GATE 2026 AE – Question 52
Isobutane (C$_4$H$_{10}$) is burnt completely in pure oxygen as per the reaction given below. Given that the standard heats of formation (in kcal/mole) of isobutane, carbon dioxide, and water vapour are -31.489, -94.052, and -60.150, respectively, the heat of reaction is ________ kcal (rounded off to 2 decimal places).
$C_4H_{10} + 6.5\, O_2 \rightarrow 4\, CO_2 + 5\, H_2O$
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Correct answer: -648.70 to -642.24
Explanation
The heat of reaction is the sum of the heats of formation of the products minus those of the reactants: $\Delta H = 4(-94.052) + 5(-60.150) - (-31.489) - 6.5(0) = -376.208 - 300.750 + 31.489 = -645.47$ kcal. The heat of formation of oxygen is zero.