JEE Main 2024 · 9 April · morning · Numerical answer · +4 / −1
The heat of solution of anhydrous CuSO₄ and CuSO₄ · 5 H₂ O are -70 kJ mol⁻¹ and +12 kJ mol⁻¹ respectively.
The heat of hydration of CuSO₄ to CuSO₄ · 5 H₂ O is -x kJ. The value of x is ________. (nearest integer).
Answer and solution
Answer 82
Solution
Chemistry ma’am · 3 steps
Dissolve directly, or hydrate first and then dissolve: both routes end at the same solution.
- Two routes. Anhydrous CuSO₄ can reach the same aqueous solution in one step, by dissolving directly. It can also take two steps: first hydrate to CuSO₄.5H₂O, then dissolve the hydrate.
- Apply Hess's law. Both routes start and end at the same states, so their enthalpy changes are equal. That gives Δ Hₛₒₗ(anhydrous) = Δ H_(hyd) + Δ Hₛₒₗ(hydrate).
Δ H_(hyd) = Δ Hₛₒₗ(CuSO₄) - Δ Hₛₒₗ(CuSO₄.5H₂O) = -70 - (+12) = -82 kJ mol⁻¹
- So. The heat of hydration is -82 kJ, so x = 82.
The common slip: 58. Adding the two values, -70 + 12, instead of subtracting the hydrate's +12 gives -58, so x = 58.
The idea to keep. Heat of hydration equals the heat of solution of the anhydrous salt minus the heat of solution of the hydrate. Keep every sign with its number.
Answer from the NTA final answer key. Solution by PrepWiser.