CH1001 2010(Language2) Notes

CH1001CH1001

CHEMISTRY: A Central ScienceCHEMISTRY: A Central Science

Department of Chemistry,School of Pharmacy & Molecular Sciences

22 The languageThe language

of chemistryof chemistry

MeasurementMeasurement

• Measurements

– always have a unit

– always an associated uncertainty

• Unit

– specific standard quantity of a particular property

– used to measure all other quantities of property

• The seven SI base units

Measurement Unit Symbollength metre m

mass kilogram kg

time second s

temperature kelvin K

amount of substance mole mol

electric current ampere A

luminous intensity candela Cd


• The SI units for ANY physical quantity can

be built from the seven base units

– For example

• area = length ! width

• length and width are measured in metres

• the unit of area is square metre, m2

• Units undergo the same kinds of

mathematical operations as the numbers to

which they are attached


EXAMPLES:

1. Volume = length x breadth x depth

The units will be metre x metre x metre = cubic metre,designated m3


2. Speed (velocity) = distance/time

The units will therefore be metre/second, designated m/s or m s-1

3. Density = mass/volume

The units will be kilogram/cubic metre, designated kg/m3 or kg m-3

• Use prefixes that divide or multiply the unit by aparticular power of ten

Prefix Symbol Factor

mega M 106

kilo k 103

deci d 10-1

centi c 10-2

milli m 10-3

micro µ 10-6

nano n 10-9

pico p 10-12

• SI or derived units are sometimes inconvenient


There is one non-SI unit that is in common

usage, particularly in an industrial and

environmental context - parts per million

(ppm)

! In solutions, it is a weight/volume measurement -

i.e. mg/litre (mg L-1)

! In solids, it is a weight/weight measurement -

i.e. mg/kg (mg kg-1)


• Uncertainties and significant figures

– Every measurement has an associated

uncertainty


24.3 ºC

297.45 K

24.32 ºC

297.47 K

– All figures up to and including the first

estimated figure are recorded

– These figures are called significant figures



24.3 ºC

297.45 K

24.32 ºC

297.47 K

– Scientific notation expresses numbers in

terms of powers of ten

• 24.3 ºC is 2.43 ! 101 ºC

• 247.45 ºC is 2.4745 ! 102 ºC



24.3 ºC

297.45 K

24.32 ºC

297.47 K

• Significant figures in calculations

– Multiplication or division of

measurements


! Addition or subtraction of measurements

3.14 x 2.751

0.64= 13

NOT 13.49709375 !

3.247

+ 41.36

+ 125.2

169.8

NOT 169.807 !

• Precision

– Signifies how reproducible a

measurement is when made a number of

times



• Accuracy

– refers to how close the value is to the

correct value

Representations of moleculesRepresentations of molecules

• Chemical formulas

– Show the relative number of each type of atom

present in a substance

• H2O

• C30H34AuBClF3N6O2P2PtW

Note: a subscripted number refers ONLY to

the atom immediately preceding it.

– A molecular formula is a chemical formula that refers to a discrete molecule

– There is a standardised way of writing chemical formulas . . .

• Binary compounds– Element further to the left of the periodic table

appears first

• KCl, Al2S3

– Hydrogen is the exception to this rule

• LiH, NH3, H2O2, HCl

– If both elements are from the same group

the lowest element appears first• SiC, BrF3

– In ionic compounds the cation is written first

followed by the anion• NaBr, MgCl2


• Writing chemical formulas for

compounds containing more than two

elements requires some knowledge of

the bonding within the compound.

– Ionic compounds

– Covalent compounds


• Ionic compounds

– Cation followed by anion

– Total charge zero

Ca(NO3)2(s) " Ca2+(aq) + 2NO3-(aq)


• Covalent compounds

– Majority are carbon-based organic

compounds

– Chemical formula often written carbon

first, followed by hydrogen and then the

remaining elements in alphabetical

order, e.g. C2H6O, C4H9BrO



• Three-dimensional structures

PICTURING MOLECULESPICTURING MOLECULES

Water

Chemical formula - showing only relative number of atoms

Electron-dot and bond-line formulas - show a bond

between atoms as either a pair of dots or a line.

The bond-line formulas are often called structural formulas.

Ball-and-Stick models - show atoms as spheres and

bonds as sticks, with accurate angles and relative sizes,

but distances are exaggerated

Space-filling models - accurately scaled-up versions

of molecules, but do not show bonds

Electron-density models - show the ball-and-stick model

within the space-filling shape, and colour the regions of

high (red) and low (blue) electron charge

PICTURINGPICTURING

MOLECULESMOLECULES

• Structural formulas

– Attempt to show the way in which theatoms in the molecule are bondedtogether

– Chemical symbols still used for eachelement present

– Constituent atoms are placed in theorder in which they are bonded together

– Bonds between neighbouring atoms arerepresented as lines


• Structural formulas

– NH3

– Structural formulas do not necessarilyshow the correct geometry of acompound


HN

H

H

NH H

H

NH H

H

• •

NomenclatureNomenclature

• Nomenclature — the system for

naming of compounds

• International Union of Pure and Applied

Chemistry (IUPAC)

• Some compounds are better known by

their common unsystematic name rather

than their systematic IUPAC name

– The systematic name of water is oxidane !

1. The element closer to the left of the

periodic table appears first.

If both elements are from the same

group of the periodic table, the lower

one appears first.


• Naming inorganic compounds

• Nonmetallic binary compounds

– Guidelines for naming:

NO NO2 N2O5

Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide

2. The element that appears first retains

its elemental name.


• Naming Inorganic compounds



NO NO2 N2O5


3. The second element begins with a

root derived from its elemental name

and ends with the suffix –ide.





NO NO2 N2O5


4. When there is more than one atom of

a given element in the formula, the

name of the element usually contains

a prefix that specifies the number of

atoms present.






NO NO2 N2O5


• Binary compounds of hydrogen

– Hydrogen requires special consideration

– It may appear first or second in the

chemical formula of a compound,

therefore first or second in the name

• Lithium hydride LiH

• Hydrogen fluoride HF

• Hydrogen sulfide H2S

• Calcium hydride CaH2


• Ionic compounds

– Binary ionic compounds are written with

the cation first and the anion, which

takes the suffix –ide, last

• Potassium iodide KI

• Calcium fluoride CaF2

– Ionic compounds may contain

polyatomic ions

• Ammonium NH4+

• Nitrate NO3-


POLYATOMIC IONSPOLYATOMIC IONS

ANIONSOH- hydroxide

NO3- nitrate

CO32- carbonate

ClO4- perchlorate

SO42- sulfate

PO43- phosphate

CATIONS

NH4+ ammonium

MnO4- permanganate

Cr2O72- dichromate

HCO3- bicarbonate

O22- peroxide

C2H3O2- acetate

1. The name has a root taken from the

name of the central atom

• Carbonate CO32-

• Nitrite NO2-

• Ionic compounds

– Anions containing a central atom

surrounded by oxygen atoms are known

as oxoanions. Naming rules:


2. When an element forms 2 different

oxoanions, the one with fewer oxygen

atoms ends in –ite, and the other

ends in –ate

• Sulfite SO32-

• Sulfate SO42-

• Ionic compounds





3. Chlorine, bromine and iodine each

form 4 different oxoanions that are

distinguished by prefixes and suffixes

• Hypobromite BrO-

• Bromite BrO2-

• Bromate BrO3-

• Ionic compounds





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CH1001 2010(Language2) Notes