---
title: "A chemist investigates the reaction of reactant \\(\\text{R}\\), which can follow two competing pathways to yield either product \\(\\text{X}\\) or product \\(\\text{Y}\\), as represented in the reaction energy profile below.  When the reaction is carried out at \\(200\\text{ K}\\) for a brief duration, product \\(\\text{X}\\) is the major product formed. When the reaction is instead carried out at \\(500\\text{ K}\\) and allowed to reach dynamic equilibrium, product \\(\\text{Y}\\) is the major product. Which of the following statements best explains why product \\(\\text{Y}\\) predominates at \\(500\\text{ K}\\)?"
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url: "https://nerd-notes.com/ubq/121425/"
date_modified: "2026-08-23T05:02:13+00:00"
---

# A chemist investigates the reaction of reactant \(\text{R}\), which can follow two competing pathways to yield either product \(\text{X}\) or product \(\text{Y}\), as represented in the reaction energy profile below.

When the reaction is carried out at \(200\text{ K}\) for a brief duration, product \(\text{X}\) is the major product formed. When the reaction is instead carried out at \(500\text{ K}\) and allowed to reach dynamic equilibrium, product \(\text{Y}\) is the major product. Which of the following statements best explains why product \(\text{Y}\) predominates at \(500\text{ K}\)?

A chemist investigates the reaction of reactant \(\text{R}\), which can follow two competing pathways to yield either product \(\text{X}\) or product \(\text{Y}\), as represented in the reaction energy profile below.

When the reaction is carried out at \(200\text{ K}\) for a brief duration, product \(\text{X}\) is the major product formed. When the reaction is instead carried out at \(500\text{ K}\) and allowed to reach dynamic equilibrium, product \(\text{Y}\) is the major product. Which of the following statements best explains why product \(\text{Y}\) predominates at \(500\text{ K}\)?

![A reaction energy profile graph plotted on bare axes without gridlines. The vertical axis is labeled Potential Energy (kJ/mol) and the horizontal axis is labeled Reaction Coordinate. A horizontal reactant plateau at 0 kJ/mol is labeled R. A solid curve for Pathway to X rises from R to an activation barrier peak at 40 kJ/mol, then descends to a product plateau at -15 kJ/mol labeled X. A dashed curve for Pathway to Y rises from R to an activation barrier peak at 85 kJ/mol, then descends to a lower product plateau at -60 kJ/mol labeled Y. A legend in the upper-right corner contains two entries: a solid line labeled Pathway to X and a dashed line labeled Pathway to Y. No other lines, curves, labels, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787461333-Z1YzbK.jpg)

- **A.** The reaction operates under thermodynamic control at \(500\text{ K}\) because sufficient thermal energy is available to overcome both forward and reverse activation energy barriers, allowing the system to reach equilibrium where the lower free-energy product \(\text{Y}\) is favored.
- **B.** The reaction operates under kinetic control at \(500\text{ K}\) because increasing the temperature increases the rate constant of the higher activation energy pathway by a greater factor, causing product \(\text{Y}\) to form at a higher initial rate than product \(\text{X}\).
- **C.** The reaction operates under thermodynamic control at \(500\text{ K}\) because the activation energy barrier for the formation of product \(\text{Y}\) decreases as the temperature is raised, allowing \(\text{Y}\) to form more rapidly than at \(200\text{ K}\).
- **D.** The reaction operates under kinetic control at \(500\text{ K}\) because the reverse reaction from \(\text{Y}\) to \(\text{R}\) is completely nonspontaneous due to the large negative value of \(\Delta G^\circ\), irreversibly trapping all converted molecules as product \(\text{Y}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/121425/*