Learning Objectives
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recall stages in the formation of an ionic solid from its elements
use a Born–Haber cycle to obtain a measured value of the lattice energy of an ionic compound
explain the term polarisation as applied to ions
understand that the polarising power of a cation depends on its charge and its radius
understand that the polarisability of an anion depends on its size
understand that the polarisation of anions by cations leads to covalent character in some ionic bonds
use both measured and calculated standard heats of formation to explain the relative stability of
different ionic compounds
The stages in the formation of an ionic solid from its elements
Recap: Energy Changes
The diagram above shows the formation of sodium chloride from its elements but it is a very
simplistic illustration. The diagram ignores the fact that when this reaction takes place
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Sodium is actually part of a giant metallic bonded structure
Energy is needed to separate the sodiums from each other (Enthalpy change of atomization)
Energy is required to remove electrons (the first ionization energy)to form sodium ions
Chlorine is actually made up of diatomic molecules
Energy is required to separate the chlorine atoms from each other ( Enthalpy change of
atomization)
Energy is required to add electrons to each chlorine atom to form a chloride ion (the
electron affinity)
Where does all this energy come from to turn sodium metal and chlorine molecules into sodium
chloride?
Ionic crystals are actually stable because of the large release of energy when oppositely charged ions
come together to form a crystal lattice. This energy is known as the Lattice Energy, ΔHɵlattice
Two German scientists Max Born(1882-1970) and Fritz Haber(1868-1934) analysed the energy
changes when ionic compounds were formed. Their work resulted in Born-Haber cycles which are
thermochemical cycles that allow us to investigate the stability and the bonding of ionic compounds.
The strength of ionic bonds are measured as lattice energies in k Jmol-1.
The energy released is greater and the force of attraction between the ions stronger if:
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The charges on the ions are large
The ions are small allowing them to get close to each other
The lattice energy is different from the enthalpy of formation (that involves the formation of the
compound from its elements)
Na(s) + 1/2Cl2(g)
NaCl(s)
ΔHɵf [NaCl(s)] = -411k J mol-1
The lattice energy ΔHɵlattice
Na+ (g) + Cl- (g)
NaCl(s)
ΔHɵlattice[NaCl(s)]= -787 k J mol-1
Born-Haber Cycles
These allow chemists to calculate lattice energies that can not be measured directly. The Born-Haber
cycle identifies all the enthalpy changes that contribute to the standard enthalpy of formation of a
compound.
These changes involve:
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The energy required to create free gaseous ions by atomizing and then ionizing the elements
The energy given out (the lattice energy) when the ions come together to form a crystal
A Born-Haber cycle is usually set out as an enthalpy level diagram. All the terms in the cycle can be
measured by experiments apart from the lattice energy. By using Hess’s law it is possible to calculate
the lattice energy.
Hess’s Law
The enthalpy change in converting reactants into products is the same regardless of the route taken,
provided the initial and final conditions are the same
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OHT of NaCl formation
Tutorial on drawing born haber cycles
http://www.rsc.org/Education/Teachers/Resources/Databook/index.htm
Starting with the elements sodium and oxygen the measured value for the enthalpy change of
formation of sodium oxide is written downwards on the cycle showing that it is exothermic (-).
Above that the values for the atomization and ionization are written upwards as endothermic (+)
processes.
Also the amount shown in the cycle is for 2 mol of Na as there is 2 mol of Na in 1 mol of NaO.
Similiarly ½ mol of O2 for 1 mol of O2-.
Applying Hess’s Law we can work out the lattice energy.
ΔHɵlattice [Na2O] = (-657-249-992-214+ -414)
=-2526k J mol-1
What effect lattice Energies p85
A closer look at lattice energies p86
Theoretical eg: NaCl2
Questions 1-5 p87