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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2:29 minutes

Problem 8a

Textbook Question

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

Verified step by step guidance

Step 1: Analyze the reaction. The reaction involves the catalytic cracking of n-decane (C₁₀H₂₂) into propene (C₃H₆) and heptane (C₇H₁₆). This process breaks a single large molecule into two smaller molecules.

Step 2: Consider the change in the number of molecules. The reactant is one molecule of n-decane, while the products are two molecules (propene and heptane). An increase in the number of molecules generally leads to an increase in entropy (ΔS° > 0).

Step 3: Evaluate the physical states of the reactants and products. n-Decane is a liquid at standard conditions, while propene is a gas and heptane is a liquid. The formation of a gaseous product (propene) from a liquid reactant contributes to an increase in entropy.

Step 4: Consider molecular complexity. Breaking a larger molecule into smaller fragments typically increases the disorder of the system, further supporting a positive ΔS°.

Step 5: Conclude the estimation. Based on the increase in the number of molecules, the formation of a gaseous product, and the increase in disorder, ΔS° for this reaction is likely positive.

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Key Concepts

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

Entropy (ΔS°)

Entropy, represented as ΔS°, is a measure of the disorder or randomness in a system. In chemical reactions, an increase in the number of gas molecules or a transition from a more ordered to a less ordered state typically results in a positive ΔS°, indicating greater disorder. Conversely, a decrease in the number of gas molecules or a transition to a more ordered state results in a negative ΔS°.

Catalytic Cracking

Catalytic cracking is a process used in the petroleum industry to break down large hydrocarbon molecules into smaller, more valuable products like gasoline and olefins. This reaction typically involves the use of heat and a catalyst, which facilitates the breaking of carbon-carbon bonds, leading to the formation of smaller alkanes and alkenes, such as propene and heptane in the given reaction.

Reaction Products and Their States

The products of a reaction can significantly influence the entropy change (ΔS°). In the provided reaction, n-decane (C10H22) is converted into propene (C3H6) and heptane (C7H16). Since the reaction produces a gaseous product (propene) alongside a liquid product (heptane), the overall disorder of the system increases, suggesting a positive ΔS° due to the generation of more gaseous molecules.

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Related Practice

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

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)

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

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.

Textbook Question

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

(b)

Textbook Question

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

(c)

Textbook Question

a. For which reaction in each set will ∆S° be more significant?

b. For which reaction will ∆S° be positive?

1. A ⇌ B or A + B ⇌ C

2. A ⇌ B + C or A + B ⇌ C + D

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