Which insoluble compound in each pair should be more soluble in nitric acid than in pure water?

PbS or lead sulfide show less solubility in water and insoluble in nitric acid.

Explanation:

PbS dissociates in water.The reaction is -

PbS(s)⇄Pb²⁺ (aq) + S²⁻(aq)

The sulfide ion which is produced in the reaction is a strong comjugated base that contain weak acid like Hydrogen sulfide.The above reaction of S²⁻ with strong acid like nitric acid decrease the concentration of sulfide ion and results in equilibrium reacion  that shifts the equation to right side and thus increases the solubility.

S²⁻(Aq) + HNO₃(Aq)⇄ HS (Aq)+ NO₃ (Aq)

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.

the answer is d.) potassium