First slide

Hess law of constant summation

Question

The heats of combustion of carbon, hydrogen and acetylene are –394K.J, – 286K.J and –1301 K.J respectively. Calculate heat of formation of C₂H₂

Moderate

Solution

Given,
$\begin{array}{l} {\Delta _C}H(C) = {\Delta _f}H(C{O_2}) = - 394KJ\,\,;\,\,{\Delta _C}H({H_2}) = {\Delta _f}H({H_2}O) = - 286KJ\\\\ {C_2}{H_2} + \frac{5}{2}{O_2} \to 2C{O_2} + {H_2}O\,,\,{\Delta _C}H({C_2}{H_2}) = - 1301KJ = {\Delta _r}H \end{array}$
$\begin{array}{l} {\Delta _r}H = [2{\Delta _f}H(C{O_2}) + {\Delta _f}H({H_2}O)] - [{\Delta _f}H({C_2}{H_2}) + \frac{5}{2}{\Delta _f}H({O_2})]\\\\ - 1301 = [2( - 394) + ( - 286)] - [{\Delta _f}H({C_2}{H_2}) + 0]\\\\ {\Delta _f}H({C_2}{H_2}) = - 788 - 286 + 1301 = + 227KJ \end{array}$

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