urgent chem - enthalpy related

15.
PbI2(s) ⇌ Pb²⁺(aq) + 2I⁻(aq)

Ksp = [Pb²⁺][I⁻]²
[Pb²⁺]o = Ksp/[I⁻]o²

When [I^] = 2[I⁻]o :
[Pb²⁺] = Ksp/(2[I⁻]o)² = (1/4)*(Ksp/[I⁻]o)= (1/4)[Pb²⁺]o
Hence, the concentration of Pb²⁺ is decreased by a factor of 4.


25.
Standard enthalpy change of formation is defined as the enthalpy change in thestandard state when one mole of the substance is formed from its elements intheir standard states.

A is not the answer because Ag⁺(aq) and Cl⁻(aq) are NOT the elements in their standard states.


26.
In a heterogeneous equilibrium, the effective reacting concentration of a solid. Therefore, its concentration term is omittedin the equilibrium expression.

Since [C]is omitted in the equilibrium expression, Cmay be a solid.
Since [A]and [B] appears in the equilibrium expression, Aand B may be in aqueous solution.


28.
Pressure can only affect gaseous components in equilibrium system. There are 3 moles of gaseous components ineither side. Therefore, pressureaffects the rate of the forward reaction and that of the backward reaction tothe same extent.


30.
2Fe³⁺(aq) + 2e⁻ ⇌ 2Fe²⁺
Sn²⁺(aq) ⇌ Sn⁴⁺(aq) + 2e⁻
The redox reaction involves "2" moles of electrons.