water probably con-tains H2[PtCl4(OH)2].
platinum–iridium An alloy of platinumcontaining up to 30% iridium. Hardnessand resistance to chemical attack increaseas the iridium content is increased. It isused in jewelry, electrical contacts, and hy-podermic needles.
platinum metals The group of transitionmetals ruthenium (Ru), osmium (Os),rhodium (Rh), iridium (Ir), palladium (Pd),
and platinum (Pt). They are sometimesclassed together as they have related prop-erties and compounds.
Plexiglas /pleks-ă-glass/ (Trademark) Awidely-used acrylic resin, polymethyl-methacrylate.
plumbane /plum-bayn/ (lead(IV) hydride;PbH4) A colorless unstable gas that can beobtained by the action of acids on a mix-ture of magnesium and lead pellets.
plumbic /plum-bik/ Designating alead(IV) compound.
plumbous /plum-bŭs/ Designating alead(II) compound.
plutonium /ploo-toh-nee-ŭm/ A radioac-tive silvery element of the actinoid series ofmetals. It is a transuranic element found onEarth only in minute quantities in uraniumores but readily obtained, as 239Pu, by neu-tron bombardment of natural uranium.The readily fissionable 239Pu is a major nu-clear fuel and nuclear explosive. Plutoniumis highly toxic because of its radioactivity;in the body it accumulates in bone.
Symbol: Pu; m.p. 641°C; b.p. 3232°C;r.d. 19.84 (25°C); p.n. 94; most stable iso-tope 244Pu (half-life 8.2 × 107 years).
PM See particulate matter.
point defect See defect.
poison 1. A substance that destroys cata-lyst activity.2. Any substance that endangers biologicalactivity, whether by physical or chemicalmeans.
polar Describing a compound with mol-ecules that have a permanent dipole mo-ment. Hydrogen chloride and water areexamples of polar compounds.
polar bond A covalent bond in which thebonding electrons are not shared equallybetween the two atoms. A bond betweentwo atoms of different electronegativity issaid to be polarized in the direction of the
217
polar bond
more electronegative atom, i.e. the elec-trons are drawn preferentially towards theatom. This leads to a small separation ofcharge and the development of a bond DI-POLE MOMENT as in, for example, hydrogenfluoride, represented as H→F or as Hδ+–Fδ–
(F is more electronegative).The charge separation is much smaller
than in ionic compounds; molecules inwhich bonds are strongly polar are said todisplay partial ionic character. The effectof the electronegative element can be trans-mitted beyond adjacent atoms, thus theC–C bonds in, for example, CCl3CH3 andCH3CHO are slightly polar. See also inter-molecular forces.
polarimeter /poh-lă-rim-ĕ-ter/ (polar-iscope) An instrument for measuring OPTI-CAL ACTIVITY.
polarizability /poh-lă-rÿ-ză-bil-ă-tee/ Theease with which an electron cloud is de-formed (polarized). In ions, an increase insize or negative charge leads to an increasein polarizability. The concept is of particu-lar use in the treatment of covalent contri-butions to predominantly ionic bondsembodied in Fajans’ Rules. Thus ions, suchas I–, Se2–, Te2–, are especially prone to co-valent character, particularly in combina-tion with ions of small size and highpositive charge (i.e. high ionic potential).Molecular polarizability is a measure ofthe ease with which electrons in the mole-cule are deformed, particularly by electro-magnetic radiation.
polarization /poh-lă-ri-zay-shŏn/ 1. Therestriction of the vibrations in a transversewave so that the vibration occurs in a sin-gle plane. Electromagnetic radiation, forinstance, is a transverse wave motion. Itcan be thought of as an oscillating electricfield and an oscillating magnetic field, bothat right angles to the direction of propaga-tion and at right angles to each other. Usu-ally, the electric vector is considered since itis the electric field that interacts withcharged particles of matter and causes theeffects. In ‘normal’ unpolarized radiation,the electric field oscillates in all possible di-rections perpendicular to the wave direc-
tion. On reflection or on transmissionthrough certain substances (e.g. Polaroid)the field is confined to a single plane. Theradiation is then said to be plane-polarized.If the tip of the electric vector describes acircular helix as the wave propagates, thelight is said to be circularly polarized.2. See polarizability.3. The reduction of current in a voltaic cell,caused by the build-up of products of thechemical reaction. Commonly, the cause isthe build-up of a layer of bubbles (e.g. ofhydrogen). This reduces the effective areaof the electrode, causing an increase in thecell’s internal resistance. It can also pro-duce a back e.m.f. Often a substance suchas manganese(IV) oxide (a depolarizer) isadded to prevent hydrogen build-up.
polar molecule A molecule in which theindividual polar bonds are not perfectlysymmetrically arranged and are thereforenot ‘in balance’. Thus the charge separa-tion in the bonds gives rise to an overallcharge separation in the molecule as for ex-ample, in water. Such molecules possess adipole moment.
polarography /poh-lă-rog-ră-fee/ An an-alytical method in which current is mea-sured as a function of potential. A specialtype of cell is used in which there is a smalleasily polarizable cathode (the droppingmercury electrode) and a large non-polar-izable anode (reference cell). The analyticalreaction takes place at the cathode and isessentially a reduction of the cations,which are discharged according to theorder of their electrode potential values.The data is expressed in the form of a po-larogram, which is a plot of current againstapplied voltage. As the applied potential isincreased a point is reached at which theion is discharged. There is a step-wise in-crease in current, which levels off becauseof polarization effects. The potential athalf the step height (called the half-wavepotential) is used to identify the ion. Mostelements can be determined by polarogra-phy. The optimum concentrations are inthe range 10–2–10–4M; modified tech-niques allow determinations in the partsper million range.
polarimeter
218
polar solvent See solvent.
pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside-effects of human industrial or socialactivities. Pollution can affect the atmos-phere, rivers, seas, and the soil.
Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by carexhausts. Among recent problems are in-dustrial emissions of sulfur(IV) oxide caus-ing acid rain, and the release into theatmosphere of chlorofluorocarbons, usedin refrigeration, aerosols, etc., has beenlinked to the depletion of ozone in thestratosphere. Carbon dioxide, produced byburning fuel and by car exhausts, is slowlybuilding up in the atmosphere, whichcould result in an overall increase in thetemperature of the atmosphere (green-house effect). Car exhausts also containcarbon monoxide and lead. The former hasnot yet reached dangerous levels, but vege-tation near main roads contains a high pro-portion of lead and levels are sufficientlyhigh in urban areas to cause concern aboutthe effects on children. Lead-free gasolineis available. Photochemical smog, causedby the action of sunlight on hydrocarbonsand nitrogen oxides from car exhausts, is aproblem in several countries. Catalyticconverters reduce harmful emissions fromcar exhausts.
Water pollutants include those that arebiodegradable, such as sewage effluent,which cause no permanent harm if ade-quately treated and dispersed, as well asthose which are nonbiodegradable, such ascertain chlorinated hydrocarbon pesticides(e.g. DDT) and heavy metals, such as lead,copper, and zinc in some industrial efflu-ents (causing heavy-metal pollution).When these accumulate in the environmentthey can become very concentrated in foodchains. The pesticides DDT, aldrin, anddieldrin are now banned in many coun-tries. Water supplies can become pollutedby leaching of nitrates from agriculturalland. The discharge of waste heat can causethermal pollution of the environment, butthis is reduced by the use of cooling towers.
In the sea, oil spillage from tankers and theinadequate discharge of sewage effluentare the main problems.
Other forms of pollution are noise fromaircraft, traffic, and industry and the dis-posal of radioactive waste.
polonium /pŏ-loh-nee-ŭm/ A radioactivemetallic element belonging to group 16 ofthe periodic table. It occurs in very minutequantities in uranium ores. Over 30 ra-dioisotopes are known, nearly all alpha-particle emitters. Polonium is a volatilemetal and evaporates with time. It is alsostrongly radioactive; a quantity of polo-nium quickly reaches a temperature of afew hundred degrees C because of thealpha emission. For this reason it has beenused as a lightweight heat supply in spacesatellites.
Symbol: Po; m.p. 254°C; b.p. 962°C;r.d. 9.32 (20°C); p.n. 84; stablest isotope209Po (half-life 102 years).
polyamide /pol-ee-am-ÿd, -id/ A syntheticpolymer in which the monomers are linkedby the group –NH–CO–. Nylon is an ex-ample of a polyamide.
polyatomic /pol-ee-ă-tom-ik/ Describinga molecule (or ion or radical) that consistsof several atoms (three or more). Examplesare benzene (C6H6) and methane (CH4).
polybasic /pol-ee-bay-sik/ Describing anacid that has two or more replaceable hy-drogen atoms. For example, phospho-rus(V) acid, H3PO4, is tribasic.
polycarbonate /pol-ee-kar-bŏ-nayt/ Athermoplastic polymer consisting of poly-esters of carbonic acid and dihydroxy com-pounds. They are tough and transparent,used for making soft-drink bottles andelectrical connectors.
polychlorinated biphenyl /pol-ee-klor-ă-nay-tid, -kloh-ră- / (PCB) A type of com-pound based on biphenyl (C6H5C6H5), inwhich some of the hydrogen atoms havebeen replaced by chlorine atoms. They areused in certain polymers used for electricalinsulators. PCBs are highly toxic and con-
219
polychlorinated biphenyl
cern has been caused by the fact that theycan accumulate in the food chain.
polychloroethene /pol-ee-klor-oh-eth-een, -kloh-roh-/ (polyvinyl chloride; PVC)A synthetic polymer made fromchloroethene. It is a strong material with awide variety of uses.
polycrystalline /pol-ee-kris-tă-lin, -lÿn/Describing a substance composed of verymany minute interlocking crystals thathave solidified together.
polycyclic /pol-ee-sÿ-klik/ Describing acompound that has two or more rings in itsmolecules.
polydioxoboric(III) acid /pol-ee-dÿ-oks-oh-bô-rik/ See boric acid.
polyene /pol-ee-een/ An alkene with morethan two double bonds in its molecules.
polyester /pol-ee-es-ter/ A synthetic poly-mer made by reacting alcohols with acids,so that the monomers are linked by thegroup –O–CO–. Synthetic fibers such asDacron are polyesters.
polyethene /pol-ee-eth-een/ (polyethylene;polythene) A synthetic polymer made fromethene. It is produced in two forms – a softmaterial of low density and a harder,higher density form, which is more rigid. Itcan be made by the Ziegler process.
polyethylene /pol-ee-eth-ă-leen/ See poly-ethene.
polyhydric alcohol /pol-ee-hÿ-drik/ Analcohol that has several –OH groups in itsmolecules.
polymer /pol-i-mer/ A compound com-posed of very large molecules made up ofrepeating molecular units (monomers). Apolymer has a repeated structural unit,known as a mer. Polymers do not usuallyhave a definite relative molecular mass, asthere are variations in the lengths of differ-ent chains. They may be natural substances(e.g. polysaccharides or proteins) or syn-
thetic materials (e.g. nylon or polyethene).The two major classes of synthetic poly-mers are thermosetting, which harden irre-versibly with increased temperature (e.g.Bakelite), and thermoplastic, which softenon heating (e.g. polyethene). See also poly-merization.
polymerization /pol-i-mer-i-zay-shŏn/The process in which one or more com-pounds react to form a POLYMER. Ho-mopolymers are formed by polymerizationof one monomer (e.g. the formation ofpolyethene from ethene). Heteropolymersor copolymers come from two or moremonomers (e.g. the production of NYLON).Heteropolymers may be of different typesdepending on the arrangement of units. Analternating copolymer of two units A and Bhas an arrangement:
–A–B–A–B–A–B–A block copolymer has an arrangement inwhich blocks of one monomer alternatewith blocks of the other; for example:
–A–A–A–B–B–B–A–A–A–In a graft copolymer there is a main choiceof one monomer (–A–A–A–A–), with shortside chains of the other monomer attachedat regular intervals (–B–B–).
Stereospecific polymers have the sub-unit repeated along the chain in a regularway. These are tactic polymers. If one par-ticular group is always on the same side ofthe chain, the polymer is said to be isotac-tic. If the group alternates in position alongthe chain the polymer is syndiotactic. Ifthere is no regular pattern, the polymer isatactic.
Polymerization reactions are also classi-fied according to the type of reaction. Ad-dition polymerization occurs when themonomers undergo addition reactions,with no other substance formed. Conden-sation polymerization involves the elimina-tion of small molecules in formation of thepolymer. See also cross linkage.
polymethanal /pol-ee-meth-ă-nal/ Seemethanal.
polymethylmethacrylate /pol-ee-meth-ăl-meth-ak-ră-layt/ See Plexiglas.
polychloroethene
220
polymorphism /pol-ee-mor-fiz-ăm/ Theability of certain chemical substances toexist in more than one physical form. Forelements this is called allotropy. The exis-tence of two forms is called dimorphism.Crystalline structures of compounds canvary with temperature as a result of differ-ent packing arrangements of the particles.There is a transition temperature betweentwo forms and usually a marked change indensity.
polynucleotide /pol-ee-new-klee-ŏ-tÿd/See nucleotide.
polypeptide /pol-ee-pep-tÿd, -tid/ A PEP-TIDE composed of a large number ofamino-acid units. PROTEINS are polypep-tides containing a few hundred amino-acidunits.
polypropene /pol-ee-proh-peen/ (poly-propylene) A synthetic polymer madefrom propene. It is similar in properties topolythene, but stronger and lighter. Thepropene is polymerized by the Zieglerprocess.
polypropylene /pol-ee-proh-pă-leen/ Seepolypropene.
polysaccharide /pol-ee-sak-ă-rÿd, -rid/High-molecular-weight polymers of themonosaccharides or sugars. They containmany repeated units in their molecularstructures. They can be broken down tosmaller polysaccharides, disaccharides,and monosaccharides by hydrolysis or bythe appropriate enzyme. Important poly-saccharides are heparin, inulin, starch,glycogen (also known as animal starch),
and cellulose. See also carbohydrates;sugar.
polystyrene /pol-ee-stÿ-reen/ A syntheticpolymer made from styrene(phenylethene). Expanded polystyrene is arigid foam used in packing and insulation.
polytetrafluoroethene /pol-ee-tet-ră-floo-ŏ-roh-eth-een/ (PTFE) A syntheticpolymer made from tetrafluoroethene (i.e.CF2:CF2). It is able to withstand high tem-peratures without decomposing and alsohas a very low coefficient of friction, henceits use in non-stick pans, bearings, etc.
polyunsaturated /pol-ee-ŭn-sach-ŭ-ray-tid/ Describing a compound that has anumber of C=C bonds in its compounds.
polyurethane /pol-ee-yoor-ă-thayn/ Asynthetic polymer containing the group–NH–CO–O– linking the monomers.Polyurethanes are made by condensationof isocyanates (–NCO) with alcohols.
polyvalent /pol-ee-vay-lĕnt, pŏ-liv-ă-/ De-scribing an atom or group that has a va-lence of more than one.
polyvinyl acetate /pol-ee-vÿ-năl/ (PVA;polyvinyl ethanoate) A thermoplasticpolymer made by the polymerization ofvinyl ethanoate, CH2:CHOOCH3. It isused as a coating for paper and cloth andin adhesives.
polyvinyl chloride See polychloroethene.
polyvinyl ethanoate See polyvinyl ac-etate.
221
polyvinyl ethanoate
isotactic syndiotactic
H H HH H H
H H HX X X
H H HH H H
HH
HX
XX
XH
Addition polymers
porcelain /por-sĕ-lin, pohr-/ A ceramictraditionally made from feldspar, kaolin(China clay), marble, and quartz.
porphyrins /por-fă-rinz/ Cyclic organicstructures that have the important charac-teristic property of forming complexeswith metal ions. Examples of such metallo-porphyrins are the iron porphyrins (e.g.heme in hemoglobin) and the magnesiumporphyrin, chlorophyll, the photosyntheticpigment in plants. In nature, the majorityof metalloporphyrins are conjugated toproteins to form a number of very impor-tant molecules, e.g. hemoglobin, myoglo-bin, and the cytochromes.
potash See potassium carbonate; potas-sium hydroxide.
potash alum See alum; aluminum potas-sium sulfate.
potassamide /pŏ-tass-ă-mÿd/ See potas-sium monoxide.
potassium /pŏ-tass-ee-ŭm/ A soft reactivemetal; the third member of the alkali met-als (group 1 of the periodic table). Theatom has the argon electronic configura-tion plus an outer 4s1 electron. The elementgives a distinct lilac color to flames but,due to the intense yellow coloration fromany trace impurity of sodium, the flamemust be viewed through a cobalt glass. Theionization potential is low and the chem-istry of potassium is largely the chemistryof the K+ ion. Potassium accounts for 2.4%of the lithosphere and occurs in large saltdeposits such as carnallite(KCl.MgCl2.6H2O) and arcanite (K2SO4).Large amounts also occur in mineral formsthat are not of much use for recovery ofpotassium, e.g., orthoclase, K2Al2Si6O10.The commercial usage of potassium ismuch smaller than that of sodium; indus-trially the element is obtained by electroly-sis of the fused hydroxide. Potassiumhydroxide is obtained by electrolysis ofcarnallite solutions (the magnesium is ini-tially precipitated as Mg(OH)2), and bothhydrogen and chlorine are recovered as by-products. The chemistry of potassium and
its compounds is very similar to that of theother group 1 elements.
Particular points of distinction fromsodium are:1. Burning in air produces the superoxide
KO2.2. Because of the larger size of the K+ ion
and hence lower lattice energies, potas-sium salts are often more soluble thancorresponding sodium salts.
3. Potassium salts are usually less heavilyhydrated than corresponding sodiumsalts.The most abundant isotope of potas-
sium is 39K (93.1%), with 41K also a stableisotope (6.8%), but there is a naturally oc-curring radioactive isotope 40K (0.11%).
Like sodium, potassium forms a num-ber of ionic alkyl- and aryl- derivatives ofthe type RK.
Symbol: K; m.p. 63.65°C; b.p. 774°C;r.d. 0.862 (20°C); p.n. 19; r.a.m. 39.0983.
potassium–argon dating A techniquefor dating rocks. It depends on the ra-dioactive decay of the radioisotope 40K to40Ar (half-life 1.27 × 1010 years) and isbased on the assumption that argon hasbeen trapped in the rock since the time thatthe rock cooled (i.e. since it formed). If theratio 40Ar/40K is measured it is possible toestimate the age of the rock. See also ra-dioactive dating.
potassium bicarbonate See potassiumhydrogencarbonate.
potassium bromide (KBr) A white solidthat is extremely soluble in water. It isformed by the action of bromine on hotpotassium hydroxide solution or by theneutralization of the carbonate with hy-drobromic acid. Potassium bromide formscolorless cubic crystals. It is used exten-sively in the manufacture of photographicplates, films, and papers and (formerly) asa sedative in medicine.
potassium carbonate (pearl ash; potash;K2CO3) A white deliquescent solid manu-factured from potassium chloride by theLeblanc process or, more often, using thePrecht process. It cannot be made by the
porcelain
222
Solvay process. In the laboratory, it can beprepared by thermal decomposition ofpotassium hydrogencarbonate. Potassiumcarbonate is very soluble in water, its solu-tions being strongly alkaline due to salt hy-drolysis. It is used in the laboratory as adrying agent and industrially in the manu-facture of soft soap, hard glass, and in thedyeing industry. Potassium carbonate crys-tallizes out between 10 and 25°C asK2CO3.3H2O; it dehydrates at 100°C toK2CO3.H2O and at 130°C to K2CO3.
potassium chlorate (KClO3) A whitesoluble solid prepared by the electrolysis ofa concentrated solution of potassium chlo-ride. Industrially, it is prepared by the frac-tional crystallization of a solutioncontaining sodium chlorate and potassiumchloride. When heated, it decomposes toyield oxygen and potassium chloride.Potassium chlorate is a powerful oxidizingagent and is used in explosives, matches,weedkillers, and fireworks, and as a disin-fectant. It oxidizes iodide ions to iodinewhen in an acidic medium. When heatedjust above its melting point, potassium per-chlorate is formed.
potassium chloride (KCl) A white ionicsolid prepared by neutralizing hydrochlo-ric acid with potassium hydroxide solu-tion. Potassium chloride occurs naturallyas the minerals sylvine and carnallite. It ismore soluble than sodium chloride in hotwater but less soluble in cold water. Onevaporation of an aqueous solution color-less cubic crystals similar to those ofsodium chloride are produced. Potassiumchloride is used as a fertilizer and in themanufacture of potassium hydroxide.
potassium chromate (K2CrO4) A brightyellow solid prepared by adding potassiumhydroxide solution to a solution of potas-sium dichromate. It is extremely soluble inwater. Addition of an acid to an aqueoussolution of potassium chromate convertsthe chromate ions into dichromate ions.The salt is used as an indicator in silver ni-trate titrations. The crystals are isomor-phous with potassium sulfate.
potassium cyanide (KCN) A white ionicsolid that is very soluble in water and ex-tremely poisonous. It is made industriallyby the Castner process and is used as asource of cyanide and hydrocyanic acid.Aqueous solutions of potassium cyanideare strongly hydrolyzed, the solutionsbeing alkaline. On standing, these solu-tions slowly evolve hydrocyanic acid.
potassium dichromate (K2Cr2O7) Anorange-red solid prepared by adding potas-sium chloride solution to a concentratedsolution of sodium dichromate and crystal-lizing out or by acidifying a solution ofpotassium chromate and evaporating. It isless soluble than sodium dichromate incold water but more soluble in hot water.Potassium dichromate is used as an oxidiz-ing agent both in volumetric analysis andin organic chemistry. The crystals are tri-clinic, anhydrous, and non-deliquescent.Addition of an alkali to a solution of thedichromate yields the chromate.
potassium hydride (KH) A white ionicsolid prepared by passing hydrogen overheated potassium, the metal being sus-pended in an inert medium. It is an excel-lent reducing agent.
potassium hydrogencarbonate (potas-sium bicarbonate; KHCO3) A white solidprepared by passing carbon dioxidethrough a saturated solution of potassium-carbonate. It occurs naturally in the min-eral calcinite. Potassium hydrogencarbonate is more soluble in water thansodium hydrogencarbonate. When heated,it undergoes thermal decomposition to givethe carbonate, water, and carbon dioxide.The salt forms monoclinic crystals. Its so-lutions are strongly alkaline as a result ofsalt hydrolysis.
potassium hydrogentartrate /hÿ-drŏ-jĕn-tar-trayt/ (cream of tartar;HOOC(CHOH)2COO–K+) A white crys-talline solid, an acid salt of tartaric acidthat occurs in grape juice. It is used as theacid ingredient of BAKING POWDER.
223
potassium hydrogentartrate
potassium hydroxide (caustic potash;KOH) A white solid manufactured by theelectrolysis of potassium chloride solutionin a mercury-cathode cell. It can also beprepared by heating either potassium car-bonate or potassium sulfate with slakedlime. The process closely resembles theGossage process for making sodium hy-droxide. In the laboratory, it can be madeby reacting potassium, potassium monox-ide, or potassium superoxide with water.Potassium hydroxide resembles sodium hy-droxide but is more soluble in water andalcohol. It is preferred to sodium hydrox-ide for the absorption of carbon dioxideand sulfur(IV) oxide because of its greatersolubility. Potassium hydroxide is used asan electrolyte in the Ni–Fe electric storagebattery and in the production of soft soaps.It forms crystalline hydrates with 1, 1½,and 2 molecules of water.
potassium iodate /ÿ-ŏ-dayt/ (KIO3) Awhite solid formed either by adding iodineto a hot concentrated solution of potas-sium hydroxide or by the electrolysis ofpotassium iodide solution. No hydrates areknown. It is a source of iodide and iodicacid. When treated with a dilute acid and areducing agent, the iodate ions are reducedto iodine.
potassium iodide (KI) A white ionicsolid readily soluble in water. It is preparedby dissolving iodine in hot concentratedpotassium hydroxide solution. Both the io-dide and iodate are formed but the latter isremoved by fractional crystallization.Potassium iodide has a sodium chloridecubic lattice. In solution with iodine itforms potassium tri-iodide. It is used inmedicine, particularly in the treatment ofgoiter resulting from iodine deficiency. Di-lute acidified solutions of potassium iodidecan act as reducing agents, manganate(VII)ions being reduced to manganese(II) ions,copper(II) ions to copper(I) ions, and io-date ions to iodine.
potassium manganate(VII) (potassiumpermanganate; KMnO4) A purple solidsoluble in water. It is prepared by oxidizingpotassium manganate(VI) with chlorine.
Potassium permanganate is used in volu-metric analysis as an oxidizing agent, as abactericide, and as a disinfectant. In aque-ous solution its behavior as an oxidizingagent depends on the pH of the solution.
potassium monoxide /mon-oks-ÿd, -id/(K2O) An ionic solid that is white whencold and yellow when hot. It is prepared byheating potassium with potassium nitrate.Potassium monoxide dissolves violently inwater to form potassium hydroxide solu-tion. The hydrate K2O.3H2O is known.Potassium monoxide dissolves in liquidammonia with the formation of potassiumhydroxide and potassamide (KNH2).
potassium nitrate (saltpeter; niter;KNO3) A white solid, soluble in water,formed by fractional crystallization ofsodium nitrate and potassium chloride so-lutions. It occurs naturally as niter (salt-peter) in rocks in India, South Africa, andBrazil. When heated it decomposes to givethe nitrite and oxygen. Unlike sodium ni-trate it is non-deliquescent. Potassium ni-trate is used in gunpowder, fertilizers, andin the laboratory preparation of nitric acid.
potassium nitrite (KNO2) A creamy del-iquescent solid that is readily soluble inwater. It reacts with cold dilute mineralacids to produce solutions of nitrous acid.Potassium nitrite is used in organic chem-istry in the process of diazotization.
potassium permanganate See potas-sium manganate(VII).
potassium sulfate (K2SO4) A white solidprepared by the neutralization of eitherpotassium hydroxide or potassium carbon-ate with dilute sulfuric acid. It occurs natu-rally in Stassfurt deposits as schönite.Potassium sulfate is soluble in water, form-ing a neutral solution. It is used as a fertil-izer and in the chemical industry in thepreparation of alums. The anhydrous saltcrystallizes in the rhombic form.
potassium sulfide (K2S) A yellowish-brown solid prepared by saturating anaqueous solution of potassium hydroxide
potassium hydroxide
224
with hydrogen sulfide and then adding anequal volume of potassium hydroxide. In-dustrially it is produced by heating potas-sium sulfate and carbon at hightemperature. The sulfide crystallizes outfrom aqueous solution as K2S.5H2O. Itsaqueous solutions undergo hydrolysis, thesolution being strongly alkaline.
potassium superoxide (K2O) A yellowparamagnetic solid prepared by burningpotassium in excess oxygen. When treatedwith cold water or dilute mineral acids, hy-drogen peroxide is produced. If heatedstrongly, it yields oxygen and potassiummonoxide. Potassium superoxide is a pow-erful oxidizing agent.
potassium thiocyanate /th’ÿ-oh-sÿ-ă-nayt/ (KSCN) A colorless hygroscopicsolid. Its solution is used in a test foriron(III) compounds, with which it turns ablood-red color.
potentiometric titration /pŏ-ten-shee-ŏ-met-rik/ A titration in which an electrodeis used in the reaction mixture. The endpoint can be found by monitoring the elec-tric potential of this during the titration.
praseodymium /pray-zee-oh-dim-ee-ŭm/A soft ductile malleable silvery element ofthe lanthanoid series of metals. It occurs inassociation with other lanthanoids.Praseodymium is used in several alloys, asa catalyst, and in enamel and yellow glassfor eye protection.
Symbol: Pr; m.p. 931°C; b.p. 3512°C;r.d. 6.773 (20°C); p.n. 59; r.a.m. 140.91.
precipitate /pri-sip-ă-tayt (vb.), pri-sip-ă-tayt, -tit (n.)/ A suspension of small parti-cles of a solid in a liquid formed by achemical reaction.
precursor A substance from which an-other substance is formed in a chemical re-action.
pressure Symbol: p The pressure on a sur-face due to forces from another surface orfrom a fluid is the force acting at 90° tounit area of the surface:
pressure = force/areaThe unit is the pascal (Pa).
primary alcohol See alcohol.
primary amine See amine.
primary cell A voltaic cell in which thechemical reaction that produces the e.m.f.is not reversible. Compare accumulator.
primary standard A substance that canbe used directly for the preparation of stan-dard solutions without reference to someother concentration standard. Primarystandards should be easy to purify, dry, ca-pable of preservation in a pure state, unaf-fected by air or CO2, of a high molecularweight (to reduce the significance of weigh-ing errors), stoichiometric, and readily sol-uble. Any likely impurities should be easilyidentifiable.
prismane /priz-mayn/ See Ladenburg ben-zene.
producer gas (air gas) A mixture of car-bon monoxide (25–30%), nitrogen(50–55%), and hydrogen (10–15%), pre-pared by passing air with a little steamthrough a thick layer of white-hot coke ina furnace or ‘producer’. The air gas is usedwhile still hot to prevent heat loss and findsuses in industrial heating, for example thefiring of retorts and in glass furnaces. Com-pare water gas.
product See chemical reaction.
proline /proh-leen, -lin/ See amino acids.
promethium /prŏ-mee-th’ee-ŭm/ A ra-dioactive element of the lanthanoid seriesof metals. It is not found naturally on Earthbut can be produced artificially by the fis-sion of uranium. It is used in some minia-ture batteries.
Symbol: Pm; m.p. 1168°C; b.p. 2730°C(approx.); r.d. 7.22 (20°C); p.n. 61; sta-blest isotope 145Pm (half-life 18 years).
promoter (activator) A substance thatimproves the efficiency of a catalyst. It does
225
promoter
proof
226
not itself catalyze the reaction but assiststhe catalytic activity. For example, aluminaor molybdenum promotes the catalytic ac-tivity of finely divided iron in the Haberprocess. The manner in which a promoterfunctions is not fully understood; no onetheory covers all the examples. See alsocoenzymes.
proof A measure of the ethanol content ofintoxicating drinks. In the United States,proof spirit contains 40% ethanol by vol-ume. In the UK, it contains 49.28% ofethanol by weight (57.1% by volume at16°C). The degree of proof gives the num-ber of parts of proof spirit per 100 parts ofthe total. 100° of proof is 50% by volume,80° of proof is 0.8 × 50%, etc.
propanal /proh-pă-nal/ (propionaldehyde;C2H5CHO) A colorless liquid aldehyde.
propane /proh-payn/ (C3H8) A gaseousalkane obtained either from the gaseousfraction of crude oil or by the cracking ofheavier fractions. The principal use ofpropane is as a fuel for heating and cook-ing, since it can be liquefied under pressure,stored in cylinders, and transported easily.Propane is the third member of the ho-mologous series of alkanes.
propanedioic acid /proh-payn-dÿ-oh-ik/(malonic acid; CH2(COOH)2) A whitecrystalline dibasic carboxylic acid.
propane-1,2,3-triol (glycerol; glycerine;CH2(OH)CH(OH)CH2(OH)) A colorlessviscous liquid obtained as a by-productfrom the manufacture of soap by the reac-tion of animal fats with sodium hydroxide.It is used as a solvent and plasticizer. Seealso glyceride.
propanoate /proh-pă-noh-ayt, -it/ A saltor ester of propanoic acid.
propanoic acid /proh-pă-noh-ik/ (propi-onic acid; C2H5COOH) A colorless liquidcarboxylic acid.
propanol /proh-pă-nol, -nohl/ Either oftwo alcohols: propan-1-ol (CH3CH2-
CH2OH) and propan-2-ol (CH3CH2-(OH)CH3). Both are colorless volatileflammable liquids.
propanone /proh-pă-nohn/ (acetone;CH3-COCH3) A colorless liquid ketone,used as a solvent and in the manufacture ofmethyl 2-methyl-propanoate (from whichpolymethylmethacrylate is produced).Propanone is manufactured from propene,either by the air-oxidation of propan-2-olor, as a by-product from the cumeneprocess.
propenal /proh-pĕ-năl/ (acrolein;CH2:CHCHO) A colorless liquid unsatu-rated aldehyde with a pungent odor. It canbe polymerized to make acrylate resins.
propene /proh-peen/ (propylene; C3H6) Agaseous alkene. Propene is not normallypresent in the gaseous crude-oil fractionbut can be obtained from heavier fractionsby catalytic cracking. This is the principalindustrial source. Propene is the organicstarting material for the production ofpropan-2-ol, required for the manufactureof propanone (acetone), and the startingmaterial for the production of polypropene(polypropylene).
propenoate /proh-pĕ-nayt/ A salt or esterof propenoic acid.
propenoic acid /proh-pĕ-noh-ik/ (acrylicacid; CH2:CHCOOH) An unsaturated liq-uid carboxylic acid with a pungent odor.The acid and its esters are used to makeACRYLIC RESINS.
propenonitrile /pro-pĕ-noh-nÿ-trăl, -tril, -trÿl/ (acrylonitrile; H2C:CH(CN)) An or-ganic compound from which acrylic-typepolymers are produced.
propenyl group /proh-pĕ-năl/ (allylgroup) The organic group,CH2=CH–CH2–, derived by removing onehydrogen atom from propene.
propionaldehyde /proh-pee-ŏ-nal-dĕ-hÿd/ See propanal.
propionic acid /proh-pee-on-ik/ Seepropanoic acid.
propylene /proh-pă-leen/ See propene.
propyl group /proh-păl/ The groupCH3CH2CH2–.
protactinium /proh-tak-tin-ee-ŭm/ Atoxic radioactive element of the actinoidseries of metals. It occurs in minute quanti-ties in uranium ores as a radioactive decayproduct of actinium.
