6. Thermodynamics and Kinetics

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

(a)

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.

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

(b)

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

(c)

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

At what temperature does the entropy change of a process not contribute to the favorability of a process?

The hydrogenation of alkenes is a reaction we study in Chapter 9.

(b) Is this reaction favored or disfavored in terms of entropy?

Through the course of this chapter, we have discussed only alkane chlorination and bromination, yet there are two other halogens we have not discussed.

(b) Is radical iodination a favorable reaction? Do you expect it to be selective? Show your calculations.

The dehydrogenation of butane to trans-but-2-ene has ΔH° = +116 kJ/mol (+27.6 kcal/mol) and ΔS° = +117J/kelvin-mol (+28.0 cal/kelvin-mol).

a. Compute the value of ΔG° for dehydrogenation at room temperature (25 °C or 298 °K). Is dehydrogenation favored or disfavored?

HINT: When you are doing synthesis problems, avoid using these high-temperature industrial methods. They require specialized equipment, and they produce variable mixtures of products.

Estimate the value of entropy (∆S > 0 or ∆S < 0) for the elimination step shown.

Would you expect ∆S to be greater than, less than, or equal to zero in the following reactions?

(b)

For each of the following processes, indicate whether you expect ∆S° to be greater than, less than, or equal to 0. Explain your answer.

(d)

For each of the following processes, indicate whether you expect ∆S° to be greater than, less than, or equal to 0. Explain your answer.

(c)

For each of the following processes, indicate whether you expect ∆S° to be greater than, less than, or equal to 0. Explain your answer.

(b) 2 C₆H₁₄(l) + 19 O₂(g) → 12 CO₂(g) + 14 H₂O(g)