Acetylene, C₂H₂(g), can undergo successive hydrogenation reactions according to the sequence below:
Step 1 C₂H₂(g) + H₂(g) → C₂H₄(g)
Step 2 C₂H₄(g) + H₂(g) → C₂H₆(g)
(The overall reaction converts a molecule with a carbon–carbon triple bond into one with a carbon–carbon single bond.)
Answer the following questions about the structural changes that occur during these steps.
(a) Describe qualitatively how the carbon–carbon bond length changes from C₂H₂ to C₂H₄ to C₂H₆.
(b) Describe qualitatively how the carbon–carbon bond energy changes over the same sequence of compounds.
(c) Using bond order as a central concept, explain why the enthalpy change for Step 1 is much more exothermic than the enthalpy change for Step 2.
(d) Predict whether the average C–H bond length in C₂H₆ is shorter, longer, or approximately equal to the average C–H bond length in C₂H₂. Justify your prediction.