Symbol: Pa; m.p. 1840°C; b.p. 4000°C(approx.); r.d. 15.4 (calc.); p.n. 91; moststable isotope 231Pa (half-life 32 500years).
protein /proh-teen, -tee-in/ A naturally oc-curring compound found in all living mat-ter, consisting of AMINO ACIDS joined intolong chains by PEPTIDE links. The primarystructure is the particular sequence ofamino acids present. The protein also has asecondary structure, with the chains coiledin a helix or held together in pleated sheets.The tertiary structure is the way in whichthe chain or sheet is arranged in space. Sec-ondary structures are held by hydrogenbonds between N–H and O=C groups.Tertiary structures are held by hydrogenbonds or cross linkages of the type –S–S–(cystine links).
proton An elementary particle with a pos-itive charge (+1.602 192 × 10–19 C) andrest mass 1.672 614 × 10–27 kg. Protonsare nucleons, found in all nuclides.
proton number (atomic number) Symbol:Z The number of protons in the nucleus ofan atom. The proton number determinesthe chemical properties of the element be-cause the electron structure, which deter-mines chemical bonding, depends on theelectrostatic attraction to the positivelycharged nucleus.
Prussian blue See cyanoferrate.
prussic acid /pruss-ik/ See hydrocyanicacid.
pseudoaromatic /soo-doh-a-rŏ-mat-ik/See aromatic compound.
pseudo-first order Describing a reactionthat appears to exhibit first-order kineticsunder special conditions, even though the‘true’ order is greater than one. For exam-ple, in the hydrolysis of an ester in the pres-ence of a large volume of water, theconcentration of water remains approxi-mately constant. The rate of reaction isthus found experimentally to be propor-tional to the concentration of the ester only(even though it also depends on theamount of water present). Such a reactionis described as ‘bimolecular of the firstorder’.
pseudohalogens /soo-doh-hal-ŏ-jĕnz/ Asmall group of simple inorganic com-pounds with symmetrical molecules thatresemble the halogens in some reactionsand compounds. For example, (CN)2
(cyanogen) has compounds analogous tohalogen compounds (e.g. HCN, KCN,CH3CN, etc.). Thiocyanogen, (SCN)2, isanother example.
PTFE See polytetrafluoroethene.
purine /pyoor-een, -in/ A simple nitroge-nous organic molecule with a double ringstructure. Members of the purine group in-clude adenine and guanine, which are con-stituents of the nucleic acids, and certainplant alkaloids, such as caffeine and theo-bromine.
PVA See polyvinyl acetate.
PVC See polychloroethene.
pyranose /pÿ-ră-nohs, pi-ran-ohs/ ASUGAR that has a six-membered ring form(five carbon atoms and one oxygen atom).
pyrazine /pÿ-ră-zeen, -zin/ (1,4-diazine;C4H4N2) A heterocyclic aromatic com-pound with a six-membered ring contain-ing four carbon atoms and two nitrogenatoms.
227
pyrazine
pyrazole /pÿ-ră-zohl/ (1,2-diazole;C3H4N2) A heterocyclic crystalline aro-matic compound with a five-memberedring containing three carbon atoms andtwo nitrogen atoms.
pyrene /pÿ-reen/ (C16H10) A solid aro-matic compound whose molecules consistof four benzene rings joined together. It iscarcinogenic.
Pyrex /pÿ-reks/ (Trademark) A particu-larly strong, heat resistant, and chemicallyinert borosilicate glass commonly used forlaboratory glassware.
pyridine /pi-ră-deen/ (C5H5N) An organicliquid of formula C5H5N. The moleculeshave a hexagonal planar ring and are iso-electronic with benzene. Pyridine is an ex-ample of an aromatic heterocycliccompound, with the electrons in the car-bon–carbon pi bonds and the lone pair ofthe nitrogen delocalized over the ring ofatoms. The compound is extracted fromcoal tar and used as a solvent and as a rawmaterial for organic synthesis.
pyrimidine /pi-rim-ă-deen, pi-ră-mă-/ Asimple nitrogenous organic moleculewhose ring structure is contained in thepyrimidine bases cytosine, thymine, anduracil, which are constituents of the nucleicacids.
pyrites /pÿ-rÿ-teez, pi-, pÿ-rÿts/ A mineralsulfide of a metal; e.g. iron pyrites FeS2.
pyrolusite /pÿ-rŏ-loo-sÿt/ See man-ganese(IV) oxide.
pyrolysis /pÿ-rol-ă-sis, pi-/ The decompo-sition of chemical compounds by subject-ing them to very high temperature.
pyrometer /pÿ-rom-ĕ-ter, pi-/ An instru-ment used in the chemical industry to mea-sure high temperature, e.g. in reactorvessels.
pyrone /pÿ-rohn, pi-rohn/ A compoundhaving a six-membered ring of five carbonatoms and one oxygen, with one of the car-bon atoms attached to a second oxygen ina carbonyl group. The pyrone ring systemhas two forms depending on the position ofthe carbonyl relative to the oxygen heteroatom. It occurs in many natural products.
pyrophoric /pÿ-rŏ-fô-rik, pi-/ 1. Describ-ing a compound that ignites spontaneouslyin air.2. Describing a metal or alloy that givessparks when struck. (Lighter flints aremade of pyrophoric alloy.)
pyrophosphoric acid /pÿ-roh-fos-fô-rik/See phosphoric(V) acid.
pyrosulfuric acid /pÿ-roh-sul-fyoor-ik/See oleum.
pyrrhole /pi-rohl/ ((CH)4NH) A hetero-cyclic liquid aromatic compound with afive-membered ring containing four carbonatoms and one nitrogen atom. It has im-portant biochemical derivatives, includingchlorophyll and heme.
pyrazole
228
NNN
Pyridine
229
quadrivalent /kwod-ră-vay-lĕnt, kwod-riv-ă-/ (tetravalent) Having a valence offour.
qualitative analysis /kwol-ă-tay-tiv/Analysis carried out with the purpose ofidentifying the components of a sample.Classical methods involved simple prelimi-nary tests followed by a carefully devisedscheme of systematic tests and procedures.Modern methods include the use of suchtechniques as infrared spectroscopy andemission spectrography. Compare quanti-tative analysis.
quanta /kwon-tă/ See quantum.
quantitative analysis /kwon-tă-tay-tiv/Analysis carried out with the purpose ofdetermining the concentration of one ormore components of a sample. Classicalwet methods include volumetric and gravi-metric analysis. A wide range of moremodern instrumental techniques are alsoused, including polarography and varioustypes of chromatography and spec-troscopy. Compare qualitative analysis.
quantized /kwon-tÿzd/ Describing a phys-ical quantity that can take only certain dis-crete values, and not a continuous range ofvalues. Thus, in an atom or molecule theelectrons around the nucleus can have cer-tain energies, E1, E2, etc., and cannot haveintermediate values. Similarly, in atomsand molecules, the electrons have quan-tized values of spin angular momentumand orbital angular momentum.
quantum /kwon-tŭm/ (pl. quanta) A defi-nite amount of energy released or absorbedin a process. Energy often behaves as if it were ‘quantized’ in this way. The quan-
tum of electromagnetic radiation is thephoton.
quantum electrodynamics The use ofquantum mechanics to describe how par-ticles and electromagnetic radiation inter-act.
quantum mechanics See quantum theory.
quantum number An integer or half in-teger that specifies the value of a quantizedphysical quantity (energy, angular momen-tum, etc.). See atom; Bohr theory; spin.
quantum states States of an atom, elec-tron, particle, etc., specified by a unique setof quantum numbers. For example, the hy-drogen atom in its ground state has an elec-tron in the K shell specified by the fourquantum numbers: n = 1, l = 0, m = 0, ms =½. In the helium atom there are two elec-trons:
n = 1, l = 0, m = 0, ms = ½n = 1, l = 0, m = 0, ms = –½
quantum theory A mathematical theoryoriginally introduced by Max Planck(1900) to explain the radiation emittedfrom hot bodies. Quantum theory is basedon the idea that energy (or certain otherphysical quantities) can be changed only incertain discrete amounts for a given sys-tem. Other early applications were the ex-planations of the photoelectric effect andthe Bohr theory of the atom.
Quantum mechanics is a system of me-chanics that developed from quantum the-ory and is used to explain the behavior ofatoms, molecules, etc. In one form it isbased on de Broglie’s idea that particles canhave wavelike properties – this branch of
Q
quantum yield
230
quantum mechanics is called wave me-chanics. See orbital.
quantum yield The average number ofreactive events per absorbed photon in aphotochemical reaction.
quartz A natural crystalline form of silica(SiO2).
quasicrystal /kway-zÿ-kris-tăl,-sÿ-, kwah-zee-, -see-/ A type of crystal structure inwhich there is long-range order but inwhich the symmetry of the repeating unit isnot one allowed in normal crystals. For ex-ample, in three dimensions, it would bepossible to form a crystal structure madeup of cubes, but not one made by joiningicosahedra. However, there are examplesof solids, such as the compound AlMn,that have icosahedral symmetry and along-range repeating order in the stucture.These are known as quasicrystals.
quaternary ammonium compound Acompound formed from an amine by addi-tion of a proton to produce a positive ion.Quaternary compounds are salts, the sim-plest example being ammonium com-pounds formed from ammonia and anacid, for example:
NH3 + HCl → NH4+Cl–
Other amines can also add protons to giveanalogous compounds. For instance,methylamine (CH3NH2) forms the com-pound
[CH3NH3]+X–
where X– is an acid radical.The formation of quarternary com-
pounds occurs because the lone pair on thenitrogen atom can form a coordinate bondwith a proton. This can also occur withheterogeneous nitrogen compounds, suchas adenine, cytosine, thymine, and gua-nine. Such compounds are known as ni-trogenous bases.
quenching A method used to alter the me-chanical properties of metals. Hot metal islowered quickly into a bath of oil, water,or brine and the rapid cooling results in afine grain structure. This treatment in-creases the hardness of a metal but oftenmakes it brittle.
quicklime See calcium oxide.
quinhydrone electrode /kwin-hÿ-drohn/See quinone.
quinine /kwÿ-nÿn, kwi-neen/ A poisonousALKALOID found in the bark of the cinchonatree of South America. It is used in treatingmalaria.
quinol /kwin-ol. -ohl/ See benzene-1,4-diol.
quinoline /kwin-ŏ-leen, -lin/ (C9H7N) Acolorless two-ring heterocyclic compoundwith an unpleasant odor, which acts as abase and forms salts with acids. First madefrom the alkaloid quinine, it is found inbone oil and coal tar and used for makingdrugs and dyestuffs.
quinone /kwă-nohn, kwin-ohn/ (cyclo-hexadiene-1,4-dione; benzoquinone;C6H4O2) A yellow crystalline organiccompound with a pungent odor. Its mole-cules contain a non-aromatic six-carbonring and it behaves as an unsaturated dike-tone with conjugated double bonds. It isused in making dyestuffs. A platinum elec-trode in an equimolar solution of quinoneand hydroquinone (benzene-1,4-diol,C6H4(OH)2) is used as a standard electrodein electrochemistry. The reaction is:
C6H4(OH)2 ˆ C6H4O2 + 2H+ + 2eThis type of electrode is called a quinhy-drone electrode.
quinquevalent /kwing-kwĕ-vay-lĕnt,kwing-kwev-ă-/ See pentavalent.
231
racemate /ray-seem-ayt, -sem-, ră-/ Seeoptical activity.
racemic mixture /ray-see-mik, ră-/ Seeoptical activity.
racemization /rass-ĕ-mi-zay-shŏn/ Theconversion of an optical isomer into anequal mixture of isomers, which is not op-tically active.
rad A unit of absorbed dose of ionizing ra-diation, defined as being equivalent to anabsorption of 10–2 joule of energy in onekilogram of material.
radian Symbol: rad The SI unit of planeangle; 2π radian is one complete revolution(360°).
radiation /ray-dee-ay-shŏn/ In general theemission of energy from a source, either aswaves (light, sound, etc.) or as moving par-ticles (beta rays or alpha rays).
radical /rad-ă-kăl/ A group of atoms in amolecule. See also free radical.
radioactive /ray-dee-oh-ak-tiv/ Describ-ing an element or nuclide that exhibits nat-ural radioactivity.
radioactive dating (radiometric dating)A technique for dating archaeological spec-imens, rocks, etc., by measuring the extentto which some radionuclide has decayed togive a product. See carbon dating; potas-sium–argon dating; rubidium–strontiumdating; uranium–lead dating.
radioactive decay series The series ofdefinite isotopes to which a given radioac-tive element is transformed as it decays.
radioactivity /ray-dee-oh-ak-tiv-ă-tee/The disintegration or decay of certain un-stable nuclides with emission of radiation.The emission of alpha particles, beta parti-cles, and gamma waves are the three mostimportant forms of radiation that occurduring decay.
radiocarbon dating /ray-dee-oh-kar-bŏn/See carbon dating.
radiochemistry /ray-dee-oh-kem-iss-tree/The chemistry of radioactive isotopes of el-ements. Radiochemistry involves such top-ics as the preparation of radioactivecompounds, the separation of isotopes bychemical reactions, the use of radioactivelabels in studies of mechanisms, and exper-iments on the chemical reactions and com-pounds of transuranic elements.
radiogenic /ray-dee-oh-jen-ik/ Caused byradioactive decay.
radioisotope /ray-dee-oh-ÿ-sŏ-tohp/ Aradioactive isotope of an element. Tritium,for instance, is a radioisotope of hydrogen.Radioisotopes are extensively used in re-search as souces of radiation and as tracersin studies of chemical reactions. Thus, if anatom in a compound is replaced by a ra-dioactive nuclide of the element (a label) itis possible to follow the course of thechemical reaction. Radioisotopes are alsoused in medicine or diagnosis and treat-ment.
radiolysis /ray-dee-ol-ă-sis/ A chemical re-action produced by high-energy radiation(x-rays, gamma rays, or particles).
radiometric dating /ray-dee-oh-met-rik/See radioactive dating.
R
radio waves A form of ELECTROMAGNETIC
RADIATION with wavelengths greater than afew millimeters.
radium /ray-dee-ŭm/ A white radioactiveluminescent metallic element of the alka-line-earth group. It has several short-livedradioisotopes and one long-lived isotope,radium-226 (half-life 1602 years). Radiumis found in uranium ores, such as the ox-ides pitchblende and carnotite. It was for-merly used in luminous paints andradiotherapy.
Symbol: Ra; m.p. 700°C; b.p. 1140°C;r.d. 5 (approx. 20°C); p.n. 88; r.a.m.226.0254 (226Ra).
radon /ray-don/ A colorless monatomicradioactive element of the rare-gas group,now known to form unstable compounds.It has 19 short-lived radioisotopes; themost stable, radon-222, is a decay productof radium-226 and itself disintegrates intoan isotope of polonium with a half-life of3.82 days. 222Rn is sometimes used in ra-diotherapy. Radon occurs in uraniummines and is also detectable in houses builtin certain areas of the country.
Symbol: Rn; m.p. –71°C; b.p. –61.8°C;d. 9.73 kg m–3 (0°C); p.n. 86.
raffinate /raf-ă-nayt/ The liquid remain-ing after the solvent extraction of a dis-solved substance. See solvent extraction.
raffinose /raf-ă-nohs/ A SUGAR occurringin sugar beet. It is a trisaccharide consistingof fructose, galactose, and glucose units.
r.a.m. See relative atomic mass.
Raman effect /rah-măn/ A change in thefrequency of electromagnetic radiationthat occurs when a photon of radiation un-dergoes an inelastic collision with a mol-ecule. The effect was first observed by theIndian physicists Sir ChandrasekharaVenkata Raman (1888–1970) and his col-league Sir Kariamanikkam Srinivasa Krish-nan in 1928. It has been used extensively inRaman spectroscopy for the determinationof molecular structure, particularly sincethe advent of the laser.
Raney nickel /ray-nee/ A catalytic formof nickel produced by treating anickel–aluminum alloy with caustic soda.The aluminum dissolves (as aluminate) andRaney nickel is left as a spongy mass,which is pyrophoric when dry. It is used es-pecially for catalyzing hydrogenation reac-tions.
Raoult’s law /rah-oolz/ A relationship be-tween the pressure exerted by the vapor ofa solution and the presence of a solute. Itstates that the partial vapor pressure of asolvent above a solution (p) is proportionalto the mole fraction of the solvent in the so-lution (X) and that the proportionalityconstant is the vapor pressure of pure sol-vent, (p0), at the given temperature: i.e. p =p0X. Solutions that obey Raoult’s law aresaid to be ideal. There are some binary so-lutions for which Raoult’s law holds overall values of X for either component. Suchsolutions are said to be perfect and this be-havior occurs when the intermolecular at-traction between molecules within onecomponent is almost identical to the at-traction of molecules of one component formolecules of the other (e.g. chlorobenzeneand bromobenzene). Because of solvationforces this behavior is rare and in generalRaoult’s law holds only for dilute solu-tions.
For solutions that are ideal but not per-fect the solute behavior is similar in thatthe partial pressure of the solute, ps, is pro-portional to the mole fraction of the solute,Xs, but in this case the proportionality con-stant, p′, is not the vapor pressure of thepure solute but must be determined exper-imentally for each system. This soluteequivalent of Raoult’s law has the form ps= p′Xs, and is called HENRY’S LAW. Becauseof intermolecular attractions p′ is usuallyless than p0. The law is named for theFrench chemist Françoise-Marie Raoult(1830–1901). See also Babo’s law.
rare earths See lanthanoids.
rare gases (noble gases; inert gases; group0 elements) A group of of monatomic,low-boiling gases classified as belonging togroup 0 of the periodic table and occupy-
radio waves
232
233
rate of reaction
ing a position between the highly elec-tronegative group 17 elements and thehighly electropositive group 1 elements.They are: helium (He), neon (Ne), argon(Ar), krypton (Kr), xenon (Xe), and radon(Rn). This complete filling of the s and plevels gives a closed-shell structure and isassociated with a general lack of chemicalreactivity and very high ionization ener-gies. It is the acquisition of a stable rare-gasconfiguration that is associated with deter-mining the valence of many covalent andionic compounds.
Prior to 1962 the rare gases were fre-quently called inert gases as no chemicalcompounds were known (there were a fewclathrates and ‘hydrates’), but the realiza-tion that the ionization potential of xenonwas sufficiently low to be accessible tochemical reaction led to the preparation ofseveral fluorides, oxides, oxyfluorides, anda hexafluoroplatinate of xenon. Severalunstable krypton and radon compoundshave been synthesized.
Argon forms about 1% of the atmos-phere but the other gases are present inonly minute traces (excluding radon). Theyare obtained by fractionation of liquid airor (in the USA) of natural gas. Consider-able amounts of argon are produced as aby-product from ammonia-plant tail gas.Their applications depend heavily on theirinertness; for example, welding (Ar), fillinglight bulbs (Ar), gas-stirring in high tem-perature metallurgy (Ar), powder technol-ogy (Ar), discharge tubes (Ne), andchemical research as inert carriers (He, Ar).Neon has been proposed as an oxygen dilu-ent for deep-sea diving because of its lowersolubility in blood. Liquid helium is exten-sively used in low-temperature work andradon is used therapeutically as a source ofα-particles.
The gases helium and argon are formedby radioactive decay in various minerals,for example argon by electron capture of40K. This mechanism may be applied toage-determination of minerals. Radon isgenerally obtained as 222Rn from decay ofradium. The half-life is only about 3.8days.
Rashig process /rash-ig/ A method for
the manufacture of chlorobenzene, andthence phenol, from benzene. Benzenevapor, hydrogen chloride, and air arepassed over a copper(II) chloride catalyst(230°C):
2C6H6 + 2HCl + O2 → 2C6H5Cl +2H2O
The conversion to phenol is by a siliconcatalyst (425°C):
C6H5Cl + H2O → HCl + C6H5OH
rate constant (velocity constant; specificreaction rate) Symbol: k The constant ofproportionality in the rate expression for achemical reaction. For example, in a reac-tion A + B → C, the rate may be propor-tional to the concentration of A multipliedby that of B; i.e.
rate = k[A][B]where k is the rate constant for this partic-ular reaction. The constant is independentof the concentrations of the reactants butdepends on temperature; consequently thetemperature at which k is recorded must bestated. The units of k vary depending onthe number of terms in the rate expression,but are easily determined rememberingthat rate has the units s–1.
rate-determining step (limiting step)The slowest step in a multistep reaction.Many chemical reactions are made up of anumber of steps in which the one with thelowest rate is the one that determines therate of the overall process.
The overall rate of a reaction cannotexceed the rate of the slowest step. For ex-ample, the first step in the reaction betweenacidified potassium iodide solution and hy-drogen peroxide is the rate-determiningstep:
H2O2 + I– → H2O + OI– (slow)H+ + OI– → HOI (fast)
HOI + H+ + I– → I2 + H2O (fast)
rate of reaction A measure of the amountof reactant consumed in a chemical reac-tion in unit time. It is thus a measure of thenumber of effective collisions between re-actant molecules. The rate at which a reac-tion proceeds can be measured by the ratethe reactants disappear or by the rate atwhich the products are formed. The princi-
pal factors affecting the rate of reaction aretemperature, pressure, concentration of re-actants, light, and the action of a catalyst.The units usually used to measure the rateof a reaction are mol dm–3 s–1. See also lawof mass action.
rationalized units A system of units inwhich the equations have a logical form re-lated to the shape of the system. SI unitsform a rationalized system of units. In itformulae concerned with circular symme-try contain a factor of 2π; those concernedwith radial symmetry contain a factor of4π.
raw material A substance from whichother substances are made. In the chemicalindustry it may be simple (such as nitrogenfrom air used to make ammonia) or com-plex (such as coal and petroleum, used tomake a wide range of products).
rayon /ray-on/ An artificial fiber formedfrom wood pulp (cellulose). There are twotypes. Viscose rayon is made by dissolvingthe cellulose in carbon disulfide andsodium hydroxide. The solution is forcedthrough a fine nozzle into an acid bath,which regenerates the fibers. Acetate rayonis made by dissolving cellulose acetate in anorganic solvent, and forcing the solutionthrough a nozzle. The solvent is evapo-rated, and the cellulose acetate thus ob-tained as fibers.
reactant /ree-ak-tănt/ A compound takingpart in a chemical reaction.
reaction See chemical reaction.
reactive dye A dye that ‘sticks’ to thefibers of a fabric by forming covalentchemical bonds with the substance of thefabric. The dyes used to color the cellulosefibers in rayon are examples of reactivedyes.
reagent /ree-ay-jĕnt/ A compound that re-acts with another (the substrate). The termis usually used for common laboratorychemicals – sodium hydroxide, hydrochlo-
ric acid, etc. – used for experiment andanalysis.
realgar /ree-al-ger/ The mineral form ofarsenic(II) sulfide, As4S4. It is bright red incolor and used as a pigment, in tanning,and in pyrotechnics.
real gas See gas laws.
rearrangement A reaction in which thegroups of a compound rearrange them-selves to form a different compound.
reciprocal proportions, law of Seeequivalent proportions, law of.
recrystallization /ree-kris-tă-li-zay-shŏn/The repeated crystallization of a com-pound to ensure purity of the sample.
rectified spirit /rek-tă-fÿd/ A constant-boiling mixture of ethanol and water thatcontains about 6% water; no more watercan be removed by further distillation. It isused as an industrial solvent.
red lead See dilead(II) lead(IV) oxide.
redox /ree-doks/ Relating to the processof oxidation and reduction, which are inti-mately connected in that during oxidationby chemical agents the oxidizing agent it-self becomes reduced, and vice versa. Thusan oxidation process is always accompa-nied by a reduction process. In electro-chemical processes this is equally true,oxidation taking place at the anode and re-duction at the cathode. These systems areoften called redox systems, particularlywhen the interest centers on both com-pounds.
Oxidizing and reducing power is indi-cated quantitatively by the redox potentialor standard electrode potential, EŠ.Redox potentials are normally expressedas reduction potentials. They are obtainedby electrochemical measurements and thevalues are referred to the H+/H2 couple forwhich EŠ is set equal to zero. Thus in-creasingly negative potentials indicate in-creasing ease of oxidation or difficulty ofreduction. Thus in a redox reaction the half
rationalized units
234
reaction with the most positive value ofEŠ is the reduction half and the half reac-tion with the least value of EŠ (or mosthighly negative) becomes the oxidationhalf. See also electrode potential.
red phosphorus See phosphorus.
reducing agent See reduction.
reduction /ri-duk-shŏn/ The gain of elec-trons by such species as atoms, molecules,or ions. It often involves the loss of oxygenfrom a compound, or addition of hydro-gen. Reduction can be effected chemically,i.e. by the use of reducing agents (electrondonors), or electrically, in which case thereduction process occurs at the cathode.For example,
2Fe3+ + Cu → 2Fe2+ + Cu2+
where Cu is the reducing agent and Fe3+ isreduced, and
2H2O + SO2 + 2Cu2+ →4H+ + SO4
2– + 2Cu+
where SO2 is the reducing agent and Cu2+
is reduced.Tin(II) chloride, sulfur(IV) oxide, the
hydrogensulfite ion, and hydroxylaminehydrochloride are common reducingagents. See also redox.
reduction potential See electrode poten-tial.
refining The process of removing impuri-ties from a substance or of extracting asubstance from a mixture.
refluxing /ri-fluks-ing/ The process ofboiling a liquid in a vessel connected to acondenser, so that the condensed liquidruns back into the vessel. By using a refluxcondenser, the liquid can be maintained atits boiling point for long periods of time,without loss. The technique is a standardmethod of carrying out reactions in or-ganic chemistry.
reforming The cyclization of straight-chain hydrocarbons from crude oil byheating under pressure with a catalyst, usu-ally platinum on alumina. For example, the
manufacture of methylbenzene from hep-tane:
C7H16 → C6H11CH3This first step is the production of methylcyclohexane, which then loses six hydro-gen atoms to give methylbenzene:
C6H11CH3 → C6H5CH3 + 3H2See also steam reforming.
refractory /ri-frak-tŏ-ree/ Describing acompound (e.g. an inorganic oxide) thathas a very high melting point.
Regnault’s method A method used forthe determination of the density of gases. Abulb of known volume is evacuated andweighed then the gas is admitted at aknown pressure (from a vacuum line) andthe bulb weighed again. The temperature isalso noted and the data corrected to STP.The method is readily applicable to the de-termination of approximate relative mol-ecular masses of gaseous samples. Themethod is named for the French physicistand chemist Henri Victor Regnault(1800–78).
relative atomic mass (r.a.m.) Symbol:Ar The ratio of the average mass per atomof the naturally occurring element to 1/12of the mass of an atom of nuclide 12C. Itwas formerly called atomic weight
relative density Symbol: d The ratio ofthe density of a given substance to the den-sity of some reference substance. The rela-tive densities of liquids are usuallymeasured with reference to the density ofwater at 4°C. Relative densities are alsospecified for gases; usually with respect toair at STP. The temperature of the sub-stance is stated or is understood to be20°C. Relative density was formerly calledspecific gravity.
relative molecular mass Symbol: MrThe ratio of the average mass per moleculeof the naturally occurring form of an ele-ment or compound to 1/12 of the mass ofan atom of nuclide 12C. This was formerlycalled molecular weight. It does not have tobe used only for compounds that have dis-crete molecules; for ionic compounds (e.g.
235
relative molecular mass
NaCl) and giant-molecular structures (e.g.BN) the formula unit is used.
relaxation The process by which an ex-cited species loses energy and falls to alower energy level (such as the groundstate).
rem /rem/ (radiation equivalent man) Aunit for measuring the effects of radiationdose on the human body. One rem is equiv-alent to an average adult male absorbingone rad of radiation. The biological effectsdepend on the type of radiation as well asthe energy deposited per kilogram.
resin /rez-in/ A yellowish insoluble or-ganic compound exuded by trees as a vis-cous liquid that gradually hardens onexposure to air to form a brittle amor-phous solid. Synthetic resins are artificialpolymers used in making adhesives, insula-tors and paints. See also rosin.
resolution (of racemates) The separationof a racemate into the two optical isomers.This cannot be done by normal methods,such as crystallization or distillation, be-cause the isomers have identical physicalproperties. The main methods are:1. Mechanical separation. Certain opti-
cally active compounds form crystalswith distinct left- and right-handedshapes. The crystals can be sorted byhand.
2. Chemical separation. The mixture is re-acted with an optical isomer. The prod-ucts are then not optical isomers of eachother, and can be separated by physicalmeans.For instance, a mixture of D- and L-
forms of an acid, acting with a pure L-base,produces two salts that can be separated byfractional crystallization and then recon-verted into the acids.3. Biochemical separation. Certain organiccompounds can be separated by using bac-teria that feed on one form only, leavingthe other.
resonance (mesomerism) The behavior ofmany compounds cannot be adequatelyexplained by a single structure using simple
single and double bonds. The bonding elec-trons of the compound have a different dis-tribution in the molecules. The actualbonding in the molecule can be regarded asa hybrid of two or more conventionalforms of the molecule, called resonanceforms or canonical forms. The result is aresonance hybrid. For example, the car-bonyl group in a ketone has negativecharge on the oxygen atom. It can be de-scribed as a resonance hybrid, somewherebetween =C=O, in which a pair of electronsis shared between the C and the O, and=C+–O–, in which the electrons are local-ized on the O atom. Note that the twocanonical forms do not contribute equallyin the hybrid. The bonding of benzene canbe represented by a resonance hybrid oftwo Kekulé structures and, to a lesser ex-tent, three Dewar structures.
resonance ionization spectroscopy(RIS) A type of spectroscopy that detectsspecific types of atoms using lasers. Thefrequency of the laser is chosen so that onlythe atoms of interest in a sample are ex-cited by the laser.
resorcinol /rez-or-să-nol, -nohl/ See ben-zene-1,3-diol.
retort A piece of laboratory apparatusconsisting of a glass bulb with a long nar-row neck. In industrial chemistry, variousmetallic vessels in which distillations or re-actions take place are called retorts.
retrosynthetic analysis /ret-roh-sin-th’ĕ-tik/ A technique for planning the synthesisof an organic molecule. The structure ofthe molecule is considered and it is dividedinto imaginary parts, which could combineby known reactions. These disconnectionsof the molecule suggest charged fragments,known as synthons, which give a guide topossible reagents for the synthesis.
reverberatory furnace /ri-ver-bĕ-ră-tor-ee, -toh-ree/ A furnace for smelting metals.It has a curved roof so that heat is reflecteddownwards, the fuel being in one part ofthe furnace and the ore in the other.
relaxation
236
reverse osmosis A process by whichwater containing a salt is separated via asemipermeable membrane into pure waterand salt water streams by subjecting the in-coming water to pressures significantlygreater than its OSMOTIC PRESSURE. Thiscauses pure water to be forced through themembrane, leaving the salt behind. Reverseosmosis is used in the purification of seawater. See osmosis.
reversible change A change in the pres-sure, volume, or other properties of a sys-tem, in which the system remains atequilibrium throughout the change. Suchprocesses could be reversed; i.e. returned tothe original starting position through thesame series of stages. They are never real-ized in practice. An isothermal reversiblecompression of a gas, for example, wouldhave to be carried out infinitely slowly andinvolve no friction, etc. Ideal energy trans-fer would have to take place between thegas and the surroundings to maintain aconstant temperature.
