JEE Main 2025 Jan 28 Shift 2, Chemistry Q22: Hess’s Law and Bond Energy

JEE Main2025Jan 28, Shift 2Chemistry

Consider the following data :

Heat of formation of $C O_{2} (g) = - 393.5$ kJ mol $^{- 1}$

Heat of formation of $H_{2} O (l) = - 286.0$ kJ mol $^{- 1}$

Heat of combustion of benzene $= - 3267.0$ kJ mol $^{- 1}$

The heat of formation of benzene is _____ kJ mol $^{- 1}$ . (Nearest integer)

Your answer

Solution

Combustion of benzene:

$C_{6} H_{6} (l) + \frac{15}{2} O_{2} (g) \to 6 C O_{2} (g) + 3 H_{2} O (l)$

Apply the relation $Δ H_{c} = \sum Δ H_{f} (products) - \sum Δ H_{f} (reactants)$ , using $Δ H_{f} (O_{2}) = 0$ :

$- 3267 = [6 (- 393.5) + 3 (- 286.0)] - Δ H_{f} (C_{6} H_{6})$

$- 3267 = [- 2361 - 858] - Δ H_{f} (C_{6} H_{6}) = - 3219 - Δ H_{f} (C_{6} H_{6})$

$Δ H_{f} (C_{6} H_{6}) = - 3219 - (- 3267) = + 48$ kJ mol $^{- 1}$ .

Concept behind this question

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