Aspects of Bonding

&

Acid Strength

CHEM 110/ 2013

Intramolecular Bonding The bonding between molecules/atoms in the solid state Ionic bonding e.g. sodium chloride Covalent bonding e.g. Graphite or diamond Metallic bonding e.g. copper/gold etc. Van der Waals bonding e.g. iodine or benzene Hydrogen bonding e.g. water and alcohols

Typically ionic solids. Have high melting and

boiling points They are brittle.

They form electrolyte solutions if they dissolve in water.

Ionic Bonding

Ions stack together in regular crystalline structures

The attractive force of attraction between a pair of oppositely charged ions which increases with increased charge on the ions & with decrease in ionic sizes

requires more energy to break up

requires less energy to break up

Ionic Solids

Na+ = 99 pm

F F

Covalent Bonding Sharing of a pair of Electrons in the area between the nuclei. Complete their octets by sharing electron pairs

E.g. Consider fluorine, F2

F F

The electron configuration of F is: 1s 22s 22p 5

These are combined to form F2.

It is a special case The more valence

electrons, the stronger the bond.

This gives rise to their properties such as malleability or ductility.

Good thermal and electrical conductors

Metallic Bonding The electrons are considered to be “delocalized”

Due to instantaneous dipoles non-conducting species

Van der Waals Bonding

Bonding in Sulfur Sulfur Sulfur does not form stable diatomic molecules, S2, like oxygen, O2.

Consider the bonding O2: Atomic radius of O is 0.74 A, (ie. not too big) sideways overlap of py orbitals is possible π bond the π bond makes O2 fairly stable

Consider the bonding S2: Atomic radius of S is 1.04 A, too big to allow good overlap of its 3py orbitals double bonds are not formed between S atoms.

Sulfur forms crown shaped S8 molecules: • Two -bonds per S atom • Stable octet (full shell of electrons)

Each S is sp3 hybridised with 2 lone pairs of electrons:

3s 3px 3py 3pz

Bonding in Sulfur

Bonding in Phosphorus Phosphorus Phosphorus does not form stable diatomic molecules, P2, like nitrogen, N2.

Bonding in N2: π bonding is favoured in N2 due to small atomic radius

and good orbital overlap.

Bonding in P:

P is too large to form PP triple

bond (ie, 2 π bonds)

Atomic radius:

Nitrogen: 0.74 A Phosphorus: 1.10 A

Phosphorus Minerals

Compounds of Oxygen (Oxides) 1 2 13 14 15 16 17 18

Li2O BeO B2O3

CO CO2

N2O, NO NO2

Na2O MgO Al2O3 SiO2

P2O3 P2O5

SO2

SO3

Cl2O ClO2

Cl2O7

K2O CaO Ga2O3 GeO2

As2O3 As2O5

SeO2

SeO3

Br2O

Rb2O SrO In2O3

SnO SnO2

Sb2O3 Sb2O5

TeO2

TeO3

I2O5

XeO3 XeO4

Cs2O BaO Tl2O3

Tl2O PbO PbO2

Bi2O5 PoO2

Basic Oxides (Metals form basic

Ionic oxides )

Amphoteric Oxides (Metalloids)

Acidic Oxides (Nonmetals form acidic

covalent oxides)

Acidity increases from left to right

Acids & Bases Arrhenius: An acid provides H+ ions (ionizable hydrogen); and

A base produces OH- ions in an aqueous solution

Brønsted-Lowry: An acid is a proton donor, and

A base is a proton acceptor

Lewis: An acid is a species that accepts an e- -pair electron

deficient (incomplete octet), and

A base is an e--pair donor electron rich (has lone

pair(s))

Lewis acid-base reaction forms an adduct via a

coordinate covalent bond

Base Anhydride Anhydride means “without water”. Metal oxides are base anhydrides & react with water to form a basic solution:

CaO(s) + H2O Ca2+(aq) + 2OH-(aq)

basic oxide calcium hydroxide The oxide ion is protonated to produce the hydroxide:

O2- + H2O 2OH-(aq) (100%)

A basic oxide reacts with acids to produce water

CaO(s) + 2H+(aq) Ca2+(aq) + H2O(l)

Basic Oxides

Acid Anhydride All nonmetal oxides react with water to form oxoacids. • Oxoacids contain H, O and one other atom. • Dissociation of an oxoacid produces H+ ions. Reaction of sulfur trioxide, SO3, with water: SO3(g) + H2O(l) H2SO4(aq) H2SO4(aq) 2H+(aq) + SO4

