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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4:19 minutes

Problem 47a

Textbook Question

Calculate the ∆H° value for the following reaction:

Verified step by step guidance

Step 1: Identify the bonds broken and formed in the reaction. In the given reaction, CH₄ reacts with Cl₂ under UV light to form CH₃Cl and HCl. Bonds broken: one C-H bond in CH₄ and one Cl-Cl bond in Cl₂. Bonds formed: one C-Cl bond in CH₃Cl and one H-Cl bond in HCl.

Step 2: Use bond dissociation energy (BDE) values to calculate the energy required to break the bonds. Look up the BDE values for the C-H bond in CH₄ and the Cl-Cl bond in Cl₂. For example, the BDE for a C-H bond in CH₄ is approximately 412 kJ/mol, and for a Cl-Cl bond, it is approximately 243 kJ/mol.

Step 3: Use bond dissociation energy values to calculate the energy released when bonds are formed. Look up the BDE values for the C-Cl bond in CH₃Cl and the H-Cl bond in HCl. For example, the BDE for a C-Cl bond is approximately 338 kJ/mol, and for an H-Cl bond, it is approximately 431 kJ/mol.

Step 4: Calculate the total energy change (∆H°) for the reaction using the formula: ∆H° = (Sum of BDEs of bonds broken) - (Sum of BDEs of bonds formed). Substitute the values from Steps 2 and 3 into this formula.

Step 5: Interpret the sign of ∆H°. If the value of ∆H° is negative, the reaction is exothermic, meaning it releases energy. If the value is positive, the reaction is endothermic, meaning it absorbs energy.

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Key Concepts

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

Enthalpy Change (∆H°)

Enthalpy change (∆H°) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ∆H° < 0) or endothermic (absorbs heat, ∆H° > 0). Calculating ∆H° for a reaction involves using standard enthalpies of formation for the reactants and products, applying the formula ∆H° = Σ(∆H°f products) - Σ(∆H°f reactants).

Bond Dissociation Energy

Bond dissociation energy is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals or atoms. In the context of the given reaction, understanding the bond energies of C-H and Cl-Cl bonds is crucial for calculating the overall energy change during the reaction. The difference in energy between the bonds broken and formed determines the reaction's enthalpy change.

Photochemical Reactions

Photochemical reactions are chemical reactions that are initiated by the absorption of light. In the provided reaction, the presence of light (hν) facilitates the breaking of the Cl-Cl bond, leading to the formation of chlorine radicals that react with methane (CH4). Understanding the role of light in these reactions is essential for analyzing the mechanism and energy changes involved.

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Related Practice

Textbook Question

Without concerning yourself with the mechanism of the reaction, calculate the equilibrium constant for the following equilibrium processes. (Assume T = 298 K.)

(a)

Textbook Question

Keto–enol tautomerism is a reaction we discuss in detail in Chapter 19. Estimate the equilibrium constant of this reaction (BDE for C―C π bond = 65 kcal/mol ; for C―O π bond = 85 kcal/mol).

Textbook Question

Calculate ∆H° for the following equilibrium processes.

(a)

Textbook Question

Calculate ∆H° for the following equilibrium processes.

(d)

Textbook Question

To this point, hydrogenation has always been an exothermic process. Using the numbers from Figure 21.6, calculate ∆H^o_hydr for each step of the reduction of benzene.

∆H₁ + ∆H₂ + ∆H₃ = ∆H_tot = ―49.5 kcal /mol (―208 kJ/mol)

Multiple Choice

Predict the sign and magnitude of ∆H^oin kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.

Multiple Choice

If ΔH° is negative, what does this say about the favorability of a reaction?

Multiple Choice

Calculate ΔH° for the following reaction:

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