In practice, all real processes are irre-versible changes in which there is not anequilibrium throughout the change. In anirreversible change, the system can still bereturned to its original state, but notthrough the same series of stages. For aclosed system, there is always an entropyincrease involved in an irreversible change.
reversible reaction A chemical reactionthat can proceed in either direction. For ex-ample, the reaction:
N2 + 3H2 ˆ 2NH3is reversible. In general, there will be anequilibrium mixture of reactants and prod-ucts.
RF value A measure of the relative distancetraveled by a sample in a chromatographyexperiment. It is obtained by dividing thedistance traveled by the solvent front intothe distance traveled by the sample. Understandard conditions, the RF value is char-acteristic of a particular substance. Seepaper chromatography; thin-layer chro-matography.
rhenium /ree-nee-ŭm/ A rare silvery tran-
sition metal that usually occurs naturallywith molybdenum (it is extracted from fluedust in molybdenum smelters). The metalis chemically similar to manganese and isused in alloys and catalysts.
Symbol: Re; m.p. 3180°C; b.p. 5630°C;r.d. 21.02 (20°C); p.n. 75; r.a.m. 186.207.
rheology /ree-ol-ŏ-jee/ The study of theways in which matter can flow. This topicis of particular interest in the study of poly-mers.
rhodium /roh-dee-ŭm/ A rare silvery hardtransition metal. It is difficult to work andhighly resistant to corrosion. Rhodium oc-curs native but most is obtained from cop-per and nickel ores. It is used in protectivefinishes, alloys, and as a catalyst.Symbol: Rh; m.p. 1966°C; b.p. 3730°C;r.d. 12.41 (20°C); p.n. 45; r.a.m.102.90550.
rhombic crystal /rom-bik/ See crystal sys-tem.
ribonucleic acid /rÿ-boh-new-klee-ik/ SeeRNA.
ribose /rÿ-bohs/ (C5H10O5) A monosac-charide; a component of RNA.
ring A closed loop of atoms in a molecule,as in benzene or cyclohexane. A fused ringis one joined to another ring in such a waythat they share two atoms. Naphthalene isan example of a fused-ring compound.
ring closure A reaction in which one partof an open chain in a molecule reacts withanother part, so that a ring of atoms isformed.
RIS See resonance ionization spec-troscopy.
RNA (ribonucleic acid) A nucleic acidfound mainly in the cytoplasm and in-volved in protein synthesis. It is a singlepolynucleotide chain similar in composi-tion to a single strand of DNA except thatthe sugar ribose replaces deoxyribose andthe pyrimidine base uracil replaces
237
RNA
rock
238
thymine. RNA is synthesized on DNA inthe nucleus and exists in three forms. Incertain viruses, RNA is the genetic mater-ial.
rock A definable part of the Earth’s crustmade up of a mixture of MINERALS. It maybe hard (for example, granite), soft (chalk),consolidated (clay), or loose (sand).
rock salt (halite) A transparent naturallyoccurring mineral form of sodium chlo-ride.
roentgen /rent-gĕn/ Symbol: R A unit ofradiation, used for x-rays and gamma rays,defined in terms of the ionizing effect onair. One roentgen induces 2.58 × 10–4
coulomb of charge in one kilogram of dryair. The unit is named for the Germanphysicist Wilhelm Konrad Roentgen(1845–1923).
roentgenium /rent-gĕn-ee-ŭm/ A radioac-tive element produced synthetically. It isnamed for Wilhelm Roentgen
Symbol: Rg; p.n. 111.
Rose’s metal A fusible alloy containing50% bismuth, 25–28% lead, and tin. Itslow melting point (about 100°C) leads toits use in fire-protection devices.
rosin A brittle yellow or brown resin thatremains after the distillation of turpentine.It is used as a flux in soldering and in mak-ing paints and varnishes. Powdered rosingives a ‘grip’ to violin bows and boxers’shoes. See also resin.
rotary dryers Devices commonly used inthe chemical industry for the drying, mix-ing, and sintering of solids. They consist es-sentially of a rotating inclined cylinder,which is longer in length than in diameter.Gases flow through the cylinder in either acountercurrent or cocurrent direction toregulate the flow of solids, which are fedinto the end of the cylinder. Rotary dryerscan be applied to both batch and continu-ous processes.
R-S convention See optical activity.
rubber A natural or synthetic polymericelastic material. Natural rubber is a poly-mer of methylbuta-1,3-diene (isoprene).Various synthetic rubbers are made bypolymerization; for example chloroprenerubber (from 2-chlorobuta-1,3-diene) andsilicone rubbers. See also vulcanization.
rubidium /roo-bid-ee-ŭm/ A soft silveryhighly reactive element of the alkali-metalgroup. Naturally occurring rubidium com-prises two isotopes, one of which, 87Rb, isradioactive (half-life 5 × 1010 years). It isfound in small amounts in several complexsilicate minerals, including lepidolite. Ru-bidium is used in vacuum tubes, photo-cells, and in making special glass.
Symbol: Rb; m.p. 39.05°C; b.p. 688°C;r.d. 1.532 (20°C); p.n. 37; r.a.m. 85.4678.
rubidium–strontium dating A tech-nique for dating rocks. It depends on theradioactive decay of the radioisotope 87Rbto 87Sr (half-life 4.7 × 1010 years). If theratio 87Sr/87Rb is measured it is possible toestimate the age of the rock. See also ra-dioactive dating.
ruby A gemstone variety of the mineralcorundum (aluminum oxide, Al2O3) col-ored red by chromium impurities. Rubiescan also be made synthetically. They areused in jewelry, as bearings in watches, andin some lasers.
rusting The corrosion of iron in air toform an oxide. Both oxygen and moisturemust be present for rusting to occur. Theprocess is electrolytic, involving electro-chemical reactions on the wet iron:
Fe(s) → Fe2+(aq) + 2e–
2H2O + O2(aq) + 4e– → 4OH–(aq)Iron(II) hydroxide precipitates and is
oxidized to the red hydrated iron(III) oxideFe2O3.H2O.
ruthenium /roo-th’ee-nee-ŭm/ A transi-tion metal that occurs naturally with plat-inum. It forms alloys with platinum thatare used in electrical contacts. Rutheniumis also used in jewelry alloyed with palla-dium.
Symbol: Ru; m.p. 2310°C; b.p.3900°C; r.d. 12.37 (20°C); p.n. 44; r.a.m.101.07.
rutherfordium /ruth-er-for-dee-ŭm/ A ra-dioactive metal not found naturally onearth. It is the first transactinide metal.Atoms of rutherfordium are produced bybombarding 249Cf with 12C or by bom-barding 248Cm with 18O.
Symbol: Rf; m.p. 2100°C (est.); b.p.5200°C (est.); r.d. 23 (est.); most stableisotope 261Rf (half-life 65s).
rutile /roo-teel, -tÿl/ A naturally occurringreddish-brown form of titanium(IV) oxide,TiO4. It is an ore of titanium and is alsoused in making ceramics. Some clear vari-
eties of rutile are used as a semipreciousgemstone.
Rydberg constant /rid-berg/ A constantthat occurs in formula for the frequenciesof spectral lines in atomic spectra. For thehydrogen atom it has the value 1.0968 ×107m–1. The value of the Rydberg constantcan be calculated from the BOHR THEORY ofthe hydrogen atom and from quantum me-chanics. These calculations showed that:
R = Μ02me4c3/8h3
where Μ0 is the magnetic constant, m ande are the mass and charge respectively of anelectron, h is the Planck constant and c isthe speed of light in a vacuum. The con-stant is named for the Swedish physicist Jo-hannes Robert Rydberg (1854–1919). Seealso hydrogen atom spectrum.
239
Rydberg constant
240
Sabatier–Senderens process A methodof hydrogenating unsaturated vegetableoils to make margarine, using hydrogenand a nickel catalyst. It is named for theFrench chemists Paul Sabatier (1854–1941) and Jean-Baptiste Senderens (1856–1937). See also hardening.
saccharide /sak-ă-rÿd, -rid/ See sugar.
saccharin /sak-ă-rin/ (C7H5NO3S) Awhite crystalline organic compound usedas an artificial sweetener; it is about 550times as sweet as sugar (sucrose). It isnearly insoluble in water and so generallyused in the form of its sodium salt. Possiblelinks with cancer in animals has restrictedits use in some countries.
Sachse reaction /sak-sĕ/ A process for themanufacture of ethyne from natural gas(methane). Part of the methane is burned intwo stages to raise the furnace temperatureto about 1500°C. Under these conditions,the rest of the methane is converted toethyne and hydrogen:
2CH4 → C2H2 + 3H2The process is important since it provides asource of ethyne from readily availablenatural gas, thus avoiding the expensivecarbide process.
sacrificial protection A method of pro-tection against electrolytic corrosion (espe-cially rusting). In protecting steel pipelines,for instance, zinc or magnesium rods areburied in the ground at points along theline and connected to the pipeline. Rustingof iron is an electrochemical process inwhich Fe2+ ions dissolve in the water incontact with the surface. A more elec-tropositive element, such as zinc, protectsagainst this because Zn2+ ions dissolve in
preference. In other words, the zinc rodsacrifices itself for the steel pipeline. Thesame effect occurs with galvanized iron. Ifthe zinc coating is scratched, the iron ex-posed does not rust until all the zinc hasdissolved away. (Note that tin, being lesselectropositive than iron, has the oppositeeffect.)
safety lamp (Davy lamp) A type of oillamp for use in coal mines designed so thatit will not ignite any methane (firedamp)present and cause an explosion. The flameis surrounded by a fine metal gauze whichdissipates the heat from the flame but neverreaches the ignition temperature ofmethane. If methane is present it burns in-side the gauze; the lamp can be used as atest for pockets of methane. The lamp isalso named for the British chemist SirHumphry Davy (1778–1829).
sal ammoniac /sal ă-moh-nee-ak/ See am-monium chloride.
salicylate /sal-ă-sil-ayt, sal-ă-sil-ayt, să-liss-ă-layt/ (hydroxybenzoate) A salt orester of salicylic acid.
salicylic acid /sal-ă-sil-ik/ (2-hydroxyben-zoic acid; C6H4(OH)COOH) A crystallinearomatic carboxylic acid. It is used in med-icines, as an antiseptic, and in the manu-facture of azo dyes. Its ethanoyl (acetyl)ester is aspirin. See aspirin; methyl salicy-late.
saline /say-lÿn/ Containing a salt, espe-cially an alkali-metal halide such as sodiumchloride.
salt A compound with an acidic and abasic radical, or a compound formed by
S
total or partial replacement of the hydro-gen in an acid by a metal. In general termsa salt is a material that has identifiablecationic and anionic components.
salt bridge An electrical contact betweentwo half cells, used to prevent mixing. Aglass U-tube filled with potassium chloridein agar is often used.
saltcake /sawlt-kayk/ An impure form ofSODIUM SULFATE, formerly produced indus-trially as a by-product of the LEBLANC
PROCESS.
salt hydrate A metallic salt in which themetal ions are surrounded by a fixed num-ber of water molecules. The water mol-ecules are bound to the metal ions byion–dipole interactions. In the solid statethe water molecules form part of the crys-tal structure. It is thus a clathrate.
Salt hydrates that contain several mol-ecules of water for each metal ion can losethem progressively (effloresce) if the pres-sure of water vapor is kept below the dis-sociation pressure of the system; eventuallyan anhydrous salt is formed. Alternatively,if the vapor pressure is continuously raisedthe system will add molecules of water(deliquesce) until a saturated solutionforms.
salting out The precipitation of a colloid(such as gelatin or soap) by adding an ionicsalt to a solution. See gelatin; soap.
saltpeter /sawlt-pee-ter/ See potassium ni-trate.
sal volatile /sal voh-lat-ă-lee/ See ammo-nium carbonate.
samarium /să-mair-ee-ŭm/ A silvery ele-ment of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids. Samarium is used in the metal-lurgical, glass, and nuclear industries.
Symbol: Sm; m.p. 1077°C; b.p.1791°C; r.d. 7.52 (20°C); p.n. 62; r.a.m.150.36.
sand A mineral form of silica (silicon(IV)oxide, SiO2) consisting of separate grainsof quartz. It results from erosion of quartz-containing rocks; the grains may berounded by the tumbling action of water orwind. Sand is used in concrete and mortarand in making glass and other ceramics.
Sandmeyer reaction /sand-mÿ-er/ Amethod of producing chloro- and bromo-substituted derivatives of aromatic com-pounds by using the DIAZONIUM SALT witha copper halide. The reaction starts with anamine, which is diazotized at low tempera-ture by using hydrochloric acid andsodium nitrite (to produce nitrous acid,HNO2). For example:
C6H5NH2 + NaNO2 + HCl → C6H5N2+
+ Cl– + OH– + Na+ + H2OThe copper halide (e.g. CuCl) acts as a cat-alyst to give the substituted benzene deriv-ative:
C6H5N2+ + Cl– → C6H5Cl + N2
This reaction was discovered by the Ger-man chemist Traugott Sandmeyer(1854–1922) in 1884. A variation of thereaction, in which the catalyst is freshlyprecipitated copper powder, was reportedin 1890 by the German chemist LudwigGatterman (1860–1920). This is known asthe Gatterman reaction (or Gat-terman–Sandmeyer reaction).
sandstone A sedimentary rock consistingof consolidated sand grains cemented to-gether with calcium carbonate, clay, or ox-ides of iron. It is used mainly as a buildingstone.
sandwich compound A type oforganometallic compound formed betweentransition metal ions and aromatic com-pounds, in which the metal ion is ‘sand-wiched’ between two rings. Four, five, six,seven, and eight-membered rings areknown to form complexes with a numberof elements including V, Cr, Mn, Co, Ni,and FE. The bonding in such compounds isnot between the metal ion and individualatoms on the ring; rather it involves bond-ing of d electrons of the ion with the pi-electron system of the ring as a whole.FERROCENE is the original example, having
241
sandwich compound
two parallel cyclopentadienyl rings sand-wiching the iron ion. There are a numberof related types of compound. Thus, half-sandwich compounds (sometimes knownas piano-stool compounds) have only onering bound to the central ion, with otherligands coordinating in the normal way.Bent sandwich compounds have two ringsthat are not parallel, and the ion is attachedto other ligands. Multidecker sandwichcompounds have more than two parallelrings (and more than two coordinatingions.
saponification /să-pon-ă-fă-kay-shŏn/The process of hydrolyzing an ester with ahydroxide. The carboxylic acid forms asalt. For instance, with sodium hydroxide:
RCOOR′ + NaOH → NaOOCR +R′OH
The saponification of fats, which are estersof 1,2,3-trihydroxypropane with long-chain carboxylic acids, forms soaps.
sapphire A gemstone variety of the min-eral corundum (aluminum oxide, Al2O3)colored blue (or other colors) by impuri-ties. Sapphires can also be made syntheti-cally.
saturated compound An organic com-pound that does not contain any double ortriple bonds in its structure. A saturatedcompound will undergo substitution reac-tions but not addition reactions since eachatom in the structure will already haveformed its maximum possible number ofsingle bonds. Compare unsaturated com-pound.
saturated solution A solution that con-tains the maximum equilibrium amount ofsolute at a given temperature. A solution issaturated if it is in equilibrium with itssolute. If a saturated solution of a solid iscooled slowly, the solid may stay tem-porarily in solution; i.e. the solution maycontain more than the equilibrium amountof solute. Such solutions are said to be su-persaturated.
saturated vapor A vapor that is in equi-librium with the solid or liquid. A satu-
rated vapor is at the maximum pressure(the saturated vapor pressure) at a giventemperature. If the temperature of a satu-rated vapor is lowered, the vapor con-denses. Under certain circumstances, thesubstance may stay temporarily in thevapor phase; i.e. the vapor contains morethan the equilibrium concentration of thesubstance. The vapor is then said to be su-persaturated.
s-block elements The elements of thefirst two groups of the periodic table; i.e.group 1 (H, Li, Na, K, Rb, Cs, Fr) andgroup 2 (Be, Mg, Ca, Sr, Ba, Ra). They areso called because their outer shells have theelectronic configurations ns1 or ns2. The s-block excludes those with inner (n – 1)dlevels occupied (i.e. it excludes transitionelements, which also have s2 and occasion-ally s1 configurations).
scandium /skan-dee-ŭm/ A lightweightsilvery element belonging to the first tran-sition series. It is found in minute amountsin over 800 minerals, often associated withlanthanoids. Scandium is used in high-in-tensity lights and in electronic devices.
Symbol: Sc; m.p. 1541°C; b.p. 2831°C;r.d. 2.989 (0°C); p.n. 21; r.a.m.44.955910.
scanning tunneling microscope (STM)An instrument that makes use of quantum-mechanical tunneling to investigate thesurface structure of a sample of material. Inthis technique a sharp conducting tip isbrought near a surface. This causes elec-trons to tunnel from the surface to the tip,with the probability of this occurring de-pending on both the distance between thesurface and the tip and the electron densityat the surface. The tunneling produces anelectrical current which is kept constant bymoving the tip up or down as it is movedover the surface. Single atoms can be ob-served using STM. See also atomic forcemicroscope.
scavenger A compound or chemicalspecies that removes a trace componentfrom a system or that removes a reactiveintermediate from a reaction.
saponification
242
243
self-assembly
Schiff’s base /shiffs/ A type of compoundformed by reacting an aldehyde or ketone(e.g. RCOR) with an aryl amine (e.g.ArNH2). The product, an N-arylimide,which is usually crystalline, has the for-mula R2C:NAr. The compound is namedfor the German chemist Hugo Schiff(1834–1915).
Schiff’s reagent An aqueous solution ofmagenta dye decolorized by reduction withsulfur(IV) oxide. It is a test for aldehydesand ketones. Aliphatic aldehydes restorethe color quickly; aliphatic ketones andaromatic aldehydes slowly; aromatic ke-tones give no reaction.
Schotten–Baumann reaction /shot-ĕnbow-mahn/ A method for the preparationof N-phenylbenzamide (benzanilide) inwhich a mixture of phenylamine (aniline)and sodium hydroxide is stirred while ben-zoyl chloride is added slowly. The N-phenylbenzamide precipitates and can berecrystallized from hot ethanol:
C6H5NH2 + C6H5COCl →C6H5NH.COC6H5 + HCl
Schottky defect /shot-kee/ See defect.
scrubber The part of a chemical plantthat removes impurities from a gas by pass-ing it through a liquid.
seaborgium /see-bor-gee-ŭm/ A radioac-tive metallic element not occurring natu-rally on Earth. It can be made bybombarding 249Cf with 18O nuclei using acyclotron. Six isotopes are known.
Symbol: Sg; most stable isotope 266Sg(half-life 27.3s).
second Symbol: s The SI base unit of time.It is defined as the duration of9 192 631 770 cycles of a particular wave-length of radiation corresponding to atransition between two hyperfine levels inthe ground state of the cesium-133 atom.
secondary alcohol See alcohol.
secondary amine See amine.
secondary cell See accumulator.
secondary structure See protein.
second-order reaction A reaction inwhich the rate of reaction is proportionalto the product of the concentrations of twoof the reactants or to the square of the con-centration of one of the reactants; i.e.
rate = k[A][B]or
rate =k[A]2
For example, the hydrolysis by dilutealkali of an ester is a second-order reaction:
rate = k[ester][alkali]The rate constant for a second-order re-
action has the units mol–1 dm3 s–1. Unlike afirst-order reaction, the time for a definitefraction of the reactants to be consumed isdependent on the original concentrations.
sedimentation The settling of a suspen-sion, either under gravity or in a centrifuge.The speed of sedimentation can be used toestimate the average size of the particles.This technique is used with an ultracen-trifuge to find the relative molecularmasses of macromolecules.
seed A small crystal added to a gas or liq-uid to assist solidification or precipitationfrom a solution. The seed, usually a crystalof the substance to be formed, enables par-ticles to pack into predetermined positionsso that a larger crystal can form.
selenium /si-lee-nee-ŭm/ A metalloid ele-ment existing in several allotropic formsand belonging to group 16 of the periodictable. It occurs in minute quantities in sul-fide ores and industrial sludges. The com-mon gray metallic allotrope is verylight-sensitive and is used in photocells,solar cells, some glasses, and in xerogra-phy. The red allotrope is unstable and re-verts to the gray form under normalconditions.
Symbol: Se; m.p. 217°C (gray); b.p.684.9°C (gray); r.d. 4.79 (gray); p.n. 34;r.a.m. 78.96.
self-assembly See supramolecular chem-istry.
Seliwanoff’s test A test for ketose SUGARS
in solution. The reagent used consists of re-sorcinol dissolved in hydrochloric acid. Afew drops are added to the solution and ared precipitate indicates a ketonic sugar.The test is named for the Russian chemistF. F. Seliwanoff.
semicarbazide /sem-ee-kar-bă-zÿd/ Seesemicarbazone.
semicarbazone /sem-ee-kar-bă-zohn/ Atype of organic compound containing theC:N.NH.CO.NH2 grouping, formed by re-action of an aldehyde or ketone with semi-carbazide (H2N.NH.CO.NH2). Thecompounds are crystalline solids withsharp melting points, which can be used tocharacterize the original aldehyde or ke-tone.
semiconductor /sem-ee-kŏn-duk-ter/ Acrystalline material which conducts onlyunder certain conditions. The conductivity,unlike in metals, increases with tempera-ture because the highest occupied and low-est unoccupied energy levels are very close.A semiconductor can be formed by intro-ducing small amounts of an impurity to anultrapure material. If these impurities canwithdraw electrons from the occupied en-ergy level, leaving ‘holes’ which permitconduction, the resultant semiconductionis known as p-type. Alternatively, these im-purities may donate electrons which enterthe unoccupied energy level, thus formingan n-type semiconductor. Semiconductorsare used extensively by the electronics in-dustry in such devices as integrated cir-cuits.
semipermeable membrane /sen-ee-per-mee-ă-băl/ A membrane that, when sepa-rating a solution from a pure solvent,permits the solvent molecules to passthrough it but does not allow the transferof solute molecules. Synthetic semiperme-able membranes are generally supportedon a porous material, such as unglazedporcelain or fine wire screens, and are com-monly formed of cellulose or related mat-erials. They are used in osmotic studies, gasseparations, and medical applications.
Equilibrium is reached at a semiperme-able membrane if the chemical potentialson both sides become identical; migrationof solvent molecules towards the solutionis an attempt by the system to reach equi-librium. The pressure required to halt thismigration is the osmotic pressure.
semipolar bond /sem-ee-poh-ler/ A coor-dinate bond.
septivalent /sep-tă-vay-lĕnt, sep-tiv-ă-/(heptavalent) Having a valence of seven.
sequence rule See CIP system.
sequestration /see-kwes-tray-shŏn/ Theformation of a complex with an ion in so-lution, so that the ion does not have its nor-mal activity. Sequestering agents are oftenchelating agents, such as edta.
serine /se-reen, -rin/ See amino acids.
sesqui- Prefix indicating a 2/3 ratio. Asesquioxide, for example, would have theformula M2O3. A sesquicarbonate is amixed carbonate and hydrogencarbonateof the type Na2CO3.NaHCO3.2H2O,which contains 2 CO3 units and 3 sodiumions.
sesquiterpene See terpene.
shell A group of electrons that share thesame principal quantum number. Earlywork on x-ray emission studies used theterms K, L, M, and these are still some-times used for the first three shells: n = 1 K-shell; n = 2 L-shell; n = 3 M-shell.
sherardizing /sh’e-răr-dÿ-zing/ A tech-nique used to obtain corrosion-resistant ar-ticles of iron or steel by heating with zincdust in a sealed rotating drum for severalhours at about 375°C. At this temperaturethe two metals combine, forming internallayers of zinc–iron and zinc–steel alloysand an external layer of pure zinc. Sher-ardizing is principally used for small parts,such as springs, washers, nuts, and bolts.
Seliwanoff’s test
244
short period See period.
SI See SI units.
side chain In an organic compound, analiphatic group or radical attached to alonger straight CHAIN of atoms in an acycliccompound or to one of the atoms in thering of a cyclic compound.
side reaction A chemical reaction thattakes place to a limited extent at the sametime as a main reaction. Thus the mainproduct of a reaction may contain smallamounts of other compounds.
siemens /see-mĕnz/ (mho) Symbol: S TheSI unit of electrical conductance, equal to aconductance of one ohm–1. The unit isnamed for the German electrical engineerErnst Werner von Siemens (1816–92).
sievert /see-vert/ Symbol: Sv The SI unit ofdose equivalent. It is the dose equivalentwhen the absorbed dose produced by ion-izing radiation multiplied by certain di-mensionless factors is 1 joule per kilogram(1 J kg–1). The dimensionless factors areused to modify the absorbed dose to takeaccount of the fact that different types ofradiation cause different biological effects.The unit is named for the Swedish physicistRolf Sievert (1898–1966).
sigma orbital See orbital.
sigmatropic rearrangement /sig-mă-trop-ik/ See pericyclic reaction.
silane /sil-ayn/ Any of a small number ofhydrides of silicon: SiH4, Si2H6, Si3H8, etc.Silanes can be made by the action of acidson magnesium silicide (Mg2Si). They areunstable compounds and ignite sponta-neously in air. Only the first few membersof the series are known and there is not therange of ‘hydrosilicon’ compounds to com-pare with the hydrocarbons.
silica /sil-ă-kă/ See silicon(IV) oxide.
silica gel A gel made by coagulatingsodium silicate sol. The gel is dried by heat-
ing and used as a catalyst support and as adrying agent. The silica gel used in desicca-tors and in packaging to remove moistureis often colored with a cobalt salt to indi-cate whether it is still active (blue = dry;pink = moist).
silicates /sil-ă-kayts, -kits/ A large numberof compounds containing metal ions andcomplex silicon–oxygen compounds. Thenegative ions in silicates are of the formSiO4
4–, Si2O76–, etc., and many are poly-
meric, containing SiO4 units linked in longchains, sheets, or three-dimensional arrays.Aluminosilicates and borosilicates are sim-ilar materials containing aluminum orboron atoms in the structure. Many sili-cates occur naturally in rocks and miner-als.
silicic acid /să-liss-ik/ A colloidal or jelly-like hydrated form of silicon(IV) oxide(SiO2), made by adding an acid (such as hy-drochloric acid) to a soluble SILICATE.
silicide /sil-ă-sÿd/ A compound of siliconwith a more electropositive element.
silicon /sil-ă-kŏn/ A hard brittle gray met-alloid element; the second element in group14 of the periodic table. It has the elec-tronic configuration of neon with four ad-ditional outer electrons; i.e., [Ne]3s23p2.
Silicon accounts for 27.7% of the massof the Earth’s crust and occurs in a widevariety of silicates with other metals, clays,micas, and sand, which is largely SiO2. Theelement is obtained on a small scale by thereduction of silicon(IV) oxide (SiO2) bycarbon or calcium carbide. For semicon-ductor applications very pure silicon isproduced by direct reaction of silicon withan HCl/Cl2 mixture to give silicon tetra-chloride (SiCl4), which can be purified bydistillation. This is then decomposed on ahot wire in an atmosphere of hydrogen.For ultra-pure samples zone refining isused. Unlike carbon, silicon does not formallotropes but has only the diamond typeof structure.
Silicon does not react with hydrogenexcept under extreme conditions in thepresence of chlorine, in which case com-
245
silicon
pounds such as trichlorosilane (HSiCl3) areobtained. The hydride SiH4 itself may beprepared in the laboratory by hydrolysis ofmagnesium silicide (a number of other hy-drides up to Si6H14 are obtained), or moreconveniently by reduction of silicon tetra-chloride with lithium tetrahydridoalumi-nate:
SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3The silanes are mildly explosive and thechloroalkyl silanes extremely so.
Silicon combines with oxygen whenheated in air to form silicon(IV) oxide. Themineral silicates constitute a vast collectionof materials with a wide range of structuresincluding chains, rings, lattices, twistedchains, sheets, and varyingly cross-linkedstructures all based on different assembliesof [SiO4]2– tetrahedra. Being a metalloid,silicon is somewhat amphoteric and conse-quently silica is weakly acidic, dissolving infused alkalis to form the appropriate sili-cate.
Silicon also forms an immense numberof organic derivatives. In many cases wheresingle bonding only is involved these com-pounds are rather similar to organicanalogs with the general distinction thatnucleophilic attachment occurs at silicon(rather than at hydrogen) and bond anglesat oxygen and nitrogen are often largerthan the organic materials. Optical activityand inversion of activity is observed. Themajor differences are due to the type of‘double bond’ available to carbon (pπ–pπ),and to silicon (p → dπ or back bonding).Thus silicon does not form conventionaldouble bonds and silicon bonds to oxygenare of the ether type (as in siloxanes) ratherthan the ketone type. The Si–OH bond isalso very much less stable than the alcohollink, R–OH, ready condensation to thesiloxane occurring in the silicon case:
2R3SiOH → R3Si–O–SiR3 + H2OSymbol: Si; m.p. 1410°C; b.p. 2355°C;
r.d. 2.329 (20°C); p.n. 14; r.a.m. 28.0855
silicon carbide (carborundum; SiC) Ablack very hard crystalline solid made byheating silicon(IV) oxide (sand) with car-bon (coke) in an electric furnace. It is usedas an abrasive.
silicon(IV) chloride (silicon tetrachlo-ride; SiCl4) A colorless fuming liquidwhich rapidly hydrolyzes in moist air orwater. It is used to produce pure silica formaking special kinds of glass.
silicon dioxide /dÿ-oks-ÿd, -id/ See sili-con(IV) oxide.
silicones /sil-ă-kohnz/ Polymeric syntheticsilicon compounds containing chains of al-ternating silicon and oxygen atoms, withorganic groups bound to the silicon atoms.Silicones are used as lubricants and waterrepellants and in waxes and varnishes. Sil-icone rubbers are superior to natural rub-bers in their resistance to both high andlow temperatures and chemicals. See alsosiloxanes.
silicon(IV) oxide (silicon dioxide; silica;SiO2) A hard crystalline compound occur-ring naturally in three crystalline forms(quartz, tridymite, and crystobalite). Sandis mostly silicon(IV) oxide. Fused silica is aglassy substance used in laboratory appa-ratus. Silica is used in the manufacture ofglass.
silicon tetrachloride /tet-ră-klor-ÿd, -id, -kloh-rÿd, -rid/ See silicon(IV) chloride.
siloxanes /să-loks-aynz/ Compounds con-taining Si–O–Si groups with organicgroups bound to the silicon atoms. The sil-icones are polymers of siloxanes.
silver A transition metal that occurs na-tive and as the sulfide (Ag2S) and chloride(AgCl). It is extracted as a by-product in re-fining copper and lead ores. Silver darkensin air due to the formation of silver sulfide.It is used in coinage alloys, tableware, andjewelry. Silver compounds are used in pho-tography.
