The hydration energies of and are and kJ/mol respectively. If lattice energy of KCl is kJ/mol, then the heat of solution of KCl is: [2025]

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Solution

Q.
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: [2025]

1 $+ x - y - z$

2 $x + y + z$

3 $z - (x + y)$

4 $- z - (x + y)$

Ans.
(3)

In question Lattice energy of KCl is given as –z. As it is negative (release of energy) it must be the energy released upon combination of gaseous $K^{+}$ and ${Cl}^{-}$ ions to give one mole solid KCl. But in the cycle below, we have used lattice energy as energy needed to dissociate one mole solid KCl into gaseous ions. Hence it must be +z.

By Hess's law:

$Δ H_{sol} (KCl) = Δ H_{LE} (KCl) + Δ H_{HE} (K^{+}) + Δ H_{HE} ({Cl}^{-})$

$= z - x - y$

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