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Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and KineticsProblem 21
Chapter 6, Problem 21

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

Verified step by step guidance
1
Step 1: Understand the concept of entropy (∆S°). Entropy is a measure of the disorder or randomness in a system. A positive ∆S° indicates an increase in disorder, while a negative ∆S° indicates a decrease in disorder.
Step 2: Analyze the first set of reactions: A → B versus A + B → C. In the first reaction, a single molecule (A) is converted into another single molecule (B), so there is no significant change in the number of particles or disorder. In the second reaction, two molecules (A and B) combine to form one molecule (C), which decreases the number of particles and reduces disorder. Therefore, the first reaction will have a more significant ∆S°.
Step 3: Determine which reaction in the first set will have a positive ∆S°. Since the first reaction (A → B) does not involve a change in the number of particles, ∆S° is likely to be close to zero or slightly positive. The second reaction (A + B → C) involves a decrease in the number of particles, so ∆S° will be negative. Thus, the first reaction is more likely to have a positive ∆S°.
Step 4: Analyze the second set of reactions: A → B + C versus A + B → C + D. In the first reaction, one molecule (A) breaks into two molecules (B and C), increasing the number of particles and disorder. In the second reaction, two molecules (A and B) combine to form two molecules (C and D), so the number of particles remains constant, and there is no significant change in disorder. Therefore, the first reaction will have a more significant ∆S°.
Step 5: Determine which reaction in the second set will have a positive ∆S°. In the first reaction (A → B + C), the number of particles increases, leading to an increase in disorder and a positive ∆S°. In the second reaction (A + B → C + D), the number of particles remains constant, so ∆S° is likely to be close to zero. Thus, the first reaction is more likely to have a positive ∆S°.

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Key Concepts

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

Entropy (∆S°)

Entropy, denoted as ∆S°, is a measure of the disorder or randomness in a system. In chemical reactions, a positive change in entropy indicates an increase in disorder, often associated with the formation of more molecules or the breaking of bonds. Understanding how to evaluate changes in entropy is crucial for predicting the spontaneity of reactions.
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Reaction Types

The two types of reactions presented are equilibrium reactions (A <--> B) and combination reactions (A + B <--> C). Equilibrium reactions involve the conversion of reactants to products and vice versa, while combination reactions involve the formation of a single product from multiple reactants. Recognizing these types helps in analyzing how entropy changes in each scenario.
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Significance of ∆S° in Reactions

The significance of ∆S° in a reaction can be assessed by considering the number of molecules involved. Generally, reactions that produce more gas molecules or involve the dissociation of a compound tend to have a greater positive ∆S°. This concept is essential for determining which reaction will have a more significant entropy change and whether that change will be positive.
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Related Practice
Textbook Question

Calculate the percentage of isopropylcyclohexane molecules that have the isopropyl substituent in an equatorial position at equilibrium. (Its ∆G° value at 25 °C is -2.1 kcal/mol.)

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Textbook Question

a. For a reaction with ∆H° = -12 kcal/mol and ∆S° = 0.01 kcal mol-1 K-1, calculate the ∆G° and the equilibrium constant at:

1. 30 °C and 2. 150 °C.

b. How does ∆G° change as T increases?

c. How does Keq change as T increases?

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Textbook Question

What alkene would you start with if you wanted to synthesize

b. ethylcyclopentane?

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Textbook Question

What alkene would you start with if you wanted to synthesize

a. pentane?

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Textbook Question

Why is the percentage of molecules with the substituent in an equatorial position greater for isopropylcyclohexane than for fluorocyclohexane?

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Textbook Question

The ∆G° for conversion of “axial” fluorocyclohexane to “equatorial” fluorocyclohexane at 25 °C is -0.25kcal/mol. Calculate the percentage of fluorocyclohexane molecules that have the fluoro substituent in an equatorial position at equilibrium.

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