Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

Example 9.13

The two reaction energy diagrams in the margin can be used to describe four reactions: A

→

B; B

→

A; C

→

D; and D

→

C. Assume that the energy scales are the

same for the two curves and that

ΔS is very small for each process.

Energy

A

B

C

D^

a) Which process would have the greatest rate constant and which the smallest?

The reaction with the greatest rate constant is the one with the lowest activation energy, D →

C. The smallest rate constant would be

observed in the reaction with the greatest

activation energy, C

→

D.

b) Which process would be most extensive? Which would be the least extensive?

Because

ΔS is small, the extent of the reaction is dictated by

ΔH. Thus, the most

exothermic reaction is the most extensive, so D

→

C is the most extensive. The least

extensive process would be the most endothermic process, C

→

D.

Example 9.13

c) Increasing the temperature would increase the rates of which processes?

Increasing the temperature increases

the rates of all processes.

d) Increasing the temperature would increase the extent of which processes?

Increasing the temperature increases the fracti

on of molecules on the higher energy side

of the reaction energy diagram, so the forma

tion of B and D are favored by an increase in

temperature. Consequently, the processes A

→

B and C

→

D are favored by an increase

in temperature. However, the A

→

B process would be affected much less than the C

→

D

process because the energy difference in the A

→

B process is small.

In the previous examples, we considered

adding and removing gases or ions and

molecules in solution. The change in concen

tration changes the ratio of concentrations

given in the equilibrium constant expression so

that it no longer equals K. The equilibrium

must then shift to change the concentrations to values that re-establish the value of K. However, changing the amount of a pure solid

or liquid does not change its concentration,

which depends only upon its dens

ity. If no change in concentra

tion results, the equilibrium

does not shift. We conclude that

the addition or removal of a pure solid or liquid

component of the equilibrium does not

affect the equilibrium concentrations

.

Example 9.14

What affect would each of the following have on the equilibrium concentration of Ag

1+ ions in the following equilibrium: AgCN(s)

U

Ag

1+(aq) + CN

1-(aq)?

a) adding an acid (H

1+) to react with CN

1- ions and produce HCN molecules.

Removal of CN

1- ions from the equilibrium would result in more AgCN dissolving to

produce Ag

1+ ions. The Ag

1+ ion concentration would increase.

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State

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