6. Thermodynamics and Kinetics

For the following equilibrium processes and the corresponding ∆G°, indicate whether you expect the equilibrium constant to be greater than, equal to, or less than 1. Justify your expectation in words.

(a)

For the following acid–base reactions studied in Assessment 5.25, draw a likely transition state. Be sure to indicate in your drawing the degree to which bonds are broken or formed.

(a)

Write the rate law for the following reaction and identify which molecules are present in the rate-determining step. Draw a possible transition state and propose a mechanism.

Calculate ∆G° for the conversion of “axial” methylcyclohexane to “equatorial” methylcyclohexane at 25 °C.

For the following acid–base reactions studied in Assessment 5.25, draw a likely transition state. Be sure to indicate in your drawing the degree to which bonds are broken or formed.

(c) H₃O⁺ + Br^– ⇌ H₂O + HBr

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

(c) ∆H° = -21.3 kcal/mol ; AS° = -51 cal/mol•K ; T = 373 K

For the following acid–base reaction, (e) calculate ∆G° at 273 K.

For the following reactions we have not seen yet, which side, if either, would be favored by increasing the temperature?

(d)

Third-order reactions are rare. Why do you think that is?

Looking ahead In Chapter 5, we explain that the equilibrium constant (K_eq) for a reaction can be calculated based on the difference in energy between reactants and products, according to the following equation:

$K_{eq}=e^{-\frac{\Delta E}{RT}}$

Using this equation, calculate the equilibrium constant for the 'reaction' shown. [For the rest of the book, if not otherwise specified, assume room temperature (298K).]

A certain process has ∆H° = 11.7 kcal/mol and AS° = +33cal/mol•K . That is, this reaction has an unfavorable enthalpy but a favorable entropy term. At what temperature will the process be neither favored nor disfavored?

For each pair of reactions, predict which will happen more quickly.

[For (a) and (b), think about the stability of the bases involved.]

(a)

(b) Mechanistically, the reaction occurs as shown below. Why is this reaction favored? Based on the stability of the anions, estimate K_eq.

In a reaction in which reactant A is in equilibrium with product B at 25 °C, what relative amounts of A and B are present at equilibrium if ∆G° at 25 °C is

a. 2.72 kcal/mol?

b. 0.65 kcal/mol?