Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

6. Thermodynamics and Kinetics

Enthalpy

Organic Chemistry

6. Thermodynamics and Kinetics

Enthalpy

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5:39 minutes

Problem 14b,c

Textbook Question

•CH₃ + HCl → CH₄ + Cl•
b. What is the activation energy for this reverse reaction?
c. What is the heat of reaction (ΔH°) for this reverse reaction?

Verified step by step guidance

Step 1: Understand the reaction. The given reaction is a radical reaction where a methyl radical (•CH3) reacts with HCl to form methane (CH4) and a chlorine radical (Cl•). The reverse reaction involves CH4 and Cl• reacting to form •CH3 and HCl.

Step 2: To determine the activation energy for the reverse reaction, recall that the activation energy (Ea) for a reverse reaction can be calculated using the relationship: Ea(reverse) = Ea(forward) - ΔH°. Here, Ea(forward) is the activation energy for the forward reaction, and ΔH° is the enthalpy change of the reaction.

Step 3: To calculate ΔH° for the reverse reaction, use the bond dissociation energies (BDEs) of the bonds involved. For the forward reaction, the bonds broken and formed are: (1) Breaking the H-Cl bond and (2) Forming the C-H bond. ΔH° = Σ(BDE of bonds broken) - Σ(BDE of bonds formed). For the reverse reaction, the sign of ΔH° will be opposite to that of the forward reaction.

Step 4: Look up the bond dissociation energies (BDEs) for the H-Cl bond and the C-H bond. Use these values to calculate ΔH° for the forward reaction, then reverse the sign to find ΔH° for the reverse reaction.

Step 5: Substitute the values of Ea(forward) and ΔH° (calculated in the previous steps) into the equation Ea(reverse) = Ea(forward) - ΔH° to find the activation energy for the reverse reaction.

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Key Concepts

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

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 context of the reverse reaction, understanding the activation energy helps predict the rate at which the reaction can proceed and is crucial for calculating the energy changes involved.

Heat of Reaction (ΔH°)

The heat of reaction, or ΔH°, is the change in enthalpy that occurs during a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). For the reverse reaction, ΔH° can be determined by the difference in energy between the products and reactants, providing insight into the energy dynamics of the process.

Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway taken by reactants to form products. It includes the sequence of elementary steps, intermediates, and transition states involved in the reaction. Understanding the mechanism is essential for analyzing the activation energy and heat of reaction, as it reveals how the reaction proceeds and the energy changes at each stage.

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Related Practice

Textbook Question

Iodination of alkanes using iodine (I₂) is usually an unfavorable reaction. (See Problem 4-17 , for example.) Tetraiodomethane (CI₄) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔH for each of the steps in your proposed mechanism.

The following bond-dissociation energies maybe helpful:

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

a. CH₃—CH₃ + I₂ → CH₃CH₂I + HI

Textbook Question

The bromination of methane proceeds through the following steps:

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

Textbook Question

a. Using bond-dissociation enthalpies from Table 4-2. (page 167), calculate the heat of reaction for each step in the free-radical bromination of methane

b. Calculate the overall heat of the reaction.

Textbook Question

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.

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

b. CH₃CH₂Cl + HI → CH₃CH₂I + HCl

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

c. (CH₃)₃C—OH + HCl → (CH₃)₃C—Cl + H₂O

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

d. CH₃CH₂CH₃ + H₂ → CH₃CH₃ + CH₄

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•CH3 + HCl → CH4 + Cl• b. What is the activation energy for this reverse reaction? c. What is the heat of reaction (ΔH°) for this reverse reaction?

Key Concepts

Activation Energy

Heat of Reaction (ΔH°)

Reaction Mechanism

Watch next

•CH₃ + HCl → CH₄ + Cl•
b. What is the activation energy for this reverse reaction?
c. What is the heat of reaction (ΔH°) for this reverse reaction?