---
title: "A student investigates the thermochemistry of the stepwise hydrogenation of ethyne, represented by the two balanced chemical equations below.  \\[ \\text{Reaction 1: } \\text{C}_2\\text{H}_2\\text{(g)} + \\text{H}_2\\text{(g)} \\rightarrow \\text{C}_2\\text{H}_4\\text{(g)} \\] \\[ \\text{Reaction 2: } \\text{C}_2\\text{H}_4\\text{(g)} + \\text{H}_2\\text{(g)} \\rightarrow \\text{C}_2\\text{H}_6\\text{(g)} \\]  The student references the following table of average bond enthalpies:  | Bond | Average bond enthalpy (\\(\\text{kJ/mol}\\)) | | :— | :— | | \\(\\text{C}-\\text{C}\\) | \\(350\\) | | \\(\\text{C}=\\text{C}\\) | \\(610\\) | | \\(\\text{C}\\equiv\\text{C}\\) | \\(840\\) | | \\(\\text{C}-\\text{H}\\) | \\(410\\) | | \\(\\text{H}-\\text{H}\\) | \\(435\\) |  Based on the data in the table, which of the following correctly compares the standard enthalpy change of Reaction 1 (\\(\\Delta H_1^\\circ\\)) to that of Reaction 2 (\\(\\Delta H_2^\\circ\\)) and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/119768/"
date_modified: "2026-08-21T08:12:17+00:00"
---

# A student investigates the thermochemistry of the stepwise hydrogenation of ethyne, represented by the two balanced chemical equations below.

\[ \text{Reaction 1: } \text{C}_2\text{H}_2\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_4\text{(g)} \]
\[ \text{Reaction 2: } \text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_6\text{(g)} \]

The student references the following table of average bond enthalpies:

| Bond | Average bond enthalpy (\(\text{kJ/mol}\)) |
| :— | :— |
| \(\text{C}-\text{C}\) | \(350\) |
| \(\text{C}=\text{C}\) | \(610\) |
| \(\text{C}\equiv\text{C}\) | \(840\) |
| \(\text{C}-\text{H}\) | \(410\) |
| \(\text{H}-\text{H}\) | \(435\) |

Based on the data in the table, which of the following correctly compares the standard enthalpy change of Reaction 1 (\(\Delta H_1^\circ\)) to that of Reaction 2 (\(\Delta H_2^\circ\)) and provides the correct justification?

A student investigates the thermochemistry of the stepwise hydrogenation of ethyne, represented by the two balanced chemical equations below.

\[ \text{Reaction 1: } \text{C}_2\text{H}_2\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_4\text{(g)} \]
\[ \text{Reaction 2: } \text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_6\text{(g)} \]

The student references the following table of average bond enthalpies:

| Bond | Average bond enthalpy (\(\text{kJ/mol}\)) |
| :--- | :--- |
| \(\text{C}-\text{C}\) | \(350\) |
| \(\text{C}=\text{C}\) | \(610\) |
| \(\text{C}\equiv\text{C}\) | \(840\) |
| \(\text{C}-\text{H}\) | \(410\) |
| \(\text{H}-\text{H}\) | \(435\) |

Based on the data in the table, which of the following correctly compares the standard enthalpy change of Reaction 1 (\(\Delta H_1^\circ\)) to that of Reaction 2 (\(\Delta H_2^\circ\)) and provides the correct justification?

- **A.** \(\Delta H_1^\circ\) is less exothermic than \(\Delta H_2^\circ\) because the \(\text{C}\equiv\text{C}\) bond in \(\text{C}_2\text{H}_2\) has a higher total bond enthalpy than the \(\text{C}=\text{C}\) bond in \(\text{C}_2\text{H}_4\), requiring more energy to break all three bonds.
- **B.** \(\Delta H_1^\circ\) is less exothermic than \(\Delta H_2^\circ\) because the shorter bond length of \(\text{C}\equiv\text{C}\) prevents \(\text{C}-\text{H}\) bonds from releasing energy during their formation.
- **C.** \(\Delta H_1^\circ\) is more exothermic than \(\Delta H_2^\circ\) because the \(\text{C}-\text{H}\) bonds formed in Reaction 1 have higher bond enthalpies than the \(\text{C}-\text{H}\) bonds formed in Reaction 2.
- **D.** \(\Delta H_1^\circ\) is more exothermic than \(\Delta H_2^\circ\) because converting the \(\text{C}\equiv\text{C}\) bond to a \(\text{C}=\text{C}\) bond requires less energy (\(230\text{ kJ/mol}\)) than converting the \(\text{C}=\text{C}\) bond to a \(\text{C}-\text{C}\) bond (\(260\text{ kJ/mol}\)).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119768/*