Question

CaCl2 has a negative heat of solution and KCI has a positive heat of solution. If this is the case,

1) What sign does lattice energy always have?

2) What sign does heat of hydration always have? How is this value different for Ca2+ vs K+.

3) Given the answer to these teo questions and the heat of solution values, which compound, CaCl2 or KCI should have a large negative heat of hydration? dHsoln=U + dH hydr

4) Which compound would be the best to melt ice on the road, LiCl or CaCl2? Please explain

Answer 1

1) Lattice energy is energy needed to separate solid into its gaseous ions. It increases with greater charge of ions and smaller atomic radius. Thus the sign of lattice energy is +ve. For the two compounds given here, the lattice energy would be higher for CaCl2. The lattice energy decreases in the order CaCl2 > KCl.

2) Heat of hydration is the amount of energy released when a ion dissolves in water. The sign of heat of hydration is -ve. As the size on ion increases, the heat of hydration goes down. A higher charge measn greater heat of hydration. Thus here the order of heat of hydration would decrease as CaCl2 > KCl with CaCl2 having higher heat of hydration than KCl.

3) CaCl2 invariably has a greater heat of hydration as compared to KCl.

4) The best compound to melt ice of the road thus would be CaCl2 beacuse it gives greater amount of energy, more -ve heat of solution.