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Ch.14 - Chemical Kinetics
All textbooksBrown 14th EditionCh.14 - Chemical KineticsProblem 75c
Chapter 14, Problem 75c

(c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?

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1
Understand the role of a catalyst: A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process.
Identify the effect on activation energy: Catalysts work by providing an alternative reaction pathway with a lower activation energy compared to the uncatalyzed reaction.
Analyze the effect on overall enthalpy change (ΔH): The overall enthalpy change for a reaction is determined by the properties of the reactants and products and is independent of the pathway taken from reactants to products.
Conclude the specific effect of catalysts: Catalysts affect the activation energy of a reaction but do not alter the overall enthalpy change of the reaction.
Apply this understanding: In practical applications, using a catalyst can make a reaction more feasible by lowering the energy barrier (activation energy), but it will not change the energy released or absorbed (enthalpy change) by the reaction.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy, allowing more reactant molecules to participate in the reaction at a given temperature.
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Catalyzed vs. Uncatalyzed Reactions

Enthalpy Change

Enthalpy change refers to the heat content change of a system during a chemical reaction at constant pressure. It is a state function that depends only on the initial and final states of the reactants and products, and is not influenced by the pathway taken, including the presence of a catalyst.
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Enthalpy of Formation

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. Catalysts lower the activation energy barrier, making it easier for reactants to convert into products, thus speeding up the reaction without altering the overall enthalpy change.
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Activity Series Chart
Related Practice
Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(a) Confirm that the elementary reactions add to give the overall reaction.

Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(b) Based on the experimentally determined rate law, which step is rate determining?

Textbook Question

(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst?

Textbook Question

(a) Most commercial heterogeneous catalysts are extremely finely divided solid materials. Why is particle size important?