Definition: The lattice dissociation enthalpy, ΔHϴL (diss) is

the enthalpy change when 1 mole of an ionic solid is separated into its gaseous ions

Example: Sodium chloride (NaCl)

NaCl (s) → Na+ (g) + Cl- (g) ΔHϴL (diss) = + 787 kJ

mol-1

Energy must be put in to break the strong ionic bonds in the lattice; therefore it is an endothermic process

The two factors which affect the value of ΔHϴL (diss) are:

1) The size of the ions

The halide ions increase in size in the order F-, Cl-, Br- and I-

The distance between the positive ion and negative halide ion increases

Therefore the electrostatic force of attraction between the ions is weaker and the ΔHϴ

L (diss) values decrease

Compound ΔHϴL (kJ mol-1)

NaF 918NaCl 780

2) The charges on the ions

The greater the charges on the ions, the stronger the electrostatic force of attraction between the ions

Therefore more energy is required to separate the ions and the ΔHϴ

L (diss) value increases

Compound ΔHϴL (kJ mol-1)

NaF 918MgO 3791

Na+ F-

Mg2+ O2-

Born-Haber cycles are used to calculate lattice enthalpies, ΔHϴ

L from experimental data (ΔHϴf, ΔHϴ

diss, ΔHϴ

i, ΔHϴea and ΔHϴ

at)

We can also calculate lattice enthalpies, ΔHϴL using a

model called the perfect ionic model

Definition: The lattice dissociation enthalpy, ΔHϴL (diss) is

the enthalpy change when 1 mole of an ionic solid is separated into its gaseous ions

The perfect ionic model assumes:

The ions are perfect spheres The ions are held together only by electrostatic forces

(pure ionic bonding)

We can compare the ΔHϴL (diss) values from the Born-

Haber cycle (calculated from experimental data) with those from the perfect ionic model (theoretical)

Compound(Alkali halides)

ΔHϴL Born-Haber (kJ mol-1)

ΔHϴL Perfect ionic

model (kJ mol-1)NaCl 787 769NaBr 747 732NaI 704 682KCl 701 690KBr 670 665KI 629 632

For the alkali halides there is good agreement between the ΔHϴ

L (diss) values from the Born-Haber cycle and the perfect ionic model

We can conclude the structure of alkali halides is just like the perfect ionic model (ions are perfect spheres and held together only by electrostatic forces)

This provides evidence to support the model works

We can conclude the structure of alkali halides is just like the perfect ionic model (ions are perfect spheres and held together only by electrostatic forces)

The type of bonding in alkali halides is ionic bonding

For the silver halides there is less agreement between the ΔHϴ

L (diss) values from the Born-Haber cycle and the perfect ionic model

Compound(Silver halides)

ΔHϴL Born-Haber (kJ mol-1)

ΔHϴL Perfect ionic

model (kJ mol-1)AgF 955 870AgCl 915 864AgBr 904 830AgI 889 808

The ions are not perfect spheres and there is some covalent bonding between the ions

The type of bonding in silver halides is ionic with covalent character

The perfect ionic model does not take into account any covalent bonding present between the ions

Ionic Ionic with some covalent character

The type of bonding in silver halides is ionic with some covalent character

More energy would be required to separate the ionic solid into gaseous ions as the bonding is stronger

This explains the higher values of ΔHϴL (diss) from the

Born-Haber cycle compared to the perfect ionic model

Silver halides ΔHϴL Born-Haber ΔHϴ

L Perfect ionic

AgCl 915 864AgBr 904 830AgI 889 808

The electron cloud of the negative ion can be (distorted) polarised by the positive ion

Negative ions are easily polarised if they are large

Iodide ions are large and are easily polarised and therefore the ionic bond has the greatest covalent character

This explains why there is the greatest difference in ΔHϴ

L (diss) values between the B-H cycle and the perfect ionic model for silver iodides