JEE Main 2025 Jan 24 Shift 2, Chemistry Q16: Hess’s Law and Bond Energy

JEE Main2025Jan 24, Shift 2Chemistry

$S (g) + \frac{3}{2} O_{2} (g) \to S O_{3} (g) + 2 x kcal$

$S O_{2} (g) + \frac{1}{2} O_{2} (g) \to S O_{3} (g) + y kcal$

The heat of formation of $S O_{2} (g)$ is given by :

Solution

Convert the heat released into enthalpy change ( $Δ H = - heat released$ ):

Reaction 1: $S (g) + \frac{3}{2} O_{2} \to S O_{3}$ , $Δ H_{1} = - 2 x = Δ H_{f} (S O_{3})$

Reaction 2: $S O_{2} + \frac{1}{2} O_{2} \to S O_{3}$ , $Δ H_{2} = - y = Δ H_{f} (S O_{3}) - Δ H_{f} (S O_{2})$

Subtract reaction 2 from reaction 1 (Hess's law):

$Δ H_{f} (S O_{2}) = Δ H_{f} (S O_{3}) - Δ H_{2} = (- 2 x) - (- y) = y - 2 x$

$∴ Δ H_{f} (S O_{2}) = (y - 2 x) kcal$ .

Correct option: (2)

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