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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5:12 minutes

Problem 41d

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

Verified step by step guidance

Identify the bonds broken and formed in the reaction. In this case, the bonds broken are one C-H bond in CH3CH2CH3 (propane) and one H-H bond in H2. The bonds formed are one C-H bond in CH3CH3 (ethane) and one C-H bond in CH4 (methane).

Refer to the bond-dissociation enthalpy table (TABLE 4-2, p. 167) to find the bond-dissociation enthalpy values for each bond involved. For example, the bond-dissociation enthalpy for a C-H bond and an H-H bond will be listed in the table.

Calculate the total energy required to break the bonds. This is done by summing the bond-dissociation enthalpy values for the bonds broken (C-H in propane and H-H in H2).

Calculate the total energy released when the new bonds are formed. This is done by summing the bond-dissociation enthalpy values for the bonds formed (C-H in ethane and C-H in methane).

Determine the overall enthalpy change (ΔH°) for the reaction by subtracting the total energy released (bonds formed) from the total energy required (bonds broken). Use the formula: ΔH° = Σ(bonds broken) - Σ(bonds formed).

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Key Concepts

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

Bond-Dissociation Enthalpy

Bond-dissociation enthalpy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals. It is a crucial concept in thermochemistry, as it helps predict the stability of molecules and the energy changes during chemical reactions. BDE values are typically expressed in kilojoules per mole (kJ/mol) and vary depending on the type of bond and the molecular environment.

Enthalpy Change (ΔH°)

The enthalpy change (ΔH°) of a reaction is the difference in enthalpy between the products and reactants under standard conditions. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). Calculating ΔH° using bond-dissociation enthalpies involves summing the BDEs of bonds broken in the reactants and subtracting the BDEs of bonds formed in the products.

Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. Understanding the mechanism is essential for predicting the products and energy changes in a reaction. In the context of the given reaction, recognizing the bonds involved and their respective BDEs allows for accurate calculation of the overall enthalpy change, reflecting the energy dynamics of the process.

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Related Practice

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?

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

If the following reaction is favorable, what can we say about the sign of ∆H°? Explain your answer.

Textbook Question

Give the approximate bond-dissociation energy for each indicated bond.

Textbook Question

Calculate ∆H° for the following alkene addition reaction, one we discuss further in Chapter 7. Predict the sign of ∆S° . (The BDE for C―C π bond is approximately 65 kcal/mol.)

Textbook Question

Reaction (c), on the other hand, is favored (∆G° < 0). Identify the bonds formed and broken and explain this result in light of (a) and (b).

(c)

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