First slide

Hess law of constant summation

Question

Heat of formation of CO and CO₂ are –26.4,- 94.0 kcal/mole respectively. What is the heat of combustion of CO in kcals ?

Easy

Solution

$\begin{array}{l} {C_{(s)}} + \frac{1}{2}{O_2}_{(g)} \to C{O_{(g)}}\,\,\Delta {H_1} = - 26.4Kcal/mole \to (1)\\\ {C_{(s)}} + {O_2}_{(g)} \to C{O_2}_{(g)}\,\,\Delta {H_2} = - 94Kcal/mole \to (2)\\\ C{O_{(g)}}\, + \frac{1}{2}{O_2}_{(g)} \to C{O_2}_{(g)}\,\,\Delta H_C = ?\, \to (3)\\\\ eq.(2) - eq.(1)\,(or)\frac{1}{{eq.(1)}} + eq.(2) \Rightarrow eq.(3)\\\\ {\Delta _C}H(CO) = \,\Delta {H_2} - \Delta {H_1} = - 94 - ( - 26.4) = - 67.6Kcal/mole \end{array}$

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