Symbol: Ag; m.p. 961.93°C; b.p.2212°C; r.d. 10.5 (20°C); p.n. 47; r.a.m.107.8682.
silver(I) bromide (AgBr) A pale yellowprecipitate obtained by adding a solublebromide solution to a solution of silver(I)nitrate. The halide dissolves in concen-
silicon carbide
246
trated ammonia solution but not in diluteammonia solution. It is used extensively inphotography for making the light-sensitiveemulsions for plates and films. Silver(I)bromide crystallizes in a similar manner tosodium(I) chloride. Unlike silver(I) chlo-ride and silver(I) iodide, silver(I) bromidedoes not absorb ammonia gas.
silver(I) chloride (AgCl) A compoundprepared as a curdy white precipitate bythe addition of a soluble chloride solutionto a solution of silver(I) nitrate. It occurs innature as the mineral ‘horn silver’. Silver(I)chloride is insoluble in water but dissolvesin concentrated hydrochloric acid and con-centrated ammonia solution. It is ratherunreactive but is affected by sunlight, beingused extensively in photography. Silver(I)chloride crystallizes in the sodium chloridelattice pattern. It absorbs ammonia gasforming ammines, e.g. AgCl.2NH3 andAgCl.3NH3.
silver(I) iodide (AgI) A pale yellow pre-cipitate prepared by the addition of a solu-ble iodide solution to a solution of silver(I)nitrate. Silver(I) iodide occurs in nature asiodoargyrite in the form of hexagonal crys-tals. It is virtually insoluble in ammonia so-lution and is used in the photographicindustry. Silver(I) iodide will absorb am-monia gas. It is trimorphic and is found tocontract when heated and expand on cool-ing.
silver-mirror test A test for the aldehydegroup. A few drops of the sample arewarmed with TOLLEN’S REAGENT. An alde-hyde reduces Ag+ to silver metal, causing abrilliant silver mirror to coat the insidewall of the tube.
silver(I) nitrate (AgNO3) A white crys-talline solid prepared by dissolving silver indilute nitric acid and crystallizing the solu-tion. Silver(I) nitrate is dimorphous, crys-tallizing in the orthorhombic andhexagonal systems. It undergoes thermaldecomposition to yield silver, nitrogen(IV)oxide, and oxygen. Probably the most im-portant silver salt, it is used in the medicalindustry for the treatment of warts and in
the photographic industry. In the labora-tory, it is used both as an analytical andvolumetric reagent. Silver(I) nitrate in solu-tion and in the solid state can oxidize or-ganic compounds, e.g. aldehydes can beoxidized to carboxylic acids; the silver ionsare reduced to silver.
silver(I) oxide (argentous oxide; Ag2O) Abrown amorphous solid formed by the ad-dition of sodium or potassium hydroxidesolution to a solution of silver nitrate. Sil-ver(I) oxide turns moist red litmus blue anddecomposes (at 160°C) to give silver andoxygen. Silver(I) oxide dissolves in concen-trated ammonia solution, forming thecomplex ion [Ag(NH3)2]+. In organicchemistry it is used when moist to convertalkyl halides to the corresponding alcoholand when dry to convert alkyl halides intoethers.
silver(II) oxide (argentic oxide; AgO) Ablack diamagnetic substance prepared bythe oxidation of either silver or silver(I)oxide using ozone. Alternatively it can beprepared as a precipitate by the addition ofa solution of potassium persulfate to one ofsilver(I) nitrate.
single bond A covalent bond betweentwo elements that involves one pair of elec-trons only. It is represented by a single line,for example H–Br, and is usually a sigmabond, although it can be a pi bond. Com-pare multiple bond. See also orbital.
sintering /sin-ter-ing/ A process in whichcertain powdered substances (e.g. metals,ceramics) coagulate into a single masswhen heated to a temperature below thesubstance’s melting point. Sintered glass isa porous material used for laboratory fil-tration.
SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. It hasseven base units and two dimensionlessunits, formerly called supplementary units.Derived units are formed by multiplicationand/or division of base units. Standard pre-fixes are used for multiples and submulti-
247
SI units
SI units
248
BASE AND DIMENSIONLESS SI UNITS
Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr*supplementary units
DERIVED SI UNITS WITH SPECIAL NAMES
Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv
DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS
Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y
ples of SI units. The SI system is a coherentrationalized system of units.
slag Glasslike compounds of compara-tively low melting point formed during theextraction of metals when the impurities inan ore react with a flux. The fact that theslag is a liquid of comparatively low den-sity means that it can be separated from theliquid metal on which it floats. In a blastfurnace, the flux used is limestone(CaCO3) and the main impurity is silica(SiO2). The slag formed is mainly calciumsilicate:
CaCO3 + SiO2 → CaSiO3It is used as railroad ballast and in fertiliz-ers and concrete.
slaked lime See calcium hydroxide.
slaking See calcium hydroxide.
slip planes The planes of weakness in acrystal, e.g. the boundaries between the oc-tahedral layers in graphite.
slurry A thin paste of suspended solid par-ticles in a liquid.
smectic crystal /smek-tik/ See liquid crys-tal.
smelting An industrial process for ex-tracting metals from their ores at high tem-peratures. Generally the ore is reducedwith carbon (for zinc and tin) or carbonmonoxide (for iron). Copper and lead areobtained by reduction of the oxide with thesulfide; for example:
2Cu2O + Cu2S → 6Cu + SO2A flux is also used to combine with impu-rities and form a slag on top of the moltenmetal (see slag).
smithsonite /smith-sŏ-nÿt/ See calamine.
SNG Substitute (or synthetic) natural gas.A mixture of hydrocarbons manufacturedfrom coal or the naphtha fraction of petro-leum for use as a fuel.
SN1 reaction See nucleophilic substitu-tion.
SN2 reaction See nucleophilic substitu-tion.
soap One of a number of sodium or potas-sium compounds of fatty acids that arecommonly used to improve the cleansingproperties of water. Soap was the earliestknown DETERGENT. It is made by first re-acting vegetable oils and animal fats with astrong solution of sodium or potassium hy-droxide to produce organic acids, such asoctadecanoic acid. The soap is then precip-itated out as the salt, e.g. sodium octade-canoate, by adding excess sodium chloride.In water, soap molecules break up to pro-duce ions, which are responsible for thecleansing properties.
soapstone (steatite) A soft type of talcwhich has a greasy feel and which is easy tocarve to make ornaments. See talc.
soda See sodium carbonate; sodium hy-droxide.
soda ash See sodium carbonate.
soda glass See glass.
soda lime A gray solid produced byadding sodium hydroxide solution to cal-cium oxide, to give a mixture of Ca(OH)2and NaOH. It is used in the laboratory asa drying agent and as an absorbent fordioxide.
sodamide /soh-dă-mÿd/ (NaNH2) Awhite ionic solid formed by passing dryammonia over sodium at 300–400°C. Thecompound reacts with water to givesodium hydroxide and ammonia. It reactswith red-hot carbon to form sodiumcyanide and with nitrogen(I) oxide to formsodium azide. Sodamide is used in theCastner process and in the explosives in-dustry.
soda water A solution of carbon dioxidedissolved in water under pressure, used infizzy drinks. When the pressure is released,the liquid effervesces with bubbles of car-bon dioxide gas. See also carbonic acid.
249
soda water
sodium /soh-dee-ŭm/ A soft reactivemetal; the second member of the alkalimetals (group 1 of the periodic table). Ithas the electronic configuration of a neonstructure plus one additional outer 3s elec-tron. Electronic excitation in flames or thefamiliar sodium lamps gives a distinctiveyellow color arising from intense emissionat the so called ‘sodium-D’ line pair. Theionization potential is rather low and thesodium atom readily loses its electron (i.e.the metal is strongly reducing). The chem-istry of sodium is largely the chemistry ofthe monovalent Na+ ion.
Sodium occurs widely as NaCl in sea-water and as deposits of halite in dried uplakes etc. (2.6% of the lithosphere). The el-ement is obtained commercially by elec-trolysis of NaCl melts in which the meltingpoint is reduced by the addition of calciumchloride; sodium is produced at the ironcathode (the Downs cell). The metal is ex-tremely reactive. It reacts vigorously withthe halogens, and also with water to givehydrogen and sodium hydroxide. Thechemistry of sodium is very similar to thatof the other members of group 1.
Nearly all sodium compounds are solu-ble in water. Sodium hydroxide is pro-duced commercially by the electrolysis ofbrine using diaphragm cells or mercury-cathode cells; chlorine is a coproduct.
Solutions of sodium metal in liquid am-monia are blue and have high electricalconductivities; the main current carrier ofsuch solutions is the solvated electron.Such solutions are used in both organic andinorganic chemistry as efficient reducingagents. Sodium also forms a number ofalkyl and aryl derivatives by reaction withthe appropriate mercury compound, e.g.:
(CH3)2Hg + 2Na → 2CH3Na + Na/HgThese materials are used as catalysts forpolymerization reactions. The metal itselfhas a body-centered structure.
Symbol: Na; m.p. 97.81°C; b.p. 883°C;r.d. 0.971 (20°C); p.n. 11; r.a.m.22.989768.
sodium acetate See sodium ethanoate.
sodium aluminate (NaAlO2) A whitesolid produced by adding excess aluminum
to a hot concentrated solution of sodiumhydroxide. Once the reaction has been ini-tiated, sufficient heat is liberated to keepthe reaction going; hydrogen is also pro-duced. In solution the aluminate ions havethe structure Al(OH)4
– and NaAl(OH)4 isknown as sodium(I) tetrahydroxoalumi-nate(III). Addition of sodium hydroxide toan aluminum salt solution produces awhite gelatinous precipitate of aluminumhydroxide, which dissolves in excess alkalito give a solution of sodium(I) tetrahy-droxoaluminate(III).
sodium azide (NaN3) A white solid pre-pared by passing nitrogen(I) oxide overheated sodamide. On heating, sodiumazide decomposes to give sodium and ni-trogen. It is used as a reagent in organicchemistry and in the preparation of leadazide (for use in detonators).
sodium benzenecarboxylate /ben-zeen-kar-boks-ă-layt/ (sodium benzoate;C6H5COONa) A white crystalline powdermade by neutralizing benzoic acid withsodium hydroxide solution and then evap-orating the solution. It is used as an urinaryantiseptic, in the dyeing industry, and as afood preservative.
sodium benzoate /ben-zoh-ayt/ Seesodium benzenecarboxylate.
sodium bicarbonate See sodium hydro-gencarbonate.
sodium bisulfate See sodium hydrogen-sulfate.
sodium bisulfite See sodium hydrogen-sulfite.
sodium borate /bor-ayt, boh-rayt/ See dis-odium tetraborate decahydrate.
sodium bromide (NaBr) A white solidformed by the action of bromine on hotsodium hydroxide solution. Alternatively,it can be made by the action of dilute hy-drobromic acid on sodium carbonate orhydroxide. Sodium bromide is soluble inwater and forms crystals similar in shape to
sodium
250
sodium chloride. It is used in medicine andphotography. Sodium bromide forms a hy-drate, NaBr.2H2O.
sodium carbonate (soda ash; Na2CO3)A white amorphous powder, which aggre-gates on exposure to air owing to the for-mation of hydrates. Industrially it isprepared by the ammonia–soda (Solvay)process. On crystallizing from aqueous so-lution large translucent crystals of the dec-ahydrate (Na2CO3.10H2O) are formed(washing soda). These effloresce formingthe monohydrate, Na2CO3.H2O. Largequantities of sodium carbonate are used inthe manufacture of sodium hydroxide.Washing soda is used as a domesticcleanser. The salt produces an alkaline so-lution in water by hydrolysis:
Na2CO3 + 2H2O → 2NaOH + H2CO3It is used in volumetric analysis to stan-dardize strong acids.
sodium chlorate(V) (NaClO3) A whitesolid formed by the action of chlorine onhot concentrated sodium hydroxide solu-tion, or by the electrolysis of a concen-trated sodium chloride solution. It issoluble in water. Sodium chlorate is a pow-erful oxidizing agent. If heated it yieldsoxygen and sodium chloride. The chlorateis used in explosives, in matches, as a weedkiller, and in the textile industry.
sodium chloride (common salt; salt;NaCl) A white solid prepared by the neu-tralization of hydrochloric acid with aque-ous sodium hydroxide. It occurs in seawater and natural brines. Natural solid de-posits of rock salt are also found in certainplaces. The compound dissolves in waterwith the absorption of heat, its solubilitychanging very little with temperature. It isused to season and preserve food. Industri-ally, it is used as a raw material in the man-ufacture of sodium carbonate (Solvayprocess), sodium hydroxide (electrolysis),and soap. Sodium chloride is almost insol-uble in alcohol.
sodium-chloride structure A form ofcrystal structure that consists of a face-cen-tered cubic arrangement of sodium ions
with chloride ions situated at the middle ofeach edge and in the center of the cube.Electrostatic attraction holds the oppo-sitely charged ions together.
The lattice can also be thought of astwo interpenetrating face-centered cubes,one composed of sodium ions and theother of chloride ions. Other compounds(e.g. sodium bromide and potassium chlo-ride) having their ions arranged in the samepositions, are also described as having thesodium-chloride structure.
sodium cyanide (NaCN) A white solidformed either by the action of carbon onsodamide at high temperature or by pass-ing ammonia over sodium at 300–400°Cto form sodamide and reacting it with car-bon. The industrial salt is prepared by theCastner process and purified by recrystal-lization from liquid ammonia. It is ex-tremely poisonous and used as a source ofcyanide and hydrocyanic acid. In thecyanide process it is used in the extractionof silver and gold. Its aqueous solutions arealkaline due to salt hydrolysis.
sodium dichromate(VI) (Na2Cr2O7) Anorange deliquescent solid that is very solu-ble in water. It is made from finely pow-dered chromite, which is heated withcalcium oxide and sodium carbonate in a
251
sodium dichromate(VI)
chloride ion
sodium ion
Sodium-chloride structure
furnace. If an alkali is added to a solutionof the dichromate, the chromate is pro-duced. At high temperatures, sodiumdichromate decomposes to give the chro-mate, chromic oxide, and oxygen. It is usedas an oxidizing agent, particularly in or-ganic chemistry and in volumetric analysisto estimate iron(II) ions and iodide ions.
sodium dihydrogen phosphate(V) /dÿ-hÿ-drŏ-jĕn/ (sodium dihydrogen or-thophosphate; NaH2PO4) A white solidprepared by titrating phosphoric acid withsodium hydroxide solution using methylorange as the indicator. On evaporationwhite crystals of the monohydrate areformed. It is used in some baking powders.The monohydrate crystallizes in the or-thorhombic system.
sodium dioxide /dÿ-oks-ÿd, -id/ Seesodium superoxide.
sodium ethanoate /eth-ă-noh-ayt/(sodium acetate; CH3COONa) A whitesolid prepared by the neutralization ofethanoic acid with either sodium carbonateor sodium hydroxide. Sodium ethanoatereacts with sulfuric acid to form sodiumhydrogensulfate and ethanoic acid; withsodium hydroxide it gives rise to sodiumcarbonate and methane. Sodium ethanoateis used in the dyeing industry.
sodium fluoride (NaF) A white solidformed by the action of hydrofluoric acidon sodium carbonate or sodium hydroxide(the reaction between metallic sodium andfluorine is too violent). Sodium fluoride issoluble in water and reacts with concen-trated sulfuric acid to yield hydrogen fluo-ride. It is used as a constituent of ceramicenamels, as an antiseptic, and as an agentto prevent fermentation.
sodium formate See sodium methanoate.
sodium hexafluoroaluminate /heks-ă-floo-ŏ-roh-ă-loo-mă-nayt/ (cryolite;Na3AlF6) A compound found in largequantities in South Greenland. It is whiteor colorless, but may be reddish or brownbecause of impurities. Sodium hexafluo-
roaluminate is used as a flux in the manu-facture of aluminum. It crystallizes in themonoclinic system but in forms that closelyresemble cubes and isometric octahedrals.
sodium hydride (NaH) A white crys-talline solid prepared by passing a purestream of dry hydrogen over sodium at350°C; the sodium is usually suspended inan inert medium. Electrolysis of fusedsodium hydride yields hydrogen at theanode, suggesting the presence of the H–
ion. Sodium hydride reacts with water toform sodium hydroxide solution and hy-drogen. It is used as a powerful reducingagent to convert water to hydrogen, con-centrated sulfuric acid to hydrogen sulfide,and iron(III) oxide to iron. Sodium hydridebursts into flames spontaneously on con-tact with the halogens at room tempera-ture. It dissolves in liquid ammonia to givesodamide, NaNH2.
sodium hydrogencarbonate (sodium bi-carbonate; baking soda; NaHCO3) Awhite solid formed either by passing an ex-cess of carbon dioxide through sodium car-bonate or hydroxide solution, or byprecipitation when cold concentrated solu-tions of sodium chloride and ammoniumhydrogencarbonate are mixed. Sodium hy-drogencarbonate decomposes on heatingto give sodium carbonate, carbon dioxide,and water. With dilute acids, it yields car-bon dioxide. It is used as a constituent ofbaking powder, in effervescent beverages,and in fire extinguishers. Its aqueous solu-tions are alkaline as a result of salt hydrol-ysis. Sodium hydrogencarbonate formsmonoclinic crystals.
sodium hydrogensulfate /hÿ-drŏ-jĕn-sul-fayt/ (sodium bisulfate; NaHSO4) A whitesolid formed either by the partial neutral-ization of sodium hydroxide solution withsulfuric acid, by the action of concentratedsulfuric acid on sodium nitrate, or by heat-ing equimolar quantities of sodium chlo-ride and concentrated sulfuric acid. Inaqueous solution sodium hydrogensulfateis strongly acidic. It crystallizes as themonohydrate (NaHSO4.H2O), which de-hydrates on warming. If heated strongly
sodium dihydrogen phosphate(V)
252
the pyrosulfate (Na2S2O7) is formed,which decomposes to give the sulfate andsulfur(VI) oxide. Sodium hydrogensulfateis used as a cheap source of sulfuric acidand in the dyeing industry.
sodium hydrogensulfite /hÿ-drŏ-jĕn-sul-fÿt/ (sodium bisulfite; NaHSO3) A whitepowder prepared by saturating a solutionof sodium carbonate with sulfur(IV) oxide.It is isolated from the aqueous solution byprecipitation with alcohol. If heated it un-dergoes thermal decomposition to givesodium sulfate, sulfur(VI) oxide, and sul-fur. Sodium hydrogensulfite is used to ster-ilize wine casks and in medicine as anantiseptic.
sodium hydroxide (caustic soda; NaOH)A white deliquescent slightly translucentsolid. In solution it is a strong alkali andelectrolyte. In the laboratory it can be pre-pared by reacting sodium, sodium monox-ide, or sodium peroxide with water.Industrially it can be prepared by the elec-trolysis of sodium chloride using a mercurycathode (Castner–Kellner) or diaphragmcell. It can also be manufactured by theGossage process. Sodium hydroxide dis-solves readily in water with the evolutionof heat. Its solutions have a soapy feel andare very corrosive. It is used to absorbacidic gases, such as carbon dioxide andsulfur(IV) oxide. Industrially it is used insoap and paper manufacture and in the pu-rification of bauxite.
sodium iodide (NaI) A white solidformed by reacting sodium carbonate orsodium hydroxide with hydroiodic acid;the solution is then evaporated. Sodium io-dide forms colorless crystals having a cubicshape. It is used as a source of iodine andin medicine and photography. Acidified so-lutions of sodium iodide exhibit reducingproperties owing to the formation of hy-droiodic acid.
sodium metasilicate /met-ă-sil-ă-kayt/See sodium silicate.
sodium methanoate /meth-ă-noh-ayt/(sodium formate; HCOONa) A white
solid produced by reacting carbon monox-ide with solid sodium hydroxide at 200°Cand 10 atmospheres pressure. Alternativelyit can be prepared by the neutralization ofmethanoic acid with sodium hydroxide so-lution.
sodium monoxide /mon-oks-ÿd, -id/(Na2O) A white solid formed by burningsodium in a deficiency of oxygen or alter-natively by reducing sodium peroxide orsodium hydroxide with the requisiteamount of sodium. It reacts violently withwater to form sodium hydroxide and withacids to form solutions of their salts.Sodium monoxide forms cubic crystals. Itdissolves in liquid ammonia to give a mix-ture of sodamide and sodium hydroxide.
sodium nitrate (Chile saltpeter; NaNO3)A white solid formed by the neutralizationof nitric acid with either sodium carbonateor sodium hydroxide. It occurs naturally inlarge quantities in South America. Impureindustrial sodium nitrate is called caliche.Sodium nitrate is very soluble in water andon crystallization forms colorless deliques-cent crystals. On heating it decomposes togive sodium nitrite and oxygen. Whenheated with concentrated sulfuric acid, ni-tric acid is produced. Sodium nitrate isused as a fertilizer and as a source of ni-trates and nitric acid. The salt crystallizesin the rhombohedral system and is isomor-phous with the iodate.
sodium nitrite (NaNO2) A yellowish-white solid formed by the thermal decom-position of sodium nitrate. It is readilysoluble in water. The anhydrous salt formsorthorhombic crystals. When treated withcold dilute hydrochloric acid, sodium ni-trite forms nitrous acid. It is used in or-ganic chemistry in the process ofdiazotization and industrially as a corro-sion inhibitor.
sodium orthophosphate /or-thoh-fos-fayt/ See trisodium phosphate(V).
sodium peroxide (Na2O2) A yellowish-white ionic solid formed by the direct com-bination of burning sodium with excess
253
sodium peroxide
oxygen. It reacts with water to formsodium hydroxide and hydrogen peroxide;the latter decomposes rapidly in the alka-line solution to give oxygen. Sodium per-oxide is used as a bleaching agent and anoxidizing agent for such materials as wooland wood pulp. Sodium peroxide can con-vert nitrogen(II) oxide to sodium nitrateand iodine to sodium iodate.
sodium silicate (sodium metasilicate;Na2SiO3.5H2O) A colorless crystallinesolid used in detergents and other cleaningcompounds, in refractories, and in cementsfor ceramics. A concentrated aqueous solu-tion is known as water glass and is used asa sizing agent and for making precipatedsilica and silica gel.
sodium sulfate (Na2SO4) A white solublesolid formed by heating a mixture ofsodium chloride and concentrated sulfuricacid. Industrially it is prepared by the firststage of the Leblanc process. Sodium sul-fate forms two hydrates, the decahydrate(Na2SO4.10H2O) and the heptahydrate(Na2SO4.7H2O). Sodium decahydrate,known as Glauber’s salt, is used in themanufacture of glass and as a purgative inmedicine. It effloresces on exposure to airto form the anhydrous salt, which is usedas a drying agent in organic chemistry. Atroom temperature sodium sulfate crystal-lizes in the orthorhombic system; around250°C there is a transition to the hexago-nal system. The decahydrate crystallizes inthe monoclinic form. Glauber’s salt isnamed for the German chemist JohannRudolf Glauber (1604–68).
sodium sulfide (Na2S) A yellow-red solidformed by the reduction of sodium sulfateusing carbon (or carbon monoxide or hy-drogen) at a high temperature. It is a cor-rosive material and deliquesces to releasehydrogen sulfide. Sodium sulfide forms hy-drates containing 4½, 5, and 9 H2O. Inaqueous solution it is readily hydrolyzed,the solution being strongly alkaline.
sodium sulfite (Na2SO3) A white solidformed by reacting the exact amount ofsulfur(IV) oxide with either sodium car-
bonate or sodium hydroxide. It is readilysoluble in water and crystallizes as color-less crystals of the heptahydrate(Na2SO3.7H2O). When treated with dilutemineral acids, sulfur(IV) oxide is evolved.At high temperatures, sodium sulfite un-dergoes thermal decomposition to givesodium sulfate and sodium sulfide.
sodium superoxide (sodium dioxide;NaO2) A pale yellow solid obtained byheating sodium peroxide in oxygen at490°C. It reacts with water to give hydro-gen peroxide, sodium hydroxide solution,and oxygen. Commercial sodium peroxidecontains 10% sodium superoxide.
sodium thiosulfate(IV) (Na2S2O3) Awhite solid prepared either by boilingsodium sulfite with flowers of sulfur or bypassing sulfur(IV) oxide into a suspensionof sulfur in boiling sodium hydroxide.Sodium thiosulfate is readily soluble inwater and crystallizes as large colorlesscrystals of the pentahydrate(Na2S2O3.5H2O). It reacts with diluteacids to give sulfur and sulfur(IV) oxide. Itis used in photography as ‘hypo’ and in-dustrially as an antichlor. In volumetricanalysis, solutions of sodium thiosulfateare usually prepared from the pentahy-drate. On heating it disproportionates togive sodium sulfate and sodium sulfide.
soft iron Iron that has a low carbon con-tent and is unable to form permanent mag-nets.
soft soap A liquid soap made by saponi-fication with potassium hydroxide (ratherthan sodium hydroxide).
soft water See hardness (of water).
sol A COLLOID consisting of solid particlesdistributed in a liquid medium. A wide va-riety of sols are known; the colors often de-pend markedly on the particle size. Theterm aerosol is used for solid or liquidphases dispersed in a gaseous medium.
solder /sod-er/ An alloy used in joiningmetals. The molten solder wets the surfaces
sodium silicate
254
to be joined, without melting them, and so-lidifies on cooling to form a hard joint. Thesurfaces must be clean and free of oxideand a flux is often used to achieve this. Softsolders consist of lead with up to 60% tinand melt in the range 183–250°C. Soft-sol-dering is used, for example, in plumbingand making electrical contacts. Brazingsolders are copper–zinc alloys that havehigher melting points and produce strongerjoints than soft solders; silver can be addedto produce silver solders.
solid The state of matter in which the par-ticles occupy fixed positions, giving thesubstance a definite shape. The particlesare held in these positions by bonds. Threekinds of attraction fix the positions of theparticles: ionic, covalent, and intermolecu-lar. Since these bonds act over short dis-tances the particles in solids are packedclosely together. The strengths of thesethree types of bonds are different and so,therefore, are the mechanical properties ofdifferent solids.
solid solution A solid composed of twoor more substances mixed together at themolecular level. Atoms, ions, or moleculesof one component in the crystal are at lat-tice positions normally occupied by theother component. Certain alloys are solidsolutions of one metal in another. Isomor-phic salts can also sometimes form solid so-lutions, as in the case of crystalline alums.
solubility /sol-yŭ-bil-ă-tee/ The amount ofone substance that could dissolve in an-other to form a saturated solution underspecified conditions of temperature andpressure. Solubilities are stated as moles ofsolute per 100 grams of solvent, or as massof solute per unit volume of solvent.
solubility product Symbol: Ks If an ionicsolid is in contact with its saturated solu-tion, there is a dynamic equilibrium be-tween solid and solution:
AB(s) ˆ A+(aq) + B–(aq)The equilibrium constant for this is givenby
[A+][B–]/[AB]
The concentration of undissolved solid[AB] is also constant, so
Ks = [A+][B–]Ks is the solubility product of the salt (at agiven temperature). For a salt A2B3, for in-stance:
Ks = [A+]2[B–]3, etc.Solubility products are meaningful only forsparingly soluble salts. If the product ofions exceeds the solubility product, precip-itation occurs.
solute /sol-yoot, soh-loot/ A material thatis dissolved in a solvent to form a solution.
solution A liquid system of two or morespecies that are intimately dispersed withineach other at a molecular level. The systemis therefore totally homogeneous. Themajor component is called the solvent(generally liquid in the pure state) and theminor component is called the solute (gas,liquid, or solid).
The process occurs because of a directintermolecular interaction of the solventwith the ions or molecules of the solute.This interaction is called solvation. Part ofthe energy of this interaction appears as achange in temperature on dissolution. Seealso solid solution; solubility.
solvation /sol-vay-shŏn/ The attraction ofa solute species (e.g. an ion) for moleculesof solvent. In water, for example, a positiveion will be surrounded by water molecules,which tend to associate around the ion be-cause of attraction between the positivecharge of the ion, and the negative part ofthe polar water molecule. The energy ofthis solvation (hydration in the case ofwater) is the ‘force’ needed to overcome theattraction between positive and negativeions when an ionic solid dissolves. The at-traction of the dissolved ion for solventmolecules may extend for several layers. Inthe case of transition metal elements, ionsmay also form complexes by coordinationto the nearest layer of molecules.
Solvay process /sol-vay/ (ammonia–sodaprocess) An industrial process for makingsodium carbonate. The raw materials arecalcium carbonate and sodium chloride
255
Solvay process
(with ammonia). The calcium carbonate isheated:
CaCO3 → CaO + CO2Carbon dioxide is bubbled into a solutionof sodium chloride saturated with ammo-nia, precipitating sodium hydrogencarbon-ate and leaving ammonium chloride insolution. The sodium hydrogencarbonateis then heated:
2NaHCO3 → Na2CO3 + H2O + CO2The ammonia is regenerated by heating theammonium chloride with the calciumoxide:
2NH4Cl + CaO → CaCl2 + 2NH3 +H2O
solvent /sol-vĕnt/ A liquid capable of dis-solving other materials (solids, liquids, orgases) to form a solution. The solvent isgenerally the major component of the solu-tion. Solvents can be divided into classes,the most important being:Polar. A solvent in which the moleculespossess a moderate to high dipole momentand in which polar and ionic compoundsare easily soluble. Polar solvents are usu-ally poor solvents for non-polar com-pounds. For example, water is a goodsolvent for many ionic species, such assodium chloride or potassium nitrate, andpolar molecules, such as the sugars, butdoes not dissolve paraffin wax.Non-polar. A solvent in which the mole-cules do not possess a permanent dipolemoment and consequently will solvatenon-polar species in preference to polarspecies. For example, benzene and tetra-chloromethane are good solvents for io-dine and paraffin wax, but do not dissolvesodium chloride.Amphiprotic. A solvent which undergoesself-ionization and can act both as a protondonator and as an acceptor. Water is agood example and ionizes according to:
2H2O = H3O+ + OH–
Aprotic. A solvent which can neither ac-cept nor yield protons. An aprotic solventis therefore the opposite to an amphiproticsolvent.
solvent extraction (liquid–liquid extrac-tion) A method of removing a substancefrom solution by shaking it with and dis-
solving it in another (better) solvent that isimiscible with the original solvent.
solvent naphtha See naphtha.
solvolysis /sol-vol-ă-sis/ A reaction be-tween a compound and the solvent inwhich it is dissolved. See also hydrolysis.
sorbitol /sor-bă-tol, -tohl/ (HOCH2-(CHOH)4CH2OH) A hexahydric alcoholthat occurs in rose hips and rowan berries.It can be synthesized by the reduction ofglucose. Sorbitol is used to make vitamin C(ascorbic acid) and surfactants. It is alsoused in medicines and as a sweetener (par-ticularly in foods for diabetics). It is an iso-mer of mannitol.
sorption /sorp-shŏn/ Absorption of gasesby solids.
specific /spĕ-sif-ik/ Denoting a physicalquantity per unit mass. For example, vol-ume (V) per unit mass (m) is called specificvolume:
V = VmIn certain physical quantities the term
does not have this meaning: for example,specific gravity is more properly called rel-ative density.
specific gravity See relative density.
specific heat capacity Symbol: c Theamount of heat needed to raise a unit massof a substance by one degree. It is measuredin joules per kilogram per Kelvin (Jkg–1K–1) in SI units.
specific rotatory power Symbol: αm Therotation of plane-polarized light in degreesproduced by a 10 cm length of solutioncontaining 1 g of a given substance per mil-liliter of stated solvent. The specific rota-tory power is a measure of the opticalactivity of substances in solution. It is mea-sured at 20°C using the D-line of sodium.
spectra See spectrum.
spectral line A particular wavelength ofelectromagnetic radiation emitted or ab-
solvent
256
sorbed by an atom, ion, or molecule. Seeline spectrum.
spectral series A group of related lines inthe absorption or emission spectrum of asubstance. The lines in a spectral seriesoccur when the transitions all occur be-tween one particular energy level and a setof different levels. See also Bohr theory.
spectrograph /spek-trŏ-graf, -grahf/ Aninstrument for producing a photographicrecord of a spectrum.
spectrographic analysis /spek-trŏ-graf-ik/ A method of analysis in which the sam-ple is excited electrically (by an arc orspark) and emits radiation characteristic ofits component atoms. This radiation ispassed through a slit, dispersed by a prismor a grating, and recorded as a spectrum,either photographically or photoelectri-cally. The photographic method waswidely used for qualitative and semiquan-titative work but photoelectric detectionalso allows wide quantitative application.
spectrometer /spek-trom-ĕ-ter/ 1. An in-strument for examining the different wave-lengths present in electromagneticradiation. Typically, spectrometers have asource of radiation, which is collimated bya system of lenses and/or slits. The radia-tion is dispersed by a prism or grating, andrecorded photographically or by a photo-cell. There are many types for producingand investigating spectra over the wholerange of the electromagnetic spectrum.Often spectrometers are called spectro-scopes.2. Any of various other instruments for an-alyzing the energies, masses, etc., of parti-cles. See mass spectrometer.
spectrophotometer /spek-troh-foh-tom-ĕ-ter/ A form of spectrometer able to mea-sure the intensity of radiation at differentwavelengths in a spectrum, usually in thevisible, infrared, or ultraviolet regions.
spectroscope /spek-trŏ-skohp/ An instru-ment for examining the different wave-
lengths present in electromagnetic radia-tion. See also spectrometer.
spectroscopy /spek-tros-kŏ-pee/ 1. Theproduction and analysis of spectra. Thereare many spectroscopic techniques de-signed for investigating the electromag-netic radiation emitted or absorbed bysubstances. Spectroscopy, in variousforms, is used for analysis of mixtures, foridentifying and determining the structuresof chemical compounds, and for investigat-ing energy levels in atoms, ions, and mol-ecules. In the visible and longer wavelengthultraviolet, transitions correspond to elec-tronic energy levels in atoms and mol-ecules. The shorter wavelength ultravioletcorresponds to transitions in ions. In the X-ray region, transitions in the inner shells ofatoms or ions are involved. The infrared re-gion corresponds to vibrational changes inmolecules, with rotational changes atlonger wavelengths.2. Any of various techniques for analysingthe energy spectra of beams of particles orfor determining mass spectra.
spectrum /spek-trŭm/ (pl. spectra) 1. Arange of electromagnetic radiation emittedor absorbed by a substance under particu-lar circumstances. In an emission spec-trum, light or other radiation emitted bythe body is analyzed to determine the par-ticular wavelengths produced. The emis-sion of radiation may be induced by avariety of methods; for example, by hightemperature, bombardment by electrons,absorption of higher-frequency radiation,etc. In an absorption spectrum a continu-ous flow of radiation is passed through thesample. The radiation is then analyzed todetermine which wavelengths are ab-sorbed. See also band spectrum; continu-ous spectrum; line spectrum.2. In general, any distribution of a prop-erty. For instance, a beam of particles mayhave a spectrum of energies. A beam ofions may have a mass spectrum (the distri-bution of masses of ions). See mass spec-trometer.
spelter Commercial zinc containing about3% impurities, mainly lead.
