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Chemistry Question 53 – JEE-MAIN 2025

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The hydration energies of K+ and Cl− are −x and −y kJ/mol respectively. If lattice energy of KCl is −z kJ/mol, then the heat of solution of KCl is :

The heat of solution is the enthalpy change when one mole of a substance dissolves in a large amount of solvent.

🥷
Ninja Strategy•Apply Hess's Law with Correct Enthalpy Signs

Understand that the heat of solution is the sum of lattice dissociation energy (endothermic, positive) and total hydration energy (exothermic, negative).

Step 1: Define the overall process and its components✦ Active

The heat of solution (ΔHsol) for an ionic compound like KCl is the sum of the energy required to break the crystal lattice (lattice dissociation energy) and the energy released when the gaseous ions are hydrated (hydration energy).

ΔHsol=ΔHlattice,dissociation+ΔHhydration
Step 2: Determine the values for lattice dissociation energy and total hydration energy○ Expand

Given:

- Hydration energy of K+ = −x kJ/mol

- Hydration energy of Cl− = −y kJ/mol

Total hydration energy, ΔHhydration=(−x)+(−y)=−(x+y) kJ/mol.

Lattice energy of KCl is −z kJ/mol. This value typically refers to the energy released when gaseous ions form the solid lattice (K+(g)+Cl−(g)→KCl(s)). Therefore, the energy required to break the lattice (lattice dissociation energy) is the negative of this value: ΔHlattice,dissociation=−(−z)=+z kJ/mol.

💡 Teacher's Secret Hint

Pay close attention to the sign convention for lattice energy. If it's given as formation energy, its dissociation energy will have the opposite sign.

Step 3: Calculate the heat of solution○ Expand

Substitute the values into the equation from Step 1:

ΔHsol=(+z)+(−(x+y))
ΔHsol=z−(x+y) kJ/mol
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