---
title: "A student investigates the thermochemical properties of three gaseous constitutional isomers with the molecular formula \\(\\text{C}_5\\text{H}_{12}\\). The standard enthalpies of formation, \\(\\Delta H_f^\\circ\\), for the isomers at \\(298 \\text{ K}\\) are given in the table below.  | Isomer | Structural formula | \\(\\Delta H_f^\\circ\\ (\\text{kJ/mol})\\) | |—|—|—| | \\(\\text{X}\\) | \\(\\text{CH}_3\\text{CH}_2\\text{CH}_2\\text{CH}_2\\text{CH}_3\\) | \\(-146\\) | | \\(\\text{Y}\\) | \\((\\text{CH}_3)_2\\text{CHCH}_2\\text{CH}_3\\) | \\(-154\\) | | \\(\\text{Z}\\) | \\(\\text{C}(\\text{CH}_3)_4\\) | \\(-168\\) |  All three isomers undergo complete combustion with excess \\(\\text{O}_2\\text{(g)}\\) according to the following equation: \\[ \\text{C}_5\\text{H}_{12}\\text{(g)} + 8\\,\\text{O}_2\\text{(g)} \\rightarrow 5\\,\\text{CO}_2\\text{(g)} + 6\\,\\text{H}_2\\text{O(l)} \\]  Which of the following correctly ranks the isomers in order of the magnitude of heat released per mole of isomer completely combusted, from greatest to least, with the correct justification?"
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url: "https://nerd-notes.com/ubq/123841/"
date_modified: "2026-09-28T12:30:30+00:00"
---

# A student investigates the thermochemical properties of three gaseous constitutional isomers with the molecular formula \(\text{C}_5\text{H}_{12}\). The standard enthalpies of formation, \(\Delta H_f^\circ\), for the isomers at \(298 \text{ K}\) are given in the table below.

| Isomer | Structural formula | \(\Delta H_f^\circ\ (\text{kJ/mol})\) |
|—|—|—|
| \(\text{X}\) | \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\) | \(-146\) |
| \(\text{Y}\) | \((\text{CH}_3)_2\text{CHCH}_2\text{CH}_3\) | \(-154\) |
| \(\text{Z}\) | \(\text{C}(\text{CH}_3)_4\) | \(-168\) |

All three isomers undergo complete combustion with excess \(\text{O}_2\text{(g)}\) according to the following equation:
\[ \text{C}_5\text{H}_{12}\text{(g)} + 8\,\text{O}_2\text{(g)} \rightarrow 5\,\text{CO}_2\text{(g)} + 6\,\text{H}_2\text{O(l)} \]

Which of the following correctly ranks the isomers in order of the magnitude of heat released per mole of isomer completely combusted, from greatest to least, with the correct justification?

A student investigates the thermochemical properties of three gaseous constitutional isomers with the molecular formula \(\text{C}_5\text{H}_{12}\). The standard enthalpies of formation, \(\Delta H_f^\circ\), for the isomers at \(298 \text{ K}\) are given in the table below.

| Isomer | Structural formula | \(\Delta H_f^\circ\ (\text{kJ/mol})\) |
|---|---|---|
| \(\text{X}\) | \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\) | \(-146\) |
| \(\text{Y}\) | \((\text{CH}_3)_2\text{CHCH}_2\text{CH}_3\) | \(-154\) |
| \(\text{Z}\) | \(\text{C}(\text{CH}_3)_4\) | \(-168\) |

All three isomers undergo complete combustion with excess \(\text{O}_2\text{(g)}\) according to the following equation:
\[ \text{C}_5\text{H}_{12}\text{(g)} + 8\,\text{O}_2\text{(g)} \rightarrow 5\,\text{CO}_2\text{(g)} + 6\,\text{H}_2\text{O(l)} \]

Which of the following correctly ranks the isomers in order of the magnitude of heat released per mole of isomer completely combusted, from greatest to least, with the correct justification?

- **A.** Isomer \(\text{Z} >\) Isomer \(\text{Y} >\) Isomer \(\text{X}\), because isomer \(\text{Z}\) has the most negative \(\Delta H_f^\circ\), indicating that its chemical bonds store the greatest chemical potential energy to be released as heat during combustion.
- **B.** Isomer \(\text{Z} >\) Isomer \(\text{Y} >\) Isomer \(\text{X}\), because isomer \(\text{Z}\) has the most branched structure, which reduces intermolecular attractions and allows the combustion reaction to proceed with a lower activation energy.
- **C.** Isomer \(\text{X} >\) Isomer \(\text{Y} >\) Isomer \(\text{Z}\), because the unbranched chain in isomer \(\text{X}\) contains weaker individual \(\text{C}-\text{C}\) bonds that require more energy to break during the combustion process.
- **D.** Isomer \(\text{X} >\) Isomer \(\text{Y} >\) Isomer \(\text{Z}\), because isomer \(\text{X}\) has the least negative \(\Delta H_f^\circ\) and is therefore the least thermodynamically stable, placing it highest in enthalpy relative to the identical combustion products.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123841/*