Discussing the difference between theoretically and experimentally derived lattice energies

15.2.4

So far we’ve learned that although lattice enthalpy cannot be directly measured, it can be determined in two ways: Theoretically using equations like this

oneΔHlat˚=(Knm)/(RMn+ + RXm- )

Experimentally using Born Haber CyclesHlat = Hf – (Hatom(for both Na and

Cl)+ Hi+ He) Once again, the epic struggle between

man’s mind and the universe that it lives in...

Q – What does Empirical Mean?Compound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

A – Experimentally based!Compound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

Q – Does the Born-Haber Model agree with the ionic model?Compound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

A – Generally the agreement is goodCompound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

Q – Deduce the relationship between the ionic/covalent nature of a bond and the agreement between the models.Compound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

A – The more ionic in nature a bond is, the more agreement there is. Compounds that are more covalent in nature have higher empirical values.Compound Empirical

value(Born-Haber Model)

Theoretical value

(Ionic Model)

% Difference

NaCl 790 769 2.7

NaBr 754 732 2.9

NaI 705 682 3.3

AgI 892 808 9.4

All values are for ΔHlat˚ in kJmol- % Diff = (Emp-Theo)/Emp

Summing up Usually there is good agreement between

empirical and theoretical values If there isn’t good agreement:

It implies that the description of the compound as ionic is inappropriate

There could be a significant degree of covalent character in the bonding (EN difference less than 1.7)

Presence of covalent character leads to an increase in ΔHo

lat

Homework

Get your A in gear and finish your IA soon. Or else…A = AttitudeIA = Internal Assessment