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

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Presentation transcript:

Discussing the difference between theoretically and experimentally derived lattice energies

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...

Q – What does Empirical Mean? CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl NaBr NaI AgI All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

A – Experimentally based! CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl NaBr NaI AgI All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

Q – Does the Born-Haber Model agree with the ionic model? CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl NaBr NaI AgI All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

A – Generally the agreement is good CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl NaBr NaI AgI All values are for ΔH lat ˚ in kJmol - % Diff = (Emp-Theo)/Emp

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 NaCl NaBr NaI AgI All values are for ΔH lat ˚ 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. CompoundEmpirical value (Born-Haber Model) Theoretical value (Ionic Model) % Difference NaCl NaBr NaI AgI All values are for ΔH lat ˚ 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 ΔH o lat

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