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

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

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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
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Related Practice
Textbook Question

The reaction 2 NO1g2 + Cl21g2¡2 NOCl1g2 was performed and the following data were obtained under conditions of constant 3Cl24:

(a) Is the following mechanism consistent with the data? NO1g2 + Cl21g2ΔNOCl21g2 1fast2 NOCl21g2 + NO1g2¡2 NOCl1g2 1slow2

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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.

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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?

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Textbook Question

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

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Open Question
In solution, chemical species as simple as H+ and OH- can serve as catalysts for reactions. Imagine you could measure the [H+] of a solution containing an acid-catalyzed reaction as it occurs. Assume the reactants and products themselves are neither acids nor bases. Sketch the [H+] concentration profile you would measure as a function of time for the reaction, assuming t = 0 is when you add a drop of acid to the reaction.
Open Question
The oxidation of SO2 to SO3 is accelerated by NO2. The reaction proceeds according to: NO2(g) + SO2(g) → NO(g) + SO3(g) 2 NO(g) + O2(g) → 2 NO2(g) (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of SO2 by O2 to give SO3. (d) Is this an example of homogeneous catalysis or heterogeneous catalysis?