6. Thermodynamics and Kinetics

Chapter 5 taught us that chemical reactions are random collisions. Experimentally, how can we make molecules collide more often?

The molecule A undergoes a three-step reaction to make B, where the first step is rate determining. Conversely, C can be produced by A in a two-step reaction where the second step is rate determining. Show a reaction coordinate diagram for each reaction, making it clear that the reaction to make C is faster than the reaction to make B.

Parts (a)–(f) of this assessment refer to the rotation around the single bond of ethane.

(b) What is the order of the reaction with regard to ethane?

Within the following pairs, pick which reaction you would expect to be faster based on having a higher value of the frequency factor (A).

(a)

Draw an approximate reaction-energy diagram showing the curves for the two possible pathways for ionic addition of HBr to 1-methylcyclohexene. (a) Formation of the major product, 1-bromo-1-methylcyclohexane, and (b) formation of the minor product, 1-bromo-2-methylcyclohexane. Point out how these curves show that 1-bromo-1-methylcyclohexane should be formed faster.

Draw a reaction coordinate diagram for a one step reaction that has the following values of E_a and ∆H. (b) E_a = 2 kcal/mol ; ∆H° = -17 kcal/mol

Using Equation 5.8, calculate the difference in transition state energies that lead to the rate differences shown in Figure 24.35.

Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( E_a) , and ∆G°, for each of the following.

(d) a slightly exothermic, three-step reaction where the third step is rate-determining.

The reaction of tert-butyl chloride with methanol

is found to follow the rate equation

rate = k_r[(CH₃)₃C—Cl]

c. What is the kinetic order overall

Treatment of tert-butyl alcohol with concentrated HCl gives tert-butyl chloride.

When the concentration of H⁺ is doubled, the reaction rate doubles. When the concentration of tert-butyl alcohol is tripled, the reaction rate triples. When the chloride ion concentration is quadrupled, however, the reaction rate is unchanged. Write the rate equation for this reaction.

The bromination of methane proceeds through the following steps:

a. Draw a complete reaction-energy diagram for this reaction.

b. Label the rate-limiting step.

How will the rate of the reaction between bromomethane and hydroxide ion be affected if the following changes in concentration are made?

b. The concentration of the alkyl halide is cut in half and the concentration of the nucleophile is not changed.

(a) On a reaction coordinate diagram, show why a general, one-step reaction is faster when a catalyst is employed.

(b) Why does a catalyst often allow a reaction to proceed at lower temperatures?

Parts (a)–(f) of this assessment refer to the rotation around the single bond of ethane.

(c) Write the rate law for this reaction.