257
spelter
spin A property of certain elementary par-ticles whereby the particle acts as if it werespinning on an axis; i.e. it has an angularmomentum. Such particles also have amagnetic moment. In a magnetic field thespins line up at an angle to the field direc-tion and precess around this direction. Cer-tain definite orientations to the fielddirection occur such that msh/2π is thecomponent of angular momentum alongthis direction. ms is the spin quantum num-ber, and for an electron it has values +1/2and –1/2. h is the Planck constant.
spontaneous combustion The self-igni-tion of a substance that has a low ignitiontemperature. It occurs when slow oxida-tion of the substance (such as a heap ofdamp straw or oily rags) builds up suffi-cient heat for ignition to take place.
square-planar See complex.
stabilization energy The difference inenergy between the delocalized structureand the conventional structure for a com-pound. For example, the stabilization en-ergy of benzene is 150 kJ per mole, whichrepresents the difference in energy betweena Kekulé structure and the delocalizedstructure: the delocalized form being oflower energy is therefore more stable. Thestabilization energy can be determined bycomparing the experimental value for theheat of hydrogenation of benzene with thatcalculated for Kekulé benzene from bond-energy data.
stabilizer A substance added to preventchemical change (i.e. a negative catalyst).
staggered conformation See conforma-tion.
stainless steel See steel.
stalactites and stalagmites /stal-ăk-tÿtsstal-ăg-mÿts, stă-lak-tÿts stă-lag-mÿys/ Pil-lars of calcium carbonate found hangingfrom the ceiling (stalactites) and standingon the floor (stalagmites) in limestone cav-erns. They form when water containingdissolved carbon dioxide forms weak car-
bonic acid, which is able to react with lime-stone rock (calcium carbonate) to give a so-lution of calcium hydrogencarbonate. Thesolution accumulates in drops on the roofof caves and as it evaporates, the reactionis reversed causing calcium carbonate to bedeposited. Over a very long period, the de-posits build up to give a stalactite hangingfrom the cavern roof. Water that dripsonto the floor from the tip of the stalactitealso evaporates to leave a deposit of cal-cium carbonate. This gradually grows toform a stalagmite. Stalagmites and stalac-tites can eventually meet to form a singlerock pillar.
standard cell A voltaic cell whose e.m.f.is used as a standard. See Clark cell; We-ston cadmium cell.
standard electrode A half cell used formeasuring electrode potentials. The hydro-gen electrode is the basic standard but, inpractice, calomel electrodes are usuallyused.
standard pressure An internationallyagreed value; a barometric height of 760mmHg at 0°C; 101 325 Pa (approximately100 kPa). This is sometimes called the at-mosphere (used as a unit of pressure). Thebar, used mainly when discussing theweather, is 100 kPa exactly.
standard solution A solution that con-tains a known weight of the reagent in adefinite volume of solution. A standardflask or volumetric flask is used for thispurpose. The solutions may be prepared bydirect weighing for PRIMARY STANDARDS. Ifthe reagent is not available in a pure formor is deliquescent the solution must bestandardized by titration against anotherknown standard solution.
standard state The standard conditionsused as a reference system in thermody-namics: pressure is 101 325 Pa; tempera-ture is 25°C (298.15 K); concentration is 1mol. The substance under investigationmust also be pure and in its usual state,given the above conditions.
spin
258
standard temperature An internation-ally agreed value for which many measure-ments are quoted. It is the meltingtemperature of water, 0°C (273.15 K). Seealso STP.
stannane /stan-ayn/ (tin(IV) hydride;SnH4) A colorless poisonous gas preparedby the action of lithium tetrahydroalumi-nate(III) on tin(II) chloride. It is unstable,decomposing immediately at 150°C. Stan-nane acts as a reducing agent.
stannate /stan-ayt/ See tin.
stannic chloride /stan-ik/ See tin(IV)chloride.
stannic compounds Compounds oftin(IV).
stannite /stan-ÿt/ See tin.
stannous compounds /stan-ŭs/ Com-pounds of tin(II).
starch A polysaccharide that occurs ex-clusively in plants. Starches are extractedcommercially from maize, wheat, barley,rice, potatoes, and sorghum. The starchesare storage reservoirs for plants; they canbe broken down by enzymes to simple sug-ars and then metabolized to supply energyneeds. Starch is a dietary component of an-imals.
Starch is not a single molecule but amixture of amylose (water-soluble, bluecolor with iodine) and amylopectin (notwater-soluble, violet color with iodine).The composition is amylose 10–20%,amylopectin 80–90%.
Stark effect The splitting of atomic spec-tral lines because of the presence of an ex-ternal electric field. This phenomenon wasdiscovered by the German physicist Jo-hannes Stark (1874–1957) in 1913.
states of matter The three physical con-ditions in which substances occur: solid,liquid, and gas. The addition or removal ofenergy (usually in the form of heat) enablesone state to be converted into another.
The major distinctions between thestates of matter depend on the kinetic ener-gies of their particles and the distances be-tween them. In solids, the particles havelow kinetic energy and are closely packed;in gases they have high kinetic energy andare very loosely packed; kinetic energy andseparation of particles in liquids are inter-mediate.
Solids have fixed shapes and volumes,i.e. they do not flow, like liquids and gases,and they are difficult to compress. In solidsthe atoms or molecules occupy fixed posi-tions in space. In most cases there is a reg-ular pattern of atoms – the solid iscrystalline.
Liquids have fixed volumes (i.e. lowcompressibility) but flow to take up theshape of the container. The atoms or mol-ecules move about at random, but they arequite close to one another and the motionis hindered.
Gases have no fixed shape or volume.They expand spontaneously to fill the con-tainer and are easily compressed. The mol-ecules have almost free random motion.
A plasma is sometimes considered to bea fourth state of matter.
stationary phase See chromatography.
statistical mechanics The branch ofphysics and chemistry associated withusing statistical methods to relate the prop-erties of a large number of microscopicparticles, such as atoms and molecules, tothe macroscopic properties of the system.
steam distillation A method of isolatingor purifying substances by exploiting Dal-ton’s law of partial pressures to lower theboiling point of the mixture. When two im-miscible liquids are distilled, the boilingpoint will be lower than that of the morevolatile component and consequently willbe below 100°C if one component is water.The method is particularly useful for re-covering materials from tarry mixtures.
steam reforming The conversion of amethane–steam mixture at 900°C with anickel catalyst into a mixture of carbonmonoxide and hydrogen. The mixture of
259
steam reforming
gases (synthesis gas) provides a startingmaterial in a number of processes, e.g. themanufacture of methanol.
stearate /stee-ă-rayt/ A salt or ester ofstearic acid; an octadecanoate.
stearic acid /stee-a-rik/ See octadecanoicacid.
steatite /stee-ă-tÿt/ See soapstone.
steel An alloy of iron with small amountsof carbon and, often, other metals. Carbonsteels contain 0.05–1.5% carbon – themore carbon, the harder the steel. Alloysteels contain carbon but also containsmall amounts of certain other elements;for example chromium, manganese, andvanadium. Their properties depend on thecomposition. Stainless steels, for instance,are resistant to corrosion. The main non-ferrous constituent is chromium (10–25%)with up to 0.7% carbon.
step An elementary stage in a chemical re-action, in which energy may be transferredfrom one molecule to another, bonds maybe broken or formed, or electrons may betransferred. For example, in the reactionbetween hypochlorite ions and iodide ionsin aqueous solution there are three distinctsteps:
Step 1OCl–(aq) + H2O(l) → HOCl(aq) +
OH–(aq)Step 2
I–(aq) + HOCl(aq) → HOI(aq) + Cl–(aq)Step 3
OH–(aq) + HOI(aq) → H2O(l) + OI–(aq)
steradian /sti-ray-dee-ăn/ Symbol: sr TheSI unit of solid angle. The surface of asphere, for example, subtends a solid angleof 4π at its center. The solid angle of a coneis the area intercepted by the cone on thesurface of a sphere of unit radius.
stereochemistry /ste-ree-oh-kem-iss-tree,steer-ee-/ The branch of chemistry con-cerned with the shapes of molecules andthe way these affect the chemical proper-ties.
stereoisomerism /ste-ree-oh-ÿ-som-ĕ-riz-ăm, steer-ee-/ See isomerism.
stereospecific /ste-ree-oh-spĕ-sif-ik, steer-ee-/ Describing a chemical reaction involv-ing an asymmetric atom (usually carbon)that results in the formation of only onegeometric isomer. See isomerism; opticalactivity.
stereospecific polymer See polymeriza-tion.
stearate
260
Steam distillation
steric effect /ste-rik, steer-ik/ An effect inwhich the shape of a molecule influences itsreactions. A particular example occurs inmolecules containing large groups, whichhinder the approach of a reactant (sterichindrance).
steric hindrance See steric effect.
steroid /steer-oid/ Any member of a groupof biochemical compounds having a com-plex basic ring structure. Examples are cor-ticosteroid hormones (produced by theadrenal gland), sex hormones (proges-terone, androgens, and estrogens), bileacids, and sterols (such as cholesterol).
sterol /steer-ol, -ohl/ A steroid with longaliphatic side chains (8–10 carbons) and atleast one hydroxyl group. They are lipid-soluble and often occur in membranes. Ex-amples are cholesterol and ergosterol.
still An apparatus for distillation.
stoichiometric coefficient /stoi-kee-ŏ-met-rik/ See chemical equation.
stoichiometry /stoi-kee-om-ĕ-tree/ Theproportions in which elements form com-pounds. A stoichiometric compound is onein which the atoms have combined in smallwhole numbers.
storage battery See accumulator.
STP (NTP) Standard temperature andpressure. Conditions used internationallywhen measuring quantities that vary withboth pressure and temperature (such as thedensity of a gas). The values are 101 325Pa (approximately 100 kPa) and 0°C(273.15 K). See also standard pressure;standard temperature.
straight chain See chain.
Strecker synthesis /strek-er/ A method ofsynthesizing amino acids. Hydrogencyanide forms an addition product(cyanohydrin) with aldehydes:
RCHO + HCN → RCH(CN)OHWith ammonia further substitution occurs:
RCH(CN)OH + NH3 →RCH(CN)(NH2) + H2O
Acid hydrolysis of the cyanide group thenproduces the amino acid
RCH(CN)(NH2) →RCH(COOH)(NH2)
The method is a general synthesis for α-amino acids (with the –NH2 and –COOHgroups on the same carbon atom).
strong acid An acid that is almost com-pletely dissociated into its component ionsin solution. Compare weak acid.
strong base A base that is completely oralmost completely dissociated into its com-ponent ions in solution.
strontia /stron-shee-ă/ See strontiumoxide.
strontium /stron-shee-ŭm, -tee-/ A softlow-melting reactive metal; the fourthmember of group 2 of the periodic tableand a typical alkaline-earth element. Theelectronic configuration is that of kryptonwith two additional outer 5s electrons.Strontium and barium are both of lowabundance in the Earth’s crust, strontiumoccurring as strontianite (SrCO3) and ce-lestine (SrSO4).
The element is produced industrially byroasting the carbonate to give the oxide(800°C) and then reducing with aluminum,
3SrO + 2Al → Al2O3 + 2SrStrontium has a low ionization poten-
tial, is large, and is therefore very elec-tropositive. The chemistry of strontiummetal is therefore characterized by high re-activity of the metal. The properties ofstrontium fall into sequence with other al-kaline earths. Thus it reacts directly withoxygen, nitrogen, sulfur, the halogens, andhydrogen to form respectively the oxideSrO, nitride Sr3N2, sulfide SrS, halidesSrX2, and hydride SrH2, all of which arelargely ionic in character. The oxide SrOand the metal react readily with water toform the hydroxide Sr(OH)2, which isbasic and mid-way between Ca(OH)2 andBa(OH)2 in solubility. The carbonate andsulfate are both insoluble.
261
strontium
As the metal is very electropositive thesalts are never much hydrolyzed in solutionand the ions are largely solvated as[Sr(H2O)6]2+.
Symbol: Sr; m.p. 769°C; b.p. 1384°C;r.d. 2.54 (20°C); p.n. 38; r.a.m. 87.62.
strontium bicarbonate See strontiumhydrogencarbonate.
strontium carbonate (SrCO3) A whiteinsoluble solid that occurs naturally as themineral strontianite. It can be prepared bypassing carbon dioxide over strontiumoxide or hydroxide or by passing the gasthrough a solution of a strontium salt.Strontium carbonate is then formed as aprecipitate. It is used as a slagging agent incertain metal furnaces and to produce a redflame in fireworks.
strontium chloride (SrCl2) A white solidobtained directly from strontium and chlo-rine or by passing chlorine over heatedstrontium oxide. It is used in fireworks togive a red flame. The hexahydrate(SrCl2.6H2O) is prepared by the neutral-ization of hydrochloric acid by strontiumhydroxide or carbonate.
strontium hydrogencarbonate (stron-tium bicarbonate; Sr(HCO3)2) A com-pound present in solutions formed by theaction of carbon dioxide on a suspension incold water of strontium carbonate, towhich it reverts on heating:
SrCO3 + CO2 + H2O = Sr(HCO3)2
strontium hydroxide (Sr(OH)2) A solidthat normally occurs as the octahydrate(Sr(OH)2.8H2O), which is prepared bycrystallizing an aqueous solution of stron-tium oxide. Strontium hydroxide is readilysoluble in water and aqueous solutions arestrongly basic. It is used in the purificationof sugar.
strontium nitrate (Sr(NO3)2) A colorlesscrystalline compound, often used in py-rotechnics to produce a bright red flame.
strontium oxide (strontia; SrO) A gray-ish-white powder prepared by the decom-
position of strontium carbonate, hydrox-ide, or nitrate. Strontium oxide is soluble inwater, forming an alkaline solution be-cause of the attraction between the oxideions and protons from the water:
O2– + H2O → 2OH–
strontium sulfate (SrSO4) A white spar-ingly soluble salt that occurs naturally asthe mineral celestine. It can be prepared bydissolving strontium oxide, hydroxide, orcarbonate in sulfuric acid. It is used as asubstitute for barium sulfate in paints.
structural formula The formula of acompound showing the numbers and typesof atoms present, together with the way inwhich these are arranged in the molecule.Often this can be done by grouping theatoms, as in the structural formula ofethanoic acid CH3.CO.OH. Compare em-pirical formula; molecular formula.
structural isomerism See isomerism.
styrene /stÿ-reen/ See phenylethene.
sublimate /sub-lă-mayt/ A solid formedby sublimation.
sublimation /sub-lă-may-shŏn/ The con-version of a solid into a vapor without thesolid first melting. For instance (at stan-dard pressure) iodine, solid carbon diox-ide, and ammonium chloride sublime. Atcertain conditions of external pressure andtemperature an equilibrium can be estab-lished between the solid phase and vaporphase.
sub-shell A subdivision of an electronshell. It is a division of the orbitals thatmake up a shell into sets of orbitals, whichare degenerate (i.e. have the same energy)in the free atom. For example, in the thirdshell or M-shell there are the 3s, 3p, and 3dsub-shells of which the latter two are 3-and 5-degenerate respectively.
substituent /sub-stich-û-ĕnt/ An atom orgroup substituted for another in a com-pound. Often the term is used for groupsthat have replaced hydrogen in organic
strontium bicarbonate
262
compounds; for example, in benzene deriv-atives.
substitution reaction A reaction inwhich an atom or group of atoms in an or-ganic molecule is replaced by another atomor group. The substitution of a hydrogenatom in an alkane by a chlorine atom is anexample. Substitution reactions fall intothree major classes depending upon the na-ture of the attacking substituent.Nucleophilic substitution: the attackingsubstituent is a nucleophile. Such reactionsare very common with alcohols and halo-gen compounds, in which the electron-de-ficient carbon atom attracts thenucleophile and the leaving group readilyexists alone. Examples are the hydrolysisof a haloalkane and the chlorination of analcohol:
C2H5Cl + OH– → C2H5OH + Cl–
C2H5OH + HCl → C2H5Cl + H2OElectrophilic substitution: the attackingsubstituent is an electrophile. Such reac-tions are common in aromatic compounds,in which the electron-rich ring attracts theelectrophile. The nitration of benzene inwhich the electrophile is NO2
+ is an exam-ple:
C6H6 + NO2+ → C6H5NO2 + H+
Free-radical substitution: a free radical isthe attacking substituent. Such reactionscan be used with compounds that are inertto either nucleophiles or electrophiles, forinstance the halogenation of an alkane:
CH4 + Cl2 → CH3Cl + HClThe term ‘substitution’ is very general
and several reactions that can be consid-ered as substitutions are more normallygiven special names (e.g. esterification, hy-drolysis, and nitration). See also elec-trophilic substitution; nucleophilicsubstitution.
substrate /sub-strayt/ A material that isacted on by a catalyst.
succinic acid /suk-sin-ik/ See butanedioicacid.
sucrose /soo-krohs/ (cane sugar;C12H22O11) A sugar that occurs in manyplants. It is extracted commercially from
sugar cane and sugar beet. Sucrose is a dis-accharide formed from a glucose unit anda fructose unit. It is hydrolyzed to a mix-ture of fructose and glucose by the enzymeinvertase. Since this mixture has a differentoptical rotation (levorotatory) than theoriginal sucrose, the mixture is called in-vert sugar.
sugar (saccharide) One of a class of sweet-tasting simple CARBOHYDRATES. Sugarshave molecules consisting of a chain of car-bon atoms with –OH groups attached, andeither an aldehyde or ketone group. Theycan exist in a chain form or in a ringformed by reaction of the ketone or alde-hyde group with an OH group. Monosac-charides are simple sugars that cannot behydrolyzed to sugars with fewer carbon
263
sugar
HOCH2
OH
OH
OH
glucose – a pyranose ring
fructose – in a furanose ring form
ribose – a furanose ring
OOO
CH2OH
OOO
OH
OH
OHHO
HOCH2
OH
HOOH
CH2OHOOO
Sugars
sulfa drugs
264
CH2OH
OOO
OH
HOCH2
OH
HOOO
OOO
H
OHHO
H
H
H
H
H
HOCH2OH
CH2OH CH2OHOOO OOO
HO
HO
HO
HO
H
H
H
H
OHOH
H
H
H
H
HOH
H
OH
α–D–glucose β–D–glucoseGlucose — a monosaccharide sugar
Sucrose — a disaccharide sugar
Sugar: monosaccharides and disaccharides
atoms. Two or more monosaccharide unitscan be linked in disaccharides, trisaccha-rides, etc.
Monosaccharides are also classified ac-cording to the number of carbon atoms: apentose has five carbon atoms and a hex-ose six. Monosaccharides with aldehydegroups are aldoses; those with ketonegroups are ketoses. Thus, an aldohexose isa hexose with an aldehyde group; a ke-topentose is a pentose with a ketone group,etc.
The ring forms of monosaccharides arederived by reaction of the aldehyde or ke-tone group with one of the carbons at theother end of the chain. It is possible to havea six-membered (pyranose) ring or a five-membered (furanose) ring. See also poly-saccharide.
sulfa drugs /sul-fă/ See sulfonamide.
sulfate /sul-fayt/ A salt or ester of sulfu-ric(VI) acid.
sulfide /sul-fÿd/ 1. A compound of sulfurwith a more electropositive element. Sim-ple sulfides contain the ion S2–. Polysul-fides can also be formed containing ionswith chains of sulfur atoms (Sx
2–).
2. See thioether.
sulfinate /sul-fă-nayt, -nit/ See dithionite.
sulfinic acid /sul-fin-ik/ See dithionousacid.
sulfite /sul-fÿt/ A salt or ester of sulfurousacid.
sulfonamide /sul-fon-ă-mÿd/ A type oforganic compound with the general for-mula R.SO2.NH2. Sulfonamides, which areamides of sulfonic acids, are active againstbacteria, and some are used in pharmaceu-ticals (‘sulfa drugs’).
sulfonate /sul-fŏ-nayt/ A salt or ester ofsulfonic acid. See sulfonic acid.
sulfonation /sul-fŏ-nay-shŏn/ A reactionintroducing the –SO2OH (sulfonic acid)group into an organic compound. Sulfona-tion of aromatic compounds is usually ac-complished by refluxing with concentratedsulfuric acid for several hours. The attack-ing species is SO3 (sulfur(VI) oxide) and thereaction is an example of electrophilic sub-stitution.
sulfonic acid /sul-fon-ik/ A type of or-ganic compound containing the –SO2.OHgroup. The simplest example is benzenesul-fonic acid (C6H5SO2OH). Sulfonic acidsare strong acids. Electrophilic substitutioncan introduce other groups onto the ben-
zene ring; the –SO2.OH group directs sub-stituents into the 3-position.
sulfonium compound /sul-foh-nee-ŭm/An organic compound of general formulaR3SX, where R is an organic radical and Xis an electronegative radical or element; itcontains the ion R3S+. An example is di-ethylmethylsulfonium chloride, (C2H5)2-CH3.S+Cl–, made by reacting diethylsulfide with chloromethane.
sulfoxide /sul-foks-ÿd, -id/ An organiccompound of general formula RSOR′,where R and R′ are organic radicals. An
265
sulfoxide
O
O
S OH
Sulfonic acid
H
H
HO
H
H
OH
O
OH
OH
O1C
2C
3C
4C
5C
6CH2OH
HO
H
H
O
H
OH
OH
2C
3C
4C
5C
6CH2OH
1CH OH
H
Glucose (an aldohexose)
straight-chain form
Fructose (a ketohexose)
6CH2OH
5C
OHH
3C
4C
HO
H
H
2C
H
OH
OH
C1
HO
α-glucose
ring forms
β-glucose
6CH2OH
5C
OHH
3C
4C
HO
H
H
2C
H
OH
C1
H
OOH
straight-chain form ring form
H
4C
5C
HO
OH
HOCH2
3C
OH
H
OH
C2
CH2OHO6 1
Sugar: straight-chain and ring forms
example is dimethyl sulfoxide, (CH3)2SO,commonly used as a solvent.
sulfur /sul-fer/ A low melting non-metallicsolid, yellow colored in its common forms;the second member of group 16 of the pe-riodic table. It has the electronic configura-tion [Ne]3s23p4.
Sulfur occurs in the elemental form inSicily and some southern states of the USA,and in large quantities in combined formssuch as sulfide ores (FeS2) and sulfate rocks(CaSO4). It forms about 0.5% of theEarth’s crust. Elemental sulfur is extractedcommercially by the Frasch process,named for the German–American chemistHerman Frasch (1851–1914), in which su-perheated steam is forced down the outerof three concentric tubes leading into thedeposit. This causes the sulfur to melt; thenair is blown down the central tube causingmolten sulfur to be forced out of the well tobe collected and cooled at the well head.The large amount of sulfur obtained fromsmelting of sulfide ores or roasting of sul-fates is not obtained as elemental sulfur butis used directly as SO2/SO3 for conversionto sulfuric acid (contact process).
Sulfur exhibits allotropy and its struc-ture in all phases is quite complex. Thecommon crystalline modification, rhombicsulfur, is in equilibrium with a triclinicmodification above 96°C. Both have struc-tures based on S8-rings but the crystals arequite different. If molten sulfur is pouredinto water a dark red ‘plastic’ form is ob-tained in a semielastic form. The structureappears to be a helical chain of S atoms. Se-lenium and tellurium both have a gray‘metal-like’ modification but sulfur doesnot have this form.
Sulfur reacts directly with hydrogen butthe position of the equilibrium at normaltemperatures precludes its use as a prepar-ative method for hydrogen sulfide. The el-ement burns readily in oxygen with acharacteristic blue flame to form sulfur(IV)oxide (SO2) and traces of sulfur(VI) oxide(SO3). Sulfur also forms a large number ofoxy-acid species, some containing peroxy-groups and some with two or more S-atoms. It forms four distinct fluorides,
S2F2, SF4, SF6, S2F10; three chlorides, S2Cl2,SCl2, SCl4; a bromide S2Br2, but no iodide.Apart from SF6, which is surprisingly sta-ble and inert, the halides are generally sus-ceptible to hydrolysis to give commonlySO2, sometimes H2S, and the hydrogenhalide. The halides SF4, S2Cl2, and SCl2 arefrequently used as selective fluorinatingagents and chlorinating agents in organicchemistry. The chlorides are also industri-ally important for hardening rubbers. Sul-fur hexafluoride may be prepared by directreaction of the elements but SF4 is preparedby fluorinating SCl2 with NaF. In a limitedsupply of chlorine, sulfur combines to givesulfur monochloride, S2Cl2; in excess chlo-rine at higher temperatures the dichlorideis formed,
2S + Cl2 → S2Cl2S2Cl2 + Cl2 → 2SCl2
In addition to these binary halides, sul-fur forms two groups of oxyhalides; thesulfur dihalide oxides (thionyl halides),SOX2 (F, Cl, and Br, but not I), and the sul-fur dihalide dioxides (sulfuryl halides),SO2X2 (F and Cl, but not Br and I).
Most metals react with sulfur to formsulfides of which there are a large numberof structural types. With electropositive el-ements of groups I and II the sulfides arelargely ionic (S2–) in the solid phase. Hy-drolysis occurs in solution thus:
S2– + H2O → SH– + OH–
Most transition metals form sulfides,which although formally treated as FeS,CoS, NiS, etc., are largely covalent and fre-quently non-stoichiometric.
In addition to the complexities of thebinary compounds with metals, sulfurforms a range of binary compounds withelements such as Si, As, and P, which maybe polymeric and generally of rather com-plex structure, e.g. (SiS4)n, (SbS3)n, N4S4,As4S4, P4S3.
Sulfur also forms a wide range of or-ganic sulfur compounds, most of whichhave the typically revolting smell of H2S.
Symbol: S; m.p. 112.8°C; b.p. 444.6°C;r.d. 2.07; p.n. 16; r.a.m. 32.066.
sulfur dichloride dioxide /dÿ-oks-ÿd, -id/(sulfuryl chloride; SO2Cl2) A colorlessfuming liquid formed by the reaction of
sulfur
266
chlorine with sulfur(IV) oxide in sunlight.It is used as a chlorinating agent.
sulfur dichloride oxide (thionyl chlo-ride; SOCl2) A colorless fuming liquidformed by passing sulfur(IV) oxide overphosphorus(V) chloride and distilling themixture obtained. Sulfur dichloride oxideis used in organic chemistry to introducechlorine atoms (for example, into ethanolto form monochloroethane), the organicproduct being easily isolated as the otherproducts are gases.
sulfur dioxide /dÿ-oks-ÿd, -id/ See sul-fur(IV) oxide.
sulfuretted hydrogen /sul-fyŭ-ret-id/ Seehydrogen sulfide.
sulfuric(IV) acid /sul-fyoor-ik/ See sul-furous acid.
sulfuric(VI) acid (oil of vitriol; H2SO4) Acolorless oily liquid manufactured by theCONTACT PROCESS. The concentrated acid isdiluted by adding it slowly to water, withcareful stirring. Concentrated sulfuric acidacts as an oxidizing agent, giving sulfur(IV)oxide as the main product, and also as adehydrating agent. The diluted acid acts asa strong dibasic acid, neutralizing basesand reacting with active metals and car-bonates to form sulfates.
