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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.4 - The Study of Chemical ReactionsProblem 17a,b
Chapter 4, Problem 17a,b

a. Using the BDEs in Table 4-2 (page 167), compute the value of ΔH° for each step in the iodination of methane.
b. Compute the overall value of ΔH° for iodination.

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1
Step 1: Understand the problem. The iodination of methane involves breaking and forming bonds. To compute ΔH° for each step, you need to use bond dissociation energies (BDEs) provided in [TABLE 4-2]. BDEs represent the energy required to break a bond in a molecule. The overall ΔH° is the sum of the ΔH° values for all steps in the reaction.
Step 2: Write the chemical equation for the iodination of methane. The reaction involves CH₄ (methane) reacting with I₂ (iodine) to form CH₃I (methyl iodide) and HI (hydrogen iodide). Break down the reaction into individual steps: (1) breaking the C-H bond in CH₄, (2) breaking the I-I bond in I₂, (3) forming the C-I bond in CH₃I, and (4) forming the H-I bond in HI.
Step 3: Use the BDEs from [TABLE 4-2] to calculate ΔH° for each step. For example: (1) Find the BDE for the C-H bond in CH₄ and use it to calculate the energy required to break this bond. (2) Find the BDE for the I-I bond in I₂ and calculate the energy required to break this bond. (3) Find the BDE for the C-I bond in CH₃I and calculate the energy released when this bond forms. (4) Find the BDE for the H-I bond in HI and calculate the energy released when this bond forms.
Step 4: Compute the ΔH° for each step using the formula ΔH° = Σ(BDE of bonds broken) - Σ(BDE of bonds formed). For each step, subtract the energy released (bonds formed) from the energy required (bonds broken). Ensure you use the correct BDE values from the table.
Step 5: Add the ΔH° values for all steps to compute the overall ΔH° for the iodination of methane. This sum represents the net energy change for the reaction. If the value is negative, the reaction is exothermic; if positive, the reaction is endothermic.

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Key Concepts

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

Bond Dissociation Energy (BDE)

Bond Dissociation Energy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals or atoms. It is a crucial concept in thermochemistry, as it helps predict the stability of molecules and the energy changes during chemical reactions. In the context of iodination of methane, BDE values are used to calculate the enthalpy changes associated with bond breaking and forming.
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Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) refers to the heat content change of a system at constant pressure during a chemical reaction. It can be calculated by summing the BDEs of bonds broken and formed in the reaction. Understanding ΔH° is essential for evaluating the thermodynamic favorability of a reaction, such as the iodination of methane, where the overall enthalpy change indicates whether the reaction is exothermic or endothermic.
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Iodination of Methane

Iodination of methane is a radical substitution reaction where iodine replaces one of the hydrogen atoms in methane (CH₄). This process typically involves the formation of methyl radicals and iodine radicals, leading to the generation of iodomethane (CH₃I). Analyzing the enthalpy changes during this reaction requires an understanding of the steps involved, including initiation, propagation, and termination, which are influenced by the BDEs of the involved bonds.
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Related Practice
Textbook Question

Using the BDEs in Table 4-2 (page 167), 

(c) Suggest two reasons why iodine does not react well with methane.

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Textbook Question

Classify each hydrogen atom in the following compounds as primary (1°), secondary (2°), or tertiary (3°).

a. butane

b. isobutane

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Textbook Question

The bromination of methane proceeds through the following steps:

d. Compute the overall value of ΔH° for the bromination

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Textbook Question

The bromination of methane proceeds through the following steps:

a. Draw a complete reaction-energy diagram for this reaction.

b. Label the rate-limiting step.

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Textbook Question

•CH3 + Cl2 → CH3Cl + Cl•

The activation energy is 4 kJ/mol (1 kcal/mol), and the overall ΔH° for the reaction is –110 kJ/mol (–27 kcal/mol).

b. Give the equation for the reverse reaction.

c. What is the activation energy for the reverse reaction?

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Textbook Question

What would be the product ratio in the chlorination of propane if all the hydrogens were abstracted at equal rates?

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