1. Discussing the difference between theoretically and experimentally derived lattice energies 15.2.4

2. 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 one ΔH lat ˚=(K nm )/(R Mn+ + R Xm- )  Experimentally using Born Haber Cycles H lat = H f – (H atom (for both Na and Cl)+ H i + H e )  Once again, the epic struggle between man’s mind and the universe that it lives in...

3. Q – What does Empirical Mean? CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

4. A – Experimentally based! CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

5. Q – Does the Born-Haber Model agree with the ionic model? CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

6. A – Generally the agreement is good CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

7. Q – Deduce the relationship between the ionic/covalent nature of a bond and the agreement between the models. CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

8. 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. CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl7907692.7 NaBr7547322.9 NaI7056823.3 AgI8928089.4 All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

9. 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 ΔH o lat

10. Homework  Get your A in gear and finish your IA soon. Or else… A = Attitude IA = Internal Ass essment