---
title: "A student investigates the decomposition of a gaseous reactant, \\(\\text{X(g)}\\), catalyzed by a solid platinum surface, \\(\\text{Pt(s)}\\), in a rigid container of fixed volume at constant temperature: \\[ \\text{X(g)} \\xrightarrow{\\text{Pt(s)}} \\text{Y(g)} + \\text{Z(g)} \\] The accepted mechanism for the reaction consists of two elementary steps: \\[ \\text{Step 1: } \\text{X(g)} + \\text{Pt(surface)} \\rightleftharpoons \\text{X}\\cdot\\text{Pt(surface)} \\quad (\\text{fast equilibrium}) \\] \\[ \\text{Step 2: } \\text{X}\\cdot\\text{Pt(surface)} \\rightarrow \\text{Y(g)} + \\text{Z(g)} + \\text{Pt(surface)} \\quad (\\text{slow}) \\] At low initial partial pressures of \\(\\text{X(g)}\\), the rate of decomposition is experimentally observed to be first order with respect to \\(\\text{X}\\). Which of the following best predicts and explains the effect on the apparent order of the reaction with respect to \\(\\text{X}\\) if the initial partial pressure of \\(\\text{X(g)}\\) is increased to extremely high values while the total surface area of \\(\\text{Pt(s)}\\) remains constant?"
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url: "https://nerd-notes.com/ubq/123652/"
date_modified: "2026-09-28T12:01:54+00:00"
---

# A student investigates the decomposition of a gaseous reactant, \(\text{X(g)}\), catalyzed by a solid platinum surface, \(\text{Pt(s)}\), in a rigid container of fixed volume at constant temperature:
\[
\text{X(g)} \xrightarrow{\text{Pt(s)}} \text{Y(g)} + \text{Z(g)}
\]
The accepted mechanism for the reaction consists of two elementary steps:
\[
\text{Step 1: } \text{X(g)} + \text{Pt(surface)} \rightleftharpoons \text{X}\cdot\text{Pt(surface)} \quad (\text{fast equilibrium})
\]
\[
\text{Step 2: } \text{X}\cdot\text{Pt(surface)} \rightarrow \text{Y(g)} + \text{Z(g)} + \text{Pt(surface)} \quad (\text{slow})
\]
At low initial partial pressures of \(\text{X(g)}\), the rate of decomposition is experimentally observed to be first order with respect to \(\text{X}\). Which of the following best predicts and explains the effect on the apparent order of the reaction with respect to \(\text{X}\) if the initial partial pressure of \(\text{X(g)}\) is increased to extremely high values while the total surface area of \(\text{Pt(s)}\) remains constant?

A student investigates the decomposition of a gaseous reactant, \(\text{X(g)}\), catalyzed by a solid platinum surface, \(\text{Pt(s)}\), in a rigid container of fixed volume at constant temperature:
\[
\text{X(g)} \xrightarrow{\text{Pt(s)}} \text{Y(g)} + \text{Z(g)}
\]
The accepted mechanism for the reaction consists of two elementary steps:
\[
\text{Step 1: } \text{X(g)} + \text{Pt(surface)} \rightleftharpoons \text{X}\cdot\text{Pt(surface)} \quad (\text{fast equilibrium})
\]
\[
\text{Step 2: } \text{X}\cdot\text{Pt(surface)} \rightarrow \text{Y(g)} + \text{Z(g)} + \text{Pt(surface)} \quad (\text{slow})
\]
At low initial partial pressures of \(\text{X(g)}\), the rate of decomposition is experimentally observed to be first order with respect to \(\text{X}\). Which of the following best predicts and explains the effect on the apparent order of the reaction with respect to \(\text{X}\) if the initial partial pressure of \(\text{X(g)}\) is increased to extremely high values while the total surface area of \(\text{Pt(s)}\) remains constant?

- **A.** The order will become second order because high partial pressures increase the frequency of gas-phase collisions between \(\text{X(g)}\) molecules prior to surface adsorption.
- **B.** The order will become zero order because the activation energy of Step 2 decreases to zero when all available surface sites are occupied.
- **C.** The order will become zero order because virtually all catalytic sites become occupied, causing the concentration of \(\text{X}\cdot\text{Pt(surface)}\) to reach a constant maximum that no longer increases with higher \(P_{\text{X}}\).
- **D.** The order will remain first order because the rate constant of the rate-determining elementary step is an intensive property that is independent of \(P_{\text{X}}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123652/*