---
title: "The potential energy profile for a two-step reaction, \\(\\text{A}(g) + 2\\,\\text{B}(g) \\rightarrow \\text{AB}_2(g)\\), is shown in the diagram. Which step is the rate-determining step of the forward reaction, and what is the correct justification?"
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url: "https://nerd-notes.com/ubq/119727/"
date_modified: "2026-08-21T08:12:10+00:00"
---

# The potential energy profile for a two-step reaction, \(\text{A}(g) + 2\,\text{B}(g) \rightarrow \text{AB}_2(g)\), is shown in the diagram. Which step is the rate-determining step of the forward reaction, and what is the correct justification?

The potential energy profile for a two-step reaction, \(\text{A}(g) + 2\,\text{B}(g) \rightarrow \text{AB}_2(g)\), is shown in the diagram. Which step is the rate-determining step of the forward reaction, and what is the correct justification?

![A reaction energy profile rendered as a solid black curve on bare, perpendicular coordinate axes. The vertical axis is labeled \(\text{Potential Energy (kJ/mol)}\) with tick marks at \(10\), \(30\), \(50\), \(110\), and \(120\). The horizontal axis is labeled \(\text{Reaction Progress}\). The curve starts at a horizontal plateau at \(50\text{ kJ/mol}\) labeled Reactants, rises smoothly to a first maximum at \(120\text{ kJ/mol}\) labeled \(\text{TS}_1\), descends to a local minimum plateau at \(30\text{ kJ/mol}\) labeled Intermediate, rises to a second maximum at \(110\text{ kJ/mol}\) labeled \(\text{TS}_2\), and finally descends to a horizontal plateau at \(10\text{ kJ/mol}\) labeled Products. Horizontal dashed reference lines extend from each feature to the vertical axis ticks. No other curves, labels, shading, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787299929-hczUQU.jpg)

- **A.** Step \(1\), because its transition state is at a higher absolute potential energy (\(120\text{ kJ/mol}\)) than that of Step \(2\) (\(110\text{ kJ/mol}\)).
- **B.** Step \(2\), because its activation energy (\(80\text{ kJ/mol}\)) is greater than the activation energy of Step \(1\) (\(70\text{ kJ/mol}\)).
- **C.** Step \(1\), because its activation energy (\(70\text{ kJ/mol}\)) is greater than the activation energy of Step \(2\) (\(60\text{ kJ/mol}\)).
- **D.** Step \(2\), because the overall reaction is exothermic with \(\Delta H^\circ = -40\text{ kJ/mol}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119727/*