First slide

Hess law of constant summation

Question

If the heats of formation of Al₂O₃ and Fe₂O₃ are -400 K.Cal and -190 K.Cal respectively, the heat of the following reaction is 2Al + Fe₂O₃ →2Fe + Al₂O₃

Easy

Solution

$\begin{array}{l} 2Al{\rm{ }} + {\rm{ }}F{e_2}{O_3} \to 2Fe{\rm{ }} + {\rm{ }}A{l_2}{O_{3{\rm{ }},}}{\Delta _r}H = ?\\\\ {\Delta _r}H = [{\Delta _f}H({\rm{ }}A{l_2}{O_3}) + 2{\Delta _f}H(Fe)] - [2{\Delta _f}H(Al) + {\Delta _f}H(F{e_2}{O_3})]\\\\ \,\,\,\,\,\,\,\,\,\, = [ - 400 + 0] - [0 - 190] = - 210Kcal \end{array}$

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