Consider the following data: Heat of formation of (g) = - 393.5 kJ Heat of formation of = - 286.0 kJ Heat of combustion of benzene = - 3267.0 kJ The heat of formation of benzene is _____ kJ . (Nearest integer) [2025]

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Solution

Q.
Consider the following data:
Heat of formation of ${CO}_{2}$ (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) [2025]

Ans.
(48)

$C_{6} H_{6} (l) + \frac{15}{2} O_{2} (g) ⟶ 6 {CO}_{2} (g) + 3 H_{2} O (l)$

$Δ H_{comb} = - 3267 {kJ mol}^{- 1}$

$For any reaction: Δ H_{r} = Δ H_{f} (products) - Δ H_{f} (reactants)$

Applying this formula on combustion reaction of benzene:

$Δ H_{comb} = (benzene) = [6 \times Δ H_{f} ({CO}_{2} (g)) + 3 \times Δ H_{f} (H_{2} O (l))]$ $- [Δ H_{f} (C_{6} H_{6} (l)) + \frac{15}{2} \times Δ H_{f} (O_{2} (g))]$

$- 3267 = [6 \times (- 393.5) + 3 \times (- 286)] - [Δ H_{f} (C_{6} H_{6} (l)) + \frac{15}{2} \times 0]$

$Δ H_{f} = (C_{6} H_{6} (l)) = 48 {kJ mol}^{- 1}$

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