Sulfuric acid is used in the laboratory todry gases (except ammonia), to prepare ni-tric acid and ethene, and to absorb alkenes.In industry, it is used to manufacture fertil-izers (e.g. ammonium sulfate), rayon, anddetergents, to clean metals, and in vehiclebatteries.
sulfur monochloride /mon-ŏ-klor-ÿd, -id, -kloh-rÿd, -rid/ See disulfur dichloride.
sulfurous acid /sul-fyoor-ŭs/ (sulfuric(IV)acid; H2SO3) A weak acid found only insolution, made by passing sulfur(IV) oxideinto water. The solution is unstable andsmells of sulfur(IV) oxide. It is a reducingagent, converting iron(III) ions to iron(II)ions, chlorine to chloride ions, and orange
dichromate(VI) ions to green chrom-ium(III) ions.
sulfur(IV) oxide (sulfur dioxide; SO2) Acolorless choking gas prepared by burningsulfur or heating metal sulfides in air, or bytreating a sulfite with an acid. It is a pow-erful reducing agent, used as a bleach. Itdissolves in water to form sulfurous acid(sulfuric(IV) acid) and combines with oxy-gen, in the presence of a catalyst, to formsulfur(VI) oxide. This latter reaction is im-portant in the manufacture of sulfuric(VI)acid.
sulfur(VI) oxide (sulfur trioxide; SO3) Afuming volatile white solid prepared bypassing sulfur(IV) oxide and oxygen overhot vanadium(V) oxide (acting as a cata-lyst) and cooling the product in ice. Sul-fur(VI) oxide reacts vigorously with waterto form sulfuric acid. See also contactprocess.
sulfur trioxide /trÿ-oks-ÿd/ See sulfur(VI)oxide.
sulfuryl /sul-fŭ-răl, -yŭ-, -reel/ Describinga compound containing the group =SO2.
sulfuryl chloride /sul-fŭ-răl, -yŭ-, -reel/See sulfur dichloride dioxide.
superacid /soo-per-ass-id/ An acid with ahigh proton-donating ability. Superacidsare substances such as HF–SbF3 andHSO3–SbF5. Sometimes known as magicacids, they are able to produce carbeniumions from some saturated hydrocarbons.
superconduction /soo-per-kŏn-duk-shŏn/The property of zero resistance exhibitedby certain metals and metallic compoundsat low temperatures. Metals conduct attemperatures close to absolute zero but anumber of compounds are now knownthat conduct at higher (liquid-nitrogen)temperatures. Synthetic organic supercon-ductors have also been produced.
superconductivity /soo-per-kon-duk-tiv-ă-tee/ The property of zero electrical resis-tance exhibited by certain metals and
267
superconductivity
metallic compounds at low temperatures.Metals show superconductivity at temper-atures close to absolute zero but a numberof ceramic metal oxides are now knownthat are superconducting at higher (94K orbetter) temperatures. Synthetic organic su-perconductors have also been produced.
supercooling /soo-per-koo-ling/ Thecooling of a liquid at a given pressure to atemperature below its melting temperatureat that pressure without solidifying it. Theliquid particles lose energy but do notspontaneously fall into the regular geomet-rical pattern of the solid. A supercooled liq-uid is in a metastable state and will usuallysolidify if a small crystal of the solid is in-troduced to act as a ‘seed’ for the forma-tion of crystals. As soon as this happens,the temperature returns to the melting tem-perature until the substance has completelysolidified.
superfluidity /soo-per-floo-id-ă-tee/ Aproperty of liquid HELIUM at very low tem-peratures. At 2.186 K liquid helium makesa transition to a superfluid state, which hasa high thermal conductivity and flowswithout friction.
superheating /soo-per-hee-ting/ The rais-ing of a liquid’s temperature above its boil-ing temperature, by increasing thepressure.
supernatant /soo-per-nay-tănt/ Denotinga clear liquid that lies above a sediment ora precipitate.
superoxide /soo-per-oks-ÿd/ An inor-ganic compound containing the O2
– ion.
superphosphate /soo-per-fos-fayt/ Amixture – mainly calcium hydrogen phos-phate and calcium sulfate – used as a fertil-izer. It is made from calcium phosphateand sulfuric acid.
supersaturated solution /soo-per-sach-ŭ-ray-tit/ See saturated solution.
supersaturated vapor See saturatedvapor.
supplementary units See dimensionlessunits.
supramolecular chemistry /soo-pră-mŏ-lek-yŭ-ler/ A branch of chemistry con-cerned with the synthesis and study of largestructures consisting of molecules assem-bled together in a definite pattern. In asupramolecule the molecular units (whichare sometimes known as synthons) arejoined by intermolecular bonds – i.e. by hy-drogen bonds or by ionic attractions. Aparticular interest in supramolecular re-search is the idea of self-assembly – i.e. thatthe molecules form well-defined structuresspontaneously as a result of their geometryand chemical properties. In this way,supramolecular chemistry is ‘chemistry be-yond molecules’.
There are various different examples ofsupramolecular structures. For instance,single layers of carboxylic acid moleculesheld by hydrogen bonds may form two-di-mensional crystalline structures with novelelectrical properties. Large organic poly-meric structures may be formed in which anumber of individual chains radiate from acentral point or region. These polymers arecalled dendrimers and they have a numberof possible applications. Another type ofsupramolecule is a helicate, which has adouble helix made of two chains ofbipyridyl units held by copper ions alongthe axis. The structure is analogous to thedouble helix of DNA. See also host–guestchemistry.
supramolecule /soo-pră-mol-ĕ-kyool/ Seesupramolecular chemistry.
surfactant /ser-fak-tănt/ A substance thatlowers surface tension and has propertiesof wetting, foaming, detergency, disper-sion, and emulsification.
suspension A system in which small par-ticles of a solid or liquid are dispersed in aliquid or gas.
symbol, chemical See chemical symbol.
syndiotactic polymer See polymeriza-tion.
supercooling
268
syn-isomer /sin ÿ-sŏ-mer/ See isomerism.
synthesis /sin-th’ĕ-sis/ The preparation ofchemical compounds from simpler com-pounds.
synthesis gas A mixture of carbonmonoxide and hydrogen produced bysteam reforming of natural gas.
CH4 + H2O → CO + 3H2
Synthesis gas is a useful starting mater-ial for the manufacture of a number of or-ganic compounds.
synthon /sin-thon/ See retrosyntheticanalysis.
Système International d’Unités /see-stem an-tair-nas-yo-nal doo-nee-tay/ See SIunits.
269
Système International d’Unités
270
tactic polymer See polymerization.
talc /tal’k/ (French chalk) An extremelysoft greenish or white mineral, a hydroussilicate of magnesium. Purified it is sold astalcum powder, and it is used as a lubri-cant, a filler (in paint and rubber) and aningredient of some ceramics.
tannic acid /tan-ik/ See tannin.
tannin /tan-in/ Any of several yellow or-ganic compounds found in vegetablesources such as bark of trees, oak galls, andtea. They are used in tanning animal skinsto make leather and as mordants in dyeing.Tannic acid (a type of tannin) is a whitesolid heterocyclic organic acid extractedfrom oak galls and used for making dyesand inks.
tantalum /tan-tă-lŭm/ A silvery transitionelement. It is strong, highly resistant to cor-rosion, and is easily worked. Tantalum isused in turbine blades and cutting toolsand in surgical and dental work.
Symbol: Ta; m.p. 2996°C; b.p. 5425 ±100°C; r.d. 16.654 (20°C); p.n. 73; r.a.m.180.9479.
tar (bitumen) A dark oily viscous liquidobtained by the destructive distillation ofcoal or the fractionation of PETROLEUM.Tars are mixtures consisting mainly ofhigh-molecular weight hydrocarbons andphenols.
tartar emetic /tar-ter i-met-ik/ (potassiumantimonyl tartrate; KSbO(C4H4O6).¼H2O) A poisonous compound of anti-mony, which is used as an emetic in medi-cine and as a mordant in dyeing and as aninsecticide.
tartaric acid /tar-ta-rik/ A crystalline hy-droxy carboxylic acid with the formula:
HOOC(CHOH)2COOHIts systematic name is 2,3-dihydroxy-
butanedioic acid. See also optical activity.
tartrate /tar-trayt/ A salt or ester of tar-taric acid.
tautomerism /taw-tom-ĕ-riz-ăm/ Iso-merism in which each isomer can convertinto the other, so that the two isomers arein equilibrium. The isomers are called tau-tomers. Tautomerism often results fromthe migration of a hydrogen atom. Seeketo–enol tautomerism.
TCCD See dioxins.
technetium /tek-nee-shee-ŭm/ A transi-tion metal that does not occur naturally onEarth. It is produced artificially by bom-barding molybdenum with neutrons andalso during the fission of uranium. It is ra-dioactive.
Symbol: Tc; m.p. 2172°C; b.p. 4877°C;r.d. 11.5 (est.); p.n. 43; r.a.m. 98.9063(99Tc); most stable isotope 98Tc (half-life4.2 × 106 years).
Teflon (Trademark) The synthetic poly-mer polytetrafluoroethane.
telluride /tel-yŭ-rÿd, -rid/ A compound ofTELLURIUM and another element.
tellurium /te-loor-ee-ŭm/ A brittle silverymetalloid element belonging to group 16 ofthe periodic table. It is found native and incombination with metals. Tellurium isused mainly as an additive to improve thequalities of stainless steel and variousmetals.
T
Symbol: Te; m.p. 449.5°C; b.p.989.8°C; r.d. 6.24 (20°C); p.n. 52; r.a.m.127.6.
temperature scale A practical scale formeasuring temperature. A temperaturescale is determined by fixed temperatures(fixed points), which are reproducible sys-tems assigned an agreed temperature. Onthe Celsius scale the two fixed points arethe temperature of pure melting ice (the icetemperature) and the temperature of pureboiling water (the steam temperature). Thedifference between the fixed points is thefundamental interval of the scale, which issubdivided into temperature units. The In-ternational Temperature Scale has 11 fixedpoints that cover the range 13.81 kelvin to1337.58 kelvin.
temporary hardness A type of waterhardness caused by the presence of dis-solved calcium, iron, and magnesium hy-drogencarbonates. This form of hardnesscan be removed by boiling the water. Tem-porary hardness arises because rainwatercombines with carbon dioxide from the at-mosphere to form weak carbonic acid.This reacts with carbonate rocks, such aslimestone, to produce calcium hydrogen-carbonate, which then goes into solution.When this solution is heated, the completereaction sequence is reversed so that cal-cium carbonate is precipitated. If this is notremoved, it will accumulate in boilers andhot-water pipes and reduce their efficiency.Compare permanent hardness. See alsowater softening.
tera- Symbol: T A prefix denoting 1012.For example, 1 terawatt (TW) = 1012 watts(W).
terbium /ter-bee-ŭm/ A soft ductile mal-leable silvery rare element of the lan-thanoid series of metals. It occurs inassociation with other lanthanoids. One ofits few uses is as a dopant in solid-state de-vices.
Symbol: Tb; m.p. 1356°C; b.p. 3123°C;r.d. 8.229 (20°C); p.n. 65; r.a.m.158.92534.
terephthalic acid /te-ref-thal-ik/ See ben-zene-1,4-dicarboxylic acid.
ternary compound /ter-nă-ree/ A chemi-cal compound formed from three elements;e.g. Na2SO4 or LiAlH4.
terpene /ter-peen/ Any of a class of nat-ural unsaturated hydrocarbons with for-mulae (C5H8)n, found in plants. Terpenesconsist of isoprene units,
CH2=C(CH3)CH=CH2.Monoterpenes have two units (C10H16),diterpenes four units (C20H32), triterpenesthree units (C30H48), etc. Sesquiterpeneshave three isoprene units (C15H24).
tertiary alcohol See alcohol.
tertiary amine See amine.
tertiary structure See protein.
tervalent /ter-vay-lĕnt/ (trivalent) Havinga valence of three.
Terylene /te-ră-leen/ (Trademark) A poly-mer made by condensing benzene-1,4-di-carboxylic acid (terephthalic acid) andethane-1,2-diol (ethylene glycol), used formaking fibers for textiles.
tesla /tess-lă/ Symbol: T The SI unit ofmagnetic flux density, equal to a flux den-sity of one weber of magnetic flux persquare meter. 1 T = 1 Wb m–2. The unit isnamed for the Croatian–American physi-cist Nikola Tesla (1856–1943).
tetracarbonyl nickel(0) /tet-ră-kar-bŏ-năl/ See nickel carbonyl.
tetrachloroethene /tet-ră-klor-oh-eth-een, -kloh-roh-/ (ethylene tetrachloride;tetrachloroethylene; CCl2:CCl2) A color-less poisonous liquid organic compound (ahaloalkene) used as a solvent in dry clean-ing and as a de-greasing agent.
tetrachloroethylene /tet-ră-klor-oh-eth-ă-leen, -kloh-roh-/ See tetrachloroethene.
tetrachloromethane /tet-ră-klor-oh-
271
tetrachloromethane
tetraethyl lead
272
meth-ayn/ (carbon tetrachloride; CCl4) Acolorless nonflammable liquid made by thechlorination of methane. Its main use is asa solvent.
tetraethyl lead /tet-ră-eth-ăl/ See leadtetraethyl.
tetrafluoroethene /tet-ră-floo-ŏ-roh-eth-een/ (CF2:CF2) A gaseous organic com-pound (a fluorocarbon and a haloalkene)used to make the plastic polytetrafluo-roethene (PTFE). See polytetrafluo-roethene.
tetragonal crystal /te-trag-ŏ-năl/ Seecrystal system.
tetrahedral compound /tet-ră-hee-drăl/A compound such as methane, in which anatom has four bonds directed towards thecorners of a regular tetrahedron. The an-gles between the bonds in such a com-pound are about 109°.
tetrahydrate /tet-ră-hÿ-drayt/ A crys-talline hydrated compound containing fourmolecules of water of crystallization permolecule of compound. See also complex.
tetrahydrofuran /tet-ră-hÿ-droh-fyoo-ran, -fyoo-ran/ (THF, C4H8O) A colorlessliquid widely used as a solvent and formaking polymers.
tetravalent /tet-ră-vay-lĕnt, te-trav-ă-/(quadrivalent) Having a valence of four.
thallium /thal-ee-ŭm/ A soft malleablegrayish metallic element belonging togroup 13 of the periodic table. It is foundin lead and cadmium ores, and in pyrites(FeS2). Thallium is highly toxic and wasused previously as a rodent and insect poi-son. Various compounds are now used inphotocells, infrared detectors, and low-melting glasses.
Symbol: Tl; m.p. 303.5°C; b.p. 1457°C;r.d. 11.85 (20°C); p.n. 81; r.a.m.204.3833.
thermal dissociation The decompositionof a chemical compound into component
atoms or molecules by the action of heat.Often it is temporary and reversible.
thermite /ther-mÿt/ (thermit) A mixture ofaluminum powder and (usually) iron(III)oxide. When ignited the oxide is reduced;the reaction is strongly exothermic andmolten iron is formed:
2Al + Fe2O3 → Al2O3 + 2FeThermite is used in incendiary bombs
and for welding steel. See also Gold-schmidt process.
thermochemistry /ther-moh-kem-iss-tree/The branch of chemistry concerned withheats of reaction, solvation, etc.
thermodynamics /ther-moh-dÿ-nam-iks/The study of heat and other forms of en-ergy and the various related changes inphysical quantities such as temperature,pressure, density, etc.
The first law of thermodynamics statesthat the total energy in a closed system isconserved (constant). In all processes en-ergy is simply converted from one form toanother, or transferred from one system toanother.
A mathematical statement of the firstlaw is:
δQ = δU + δWHere, δQ is the heat transferred to the
system, δU the change in internal energy(resulting in a rise or fall of temperature),and δW is the external work done by thesystem.
The second law of thermodynamics canbe stated in a number of ways, all of whichare equivalent. One is that heat cannot passfrom a cooler to a hotter body withoutsome other process occurring. Another isthe statement that heat cannot be totallyconverted into mechanical work, i.e. a heatengine cannot be 100% efficient.
The third law of thermodynamics statesthat the entropy of a substance tends tozero as its thermodynamic temperature ap-proaches zero.
Often a zeroth law of thermodynamicsis given: that if two bodies are each in ther-mal equilibrium with a third body, thenthey are in thermal equilibrium with eachother. This is considered to be more funda-
mental than the other laws because they as-sume it. See also Carnot cycle; entropy.
thermodynamic temperature Symbol: TA temperature measured in kelvins. Seealso absolute temperature.
thermoplastic polymer /ther-moh-plas-tik/ See polymer.
thermosetting polymer /ther-moh-set-ing/ See polymer.
THF See tetrahydrofuran.
thiazine /th’ÿ-ă-zeen/ (C4H4NS) Any of agroup of heterocyclic organic compoundsthat have a six-membered ring containingfour carbon atoms, one nitrogen atom, andone sulfur atom.
thiazole /th’ÿ-ă-zohl/ (C3H3NS) A color-less volatile liquid, a beterocyclic com-pound with a five-membered ringcontaining three carbon atoms, one nitro-gen atom, and one sulfur atom. It resem-bles PYRIDINE in its reactions.
thin-layer chromatography A tech-nique widely used for the analysis of mix-tures. Thin-layer chromatography employsa solid stationary phase, such as alumina orsilica gel, spread evenly as a thin layer on aglass plate. A base line is carefullyscratched near the bottom of the plateusing a needle, and a small sample of themixture is spotted onto the base line usinga capillary tube. The plate is then stood up-right in solvent, which rises up to the baseline and beyond by capillary action. Thecomponents of the spot of the sample willdissolve in the solvent and tend to be car-ried up the plate. However, some of thecomponents will cling more readily to thesolid phase than others and will not moveup the plate so rapidly. In this way, differ-ent fractions of the mixture eventually be-come separated. When the solvent hasalmost reached the top, the plate is re-moved and quickly dried. The plate is de-veloped to locate the positions of colorlessfractions by spraying with a suitable chem-ical or by exposure to ultraviolet radiation.
The components are identified by compar-ing the distance they have traveled up theplate with standard solutions that havebeen run simultaneously, or by computingan RF value.
thio alcohol /th’ÿ-oh/ See thiol.
thiocarbamide /th’ÿ-oh-kar-bă-mÿd, -mid/ (thiourea; NH2CSNH2) A colorlesscrystalline organic compound (the sulfuranalog of urea). It is converted to the inor-ganic compound ammonium thiocyanateon heating. It is used as a sensitizer in pho-tography and in medicine.
thioether /th’ÿ-oh-ee-ther/ An organicsulfide; a compound of the type RSR′.
thiol /th’ÿ-ol, -ohl/ (thio alcohol; mercap-tan) A compound of the type RSH, similarto an alcohol with the oxygen atom re-placed by sulfur.
thionyl /th’ÿ-ŏ-năl/ Describing a com-pound containing the group =SO.
thionyl chloride /th’ÿ-ŏ-năl/ See sulfurdichloride oxide.
thiophene /th’ÿ-ŏ-feen/ (C4H4S) A color-less liquid that smells like benzene, a hete-rocyclic compound with a five-memberedring containing four carbon atoms and onesulfur atom. It occurs as an impurity incommercial benzene and is used as a sol-vent and in organic syntheses.
thiosulfate /th’ÿ-oh-sul-fayt/ A salt con-taining the ion S2O3
2–. See sodium thiosul-fate(IV).
thiourea /th’ÿ-oh-yû-ree-ă/ See thiocar-bamide.
thixotropy /thiks-ot-rŏ-pee/ The changeof viscosity with movement. Fluids that un-dergo a decrease in viscosity with increas-ing velocity are said to be thixotropic.Non-drip paint is an example of athixotropic fluid.
273
thixotropy
thoria /thor-ee-ă, thoh-ree-/ See thoriumdioxide.
thorium /thor-ee-ŭm, thoh-ree-/ A toxicradioactive element of the actinoid seriesthat is a soft ductile silvery metal. It hasseveral long-lived radioisotopes found in avariety of minerals including monazite.Thorium is used in magnesium alloys, in-candescent gas mantles, and nuclear fuelelements.
Symbol: Th; m.p. 1780°C; b.p. 4790°C(approx.); r.d. 11.72 (20°C); p.n. 90;r.a.m. 232.0381.
thorium dioxide /dÿ-oks-ÿd, -id/ (thoria;ThO2) A white insoluble compound, usedas a refractory, in gas mantles, and as a re-placement for silica in some types of opti-cal glass.
threonine /three-ŏ-neen, -nin/ See aminoacids.
thulium /thoo-lee-ŭm/ A soft malleableductile silvery element of the lanthanoid se-ries of metals. It occurs in association withother lanthanoids.
Symbol: Tm; m.p. 1545°C; b.p.1947°C; r.d. 9.321 (20°C); p.n. 69; r.a.m.168.93421.
thymidine /th’ÿ-mă-din, -deen/ The NU-CLEOSIDE formed when thymine is linked toD-ribose by a β-glycosidic bond.
thymine /th’ÿ-min, -meen/ A nitrogenousbase found in DNA. It has a PYRIMIDINE
ring structure.
tin A white lustrous metal of low meltingpoint; the fourth member of group 14 ofthe periodic table. Tin itself is the first dis-tinctly metallic element of the group eventhough it retains some amphoteric proper-ties. Its electronic structure has outer s2p2
electrons ([Kr]4d105s25p2). The element isof low abundance in the Earth’s crust(0.004%) but is widely distributed, largelyas cassiterite (SnO2). The metal has beenknown since early bronze age civilizationswhen the ores used were relatively rich butcurrently worked ores are as low as 1–2%
and considerable concentration must becarried out before roasting. The metal itselfis obtained by reduction using carbon,
SnO2 + C → Sn + CO2Tin is an expensive metal and several
processes are used for recovering tin fromscrap tin-plate. These may involve chlori-nation (dry) to the volatile SnCl4, or elec-trolytic methods using an alkalineelectrolyte:
Sn + 4OH– → Sn(OH)42– + 2e– (anode)
Sn(OH)42– → Sn2+ + 4OH– (cathode)
Sn2+ + 2e → Sn (cathode)Tin does not react directly with hydro-
gen but an unstable hydride, SnH4, can beprepared by reduction of SnCl4. The lowstability is due to the rather poor overlap ofthe diffuse orbitals of the tin atom with thesmall H-orbitals. Tin forms both tin(II)oxide and tin(IV) oxide. Both are ampho-teric, dissolving in acids to give tin(II) andtin(IV) salts, and in bases to form stannitesand stannates,
SnO + 4OH– → [SnO3]4– + 2H2Ostannite (relatively unstable)
SnO2 + 4OH– → [SnO4]4– + 2H2Ostannate
The halides, SnX2, may be prepared bydissolving tin metal in the hydrogen halideor by the action of heat on SnO plus the hy-drogen halide. Tin(IV) halides may be pre-pared by direct reaction of halogen withthe metal. Although tin(II) halides are ion-ized in solution their melting points are alllow suggesting considerable covalence inall but the fluoride. The tin(IV) halides arevolatile and essentially covalent with slightpolarization of the bonds. Tin(II) com-pounds are readily oxidized to tin(IV) com-pounds and are therefore good reducingagents for general laboratory use.
Tin has three crystalline modificationsor allotropes, α-tin or ‘gray tin’ (diamondstructure), β-tin or ‘white tin’, and γ-tin;the latter two are metallic with closepacked structures. Tin also has several iso-topes. It is used in a large number of alloysincluding Babbit metal, bell metal, Britan-nia metal, bronze, gun metal, and pewteras well as several special solders.
Symbol: Sn; m.p. 232°C; b.p. 2270°C;r.d. 7.31 (20°C); p.n. 50; r.a.m. 118.710.
thoria
274
tincal /tink-ăl/ A naturally occurring formof borax. See disodium tetraborate decahy-drate.
tin(II) chloride (stannous chloride; SnCl2)A transparent solid made by dissolving tinin hydrochloric acid. Tin(II) chloride is areducing agent, it combines with ammonia,and forms hydrates with water. It is used asa mordant.
tin(IV) chloride (stannic chloride; SnCl4)A colorless fuming liquid. Tin(IV) chlorideis soluble in organic solvents but is hy-drolyzed by water. It dissolves sulfur,phosphorus, bromine, and iodine, and itdissolves in concentrated hydrochloric acidto give the anion SnCl62–.
tincture A solution in which alcohol(ethanol) is the solvent (e.g., tincture of io-dine).
tin(IV) hydride See stannane.
tin(II) oxide (stannous oxide; SnO) Adark green or black solid. It can also be ob-tained in an unstable red form, which turnsblack on exposure to air. Tin(II) oxide canbe made by precipitating the hydratedoxide from a solution containing tin(II)ions and dehydrating the product at100°C.
tin(IV) oxide (stannic oxide; tin dioxide;SnO2) A colorless crystalline solid, whichis usually discolored owing to the presenceof impurities. It exists as hexagonal orrhombic crystals or in an amorphous form.Tin(IV) oxide is insoluble in water. It isused for polishing glass and metal.
tin(II) sulfide (stannous sulfide; SnS) Agray solid that can be prepared from tinand sulfur. Above 265°C tin(II) sulfideslowly turns into tin(IV) sulfide and tin:
2SnS(s) → SnS2(s) + Sn(s)
tin(IV) sulfide (stannic sulfide; SnS2) Ayellowish solid that can be precipitated byreacting hydrogen sulfide with a solutionof a soluble tin(IV) salt. A crystalline formsometimes known as mosaic gold is ob-
tained by heating a mixture of tin filings,sulfur, and ammonium chloride. Tin(IV)sulfide is used as a pigment.
titania /tÿ-tay-nee-ă, ti-/ See titanium(IV)oxide.
titanic chloride /tÿ-tan-ik/ Seetitanium(IV) chloride.
titanic oxide See titanium(IV) oxide.
titanium /tÿ-tay-nee-ŭm, ti-/ A silverytransition metal that occurs in various oresas titanium(IV) oxide and also in combina-tion with iron and oxygen. It is extractedby conversion of titanium(IV) oxide to thechloride, which is reduced to the metal byheating with sodium. Titanium is reactiveat high temperatures. It is used in the aero-space industry as it is strong, resistant tocorrosion, and has a low density. It formscompounds with oxidation states +4, +3,and +2, the +4 state being the most stable.
Symbol: Ti; m.p. 1660°C; b.p. 3287°C;r.d. 4.54 (20°C); p.n. 22; r.a.m. 47.867.
titanium(IV) chloride (titanic chloride;titanium tetrachloride; TiCl4) A volatilecolorless liquid formed by heating tita-nium(IV) oxide with carbon in a stream ofdry chlorine at 700°C. The product is puri-fied by fractional distillation. Titanium(IV)chloride fumes in moist air forming theoxychlorides of titanium. It undergoes hy-drolysis in water but this can be preventedby the presence of excess hydrochloricacid. Crystallization of such a solutiongives crystals of the di- and pentahydrates.Titanium(IV) chloride is used in the prepa-ration of pure titanium and as an interme-diate in the production of titaniumcompounds from minerals.
titanium dioxide /dÿ-oks-ÿd/ See tita-nium(IV) oxide.
titanium(IV) oxide (titanic oxide; tita-nium dioxide; TiO2) A white ionic solidthat occurs naturally in three crystallineforms: rutile (tetragonal), brookite (or-thorhombic), and anatase (tetragonal). Itreacts slowly with acids and is attacked by
275
titanium(IV) oxide
the halogens at high temperatures. Onheating it decomposes to give titanium(III)oxide and oxygen. Titanium(IV) oxide isamphoteric; it is used as a white pigment.With hot concentrated sulfuric acid, itforms titanyl sulfate, TiOSO4. When fusedwith alkalis, it forms titanates.
titanium tetrachloride /tet-ră-klor-ÿd, -id, -kloh-rÿd, -rid/ See titanium(IV) chlo-ride.
titrant /tÿ-trănt/ See titration.
titration /tÿ-tray-shŏn/ A procedure inVOLUMETRIC ANALYSIS in which a solutionof known concentration (called the titrant)is added to a solution of unknown concen-tration from a buret until the equivalencepoint or end point of the titration isreached.
TNT See trinitrotoluene.
Tollen’s reagent /tol-ĕn/ A solution ofthe complex ion Ag(NH3)2
+ produced byprecipitation of silver oxide from silver ni-trate with a few drops of sodium hydrox-ide solution, and subsequent dissolution ofthe silver oxide in aqueous ammonia. Tol-len’s reagent is used in the SILVER-MIRROR
TEST for aldehydes, where the Ag+ ion is re-duced to silver metal. It is also a test foralkynes with a triple bond in the 1-posi-tion. A yellow precipitate of silver carbideis formed in this case.
RCCH + Ag+ → RCC–Ag+ + H+
The reagent is named for Bernhard Tol-lens (1841–1918).
toluene /tol-yoo-een/ See methylbenzene.
toluidine /tŏ-loo-ă-deen, -din/ (methylani-line; aminotoluene; CH3C6H4NH2) Anaromatic amine used in making dyestuffsand drugs. There are three isomers; the 1,2-(ortho-aminotoluene) and 1,3- (meta-)forms are liquids, the 1,4- (para-) isomer isa solid.
tonne /tun/ Symbol: t A unit of mass equalto 103 kilograms (i.e. one megagram).
topaz A hydrous aluminosilicate mineralcontaining some fluoride ions, used formaking refractories, glasses, and glazes. Aclear pale yellow or brown form is used asa gemstone.
torr A unit of pressure equal to a pressureof 101 325/760 pascals (133.322 Pa). Onetorr is equal to one mmHg. The unit isnamed for the Italian physicist EvangelistaToricelli (1608–47).
tracer An ISOTOPE of an element used toinvestigate chemical reactions or physicalprocesses (e.g. diffusion).
trans- Designating an isomer with groupsthat are on opposite sides of a bond orstructure. See isomerism.
transactinide elements /tran-sak-tă-nÿd,-zak-, -nid/ Those elements following103Lw (i.e. following the actinide series).To date, element 104 (rutherfordium)through to element 112 have been synthe-sized; so far, elements 110, 111, and 112are unnamed. All are unstable and havevery short half-lives. There is speculationthat islands of stable nuclei with very highmass numbers exist since theoretical calcu-lations predict unusual stability for ele-ments 114 and 184.
trans-fat See hardening.
transient species A short-lived interme-diate in a chemical reaction.
transition elements /tran-zish-ŏn/ A classof elements occurring in the periodic tablein four series: from scandium to zinc; fromyttrium to cadmium; from lanthanum tomercury; and from actinium to element112. The transition elements are all metals.They owe their properties to the presenceof d electrons in the atoms. In the first tran-sition series, calcium has the configuration3s23p64s2. The next element scandium has3s23p63d14s2, and the series is formed byfilling the d levels up to zinc at 3d104s2. Theother transition series occur by filling ofthe 4d, 5d, and 6d levels. Often the ele-ments scandium, yttrium, and lanthanum
titanium tetrachloride
276
are considered with the lanthanoids ratherthan the transition elements, and zinc, cad-mium, and mercury are also treated as aseparate group. Transition elements havecertain characteristic properties resultingfrom the existence of d levels:1. They have variable valences – i.e. they
can form compounds with the metal indifferent oxidation states (Fe2+ and Fe3+,etc.).
2. They form a vast number of inorganiccomplexes, in which coordinate bondsare formed to the metal atom or ion.
3. Compounds of transition elements areoften colored.See also d-block elements; lanthanoids;
metals; transactinide elements; zinc group.
transition state (activated complex) Sym-bol: ‡ A short-lived high-energy molecule,radical, or ion formed during a reaction be-tween molecules possessing the necessaryactivation energy. The transition state de-composes at a definite rate to yield eitherthe reactants again or the final products.The transition state can be considered to beat the top of the energy profile.
For the reaction,X + YZ = X…Y…Z‡ → XY+ Z
the sequence of events is as follows. X ap-proaches YZ and when it is close enoughthe electrons are rearranged producing aweakening of the bond between Y and Z.A partial bond is now formed between Xand Y producing the transition state. De-pending on the experimental conditions,the transition state either breaks down toform the products or reverts back to the re-actants.
transition temperature A temperature atwhich some definite physical change oc-curs in a substance. Examples of such tran-sitions are change of state, change ofcrystal structure, and change of magneticbehavior.
transmutation /trans-myoo-tay-shŏn,tranz-/ A change of one element into an-other by radioactive decay or by bombard-ment of the nuclei with particles.
transport number Symbol: t In an elec-
trolyte, the transport number of an ion isthe fraction of the total charge carried bythat type of ion in conduction.
transuranic elements /trans-yû-ran-ik,tranz-/ Those elements of the actinoid se-ries that have higher atomic numbers thanuranium. The transuranic elements areformed by adding neutrons to the loweractinoids by high-energy bombardmentand they generally have such short half-lives that macroscopic isolation is impossi-ble. Neptunium and plutonium are formedin trace quantities by natural neutron bom-bardment from spontaneous fission of235U.
triaminotriazine /trÿ-am-ă-noh-trÿ-ă-zeen/ See melamine.
triammonium phosphate /trÿ-ă-moh-nee-ŭm/ See ammonium phosphate.
triatomic /trÿ-ă-tom-ik/ Denoting a mol-ecule, radical, or ion consisting of threeatoms. For example, O3 and H2O are tri-atomic molecules.
triazine /trÿ-ă-zeen, -zin/ (C3H3N3) A het-erocyclic organic compound with a six-membered ring containing three carbonatoms and three nitogen atoms. There arethree isomers, used as dyestuffs and herbi-cides. See also melamine.
triazole /trÿ-ă-zohl/ (C2H3N3) A hetero-cyclic organic compound with a five-mem-bered ring containing two carbon atomsand three nitrogen atoms. There are twoisomers.
tribasic acid /trÿ-bay-sik/ An ACID withthree replaceable hydrogen atoms (such asphosphoric(V) acid, H3PO4).
tribromomethane /trÿ-broh-moh-meth-ayn/ (bromoform; CHBr3) A colorless liq-uid compound. See haloform.
trichloroacetic acid /trÿ-klor-oh-ă-see-tik, -set-ik, -kloh-roh-/ See chloroethanoicacid.
