---
title: "A proposed three-step mechanism for the gas-phase reaction \\[ \\text{X}(g) \\rightarrow \\text{Z}(g) \\] proceeds via two reaction intermediates, \\( \\text{Y}_1(g) \\) and \\( \\text{Y}_2(g) \\), through transition states \\( \\text{TS}_1 \\), \\( \\text{TS}_2 \\), and \\( \\text{TS}_3 \\). The potential energy profile for the reaction is shown below.  Which of the following correctly ranks the species from greatest to least thermodynamic stability, and identifies the rate-determining step for the forward reaction?"
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url: "https://nerd-notes.com/ubq/123708/"
date_modified: "2026-09-28T12:02:05+00:00"
---

# A proposed three-step mechanism for the gas-phase reaction \[ \text{X}(g) \rightarrow \text{Z}(g) \] proceeds via two reaction intermediates, \( \text{Y}_1(g) \) and \( \text{Y}_2(g) \), through transition states \( \text{TS}_1 \), \( \text{TS}_2 \), and \( \text{TS}_3 \). The potential energy profile for the reaction is shown below.

Which of the following correctly ranks the species from greatest to least thermodynamic stability, and identifies the rate-determining step for the forward reaction?

A proposed three-step mechanism for the gas-phase reaction \[ \text{X}(g) \rightarrow \text{Z}(g) \] proceeds via two reaction intermediates, \( \text{Y}_1(g) \) and \( \text{Y}_2(g) \), through transition states \( \text{TS}_1 \), \( \text{TS}_2 \), and \( \text{TS}_3 \). The potential energy profile for the reaction is shown below.

Which of the following correctly ranks the species from greatest to least thermodynamic stability, and identifies the rate-determining step for the forward reaction?

![A grayscale reaction energy profile plotted on a two-dimensional Cartesian plane. The horizontal axis is labeled 'Reaction Coordinate' and has no numerical ticks. The vertical axis is labeled 'Potential Energy (kJ/mol)' with numerical tick marks at 0, 20, 40, 60, 80, 100, and 120. A single continuous solid black curve depicts a three-step mechanism: the curve begins with a flat plateau at 40 labeled '\( \text{X} \)', rises to a first smooth peak at 110 labeled '\( \text{TS}_1 \)', descends to a local minimum at 80 labeled '\( \text{Y}_1 \)', rises to a second peak at 100 labeled '\( \text{TS}_2 \)', descends to a second local minimum at 20 labeled '\( \text{Y}_2 \)', rises to a third peak at 110 labeled '\( \text{TS}_3 \)', and descends to a final flat plateau at 0 labeled '\( \text{Z} \)'. No other curves, gridlines, labels, text, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790596925-jjPrhB.jpg)

- **A.** Stability: \( \text{Y}_1 > \text{X} > \text{Y}_2 > \text{Z} \) Rate-determining step: Step 1 (\( \text{X} \rightarrow \text{Y}_1 \)) because it is the first elementary step in the reaction pathway.
- **B.** Stability: \( \text{Y}_1 > \text{X} > \text{Y}_2 > \text{Z} \) Rate-determining step: Step 3 (\( \text{Y}_2 \rightarrow \text{Z} \)) because it has the largest forward activation energy barrier.
- **C.** Stability: \( \text{Z} > \text{Y}_2 > \text{X} > \text{Y}_1 \) Rate-determining step: Step 3 (\( \text{Y}_2 \rightarrow \text{Z} \)) because it has the largest forward activation energy barrier.
- **D.** Stability: \( \text{Z} > \text{Y}_2 > \text{X} > \text{Y}_1 \) Rate-determining step: Step 1 (\( \text{X} \rightarrow \text{Y}_1 \)) because it is the first elementary step in the reaction pathway.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123708/*