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

Entropy

Organic Chemistry

6. Thermodynamics and Kinetics

Entropy

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Problem 25b

Textbook Question

Calculate ∆G°, ∆H°, and ∆S° for the following acid–base reactions. Rationalize the value of ∆H° based on the structure of the conjugate bases. [Assume T = 298 K.]
(b)

Verified step by step guidance

Step 1: Identify the acid and base in the reaction. The molecule on the left (with the -OH group) acts as the acid, donating a proton to form the conjugate base (on the right, with the -O⁻ group). The molecule on the right (with the -NH₃⁺ group) acts as the conjugate acid formed from the base.

Step 2: Write the expression for ∆G° using the equation ∆G° = ∆H° - T∆S°. Here, T is given as 298 K. To calculate ∆G°, you need the values of ∆H° and ∆S°, which can be determined experimentally or provided in the problem.

Step 3: Rationalize the value of ∆H° based on the structure of the conjugate bases. The conjugate base formed (with the -O⁻ group) is stabilized by resonance, as the negative charge can delocalize onto the carbonyl oxygen. This stabilization lowers the energy of the conjugate base, contributing to the enthalpy change (∆H°).

Step 4: Consider the entropy change (∆S°). The reaction involves the transfer of a proton, which may increase disorder in the system. Evaluate the molecular changes to determine whether ∆S° is positive or negative.

Step 5: Combine the calculated or provided values of ∆H° and ∆S° into the equation ∆G° = ∆H° - T∆S° to determine the Gibbs free energy change for the reaction. This will indicate whether the reaction is thermodynamically favorable under standard conditions.

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Key Concepts

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

Gibbs Free Energy (∆G°)

Gibbs Free Energy (∆G°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It indicates the spontaneity of a reaction; a negative ∆G° suggests that the reaction can occur spontaneously, while a positive value indicates non-spontaneity. The relationship between ∆G°, enthalpy (∆H°), and entropy (∆S°) is given by the equation ∆G° = ∆H° - T∆S°, where T is the temperature in Kelvin.

Enthalpy (∆H°)

Enthalpy (∆H°) is a measure of the total heat content of a system and reflects the energy changes during a chemical reaction. It can be either exothermic (releasing heat, ∆H° < 0) or endothermic (absorbing heat, ∆H° > 0). The value of ∆H° can be rationalized by examining the stability and structure of the reactants and products, particularly the strength of bonds formed and broken during the reaction, which influences the overall energy change.

Entropy (∆S°)

Entropy (∆S°) is a measure of the disorder or randomness in a system. In chemical reactions, an increase in entropy (∆S° > 0) typically corresponds to a greater number of microstates or configurations available to the system, often seen in reactions that produce gases from solids or liquids. Understanding the changes in entropy is crucial for predicting the spontaneity of reactions, as it directly influences the Gibbs Free Energy equation.

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Related Practice

Textbook Question

(ii) Which side of the reaction is favored by entropy? (iii) If ∆S° = 0 for these reactions, calculate ∆G° (Assume T = 298 K) [BDE for O―H = 110 kcal /mol.]

(b)

Textbook Question

For the following values of ∆H° , ∆S°, and T, tell whether the process would be favored.

(b) ∆H° = +7.34 kcal/mol ; ∆S° = +43 cal/mol•K ; T = 325 K

Textbook Question

Considering the process described in Assessment 5.13, will it be favored or disfavored at a temperature higher than the one you calculated? How about at a temperature below what you calculated?

Textbook Question

For the following values of ∆H° , ∆S°, and T, tell whether the process would be favored.

(d) ∆H° = -8.3 kcal/mol ; ∆S° = -12 cal/mol•K ; T = 298 K

Textbook Question

When ethene is treated in a calorimeter with H₂ and a Pt catalyst, the heat of reaction is found to be –137 kJ/mol (–32.7 kcal/mol), and the reaction goes to completion. When the reaction takes place at 1400 K, the equilibrium is found to be evenly balanced, with K_eq =1. Compute the value of ΔS for this reaction.

Textbook Question

For the following values of ∆H° , ∆S°, and T, tell whether the process would be favored.

(a) ∆H° = -15 kcal/mol ; ∆S° = +37 cal/mol•K ; T = 273 K

Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(a)

Textbook Question

When ethene is mixed with hydrogen in the presence of a platinum catalyst, hydrogen adds across the double bond to form ethane. At room temperature, the reaction goes to completion. Predict the signs of ΔH° and ΔS° for this reaction. Explain these signs in terms of bonding and freedom of motion.

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Calculate ∆G°, ∆H°, and ∆S° for the following acid–base reactions. Rationalize the value of ∆H° based on the structure of the conjugate bases. [Assume T = 298 K.](b)

Key Concepts

Gibbs Free Energy (∆G°)

Enthalpy (∆H°)

Entropy (∆S°)

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Calculate ∆G°, ∆H°, and ∆S° for the following acid–base reactions. Rationalize the value of ∆H° based on the structure of the conjugate bases. [Assume T = 298 K.]
(b)