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Ch.15 - Chemical Kinetics
All textbooksTro 5th EditionCh.15 - Chemical KineticsProblem 96
Chapter 15, Problem 96

Is the overall reaction exothermic or endothermic in the mechanism where HCl adds across the double bond of ethene to form H3C¬CH2Cl, with the energy diagram indicating step 1 as HCl + H2C“CH2 → H3C“CH2+ + Cl⁻ and step 2 as H3C“CH2+ + Cl⁻ → H3C¬CH2Cl?

Verified step by step guidance
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insert step 1> Identify the type of reaction: The reaction involves the addition of HCl across the double bond of ethene, which is a typical electrophilic addition reaction.
insert step 2> Analyze the energy diagram: Look at the energy levels of the reactants, intermediates, and products. The energy diagram will show whether the overall energy of the products is lower or higher than the reactants.
insert step 3> Determine the energy change for each step: Step 1 involves the formation of a carbocation intermediate (H3C-CH2+), and Step 2 involves the combination of this carbocation with Cl⁻ to form the final product (H3C-CH2Cl).
insert step 4> Assess the energy of the intermediate: The formation of a carbocation is typically endothermic, as it requires energy to break the π bond and form the carbocation.
insert step 5> Evaluate the overall energy change: Compare the energy of the final product (H3C-CH2Cl) with the initial reactants (HCl + H2C=CH2). If the energy of the products is lower, the reaction is exothermic; if higher, it is endothermic.

Key Concepts

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

Exothermic and Endothermic Reactions

Exothermic reactions release energy, usually in the form of heat, to the surroundings, resulting in a temperature increase. In contrast, endothermic reactions absorb energy, leading to a temperature decrease. The classification of a reaction as exothermic or endothermic depends on the overall energy change from reactants to products.
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Energy Diagrams

Energy diagrams visually represent the energy changes during a chemical reaction. They typically show the energy of reactants and products, as well as the activation energy required for the reaction to proceed. The shape of the diagram helps determine whether the reaction is exothermic or endothermic based on the relative energy levels of the reactants and products.
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Reaction Mechanism

A reaction mechanism outlines the step-by-step sequence of elementary reactions that occur during a chemical transformation. Each step may involve the formation of intermediates and transition states, which can influence the overall energy change of the reaction. Understanding the mechanism is crucial for determining the energy profile and whether the overall reaction is exothermic or endothermic.
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Related Practice
Textbook Question

Consider this energy diagram:

a. How many elementary steps are involved in this reaction?

Textbook Question

Consider this energy diagram:

d. Is the overall reaction endothermic or exothermic?

Textbook Question

Consider the reaction in which HCl adds across the double bond of ethene: HCl + H2C=CH2 → H3C-CH2Cl The following mechanism, with the accompanying energy diagram, has been suggested for this reaction:

Step 1 HCl + H2C=CH2 → H3C=CH2+ + Cl-

Step 2 H3C=CH2+ + Cl- → H3C-CH2Cl

a. Based on the energy diagram, determine which step is rate limiting.

Textbook Question

Consider the reaction in which HCl adds across the double bond of ethene: HCl + H2C=CH2 → H3C-CH2Cl The following mechanism, with the accompanying energy diagram, has been suggested for this reaction:

Step 1 HCl + H2C=CH2 → H3C=CH2+ + Cl-

Step 2 H3C=CH2+ + Cl- → H3C-CH2Cl

b. What is the expected order of the reaction based on the proposed mechanism?