277
trichloroacetic acid
trichloroethanal /trÿ-klor-oh-eth-ă-nal, -kloh-roh-/ (chloral; CCl3CHO) A color-less liquid aldehyde made by chlorinatingethanal. It was used to make the insecticideDDT. It can be hydrolyzed to give 2,2,2-trichloroethanediol (chloral hydrate,CCl3CH(OH)2). Most compounds withtwo –OH groups on the same carbon atomare unstable. However, in this case the ef-fect of the three chlorine atoms stabilizesthe compound. It is used as a sedative.
2,2,2-trichloroethanediol /trÿ-klor-oh-eth-ayn-dÿ-ol, -ohk, -kloh-roh-/ Seetrichloroethanal.
trichloroethanoic acid /trÿ-klor-oh-eth-ă-noh-ik, -kloh-roh-/ See chloroethanoicacid.
trichloromethane /trÿ-klor-oh-meth-ayn,-kloh-roh-/ (chloroform; CHCl3) A color-less volatile liquid formerly used as ananesthetic. Now its main use is as a solventand raw material for making other chlori-nated compounds. Trichloromethane ismade by reacting ethanal, ethanol, orpropanone with chlorinated lime.
triclinic crystal /trÿ-klin-ik/ See crystalsystem.
triglyceride /trÿ-gliss-ĕ-rÿd/ A GLYCERIDE
in which esters are formed with all three–OH groups of glycerol.
trigonal bipyramid /trig-ŏ-năl bÿ-pi-ră-mid/ See complex.
trigonal crystal See crystal system.
trihydrate /trÿ-hÿ-drayt/ A crystalline hy-drated compound that contains three mol-ecules of water of crystallization permolecule of compound.
trihydric alcohol /trÿ-hÿ-drik/ See triol.
triiodomethane /trÿ-ÿ-ŏ-doh-meth-ayn/(iodoform; CHI3) A yellow crystallinecompound made by warming ethanal withan alkaline solution of an iodide:
CH3CHO + 3I– + 4OH– → CHI3 +HCOO– + 3H2O
The reaction also occurs with all ketones ofgeneral formula CH3COR (R is an alkylgroup) and with secondary alcoholsCH3CH(OH)R. Iodoform is used as a testfor such reactions (the iodoform reaction).
triiron tetroxide /trÿ-ÿ-ern tet-roks-ÿd/(ferrosoferric oxide; magnetic iron oxide;Fe3O4) A black solid prepared by passingeither steam or carbon dioxide over red-hot iron. It may also be prepared by pass-ing steam over heated iron(II) sulfide.Triiron tetroxide occurs in nature as themineral magnetite. It is insoluble in waterbut will dissolve in acids to give a mixtureof iron(II) and iron(III) salts in the ratio1:2. Generally it is chemically unreactive; itis, however, a fairly good conductor ofelectricity.
trimer /trÿ-mer/ A molecule (or com-pound) formed by addition of three identi-cal molecules. See ethanal; methanal.
trimethylaluminum /trÿ-meth-ăl-ă-loo-mă-nŭm/ (aluminum trimethyl; (CH3)3Al)A colorless liquid produced by the sodiumreduction of dimethyl aluminum chloride.It ignites spontaneously on contact with airand reacts violently with water, acids,halogens, alcohols, and amines. Aluminumalkyls are used in the Ziegler process forthe manufacture of high-density poly-ethene.
trimolecular /trÿ-mŏ-lek-yŭ-ler/ Describ-ing a reaction or step that involves threemolecules interacting simultaneously withthe formation of a product. For example,the final step in reaction between hydrogenperoxide and acidified potassium iodide istrimolecular:
HOI + H+ + I– → I2 + H2OIt is uncommon for reactions to take
place involving trimolecular steps. The ox-idation of nitrogen(II) oxide tonitrogen(IV) oxide,
2NO + O2 → 2NO2is often classified as a trimolecular reactionbut many believe it to involve two bimol-ecular reactions.
trichloroethanal
278
trinitroglycerine /trÿ-nÿ-troh-gliss-ĕ-rin, -reen/ See nitroglycerine.
trinitrophenol /trÿ-nÿ-troh-fee-nol, -nohl/See picric acid.
trinitrotoluene /trÿ-nÿ-troh-tol-yoo-een/(TNT; CH3C6H2(NO2)3) A yellow crys-talline solid. It is a highly unstable sub-stance, used as an explosive. Thecompound is made by nitrating methylben-zene and the nitro groups are in the 2, 4,and 6 positions.
triol /trÿ-ol, -ohl/ (trihydric alcohol) Analcohol that has three hydroxyl groups (-OH) per molecule of compound.
triose /trÿ-ohs/ A SUGAR that containsthree carbon atoms.
trioxoboric(III) acid /trÿ-oks-ŏ-bô-rik/See boric acid.
trioxosulfuric(IV) acid / trÿ-oks-oh- sul-fyoor-ik / See sulfurous acid.
trioxygen /trÿ-oks-ă-jĕn/ See ozone.
triple bond A covalent bond formed be-tween two atoms in which three pairs ofelectrons contribute to the bond. One pairforms a sigma bond (equivalent to a singlebond) and two pairs give rise to two pibonds. It is conventionally represented asthree lines, thus H–C≡C–H. See multiplebond.
triple point The only point at which thegas, solid, and liquid phases of a substancecan coexist in equilibrium. The triple pointof water (273.16 K at 101 325 Pa) is usedto define the kelvin.
trisodium phosphate(V) /trÿ-soh-dee-ŭm/ (sodium orthophosphate; Na3PO4) Awhite solid prepared by adding sodium hy-droxide to disodium hydrogenphosphate.On evaporation white hexagonal crystalsof the dodecahydrate (Na3PO4.12H2O)may be obtained. These crystals do not ef-floresce or deliquesce. They dissolve read-ily in water to produce alkaline solutions
owing to salt hydrolysis. Trisodium phos-phate is used as a water softener.
triterpene See terpene.
tritiated compound A compound inwhich one or more 1H atoms have been re-placed by tritium (3H) atoms.
tritium /trit-ee-ŭm, trish-/ Symbol: T, 3HA radioactive isotope of hydrogen of massnumber 3. The nucleus contains 1 protonand 2 neutrons. Tritium decays with emis-sion of low-energy beta radiation to give3He. The half-life is 12.3 years. It is usefulas a tracer in studies of chemical reactions.Compounds in which 3H atoms replace theusual 1H atoms are said to be tritiated. Apositive tritium ion, T+, is a triton.
triton /trÿ-tŏn/ See tritium.
trivalent /trÿ-vay-lĕnt, triv-ă-/ (tervalent)Having a valence of three.
tropylium ion /trŏ-pil-ee-ŭm/ The posi-tive ion C7H7
+, having a symmetricalseven-membered ring of carbon atoms.The tropylium ion ring shows non-ben-zenoid aromatic properties.
tryptophan /trip-tŏ-fan/ See amino acids.
tungsten /tung-stĕn/ A transition metaloccurring naturally in wolframite((Fe,Mn)WO4) and scheelite (CaWO4). Itwas formerly called wolfram. It is used asthe filaments in electric lamps and in vari-ous alloys.
Symbol: W; m.p. 3410 ± 20°C; b.p.5650°C; r.d. 19.3 (20°C); p.n. 74; r.a.m.183.84.
tungsten carbide Either of two carbides(W2C and WC) produced by heating pow-dered tungsten with carbon. The carbidesare extremely hard and are used in industryto make cutting tools or as an abrasive.WC has a very high melting point(2770°C) and will conduct electricity. W2Calso has a very high melting point(2780°C) but is a less efficient conductor ofelectricity. It is very resistant to chemical
279
tungsten carbide
attack and behaves in a manner very simi-lar to that of tungsten. It is strongly at-tacked by chlorine to give tungstenhexachloride, WCl6.
turpentine /ter-pĕn-tÿn/ (pine-cone oil) Ayellow viscous resin obtained from conifer-ous trees. It can be distilled to produce tur-pentine oil (also known simply asturpentine), used in medicine and as a sol-
vent in paints, polishes, and varnishes. Seeresin.
turquoise /ter-koiz, -kwoiz/ A naturallyoccurring hydrated copper aluminumphosphate, used as a green-blue gemstone.
tyrosine /tÿ-rŏ-seen, -sin, ti-/ See aminoacids.
turpentine
280
281
ultracentrifuge /ul-tră-sen-tră-fyooj/ Ahigh-speed centrifuge used for separatingout very small particles. The sedimentationrate depends on the particle size, and theultracentrifuge can be used to measure themass of colloidal particles and large mol-ecules (e.g. proteins).
ultrahigh vacuum See vacuum.
ultramarine /ul-tră-mă-reen/ See lazurite.
ultraviolet /ul-tră-vÿ-ŏ-lĕt/ (UV) A formof ELECTROMAGNETIC RADIATION, shorter inwavelength than visible light. Ultravioletwavelengths range between about 1 nmand 400 nm. Ordinary glasses are nottransparent to these waves; quartz is amuch more effective material for makinglenses and prisms for use with ultraviolet.Like light, ultraviolet radiation is producedby electronic transitions between the outerenergy levels of atoms. However, having ahigher frequency, ultraviolet photons carrymore energy than those of light and can in-duce photolysis of compounds.
unimolecular /yoo-nă-mŏ-lek-yŭ-ler/ De-scribing a reaction (or step) in which onlyone molecule is involved. For example, ra-dioactive decay is a unimolecular reaction:
Ra → Rn + αOnly one atom is involved in each disinte-gration.
In a unimolecular chemical reaction,the molecule acquires the necessary energyto become activated and then decomposes.The majority of reactions involve only uni-or bimolecular steps. The following reac-tions are all unimolecular:
N2O4 → 2NO2PCl5 → PCl3 + Cl2
CH3CH2Cl → C2H4 + HCl
unit A reference value of a quantity usedto express other values of the same quan-tity. See also SI units.
unit cell The smallest group of atoms,ions, or molecules that, when repeated atregular intervals in three dimensions, willproduce the lattice of a crystal system.There are seven basic types of unit cells,which result in the seven crystal systems.
unit processes (chemical conversions)The recognized steps used in chemicalprocesses, e.g. alkylation, distillation, hy-drogenation, pyrolysis, nitration, etc. In-dustrial processing and the economics,design, and use of the equipment are basedon these unit processes rather than consid-eration of each reaction separately.
univalent /yoo-nă-vay-lĕnt, yoo-niv-ă-/(monovalent) Having a valence of one.
universal indicator (multiple-range indi-cator) A mixture of indicator dyestuffsthat shows a gradual change in color overa wide pH range. A typical formulationcontains methyl orange, methyl red, bro-mothymol blue, and phenolphthalein andchanges through a red, orange, yellow,green, blue, and violet sequence betweenpH 3 and pH 10. Several commercialpreparations are available as both solu-tions and test papers.
unnil- A prefix used in the names of newchemical elements. Synthetic elements withhigh proton numbers were given tempo-rary names based on the proton number,pending official agreement on the actual
U
name to be used. These names were formedfrom the word elements in the next table.
The suffix -ium was also used (because theelements were metals). So element 104, forexample, had the temporary systematicname unnilquadium (un + nil + quad +ium). The names for elements using thissystem are shown in the table below.
unsaturated compound An organiccompound that contains at least one dou-ble or triple bond between two of its car-bon atoms. The multiple bond is relativelyweak; consequently unsaturated com-pounds readily undergo addition reactionsto form single bonds. Compare saturatedcompound.
unsaturated solution See saturated solu-tion.
unsaturated vapor See saturated vapor.
UPVC Unplasticized PVC; a hard-wear-ing form of PVC used in building work(e.g. for window frames).
uracil /yoor-ă-săl/ A nitrogenous base thatis found in RNA, replacing the thymine ofDNA. It has a pyrimidine ring structure.
uraninite /yoor-ă-nÿt/ See pitchblende.
uranium /yû-ray-nee-ŭm/ A toxic ra-dioactive silvery element of the actinoid se-ries of metals. Its three naturally occurringradioisotopes, 238U (99.283% in abun-dance), 235U (0.711%), and 234U(0.005%), are found in numerous mineralsincluding the uranium oxides pitchblende,uraninite, and carnotite. The readily fis-sionable 235U is a major nuclear fuel andnuclear explosive, while 238U is a source offissionable 239Pu.
Symbol: U; m.p. 1132.5°C; b.p.3745°C; r.d. 18.95 (20°C); p.n. 92; r.a.m.238.0289.
uranium hexafluoride /heks-ă-floo-ŏ-
unsaturated compound
282
Element Number Symbolnil 0 nun 1 ubi 2 btri 3 tquad 4 qpent 5 phex 6 hsept 7 soct 8 0
P.N. Temporary name Symbol Name Symbol
101 unnilunium Unu mendelevium Md102 unnilbium Unb nobelium No103 unniltrium Unt lawrencium Lr104 unnilquadium Unq rutherfordium Rf105 unnilpentium Unq dubnium Db106 unnilhexium Unh seaborgium Sg107 unnilseptium Uns bohrium Bh108 unniloctium Uno hassium Hs109 unnilennium Une meitnerium Mt110 ununnilium Uun darmstadtium Ds111 unununium Uuu roentgenium Rg112 ununbium Uub unnamed113 ununtrium Uut unnamed114 ununquadium Uuq unnamed115 ununpentium Uup unnamed116 ununhexium Uuh unnamed
283
UV
rÿd/ (UF6) A crystalline volatile com-pound, used in separating uranium iso-topes by differences in the rates of gasdiffusion.
uranium–lead dating A technique fordating certain rocks depending on thedecay of the radioisotope 238U to 206Pb(half-life 4.5 × 109 years) or of 235U to207Pb (half-life 7.1 × 108 years). The decayof 238U releases alpha particles and an est-imate of the age of the rock can be made bymeasuring the amount of helium present.Another method is to measure the amountof radioactive lead present to the amountof nonradioactive lead. See also radioactivedating.
uranium(IV) oxide (uranium dioxide;urania; UO2) An extremely poisonous ra-dioactive black crystalline solid. It occursin pitchblende (uraninite) and is used in ce-ramics, photographic chemicals, pigments,and as a nuclear fuel.
uranium(VI) oxide (uranium trioxide;UO3) An extremely poisonous radioactiveorange solid, used as a pigment in ceramicsand in uranium refining.
uranyl /yoor-ă-năl/ The inorganic radicalUO2
2+, as in uranyl nitrate UO2(NO3)2.
urea /yû-ree-ă/ (carbamide; CO(NH2)2) Awhite crystalline compound made fromammonia and carbon dioxide. It is used inthe manufacture of urea–formaldehyde(methanal) resins. Urea is the end productof metabolism in many animals and is pre-sent in urine.
urethane /yoor-ĕ-thayn/ (ethyl carbamate;CO(NH2)OC2H5) A poisonous flammableorganic compound, used in medicine, as asolvent, and as an intermediate in the man-ufacture of polyurethane resins.
uric acid /yoor-ik/ A nitrogen compoundproduced from purines. In certain animals(e.g. birds and reptiles), it is the main ex-cretory product resulting from breakdownof amino acids. In humans, uric acid crys-tals in the joints are the cause of gout.
uridine /yoor-ă-din, -deen/ The nucleosideformed when uracil is linked to D-ribose bya β-glycosidic bond.
UV See ultraviolet.
284
vacancy See defect.
vacuum (pl. vacuums or vacua) A spacecontaining gas below atmospheric pres-sure. A perfect vacuum contains no matterat all, but for practical purposes soft (low)vacuum is usually defined as down toabout 10–2 pascal, and hard (high) vacuumas below this. Ultrahigh vacuum is lowerthan 10–7 pascal.
vacuum distillation The distillation ofliquids under a reduced pressure, so thatthe boiling point is lowered. Vacuum dis-tillation is a common laboratory techniquefor purifying or separating compoundsthat would decompose at their ‘normal’boiling point.
vacuum flask See Dewar flask.
valence /vay-lĕns/ (valency) The combin-ing power of an element or radical, equalto the number of hydrogen atoms that willcombine with or displace one atom of theelement. For simple covalent molecules thevalence is obtained directly, for example Cin CH4 is tetravalent; N in NH3 is trivalent.For ions the valence is regarded as equiva-lent to the magnitude of the charge; for ex-ample Ca2+ is divalent, CO3
2– is a divalentradical. The rare gases are zero-valent be-cause they do not form compounds undernormal conditions. As the valence formany elements is constant, the valence ofsome elements can be deduced without ref-erence to compounds formed with hydro-gen. Thus, as the valence of chlorine in HClis 1, the valence of aluminum in AlCl3 is 3;as oxygen is divalent (H2O) silicon in SiO2is tetravalent. The product of the valenceand the number of atoms of each elementin a compound must be equal. For exam-
ple, in Al2O3 for the two aluminum atoms(valence 3) the product is 6 and for thethree oxygen atoms (valence 2) the productis also 6.
The valence of an element is generallyequal to either the number of valence elec-trons or eight minus the number of valenceelectrons. Transition metal ions displayvariable valence.
valence band The highest ENERGY LEVEL
in a SEMICONDUCTOR or nonconductor thatcan be occupied by electrons.
valence-bond theory A technique forcalculating the electronic structure of mol-ecules. In valence-bond theory the startingpoint is to consider the atoms in a moleculeand the ways in which valence electrons inthe atoms can pair up in chemical bonds.Each way of pairing up the electrons iscalled a valence bond structure. The over-all structure for the molecule is determinedby a linear combination of the wavefunc-tions for the valence-bond structures. Seeresonance.
valence electron An outer electron in anatom that can participate in forming chem-ical bonds.
valency /vay-lĕns/ See valence.
valeric acid /vă-le-rik, -leer-ik/ See pen-tanoic acid.
valine /val-een, -in/ See amino acids.
vanadium /vă-nay-dee-ŭm/ A silvery tran-sition element occurring in complex ores insmall quantities. It is used in alloy steels.Vanadium forms compounds with oxida-tion states +5, +4, +3, and +2.
V
It forms colored ions.Symbol: V; m.p. 1890°C; b.p. 3380°C;
r.d. 6.1 (20°C); p.n. 23; r.a.m. 50.94.
vanadium(V) oxide (vanadium pentox-ide; V2O5) An oxide of vanadium exten-sively used as a catalyst in oxidationprocesses, as in the contact process.
van der Waals equation /van-der-wawlz/An equation of state for real gases. For nmoles of gas the equation is
(p + n2a/V2)(V – nb) = nRTwhere p is the pressure, V the volume, andT the thermodynamic temperature. a and bare constants for a given substance and R isthe gas constant. The equation gives a bet-ter description of the behavior of real gasesthan the perfect gas equation (pV = nRT).
The equation contains two corrections:b is a correction for the non-negligible sizeof the molecules; a/V2 corrects for the factthat there are attractive forces between themolecules, thus slightly reducing the pres-sure from that of an ideal gas. The equa-tion is named for the Dutch physicistJohannes Diderik van der Waals(1837–1923). See also gas laws; kinetictheory.
van der Waals force An intermolecularforce of attraction, considerably weakerthan chemical bonds and arising fromweak electrostatic interactions betweenmolecules (the energies are often less than1 J mol–1).
The van der Waals interaction containscontributions from three effects; perma-nent dipole–dipole interactions found forany polar molecule; dipole–induced dipoleinteractions, where one dipole causes aslight charge separation in bonds that havea high polarizability; and dispersion forces,which result from temporary polarity aris-ing from an asymmetrical distribution ofelectrons around the nucleus. Even atomsof the rare gases exhibit dispersion forces.
van’t Hoff factor /vant-hoff/ Symbol: iThe ratio of the number of particles presentin a solution to the number of undissoci-ated molecules added. It is used in studiesof colligative properties, which depend on
the number of entities present. For exam-ple, if n moles of a compound are dissolvedand dissociation into ions occurs, then thenumber of particles present will be in. Os-motic pressure (π), for instance, will begiven by the equation
πV = inRTThe factor is named for the Dutch theo-
retical chemist Jacobus Henricus Van’tHoff (1852–1911).
van’t Hoff isochore The equation:d(logeK)/dT = ∆H/RT2
showing how the equilibrium constant, K,of a reaction varies with thermodynamictemperature, T. ∆H is the enthalpy of reac-tion and R is the gas constant.
vapor A gas formed by the vaporizationof a solid or liquid. Some particles near thesurface of a liquid acquire sufficient energyin collisions with other particles to escapefrom the liquid and enter the vapor; someparticles in the vapor lose energy in colli-sions and re-enter the liquid. At a giventemperature an equilibrium is established,which determines the vapor pressure of theliquid at that temperature.
vapor density The ratio of the mass of acertain volume of a vapor to the mass of anequal volume of hydrogen (measured at thesame temperature and pressure). Determi-nation of vapor densities is one method offinding the relative molecular mass of acompound (equal to twice the vapor den-sity). Victor Meyer’s method, Dumas’method, or Hofmann’s method can beused.
vaporization The process by which a liq-uid or solid is converted into a gas or vaporby heat. Unlike boiling, which occurs at afixed temperature, vaporization can occurat any temperature. Its rate increases as thetemperature rises.
vapor pressure The pressure exerted by avapor. The saturated vapor pressure is thepressure of a vapor in equilibrium with itsliquid or solid. It depends on the nature ofthe liquid or solid and the temperature.
285
vapor pressure
vat dyes
286
vat dyes A class of insoluble dyes appliedby first reducing them to derivatives thatare soluble in dilute alkali. In this conditionthey have a great attraction for certainfibers, such as cotton. The solution is ap-plied to the material and the insoluble dyeis regenerated in the fibers by atmosphericoxidation. Indigo and indanthrene are ex-amples of vat dyes.
verdigris /ver-di-grees, -gris, -gree/ Any ofvarious greenish basic salts of copper. Trueverdigris is basic copper acetate, a green orblue solid of variable composition used asa paint pigment. The term is often appliedto green patinas formed on metallic cop-per. Copper cooking vessels can form acoating of basic copper carbonate,CuCO3.Cu(OH)2. The verdigris formed oncopper roofs and domes is usually basiccopper sulfate, CuSO4.3Cu(OH)2.H2O,while basic copper chloride,CuCl2.Cu(OH)2, may form in regions nearthe sea.
vermilion See mercury(II) sulfide.
vesicant /ves-ă-kănt/ A substance thatcauses blistering of the skin. Mustard gas isan example.
vicinal positions /vis-ă-năl/ Positions in amolecule at adjacent atoms. For example,in 1,2-dichloroethane the chlorine atomsare in vicinal positions, and this compoundcan thus be named vic-dichloroethane.
Victor Meyer’s method /vik-ter mÿ-erz/A method for determining vapor densitiesin which a given weight of sample is va-porized and the volume of air displaced byit is measured. In practice, a bulb in a heat-ing bath is connected via a fairly long tubeto a water-bath gas-collection arrange-ment. The system is brought to equilibriumand the sample is then added (withoutopening the apparatus to the atmosphere).As the air displaced by gas is collected overwater a correction for the vapor pressure ofwater is necessary and the method may failif the vapor is soluble in water. The methodis named for the German chemist Victor
Meyer (1848–97). See also Dumas’method; Hofmann’s method.
vinegar A dilute solution (about 4% byvolume) of ETHANOIC ACID (acetic acid),often with added coloring and flavoringsuch as caramel. Natural vinegar is pro-duced by the bacterial fermentation ofcider or wine; it can also be made synthet-ically.
vinylation /vÿ-nă-lay-shŏn/ A catalytic re-action in which a compound adds acrossthe triple bond of ethyne (acetylene) toform an ethenyl (vinyl) compound. For ex-ample, an alcohol can add as follows:
ROH + HC≡CH → RHC=CH(OH)
vinyl benzene /vÿ-năl/ See phenylethene.
vinyl chloride See chloroethene.
vinyl group The group CH2:CH–.
viscose rayon /viss-kohs/ See rayon.
viscosity /vis-koss-ă-tee/ Symbol: η Theresistance to flow of a fluid.
visible radiation See light.
vitreous /vit-ree-ŭs/ Resembling glass, orhaving the structure of a glass.
volatile Easily converted into a vapor.
volt Symbol: V The SI unit of electricalpotential, potential difference, and e.m.f.,defined as the potential difference betweentwo points in a circuit between which aconstant current of one ampere flows whenthe power dissipated is one watt. 1 V = 1 JC–1. The unit is named for the Italian physi-cist Count Alessandro Volta (1745–1827).
voltaic cell /vol-tay-ik/ See cell.
volume strength See hydrogen peroxide.
volumetric analysis /vol-yŭ-met-rik/ Oneof the classical wet methods of quantitativeanalysis. It involves measuring the volumeof a solution of accurately known concen-
tration that is required to react with a so-lution of the substance being determined.The solution of known concentration (thestandard solution) is added in small por-tions from a buret. The process is called atitration and the equivalence point is calledthe end point. End points are observedwith the aid of indicators or by instrumen-tal methods, such as conduction or lightabsorption. Volumetric analysis can alsobe applied to gases. The gas is typicallyheld over mercury in a graduated tube, andvolume changes are measured on reactionor after absorption of components of amixture.
VSEPR (valence-shell electron-pair repul-sion) A method of predicting the shape ofmolecules. In this theory one atom is takento be the central atom and pairs of valenceelectrons are drawn round the centralatom. The shape of the molecule is deter-
mined by minimizing the Coulomb repul-sion between pairs and the repulsion be-tween pairs due to the Pauli exclusionprinciple. Thus, for three pairs of electronsan equilateral triangle is favored and forfour pairs of electrons a tetrahedron is fa-vored. The VSEPR theory has had consid-erable success in predicting molecularshapes. See lone pair.
vulcanite /vul-kă-nÿt/ (ebonite) A hardblack insulator made by vulcanizing rub-ber with a large amount of sulfur.
vulcanization /vul-kă-ni-zay-shŏn/ Aprocess of improving the quality of rubber(hardness and resistance to temperaturechanges) by heating it with sulfur (about150°C). Accelerators are used to speed upthe reaction. Certain sulfur compoundscan also be used for vulcanization.
287
vulcanization
288
Wacker process /wak-er/ An industrialprocess for making ethanal (and other car-bonyl compounds). To produce ethanal,ethene and air are bubbled through an acidsolution of palladium(II) chloride and cop-per(II) chloride (20–60°C and moderatepressure):
C2H4 + Pd2+ + O2 → CH3CHO + Pd +2H+
The reaction involves an intermediatecomplex between palladium(II) ions andethene. The purpose of the copper(II) chlo-ride is to oxidize the palladium back toPd2+ ions:
Pd + 2Cu2+ → Pd2+ + 2Cu+
The copper(I) ions spontaneously oxidizeto copper(II) ions in air. The process pro-vides a cheap source of ethanal (and, byoxidation, ethanoic acid) from the readilyavailable ethene.
Walden inversion /wawl-dĕn/ A reactionin which an optically active compound re-acts to give an optically active product inwhich the configuration has been inverted.This happens in the SN2 mechanism. Theinversion is named for the Russian–Ger-man chemist Paul Walden (1863–1957).See nucleophilic substitution.
washing soda See sodium carbonate.
water (H2O) A colorless liquid thatfreezes at 0°C and, at atmospheric pres-sure, boils at 100°C. In the gaseous statewater consists of single H2O molecules.Due to the presence of two lone pairs theatoms do not lie in a straight line, the anglebetween the central oxygen atom and thetwo hydrogen atoms being 105°; the dis-tance between each hydrogen atom and theoxygen atom is 0.099 nm. When ice forms,hydrogen bonds some 0.177 nm long de-
velop between the hydrogen atom and oxy-gen atoms in adjacent molecules, giving iceits tetrahedral crystalline structure with adensity of 916.8 kg m–3 at STP. Differentice structures develop under higher pres-sures. When ice melts to form liquid water,the tetrahedral structure breaks down, butsome hydrogen bonds continue to exist;liquid water consists of groups of associ-ated water molecules, (H2O)n, mixed withsome monomers and some dimers. Thismixture of molecular species has a higherdensity than the open-structured crystals.The maximum density of water, 999.97 kgm–3, occurs at 3.98°C. This accounts forthe ability of ice to float on water and forthe fact that water pipes burst on freezing.
Although water is predominantly a co-valent compound, a very small amount ofionic dissociation occurs (H2O ˆ H+ +OH–). In every liter of water at STP there isapproximately 10–7 mole of each ionicspecies. It is for this reason that, on the pHscale, a neutral solution has a value of 7.
As a polar liquid, water is the mostpowerful solvent known. This is partly aresult of its high dielectric constant andpartly its ability to hydrate ions. This latterproperty also accounts for the incorpora-tion of water molecules into some ioniccrystals as water of crystallization.
Water is decomposed by reactive metals(e.g. sodium) when cold and by less activemetals (e.g. iron) when steam is passedover the hot metal. It is also decomposedby electrolysis.
water gas A mixture of carbon monoxideand hydrogen produced when steam ispassed over red-hot coke or made to com-bine with hydrocarbons, e.g.
C(s) + H2O(g) → CO(g) + H2(g)CH4(g) + H2O(g) → CO(g) + 3H2(g)
W
The production of water gas usingmethane is an important step in the prepa-ration of hydrogen for ammonia synthesis.Compare producer gas.
water glass See sodium silicate.
water of crystallization Water presentin definite proportions in crystalline com-pounds. Compounds containing water ofcrystallization are called hydrates. Exam-ples are copper(II) sulfate pentahydrate(CuSO4.5H2O) and sodium carbonate dec-ahydrate (Na2CO3.10H2O). The water canbe removed by heating.
When hydrated crystals are heated thewater molecules may be lost in stages. Forexample, copper(II) sulfate pentahydratechanges to the monohydrate (CuSO4.H2O)at 100°C, and to the anhydrous salt(CuSO4) at 250°C. The water molecules incrystalline hydrates may be held by hydro-gen bonds (as in CuSO4.H2O) or, alterna-tively, may be coordinated to the metal ionas a complex aquo ion.
water softening The removal from waterof dissolved calcium, magnesium, and ironcompounds, thus reducing the hardness ofthe water. The compounds are potentiallydamaging because they can accumulate inpipes and boilers and they react with, andtherefore waste, soap. Temporary hardnesscan be removed by boiling the water. Per-manent hardness can be removed in a num-ber of ways: by distillation; by the additionof sodium carbonate, which causes dis-solved calcium, for example, to precipitateout as calcium carbonate; and by the use ofion-exchange products such as Permutitand Calgon. See also hardness (of water).
watt /wot/ Symbol: W The SI unit ofpower, defined as a power of one joule persecond. 1 W = 1 J s–1. The unit is named forthe British instrument maker and inventorJames Watt (1736–1819).
wavefunction /wayv-fung-shŏn/ Seequantum theory; orbital.
waveguides /wayv-gÿdz/ See microwaves.
wavelength Symbol: λ The distance be-tween the ends of one complete cycle of awave. Wavelength is related to the speed(c) and frequency (v) thus:
c = vλ
wave mechanics See quantum theory.
wave number Symbol: σ The reciprocalof the wavelength of a wave. It is the num-ber of wave cycles in unit distance, and isoften used in spectroscopy. The unit is themeter–1 (m–1). The circular wave number(symbol: k) is given by:
k = 2πσ
wax A soft amorphous water-repellant or-ganic material. The term is particularlyused for fatty-acid esters of monohydric al-cohols.
weak acid (or base) An acid or base thatis not fully dissociated in solution.
weak base A base that is only partly or in-completely dissociated into its componentions in solution.
weber /vay-ber/ Symbol: Wb The SI unitof magnetic flux, equal to the magneticflux that, linking a circuit of one turn, pro-duces an e.m.f. of one volt when reduced tozero at uniform rate in one second. 1 Wb =1 V s. The unit is named for the Germanphysicist Wilhelm Eduard Weber(1804–91).
