First slide

Enthalpy of formation

Question

ΔH_f of C₂H₂, C₂H₄ and C₂H₆ respectively are +226, +56 and -84 kJ/mole. The difference between heats of hydrogenation of ethyne and ethene forming ethane is (in kilo joules)

Moderate

Solution

$\begin{gathered} {C_2}{H_2} + 2{H_2} \to {C_2}{H_6},\,\Delta {H_1} = heat\,of\,hydrogenation\rightarrow (1) \\ {C_2}{H_4} + {H_2} \to {C_2}{H_6},\,\Delta {H_2} = heat\,of\,hydrogenation\rightarrow (2) \\eq.(1) + \frac{1}{{eq.(2)}} \\ \Rightarrow \,{C_2}{H_2} + {H_2} \to {C_2}{H_4},\,\Delta {H_r}\, = \Delta {H_1} - \Delta {H_2} = ? \\ \Delta {H_r} = \Delta {H_f}({C_2}{H_4}) - [\Delta {H_f}({C_2}{H_2}) + \Delta {H_f}({H_2})] \\ \,\,\,\,\,\,\,\,\,\,\,\,\, = + 56 - [226 + 0] = - 170KJ \\ \end{gathered}$

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