past paper questions

Question 1

^H sol: -87 kJ/mol
^S system: -210 J/mol

explain why the dissolving of magnesium sulfate in water is exothermic by considering the enthalpy changes involved. (2)

Answer 1

1- breaking lattice is endothermic & hydration of ions is exothermic
2- enthalpy of hydration is larger than lattice energy

Question 2

BaSO4:
- ^H sol: +19 kJ/mol
- T^S system: -31 kJ/mol

CaSO4:
- ^H sol: -18 kJ/mol
- T^S system: -43 kJ/mol

comment on the relative solubility in water of barium sulfate & calcium sulfate at 298K, using the data. (6)

Answer 2

1- positive ^H & -T^S for BaSO4
2- negative ^H & positive -T^S for CaSO4
3- magnitude of -T^S > ^H
4- both have positive ^G
5- ^G positive = slightly soluble only
6- BaSO4 less soluble than CaSO4 as it has a more positive ^G