First slide

Hess law of constant summation

Question

What is the heat of formation of C₆H₆, given that the heats of combustion of benzene, carbon and hydrogen are 782, 94 and 68K. Cal respectively ?

Moderate

Solution

$C (s) + O_{2} (g) \to {CO}_{2} (g), ∆_{c} H_{C} = ∆_{f} H_{{CO}_{2}} = - 94 kCals - - (1); H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l), ∆_{c} H_{H_{2}} = ∆_{f} H_{H_{2}} O = - 68 kCals - (2); C_{6} H_{6} (l) + \frac{15}{2} O_{2} (g) \to 6 {CO}_{2} (g) + 3 H_{2} O (l), ∆_{c} H_{C_{6} H_{6}} = - 782 kCals - (3);$
$\begin{array}{l} {\Delta _r}H = [6{\Delta _f}H(C{O_2}) + 3{\Delta _f}H({H_2}O)] - [{\Delta _f}H({C_6}{H_6}) + \frac{{15}}{2}{\Delta _f}H({O_2})]\\\\ - 782 = [6( - 94) + 3( - 68)] - [{\Delta _f}H({C_6}{H_6}) + 0]\\\\ {\Delta _f}H({C_6}{H_6}) = - 564 - 204 + 782 = + 14Kcal \end{array}$

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