Fondamenti di Chimica & Educazione Ambientale Lecture 3 · Fondamenti di Chimica Determining...

Maurizio Casarin A.A. 2016 - 2017

Fondamenti di Chimica

Fondamenti di Chimica & Educazione Ambientale Lecture 3

Compounds and Molecules Pure substances composed of two or more different

elements are called compounds.

Compounds have definite percent composition of their combining elements.

Molecular compounds

Standard color scheme

H2O2 CH3CH2Cl P4O10

CH3CH(OH)CH3 HCO2H

Some molecules

z  Atoms of almost all elements can gain or lose electrons to form charged species called ions.

z  Compounds composed of ions are known as ionic compounds.

u Metals tend to lose electrons to form positively charged ions called cations.

u Non-metals tend to gain electrons to form negatively charged ions called anions.

Ionic compounds

Extended array of Na+ and Cl- ions

Sodium chloride

Simplest formula unit is NaCl

Inorganic molecules

Halothane C2HBrClF3

M(C2HBrClF3) = 2MC + MH + MBr + MCl + 3MF

= (2 × 12.01) + 1.01 + 79.90 + 35.45 + (3 × 19.00) = 197.38 g/mol

Mole ratio nC/nhalothane

Mass ratio mC/mhalothane

Chemical Composition

Calculating the Mass Percent Composition of a Compound

Calculate the molecular mass M(C2HBrClF3

) = 197.38 g/mol

For one mole of compound, formulate the mass ratio and convert to percent:

%C = (2×12.01)g

197.38g ×100% =12.17%

%C =(2 ×12.01)g

197.38g×100% = 12.17%

%H =1.01g

197.38g×100% = 0.51%

%Br = 79.90g197.38g

×100% = 40.48%

%Cl = 35.45g197.38g

×100% = 17.96%

%F =(3×19.00)g

197.38g×100% = 28.88%

Calculating the Mass Percent Composition of a Compound

1.  Choose an arbitrary sample size (100g). 2.  Convert masses to amounts in moles. 3.  Write a formula. 4.  Convert formula to small whole numbers. 5.  Multiply all subscripts by a small whole number to

make the subscripts integral.

5 Step approach:

Empirical formula

Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition.

Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 u. What are the empirical and molecular formulas of dibutyl succinate?

Step 1: Determine the mass of each element in a 100g sample.

C 62.58 g

O 27.79 g H 9.63 g

Step 2: Convert masses to amounts in moles.

Step 3: Write a tentative formula.

Step 4: Convert to small whole numbers.

C5.21H9.55O1.74

C2.99H5.49O

nC = 62.58gC×1molC

12.011gC= 5.210molC

nH = 9.63gH×1molH

1.008gH= 9.55molH

nO = 27.79gO×1molO

15.999gO= 1.737molO

Step 5: Convert to a small whole number ratio.

Multiply × 2 to get C5.98H10.98O2

The empirical formula is C6H11O2

Step 6: Determine the molecular formula.

Empirical formula mass is 115 u. Molecular formula mass is 230 u.

The molecular formula is C12H22O4

Combustion analysis

Metals tend to lose electrons.

Na D Na+ + e-

Non-metals tend to gain electrons.

Cl + e- D Cl-

Reducing agents Oxidizing agents

We use the Oxidation State to keep track of the number of electrons that have been gained or lost by an element.

Oxidation States

1.  The oxidation state (OS) of an individual atom in a free element is 0. 2.  The total of the OS in all atoms in:

i.  Neutral species is 0. ii.  Ionic species is equal to the charge on the ion.

3.  In their compounds, the alkali metals and the alkaline earths have OS of +1 and +2 respectively.

4.  In compounds the OS of fluorine is always –1 5.  In compounds, the OS of hydrogen is usually +1 6.  In compounds, the OS of oxygen is usually –2. 7.  In binary (two-element) compounds with metals:

i.  Halogens have OS of –1, ii.  Group 16 have OS of –2 and iii.  Group 15 have OS of –3.

Rules for Oxidation States

Anions (- Q):Groups 15, 16, 17 (VA, VIA, VIIA) Cations (+ Q):Groups 1, 2, 3 (IA, IIA, IIIA)

Assigning Oxidation States.

What is the oxidation state of the underlined element in each of the following? a) P4; b) Al2O3; c) MnO4

-; d) NaH

a)  P4 is an element. P OS = 0 .

b)  Al2O3: O is –2. O3 is –6. Since (+6)/2=(+3), Al OS = +3.

c)  MnO4-: net OS = -1, O4 is –8. Mn OS = +7.

d)  NaH: net OS = 0, rule 3 beats rule 5, Na OS = +1 and H OS = -1.

