Question

For the reaction of dissolution of lithium sulfate in water the equilibrium constant is positive (K > 1).

Li₂SO₄(s) ⇌ 2 Li⁺(aq) + SO₄^-2(aq)  

Δ_rH^o = -30.5 kJ/mol

Which of the following statements is correct?

• A. You can dissolve a larger amount of lithium sulfate in hot water than in cold water.
• B. The entropy of the system increases when lithium sulfate dissolves in water.
• C. The reaction as written is not spontaneous at standard conditions.
• D. Dissolution of lithium sulfate in water is an endothermic process.

Answer 1

Since solids have well ordered arrangement , entropy of Li₂SO₄(s) is almost zero . But on dissolving in water , it converts to aqueous ions in solution & disorderness or randomness of ions increases hence entropy of the system also increases. Thus option B is correct.

Since the reaction is exothermic ( Δ_rH^o is negative) the forward reaction is favoured by low temperature. Thus Li₂SO₄  is more soluble in cold water than hot water . Thus option A is incorrect.

Condition for spontaniety of reaction is Δ_rG  of reaction should be negative .But Δ_rG  = Δ_rH - TΔ_rS  . For above reaction Δ_rH= negative & Δ_rS = is positive. Hence Δ_rG become negative . So option C is wrong ( this can be also explained based on the value of equillibrium constant K. if K>1 reaction is spontaneous , if K= 1 then reaction is is in equilibrium & if K<1 then reaction is nonspontaneous )

Option D is incorrect since above process is exothermic ( because Δ_rH^o is negative).