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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4:44 minutes

Problem 1b(i)

Textbook Question

(i) Using bond dissociation energies, calculate ∆H° for the following reactions. [BDE for O―H = 110 kcal /mol.]
(b)

Verified step by step guidance

Step 1: Understand the concept of bond dissociation energy (BDE). BDE is the energy required to break a bond in a molecule into its individual atoms in the gas phase. It is used to estimate the enthalpy change (∆H°) of a reaction by considering the bonds broken and formed during the reaction.

Step 2: Write down the chemical equation for the reaction. Identify all the bonds that are broken in the reactants and all the bonds that are formed in the products.

Step 3: Use the given bond dissociation energy (BDE) values. For example, the BDE for O―H is provided as 110 kcal/mol. Look up or use additional BDE values for other bonds involved in the reaction if necessary.

Step 4: Calculate the total energy required to break the bonds in the reactants. This is done by summing the BDE values for all the bonds broken.

Step 5: Calculate the total energy released when new bonds are formed in the products. Subtract the energy released (from bond formation) from the energy required (from bond breaking) to determine the overall ∆H° for the reaction.

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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 is the energy required to break a specific bond in a molecule, resulting in the formation of two radicals. It is a crucial concept in thermochemistry as it helps predict the stability of molecules and the energy changes during chemical reactions. Higher BDE values indicate stronger bonds, which require more energy to break.

Enthalpy Change (∆H°)

Enthalpy change (∆H°) refers to the heat content change of a system at constant pressure during a chemical reaction. It can be calculated using bond dissociation energies by summing the energies of bonds broken and subtracting the energies of bonds formed. Understanding ∆H° is essential for predicting whether a reaction is exothermic or endothermic.

Thermochemical Equations

Thermochemical equations represent the relationship between chemical reactions and the heat changes associated with them. They provide a way to quantify the energy changes in reactions, allowing chemists to calculate ∆H° using bond dissociation energies. These equations are vital for understanding reaction mechanisms and energy profiles.

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Related Practice

Textbook Question

Calculate ∆H° for the following equilibrium processes.

(b)

Textbook Question

The combustion of alkanes is exothermic (∆H° < 0) . Would you expect the combustion of butane or cyclobutane to be more exothermic?

Textbook Question

Reactions (a) and (b) are disfavored overall (∆G° > 0), yet they are favored based on ∆H°. Identify the bonds formed and broken for (a) and (b).

(b)

Textbook Question

a. Propose a mechanism for the following reaction:

b. Use the bond-dissociation enthalpies given in Table 4-2 (page 167) to calculate the value of ΔH° for each step shown in your mechanism. (The BDE for CH₂=CHCH₂―Br is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ΔH° for the reaction. Are these values consistent with a rapid free-radical chain reaction?

Textbook Question

(a) Using bond-dissociation energies (Table 5.6), which of the indicated bonds should break most easily?

(b) How does that help you explain the results shown in Figure 11.40?

Textbook Question

Given that ∆H° for the reaction is -42 kcal/mol and the bond dissociation enthalpies for the C−H, C−Cl, and O−H bonds are 101, 85, and 105 kcal/mol respectively, calculate the bond dissociation enthalpy of the O−Cl bond.

Textbook Question

Calculate ∆H° for the following reactions.

(b) CH₃Br + HCl → CH₃Cl + HBr

Textbook Question

Calculate ∆H° for the following reactions.

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