Periodic Trends In Oxidation States

Naming Compounds

Trivial names are used for common compounds. A systematic method of naming compounds is known as a system of nomenclature. Organic compounds Inorganic compounds

Binary Compounds of Metals and Nonmetals

NaCl = sodium chloride

name is unchanged

“ide” ending electrically neutral

MgI2 = magnesium iodide

Al2O3 = aluminum oxide

Na2S = sodium sulfide

Inorganic Nomenclature

Molecular compounds usually write the positive OS element first. HCl hydrogen chloride

mono 1 penta 5

di 2 hexa 6

tri 3 hepta 7

tetra 4 octa 8

Some pairs form more than one compound

Binary Compounds of Two Non-metals

Naming Binary Compounds of the Non-metals

Named in order of increasing group number

N2O4 Dinitrogen tetroxide

Sulfur hexafluoride

N2O Dinitrogen oxide

Emphasize the fact that a molecule is an acid by altering the name.

HCl hydrogen chloride hydrochloric acid

HF hydrogen fluoride hydrofluoric acid

Acids produce H+ when dissolved in water.

They are compounds that ionize in water.

Binary Acids

Polyatomic ions are very common. Here are a few:

ammonium ion NH4+ acetate ion C2H3O2

carbonate ion CO32- hydrogen carbonate HCO3

hypochlorite ClO- phosphate PO43-

chlorite ClO2- hydrogen phosphate HPO4

chlorate ClO3- sulfate SO4

perchlorate ClO4- hydrogensulfate HSO4

Polyatomic Ions

Gaining (anion) and Losing (cation) Electrons to attain Inert gas electronic

structure

Ionic Bonding:

When an atom gains electrons it becomes an anion. Anion are larger than the neutral atom.

W h e n a n a t o m loses electrons it becomes a cation. Cations are smaller than the neutral atom.

The attractive force between + and - charges is called a coulombic attraction

Force of attraction =

KQ1×Q2

Predict the ionic bond strength of the following ionic bonds.

Consider a cation and an anion

Mg Cl Cl

How to write formulas for ionic compounds

1. Identify ions

2.“Cross” charges

3. Write formula 1 2

Charge is Conserved: The + = - !

Naming Ionic Compounds

Cations are named as the metal + “ion”

Na+ = sodium ion

Multicharged ions

Stock System Metal (Charge in Roman Numerals)

“ion” Cu1+ Cu2+

Copper(I) ion Copper(II) ion

MgBr2 Magnesium bromide

Fe2O3 Iron(III) oxide

Ca3(PO4)2 Calcium phosphate

Naming Ionic Compounds

More Examples

Mg(NO3)2

NaHCO3

Compounds and Molecules

Some compounds are composed of molecules.

Sharing electrons to attain inert gas electron configurations. E

Covalent Bonding:

Organic compounds abound in nature Fats, carbohydrates and proteins are foods.

Propane, gasoline, kerosene, oil.

Drugs and plastics

Carbon atoms form chains and rings and act as the framework of molecules.

Naming Organic Compounds

Molecular Structure

The arrangement of atoms in a molecule

Isomers:

Same number of constituent atoms BUT

different arrangements.

Visualizations of some hydrocarbons

Functional Groups Carboxylic Acid

Functional Groups Alcohol

Isomerism

Structural

Stereo

Optical

Geometrical

Positional

Functional

Chain Isomers

n-pentane

2-methylbutane

2,2-dimethylpropane

Positional Isomers

Propan-1-olo

Propan-2-olo

C3H7OH

Positional Isomers

2-Methylphenol 4-Methylphenol 3-Methylphenol

C7H7OH

Functional Isomers

Ethanol Methoxymethane

Alcohols have the hydroxyl group, –OH Ethers have the functional group R–O–R'

Functional Isomers

Propanal

Aldehydes and ketones have the carbonyl group C=O. In aldehydes C is attached either to 1 or 2 H atoms; in ketones C is attached to two carbon atoms.

Propanone

Isomerism

Structural

Stereo

Optical

Geometrical

Positional

Functional

Geometrical Isomers

Geometrical isomerism is an example of stereo-isomerism. This occurs when substances have the same molecular formula, but a different arrangement of their atoms in space.

Geometrical isomerism can happen if: i)  There is a C=C bond in the molecule ii)  The molecule has a ring iii) The molecule has a >C=N bond

Geometrical Isomers

Cis-But-2-ene Trans-But-2-ene

trans-Pt(NH3)2Cl2

cis-Pt(NH3)2Cl2

Geometrical Isomers

Pt(NH3)Cl3

No isomers

Geometrical Isomers

cis-Pt(NH3)Cl2Br trans-Pt(NH3)Cl2Br

Two isomers

Geometrical Isomers

trans Two isomers

Geometrical Isomers

fac Two isomers

Geometrical Isomers

Optical Isomers

Optical isomerism is an example of stereo-isomerism. It occurs when substances have the same molecular and structural formula, but one cannot be superimposed on the other. The different forms are called enantioners.

optical isomerism

non-superimposable mirror images

Optical Isomers

Optical isomers can occur when there is an asymmetric carbon atom; i.e. a C atom bonded to four different groups.

Lecture 3 End

Fondamenti di Chimica & Educazione Ambientale Lecture 3 · Fondamenti di Chimica Determining...

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