Weston cadmium cell A standard cellthat produces a constant e.m.f. of 1.0186volts at 20°C. It consists of an H-shapedglass vessel containing a negative cad-mium-mercury amalgam electrode in oneleg and a positive mercury electrode in theother. The electrolyte – saturated cadmiumsulfate solution – fills the horizontal bar ofthe vessel to connect the two electrodes.The e.m.f. of the cell varies very little withtemperature, being given by the equation E= 1.0186 – 0.000 037 (T – 293), where T isthe thermodynamic temperature.
wet cell A type of cell, such as a car bat-
289
wet cell
tery, in which the electrolyte is a liquid so-lution. See also Leclanché cell.
white arsenic See arsenic(III) oxide.
white lead See lead(II) carbonate hydrox-ide.
white phosphorus See phosphorus.
white spirit A liquid hydrocarbon resem-bling kerosene obtained from petroleum,used as a solvent and in the manufacture ofpaints and varnishes.
Williamson’s continuous process Amethod of preparing simple ethers by de-hydration of alcohols with concentratedsulfuric acid. The reaction is carried out at140°C under reflux with an excess of thealcohol:
2ROH → ROR + H2OThe concentrated sulfuric acid both cat-alyzes the reaction and displaces the equi-librium to the right. The product, ether, istermed ‘simple’, because the R groups areidentical.
Williamson’s synthesis A method forthe preparation of mixed ethers by nucle-ophilic substitution. A haloalkane is re-fluxed with an alcoholic solution ofsodium alkoxide (from sodium dissolved inalcohol):
R′Cl + RO–Na+ → R′OR + NaClThe product ether is termed ‘mixed’ if thealkyl groups R and R′ are different. Thissynthesis can produce both simple andmixed ethers. The synthesis is named forthe British chemist Alexander WilliamWilliamson (1824–1904).
will-o’-the-wisp See ignis fatuus.
witherite /with-ĕ-rÿt/ A mineral form ofbarium carbonate, BaCO3.
Wöhler’s synthesis /voh-lerz/ A synthesisof urea from inorganic compounds(Friedrich Wöhler, 1828). The urea was
produced by evaporating NH4NCO (am-monium cyanate). The experiment was im-portant as it demonstrated that ‘organic’chemicals (i.e. ones produced in living or-ganisms) could be made from inorganicstarting materials. The synthesis is namedfor the German chemist Friedrich Wöhler(1800–82).
wolfram /wûlf-răm/ A former alternativename for tungsten.
wood alcohol See methanol.
Wood’s metal A fusible alloy containing50% bismuth, 25% lead, and 12.5% tinand cadmium. Its low melting point (70°C)leads to its use in fire-protection devices.The alloy is named for the English scientistWilliam Wood (1671–1730).
work function See photoelectric effect.
wrought iron A low-carbon steel ob-tained by refining the iron produced in ablast furnace. Wrought iron is processedby heating and hammering to reduce theslag content and to ensure its even distrib-ution.
Wurtz reaction /voorts/ A reaction forpreparing alkanes by refluxing ahaloalkane (RX) with sodium metal in dryether:
2RX + 2Na → RR + 2NaXThe reaction involves the coupling of twoalkyl radicals. The Fittig reaction is a simi-lar process for preparing alkyl-benzene hy-drocarbons by using a mixture of halogencompounds. For example, to obtainmethylbenzene:
C6H5Cl + CH3Cl + 2Na → C6H5CH3 +2NaCl
In this mixed reaction phenylbenzene(C6H5C6H5) and ethane (CH3CH3) arealso produced by side reactions. The Wurtzreaction is named for the French chemistCharles Adolphe Wurtz (1817–84) and theFittig reaction is named for the German or-ganic chemist Rudolph Fittig (1835–1910).
white arsenic
290
xanthate /zan-thayt/ (-SCS(OR)) A salt orester of XANTHIC ACID (where R is an or-ganic group). Cellulose xanthate is used tomake RAYON.
xanthene /zan-theen/ (CH2(C6H4)2O) Ayellow crystalline organic compound, usedin making dyestuffs and fungicides.
xanthic acid /zan-thik/ (HSCS(OR)) Anyof several unstable organic acids (where Ris an organic group), whose esters and saltshave various industrial applications. Seexanthate.
xanthine /zan-theen, -th’ÿn/ (2,6-dioxy-purine; C5H4N2O2) A poisonous colorlesscrystalline organic compound that occursin blood, coffee beans, potatoes, and urine.It is used as a chemical intermediate.
xanthone /zan-thohn/ (dibenzo-4-pyrone;CO(C6H4)2O) A colorless crystalline or-ganic compound found as a pigment ingentians and other flowers. It is used as aninsecticide and in making dyestuffs.
xenon /zen-on/ A colorless odorlessmonatomic element of the rare-gas group.It occurs in trace amounts in air. Xenon isused in electron tubes and strobe lighting.
Symbol: Xe; m.p. –111.9°C; b.p.–107.1°C; d. 5.8971 (0°C) kg m–3; p.n. 54;r.a.m. 131.29.
x-radiation An energetic form of electro-magnetic radiation. The wavelength rangeis 10–11 m to 10–8 m. X-rays are normallyproduced by absorbing high-energy elec-trons in matter. The radiation can passthrough matter to some extent (hence its
use in medicine and industry for investigat-ing internal structures). It can be detectedwith photographic emulsions and deviceslike the Geiger–Müller tube.
X-ray photons result from electronictransitions between the inner energy levelsof atoms. When high-energy electrons areabsorbed by matter, an x-ray line spectrumresults. The structure depends on the sub-stance and is thus used in x-ray spec-troscopy. The line spectrum is alwaysformed in conjunction with a continuousbackground spectrum. The minimum (cut-off) wavelength λ0 corresponds to the max-imum x-ray energy, Wmax. This equals themaximum energy of electrons in the beamproducing the x-rays. Wavelengths in thecontinuous spectrum above λ0 are causedby more gradual energy loss by the elec-trons, in the process called bremsstrahlung(braking radiation).
x-ray crystallography The study of theinternal structure of crystals using the tech-nique of x-ray diffraction.
x-ray diffraction A technique used to de-termine crystal structure by directing x-rays at the crystals and examining thediffraction patterns produced. At certainangles of incidence a series of spots are pro-duced on a photographic plate; these spotsare caused by interaction between the x-rays and the planes of the atoms, ions, ormolecules in the crystal lattice. The posi-tions of the spots are consistent with theBragg equation nλ = 2dsinθ.
x-rays See x-radiation.
xylene /zÿ-leen/ See dimethylbenzene.
291
X
292
yellow phosphorus See phosphorus.
yocto- Symbol: y A prefix denoting 10–24.For example, 1 yoctometer (ym) = 10–24
meter (m).
yotta- Symbol: Y A prefix denoting 1024.For example, 1 yottameter (Ym) = 1024
meter (m).
ytterbium /i-ter-bee-ŭm/ A soft malleablesilvery element having two allotropes andbelonging to the lanthanoid series of met-als. It occurs in association with other lan-thanoids. Ytterbium has been used toimprove the mechanical properties of steel.
Symbol: Yb; m.p. 824°C; b.p. 1193°C;r.d. 6.965 (20°C); p.n. 70; r.a.m. 173.04.
yttrium /it-ree-ŭm/ A silvery metallic ele-ment belonging to the second transition se-ries. It is found in almost every lanthanoidmineral, particularly monazite. Yttrium isused in various alloys, in yttrium–alu-minum garnets used in the electronics in-dustry and as gemstones, as a catalyst, andin superconductors. A mixture of yttriumand europium oxides is widely used as thered phosphor on television screens.
Symbol: Y; m.p. 1522°C; b.p. 3338°C;r.d. 4.469 (20°C); p.n. 39; r.a.m.88.90585.
Zeeman effect /zay-mahn/ The splittingof atomic spectral lines by an external ap-plied magnetic field. It was first reportedby the Dutch physicist Pieter Zeeman(1865–1943) in 1896.
Zeisel reaction /zÿ-sĕl/ The reaction of anether with excess concentrated hydroiodicacid. On refluxing, a mixture of iodoalka-nes is formed:
ROR′ + 2HI → H2O + RI + R′IAnalysis to identify the iodoalkanes
gives information about the compositionof the original ether.
Zeise’s salt /zÿ-sĕz, tsÿ-/ A complex ofplatinum and ethane (ethylene) first syn-thesized by W. C. Zeise in 1827. It has theformula PtCl3(CH2CH2) and was the firstcomplex in which the metal ion coordi-nated to a pi-electron system rather than toindividual atoms.
zeolite /zee-ŏ-lÿt/ A member of a group ofhydrated aluminosilicate minerals, whichoccur in nature and are also manufacturedfor their ion-exchange and selective-ab-sorption properties. They are used forwater softening and for sugar refining. Thezeolites have an open crystal structure andcan be used as MOLECULAR SIEVES. See alsoion exchange.
zepto- Symbol: z A prefix denoting 10–21.For example, 1 zeptometer (zm) = 10–21
meter (m).
zero order Describing a chemical reactionin which the rate of reaction is independentof the concentration of a reactant; i.e.
rate = k[X]0
The concentration of the reactant re-mains constant for a period of time al-though other reactants are beingconsumed. The hydrolysis of 2-bromo-2-methylpropane using aqueous alkali has arate expression,
rate = k[2-bromo-2-methylpropane]i.e. the reaction is zero order with respectto the concentration of the alkali (it doesnot depend on the alkali concentration).The rate constant for a zero reaction hasthe units mol dm–3 s–1.
YZ
zero point energy The energy possessedby the atoms and molecules of a substanceat absolute zero (0 K).
zetta- Symbol: Z A prefix denoting 1021.For example, 1 zettameter (Zm) = 1021
meter (m).
Ziegler process /zee-gler/ A method forthe manufacture of high-density poly-ethene using a catalyst of titanium(IV]]chloride and triethyl aluminum(Al(C2H5)3) under slight pressure. Themechanism involves formation of titaniumalkyls
TiCl3(C2H5)which coordinate the σ-orbitals of tita-nium with the π bond of ethene. The chain-length, and hence the density, of thepolymer can be controlled. The process isnamed for the German chemist KarlZiegler (1898–1973).
zinc A bluish-white transition metal oc-curring naturally as the sulfide (zincblende) and carbonate (smithsonite). It isextracted by roasting the ore in air andthen reducing the oxide to the metal usingcarbon. Zinc is used to galvanize iron, inalloys (e.g. brass), and in dry batteries. Itreacts with acids and alkalis but only cor-rodes on the surface in air. Zinc ions areidentified in solution by forming white pre-cipitates with sodium hydroxide or ammo-nia solution, both precipitates beingsoluble in excess reagent.
Symbol: Zn; m.p. 419.58°C; b.p.907°C; r.d. 7.133 (20°C); p.n. 30; r.a.m.65.39.
zincate /zink-ayt/ A salt containing the ionZnO2
2–.
zinc-blende (sphalerite) structure Aform of crystal structure that consists of azinc atom surrounded by four sulfur atomsarranged tetrahedrally; each sulfur atom issimilarly surrounded by four atoms of zinc.Zinc sulfide crystallizes in the cubic sys-tem. Covalent bonds of equal strength andlength result in the formation of a giantmolecular structure. If the zinc and sulfuratoms are replaced by carbon atoms the di-
amond structure is produced. Wurtzite(another form of zinc sulfide) is similar butbelongs to the hexagonal crystal system.
zinc-blende structure A form of crystalstructure that consists of a zinc atom sur-rounded by four sulfur atoms arrangedtetrahedrally; each sulfur atom is similarlysurrounded by four atoms of zinc. Zinc sul-fide crystallizes in the cubic system. Cova-lent bonds of equal strength and lengthresult in the formation of a giant molecularstructure. If the zinc and sulfur atoms arereplaced by carbon atoms the diamondstructure is produced. Wurtzite (anotherform of zinc sulfide) is similar but belongsto the hexagonal crystal system.
zinc carbonate See calamine.
zinc chloride (ZnCl2) A white crystallinesolid prepared by the action of dry hydro-gen chloride or chlorine on heated zinc.The anhydrous product undergoes subli-mation. The hydrated salt may be preparedby the addition of excess zinc to dilute hy-drochloric acid and crystallization of thesolution. Hydrates with 4, 3, 5/2, 3/2, and1 molecule of water exist. Zinc chloride isextremely deliquescent and dissolves easilyin water. It is used in dentistry, Leclanchécells, as a flux, and as a dehydrating agentin organic reactions.
zinc group A group of metallic elementsin the periodic table, consisting of zinc(Zn), cadmium (Cd), and mercury (Hg).The elements all occur at the ends of thethree transition series and all have outerd10s2 configurations; Zn [Ar]3d104s2, Cd[Kr]4d105s2, Hg[Xn]5d106s2. The elementsall use only outer s-electrons in reactionand in combination with other elementsunlike the coinage metals, which immedi-ately precede them. Thus all form diposi-tive ions, and oxidation states higher than+2 are not known. In addition mercuryforms the mercurous ion, sometimes writ-ten as Hg(I]], but actually the ion +Hg-Hg+.Although the elements fall naturally at theend of the transition series their propertiesare more like those of main-group elementsthan of transition metals:
293
zinc group
1. The melting points of the transition ele-ments are generally high whereas thoseof the zinc group are low (Zn 419.5°C,Cd 329.9°C, Hg –38.87°C).
2. The zinc-group ions are diamagneticand colorless whereas most transition el-ements have colored ions and manyparamagnetic species.
3. The zinc group does not display the vari-able valence associated with transitionmetal ions. However the group doeshave the transition-like property offorming many complexes or coordina-tion compounds, such as [Zn(NH3)4]2+
and [Hg(CN]]4]2–.Within the group, mercury is anom-
alous because of the Hg22+ ion mentioned
above and its general resistance to reac-tion. Zinc and cadmium are electropositiveand will react with dilute acids to releasehydrogen whereas mercury will not. Whenzinc and cadmium are heated in air theyburn to give the oxides, MO, which are sta-ble to further strong heating. In contrastmercury does not react readily with oxygen(slow reaction at the boiling point), andmercuric oxide, HgO, decomposes to themetal and oxygen on further heating. Allmembers of the group form dialkyls and di-aryls, R2M.
zincite /zink-ÿt/ (spartalite) A red-orangemineral form of ZINC OXIDE, ZnO, oftencontaining also some manganese. It is animportant ore of zinc.
zinc oxide (ZnO) A compound preparedby the thermal decomposition of zinc ni-trate or carbonate; it is a white powderwhen cold, yellow when hot. Zinc oxide isan amphoteric oxide, almost insoluble inwater, but dissolves readily in both acidsand alkalis. If mixed with powdered car-bon and heated to red heat, it is reduced tothe metal. Zinc oxide is used in the paintand ceramic industries. Medically it is usedin zinc ointment.
zinc sulfate (ZnSO4) A white crystallinesolid prepared by the action of dilute sulfu-ric acid on either zinc oxide or zinc car-
bonate. On crystallization the hydratedsalt is formed. The heptahydrate(ZnSO4.7H2O) is formed below 30°C, thehexahydrate (ZnSO4.6H2O) above 30°C,and the monohydrate (ZnSO4.H2O) at100°C; at 450°C the salt is anhydrous.Zinc sulfate is extremely soluble in water.It is used in the textile industry.
zinc sulfide (ZnS) A compound that canbe prepared by direct combination of zincand sulfur. Alternatively it can be preparedas a white amorphous precipitate by bub-bling hydrogen sulfide through an alkalinesolution of a zinc salt or by the addition ofammonium sulfide to a soluble zinc salt so-lution. Zinc sulfide occurs naturally as themineral zinc blende. It dissolves in diluteacids to yield hydrogen sulfide. Impurezinc sulfide is phosphorescent. It is used asa pigment and in the coatings on lumines-cent screens.
zircon /zer-kon/ See zirconium.
zirconium /zer-koh-nee-ŭm/ A hard lus-trous silvery transition element that occursin a gemstone, zircon (ZrSiO4). It is used insome strong alloy steels.
Symbol: Zr; m.p. 1850°C; b.p. 4380°C;r.d. 6.506 (20°C); p.n. 40; r.a.m. 91.224.
zwitterion /tsvit-er-ÿ-on, -ŏn, zwit-/ (am-pholyte ion) An ion that has both a posi-tive and negative charge on the samespecies. Zwitterions occur when a mole-cule contains both a basic group and anacidic group; formation of the ion can beregarded as an internal acid-base reaction.For example, amino-ethanoic acid(glycine) has the formulaH2+N.CH2.COOH. Under neutral condi-tions it exists as the zwitterionH3N.CH2.COO–, which can be formed bytransfer of a proton from the carboxylgroup to the amine group. At low pH(acidic conditions) the ion+H3N.CH2.COOH is formed; at high pH(basic conditions) H2N.CH2.COO– isformed. See also amino acid.
zincite
294
APPENDIXES
In the examples below, the systematic name is given first, followed by the trivial (com-mon) name.
Simple saturated monocarboxylic acids:
methanoic formic HCOOHethanoic acetic CH3COOHpropanoic proprionic C2H5COOHbutanoic butyric C3H7COOHpentanoic valeric C4H9COOHhexanoic caproic C5H11COOHheptanoic enathic C6H13COOHoctanoic caprylic C7H15COOHnonanoic pelargonic C8H17COOHdecanoic capric C9H19COOH
Other simple saturated acids are found in naturally occurring glycerides. They all containeven numbers of carbon atoms:
dodecanoic lauric C11H23COOHtetradecanoic myristic C13H27COOHhexadecanoic palmitic C15H31COOHoctadecanoic stearic C17H35COOHeicosanoic arachidic C19H39COOHdocosanoic behenic C21H43COOHtetracosanoic lignoceric C23H47COOHhexacosanoic cerotic C25H51COOH
Certain important unsaturated monocarboxylic acids occur naturally:
octadec-9-enoic oleic C8H17CH=CHC7H14COOHoctadeca-9,12- dienoic linoleic C5H11CH=CHCH2CH=CHC7H1 4COOHoctadeca-9,12,15- trienoic linolenic C2H5CH=CHCH2CH=CHCH2CH=CH-
C 7H14COOH
There are a number of common saturated dicarboxylic acids:
ethanedioic oxalic HOOCCOOHpropanedioic malonic HOOCCH2COOHbutanedioic succinic HOOCC2H4COOHpentanedioic glutaric HOOCC3H6COOHhexanedioic adipic HOOCC4H8COOH
297
Carboxylic Acids
Appendix I
Examples of unsaturated dicarboxylic acids are:
cis-butenedioic maleic HOOCCH=CHCOOHtrans-butenedioic fumaric HOOCCH=CHCOOH
Certain hydroxy acids occur naturally:
hydroxyethanoic glycolic CH2(OH)COOH2-hydroxypropanoic lactic CH3CH(OH)COOHhydroxybutanedioic malic CH(OH)CH2(COOH)22-hydroxy-propane-1,2,3-tricarboxylic citric (CH2)2C(OH)(COOH)3
Naturally occurring amino carboxylic acids are shown in Appendix II.
Appendix I
298
299
Amino Acids
Appendix II
NH2
CH2
COOH
glutamine glycineNH2
CHC
H2
CH2
O
NH2COOH
NH2
CH
CH3 COOH
NH2
CHC
H2
CH2
CH2
NH
NH
NH2
COOH
alanine arginine
NH2
CHC
H2
SHCOOH
cysteine glutamic acid
NH2
CHC
H2
CH2
COOH
HOOC
NH2
CHC
H2
O
NH2
COOH
asparagine
NH2
CHC
H2
O
OH
COOH
aspartine
alanine arginine
asparagine aspartic acid
cysteine glutamic acid
glutamine glycine
Appendix II
300
NH2
CHC
H2
CHCH3
CH3
COOH
NH2
CHC
H2
CH2
CH2
CH2
NH2
COOH
leucine lysine
NH2
CHC
H2
CH2
SCH3 COOH
methionine
NH2
CH
CH2
CHCH
CH
CHCH
COOH
phenylalanine
NH
CH
CH2
CH2
CH2
COOH
NH2
CHC
H2
OHCOOH
proline serine
NH2
CHC
HCH2
O
CH3
CH3
COOH
NH2
CH
O
CH2
N
CHCH
NH
COOH
histidine isoleucine
Amino Acids
histidine isoleucine
leucine lysine
phenylalaninemethionine
serineproline
301
Appendix II
Amino Acids
CHC
H
NH2
CH3
OH
COOH
NH2
CHC
H2
CHNH
CH
CH
CH
CH
COOH
threonine tryptophan
NH2
CH
CH2
CHCH
CHCH
OH
COOH
NH2
CHC
HCH3
CH3
COOH
tyrosine valine
threonine tryptophan
tyrosine valine
302
Sugars
Some simple monosaccharides. The β-D-form is shown in each case
Appendix III
O
H
OH
H
HH
OH H
HO
H
OH
arabinose
OHOCH2
CH2OH
OH
HO
OH
βfructose
O
OH
HOCH2
OH
HO OH
galactose
O
OH
HO
HOCH2
OH
OH
glucose
O
OH OH
HOCH2 OH
ribose
O
OH
OH
HO
OH
xylose
303
Nitrogenous Bases and Nucleosides
N
N N
N
NH2
OHOCH2
OH OH
adenosine
NH
NHOCH2 O
H3C
O
O
OH
thymidine
NH
NH
O
O
H3C
thymine
HN
N
N
NH
O
H2N
6543
21 7
89
guanine
HN
N N
N
H2N
HOCH2
OHOH
O
O
guanosine
Appendix IV
N
N N
N
NH2
H
71
3
6
9
adenine
304
N
N
NH2
O
3
1
H
cytosine
O
OH OH
HOCH2N
NH2
O
N
cytidine
HN
NH
O
O
uracil
HN
N
O
O
O
OH OH
HOCH2
uridine
Nitrogenous Bases and Nucleosides
Appendix IV
305
Appendix VThe Chemical Elements
(* indicates the nucleon number of the most stable isotope)
Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m
actinium Ac 89 227*
aluminum Al 13 26.982
americium Am 95 243*
antimony Sb 51 112.76
argon Ar 18 39.948
arsenic As 33 74.92
astatine At 85 210
barium Ba 56 137.327
berkelium Bk 97 247*
beryllium Be 4 9.012
bismuth Bi 83 208.98
bohrium Bh 107 262*
boron B 5 10.811
bromine Br 35 79.904
cadmium Cd 48 112.411
calcium Ca 20 40.078
californium Cf 98 251*
carbon C 6 12.011
cerium Ce 58 140.115
cesium Cs 55 132.905
chlorine Cl 17 35.453
chromium Cr 24 51.996
cobalt Co 27 58.933
copper Cu 29 63.546
curium Cm 96 247*
darmstadtium Ds 110 269*
dubnium Db 105 262*
dysprosium Dy 66 162.50
einsteinium Es 99 252*
erbium Er 68 167.26
europium Eu 63 151.965
fermium Fm 100 257*
fluorine F 9 18.9984
francium Fr 87 223*
gadolinium Gd 64 157.25
gallium Ga 31 69.723
germanium Ge 32 72.61
gold Au 79 196.967
hafnium Hf 72 178.49
hassium Hs 108 265*
helium He 2 4.0026
holmium Ho 67 164.93
hydrogen H 1 1.008
indium In 49 114.82
iodine I 53 126.904
iridium Ir 77 192.217
iron Fe 26 55.845
krypton Kr 36 83.80
lanthanum La 57 138.91
lawrencium Lr 103 262*
lead Pb 82 207.19
lithium Li 3 6.941
lutetium Lu 71 174.967
magnesium Mg 12 24.305
manganese Mn 25 54.938
meitnerium Mt 109 266*
mendelevium Md 101 258*
mercury Hg 80 200.59
molybdenum Mo 42 95.94
neodymium Nd 60 144.24
306
Appendix V
The Chemical ElementsElement Symbol p.n. r.a.m Element Symbol p.n. r.a.m
neon Ne 10 20.179
neptunium Np 93 237.048
nickel Ni 28 58.69
niobium Nb 41 92.91
nitrogen N 7 14.0067
nobelium No 102 259*
osmium Os 76 190.23
oxygen O 8 15.9994
palladium Pd 46 106.42
phosphorus P 15 30.9738
platinum Pt 78 195.08
plutonium Pu 94 244*
polonium Po 84 209*
potassium K 19 39.098
praseodymium Pr 59 140.91
promethium Pm 61 145*
protactinium Pa 91 231.036
radium Ra 88 226.025
radon Rn 86 222*
rhenium Re 75 186.21
rhodium Rh 45 102.91
roentgenium Rg 111 272*
rubidium Rb 37 85.47
ruthenium Ru 44 101.07
rutherfordium Rf 104 261*
samarium Sm 62 150.36
scandium Sc 21 44.956
seaborgium Sg 106 263*
selenium Se 34 78.96
silicon Si 14 28.086
silver Ag 47 107.868
sodium Na 11 22.9898
strontium Sr 38 87.62
sulfur S 16 32.066
tantalum Ta 73 180.948
technetium Tc 43 99*
tellurium Te 52 127.60
terbium Tb 65 158.925
thallium Tl 81 204.38
thorium Th 90 232.038
thulium Tm 69 168.934
tin Sn 50 118.71
titanium Ti 22 47.867
tungsten W 74 183.84
ununbium Uub 112 285*
ununtrium Uut 113 284*
ununquadium Uuq 114 289*
ununpentium Uup 115 288*
ununhexium Uuh 116 292*
uranium U 92 238.03
vanadium V 23 50.94
xenon Xe 54 131.29
ytterbium Yb 70 173.04
yttrium Y 39 88.906
zinc Zn 30 65.39
zirconium Zr 40 91.22
307
Appendix VI1
1
H2
He
1 2 3 4 5 6
3
Li
4
Be
5
B6
C7
N8
O9
F1
0 Ne
11 N
a1
2 Mg
13 A
l1
4 Si1
5
P1
6
S1
7 Cl
18 A
r
19 K
20 C
a2
1 Sc2
2 Ti
23 V
24 C
r2
5 Mn
26 F
e2
7 Co
28 N
i2
9 Cu
30 Z
n3
1 Ga
32 G
e3
3 As
34 Se
35 B
r3
6 Kr
37 R
b3
8 Sr3
9 Y4
0 Zr
41 N
b4
2 Mo
43 T
c4
4 Ru
45 R
h4
6 Pd
47 A
g4
8 Cd
49 In
50 Sn
51 Sb
52 T
e5
3
I5
4 Xe
55 C
s5
6 Ba
57-7
1
La-
Lu
72 H
f7
3 Ta
74 W
75 R
e7
6 Os
77 Ir
78 P
t7
9 Au
80 H
g8
1 Tl
82 P
b8
3 Bi
84 P
o8
5 At
86 R
n
87 F
r8
8 Ra
89-1
03
Ac-
Lr
10
4 Rf
10
5 Db
10
6 Sg1
07 Bh
10
8 Hs
10
9 Mt
11
0 Ds
11
1
Rg
11
2
Uu
b1
13
Uut
11
4
Uu
q1
15
Uu
p1
16
Uu
h
23
45
67
89
10
11
12
13
14
15
16
17
18
Peri
odic
Tab
le o
f th
e E
lem
ents
- gi
vin
g gr
ou
p,
ato
mic
nu
mb
er,
and
ch
emic
al s
ymb
ol
Lan
than
ides
Act
inid
es
Mo
der
n f
orm
Eu
rop
ean
con
ven
tio
nN
. A
mer
ican
con
ven
tio
n
Th
e ab
ove
is
the
mo
der
n r
eco
mm
end
ed f
orm
of
the
tab
le u
sin
g 1
8gr
ou
ps.
Old
er g
rou
p d
esig
nat
ion
s ar
e sh
ow
n b
elo
w.
Period
6 7
57 L
a
89 A
c
58 C
e
90 T
h
59 P
r
91 P
a
60 N
d
92 U
61 P
m
93 N
p
62 Sm
94 P
u
63 E
u
95 A
m
64 G
d
96 C
m
65 T
b
97 B
k
66 D
y
98 C
f
67 H
o
99 E
s
68 E
r
10
0 Fm
69 T
m
10
1 Md
70 Y
b
10
2 No
71 L
u
10
3 Lr
12
34
56
78
91
01
11
21
31
41
51
61
71
8
IAII
AII
IAIV
AV
AV
IAV
IIA
VII
I (o
r V
IIIA
)IB
IIB
IIIB
IVB
VB
VIB
VII
B0
(o
r V
IIIB
)
IAII
AII
IBIV
BV
BV
IBV
IIB
VII
I (o
r V
IIIB
)IB
IIB
IIIA
IVA
VA
VIA
VII
AV
IIIA
(or
0)
7
308
The Greek Alphabet
A α alphaB β betaΓ γ gamma∆ δ deltaE ε epsilonZ ζ zetaH η etaΘ θ thetaI ι iotaK κ kappaΛ λ lambdaM µ mu
N ν nuΞ ξ xiO ο omikronΠ π piP ρ rhoΣ σ sigmaT τ tauΥ υ upsilonΦ φ phiX χ chiΨ ψ psiΩ ω omega
Appendix VII
309
Chemical society web sites:
American Chemical Society www.chemistry.org
Royal Society of Chemistry www.rsc.org
The International Union of Pure and www.iupac.orgApplied Chemistry
Information on nomenclature:
Queen Mary College, London www.chem.qmul.ac.uk/iupac
Advanced Chemistry Development, Inc www.acdlabs.com/iupac/nomenclature
An extensive set of organic chemistry links:
WWW Virtual Library, Chemistry Section www.liv.ac.uk/Chemistry/Links
The definitive site for the chemical elements:
WebElements Periodic Table www.webelements.com
Other resources:
Chemical Education Resource Shelf, www.umsl.edu/~chemist/booksJournal of Chemical Education, University of Missouri-St. Louis
Molecule of the Month www.chm.bris.ac.uk/motm/motm.htm
The Chemistry Hypermedia Project www.chem.vt.edu/chem-ed/at Virginia Tech vt-chem-ed.html
Biographies:
Nobel Prizes http://nobelprize.org
WWW Virtual Library www.liv.ac.uk/Chemistry/Links/refbiog.html
Web Sites
Appendix VIII
Comprehensive texts covering organic chemistry:
Carey, Francis and Richard J. Sundberg. Advanced Organic Chemistry: Structure andMechanism. 4th ed. New York: Plenum, 2000.
Carey, Francis and Richard J. Sundberg. Advanced Organic Chemistry: Reactions. 4th ed.New York: Plenum, 2000.
Clayden, Jonathon; Nick Greeves; Stuart Warren; and Peter Wothers. Organic Chem-istry. Oxford, U.K.: Oxford University Press, 2000.
McMurray, John. Organic Chemistry. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2004.
March, Jerry. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 4thed. New York: Wiley, 1992.
Volhardt, Peter K. and Neil E. Schore. Organic Chemistry: Structure and Function. 4thed. New York: W. H. Freeman, 2003.
Comprehensive texts covering inorganic chemistry:
Cotton, F. A.; C. Murillo; G. Wilkinson; M. Bochmann; and R. Grimes. Advanced Inor-ganic Chemistry. 6th ed. New York: Wiley, 1999.
Greenwood, N. N. and A. Earnshaw. Chemistry of the Elements. Oxford, U.K.: Butter-worth-Heinemann, 1997.
Shriver, D. F. and P. W. Atkins. Inorganic Chemistry. 3rd ed. Oxford, U.K.: Oxford Uni-versity Press, 1999.
Additional useful sources:
Emsley, J. Nature's Building Blocks – An A-Z Guide to the Elements. Oxford, U.K.:Oxford University Press, 2001.
King, R.B. (ed.) Encyclopedia of Inorganic Chemistry. New York: Wiley, 1994.
An advanced text on biochemistry:
Nelson, David L. and Michael M. Cox. Lehninger Principles of Biochemistry. 3rd ed.New York: Worth Publishers, 2000.
The definitive book for physical chemistry:
Atkins, Peter and Julio de Paula. Atkins’ Physical Chemistry, 7th ed. Oxford, UK: OxfordUniversity Press, 2004.
310
Bibliography
