The complete catalytic hydrogenation of ethyne (acetylene) to ethane occurs in two…

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

The complete catalytic hydrogenation of ethyne (acetylene) to ethane occurs in two sequential additions of hydrogen gas.

\mathrm{C_2H_2\; +\; 2\,H_2\; \longrightarrow\; C_2H_6}

Average bond energies (in kJ mol

^{-1}

) are given below.

Bond type	$E_{\text{avg}}$
C≡C	839
C=C	614
C–C	348
C–H	413
H–H	432

Assume that no other bond types are present. (a) Write separate balanced equations for the first hydrogen addition (conversion of ethyne to ethene) and the second hydrogen addition (conversion of ethene to ethane). (b) Using the bond energies provided, calculate the approximate enthalpy change for each of the two individual steps. (c) Determine the overall enthalpy change,

\Delta H_{\text{rxn}}

, for the complete hydrogenation of ethyne to ethane. (d) Identify which step is more exothermic and explain how bond order and orbital hybridization contribute to this difference.

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