JEE Main 2025 Apr 7 Shift 2, Chemistry Q3: Hess’s Law and Bond Energy

JEE Main2025Apr 7, Shift 2Chemistry

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 :

A
$+ x - y - z$
B
$x + y + z$
C
$z - (x + y)$
D
$- z - (x + y)$

Solution

Dissolving KCl can be split into two hypothetical steps using Hess's law: breaking the ionic lattice apart into gaseous ions, then hydrating those gaseous ions.

Since the lattice energy of formation is $- z$ kJ/mol, the reverse process (breaking the lattice) requires $+ z$ kJ/mol.

The hydration steps release energy: $- x$ kJ/mol for K $_{(g)}^{+}$ and $- y$ kJ/mol for Cl $_{(g)}^{-}$ .

Adding the three steps: $Δ H_{so l u t i o n} = z + (- x) + (- y) = z - (x + y)$ .

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JEE Main 2025 Apr 7 Shift 2, Chemistry Q3: Hess’s Law and Bond Energy

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