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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8:07 minutes

Problem 7

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)

Verified step by step guidance

Step 1: Understand the problem. The hydrogenation of benzene occurs in three steps, each involving the addition of H₂ to reduce one double bond. The total enthalpy change (∆Htot) for the complete reduction of benzene to cyclohexane is given as -49.5 kcal/mol (-208 kJ/mol). We need to calculate the enthalpy change (∆H₁, ∆H₂, ∆H₃) for each step.

Step 2: Recognize that the total enthalpy change is the sum of the enthalpy changes for each step: ∆H₁ + ∆H₂ + ∆H₃ = ∆Htot. This equation will guide the calculation.

Step 3: Note that benzene has resonance stabilization energy, which makes the first hydrogenation step (∆H₁) less exothermic compared to the subsequent steps. This is because breaking the aromaticity of benzene requires additional energy.

Step 4: Use the data from Figure 21.6 (if available) to determine the individual enthalpy changes for ∆H₁, ∆H₂, and ∆H₃. If the values for ∆H₁ or any other step are provided in Section 23.2.1, use them to solve for the remaining enthalpy changes.

Step 5: Rearrange the equation ∆H₁ + ∆H₂ + ∆H₃ = ∆Htot to solve for the unknown values. For example, if ∆H₁ is known, subtract it from ∆Htot to find the sum of ∆H₂ and ∆H₃, and then use additional data to determine their individual values.

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Key Concepts

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

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) to an unsaturated compound, typically alkenes or alkynes, converting them into saturated hydrocarbons. In the context of benzene, hydrogenation reduces the aromatic ring to form cyclohexane. This process is generally exothermic, meaning it releases energy, which is crucial for calculating the enthalpy changes involved.

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). In the hydrogenation of benzene, the total enthalpy change is the sum of the individual steps (ΔH₁, ΔH₂, ΔH₃), which must equal the overall change provided in the question.

Thermochemical Equations

Thermochemical equations represent the relationship between the heat absorbed or released during a chemical reaction and the stoichiometry of the reaction. They allow for the calculation of enthalpy changes for each step in a multi-step reaction, such as the hydrogenation of benzene. By knowing the total enthalpy change and the individual steps, one can solve for the unknown enthalpy changes (ΔH₁, ΔH₂, ΔH₃) in the reaction sequence.

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