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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