Ch.15 - Chemical Kinetics

Tro - Chemistry: A Molecular Approach 6th Edition

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Tro 6th Edition

Ch.15 - Chemical Kinetics

Problem 86

Chapter 15, Problem 86

Suppose that the reaction A¡products is exothermic and has an activation barrier of 75 kJ/mol. Sketch an energy diagram showing the energy of the reaction as a function of the progress of the reaction. Draw a second energy curve showing the effect of a catalyst.

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Start by drawing a graph with the y-axis labeled as 'Energy' and the x-axis labeled as 'Reaction Progress'.

Plot the energy of the reactants (A) at a certain level on the y-axis, indicating the starting point of the reaction.

Draw a curve that rises to a peak, representing the activation energy barrier of 75 kJ/mol, and then falls to a lower energy level than the reactants, indicating an exothermic reaction.

Label the peak of the curve as the 'Transition State', and the difference in energy between the reactants and the products as the 'Enthalpy Change (ΔH)', which is negative for an exothermic reaction.

Draw a second curve with a lower peak than the first, representing the effect of a catalyst, which lowers the activation energy but does not change the initial and final energy levels.

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Key Concepts

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

Exothermic Reactions

Exothermic reactions are chemical processes that release energy, usually in the form of heat, to the surroundings. This occurs when the total energy of the products is lower than that of the reactants, resulting in a negative change in enthalpy (ΔH). In the context of the given question, the reaction A → products releases energy, indicating that the products are more stable than the reactants.

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to transform into products. In the question, the activation barrier of 75 kJ/mol indicates the energy needed to initiate the exothermic reaction, which is depicted as a peak in the energy diagram.

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. In the energy diagram, the effect of a catalyst is shown as a second curve that has a lower peak than the uncatalyzed reaction, illustrating that the catalyst reduces the energy barrier for the reaction to proceed.

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Catalyzed vs. Uncatalyzed Reactions

Related Practice

Textbook Question

Many heterogeneous catalysts are deposited on high-surfacearea supports. Why?

Consider this three-step mechanism for a reaction:

Cl₂ (g) k₁⇌k₂ 2 Cl (g) Fast

Cl (g) + CHCl₃ (g) →k₃ HCl (g) + CCl₃ (g) Slow

Cl (g) + CCl₃ (g) →k₄ CCl₄ (g) Fast

c. What is the predicted rate law?

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Textbook Question

The activation barrier for the hydrolysis of sucrose into glucose and fructose is 108 kJ/mol. If an enzyme increases the rate of the hydrolysis reaction by a factor of 1 million, how much lower must the activation barrier be when sucrose is in the active site of the enzyme? (Assume that the frequency factors for the catalyzed and uncatalyzed reactions are identical and a temperature of 25 °C.)

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