1 (6 points, 2 points for each part) For the cation, Se⁴⁺

a) Give the formula for the oxide. SeO₂

b) State whether or not the oxide is soluble in water; explain your answer.

The cation has Z²/r = 16/ 64 = 0.25, making it very strongly acidic. The anion, O2-, is very strongly basic. Thus we would predict the oxide to be insoluble. (In fact, it is a gas like SO2, and is soluble)

c) State whether the oxide is basic, amphoteric, or acidic; explain your choice.

This is an acidic oxide, like SO2, because the intense acidity of the cation (Se4+) overwhelms the basicity of two oxide anions. 

2 (8 points) Consider the following element oxides:

Ga₂O₃ CaO ZrO₂ Na₂O

a) Estimate the lattice energy of Ga₂O₃ using the Kapustinskii equation.

U = 1.202*105*(3+)(2-)*5*(1-34.5/(76+126))/(76+126) = -14802 kJ/mole 

b) Using your value for Ga₂O₃, estimate the lattice energies for the remaining oxides.

The major variation from compound to compound is in the charges of the ions and in the number of ions per formula unit:

Ga2O3. Charge*Charge*#ions = 30

CaO. Charge*Charge*#ions = 8 

ZrO2. Charge*Charge*#ions = 24

Na2O. Charge*Charge*#ions = 6

Approximate lattice enthalpies are estimated by scaling the Ga2O3 value according to these numbers:

CaO. 8/30 * 14800 = 3950

ZrO2. 24/30 * 14800 = 11800

Na2O. 6/30 * 14800 = 3000

3) (6 points) Which precipitation reactions are driven by an increase in entropy? Explain.

a) Ag⁺(aq) + Cl^-(aq) ® AgCl(s)

b) Fe³⁺(aq) + S^2-(aq) ® Fe₂S₃(s)

c) Al³⁺(aq) + PO₄^3-(aq) ® AlPO₄(s)

Wulfsberg claims that precipitation reactions involving an acidic cation and a basic anion are entropy driven. Thus b and c are entropy driven.