2-(aq)

Reaction of sulfur dioxide, SO2, with water: SO2(g) + H2O(l) H2SO3(aq) H2SO3(aq) 2H+(aq) + SO3

2-(aq)

Acidic Oxides

Sulfurous acid

Sulfuric acid

Metal/metalloid oxides with both acidic and basic properties They are generally insoluble in water. Reaction as a Base: Al2O3(s) + 6HCl(aq) 2AlCl3(aq) + 3H2O(l)

Base Acid

Reaction as an Acid: Al2O3(s) + 3H2O(l) + 2OH-(aq) 2[Al(OH)4]-(aq)

Acid Base

Under basic conditions, the soluble complex ion: tetra(hydroxo)aluminate(III) ion, [Al(OH)4]-(aq) is formed. With excess OH-, the octahedral soluble [Al(OH)6]3- complex ion is formed.

Amphoteric Oxides

Exercise 1. Write a balanced equation for Zn(OH)2 with:

a) an acid

b) a base

2. Do the following species act as acids, bases or amphoteric species?

a) PO43-

b) CH3NH3+

Neutral oxides Relatively few exist;

Insoluble in water; &

Do not reaction with either acids or bases.

Examples include:

Carbon monoxide, CO

Dinitrogen monoxide, N2O

Neutral Oxides

Oxoacids are ternary compounds containing hydrogen, oxygen and one other element. Halogens, include chlorine, bromine and iodine: eg HClO4 - Perchloric acid HClO3 - Chloric acid HClO2 - Chlorous acid HOCl - Hypochlorous acid

Group 6 elements including S, Se and Te*: eg H2SO4 - Sulfuric acid H2SO3 - Sulfurous acid

Group 5 elements including N and P: eg HNO3 - Nitric acid HNO2 - Nitrous acid H3PO4 - Phosphoric acid (Orthophosphoric acid)

Oxoacids

HOCl - Hypochlorous acid (Weak acid)

Halogen Oxoacids

HClO3 - Chloric acid

HClO2 - Chlorous acid

HClO4 - Perchloric acid (Strong acid)

The strength of the oxoacid increases with increasing number of terminal oxygen atoms

Halogen Oxoacids as Oxidising Agents

Halogen oxides and oxoacids are strong oxidising agent, especially in acidic medium, due to formation of the stable halide anion

OCl(aq) + 2H+(aq) + 2e- Cl-(aq) + H2O(l)

EoCell = +1.61 V

This strong oxidizing ability makes them effective

bleaches and disinfectants

Most common are Ca(OCl)2, and

NaOCl

Polyprotic acids Polyprotic acids contain more than one ionizable hydrogen atom per acid molecule

Examples include:

Phosphoric acid H3PO4 Triprotic

Sulfuric acid H2SO4

Sulfurous acid H2SO3

Carbonic acid H2CO3

Dissociate in a stepwise manner:

H2SO4(aq) H+(aq) + HSO4-(aq) Strong

HSO4-(aq) ⇌ H+(aq) + SO4

2-(aq) Weak

H3PO4, H2SO3, & H2CO3 - all dissociations are weak

Ka1 > Ka2 > Ka3

Diprotic

Strength of Oxoacids The acid strength of an oxoacid is dependent upon two factors

1. The electronegativity of the central atom

2. The number of terminal O atoms in the acid

molecule

Strength of Oxoacids 1. The Electronegativity of the Central Atom

HOCl is stronger acid than HOBr because Cl is more electronegative than Br, stronger pull on the electrons towards Cl

O ClH

ENeg(Cl) = 3.0

Ka = 2.9 x 10-8

O BrH

ENeg(Cl) = 2.8

Ka = 2.1 x 10-9

Acid Strength of Oxoacids 2. The Number of Terminal Oxygen Atoms in

the Acid Molecule

The acid strength of an oxoacid increases with: increasing number of terminal oxygen atoms, due to the stability of the conjugate base as a result of

resonance delocalisation

S OO

O

HH

Ka = 1.3 x 10-3

S O

O

O

O

HH

Ka = 103

Acid Strength of Oxoacids 2. The Number of Terminal Oxygen Atoms in

the Acid Molecule

In SO42- the negative charge is spread over more oxygen

atoms than in SO32-, reducing the partial negative

charge on each of the oxygen atoms.

Explanation

S OO

O-2

S O

O

O

O -2

The lower the partial negative charge, the more stable the anion, the stronger the acid!

Exercise

Which acid is the stronger acid?

Give reasons.

ClO O

O

H IO O H

O