Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

TEMPERATURE DEPENDENCE OF K Increasing the temperature increases thermal energy, which increases the fraction of molecules at higher energy. The reactants are at higher energy in an exothermic reaction (Figure 9.9a), but the products are at higher energy in an endothermic reaction (Figure 9.9b). Thus, increasing the temperature of an endothermic reaction increases the concentration of the products and decrease

s that of the reactants. Increasing the

temperature of an exothermic reaction incr

eases the concentration of the reactants and

decreases that of the products. Products ar

e in the numerator and reactants in the

denominator of K, so we conclude that

reducing the temperature increases K for an

exothermic reaction, but decreases

K for an endothermic reaction

. Note that the

activation energy dictates the speed with which

equilibrium is attained (kinetics), while the

enthalpy change dictates the extent of reaction when

S is negligible (thermodynamics). Δ

Reactant

Reactant

Product

Product

Higher energyfavored byincreasing T

Higher energyfavored byincreasing T

Lower energy favored by decreasing T

Energy

(a) Exothermic Reaction

(b) Endothermic Reaction

Figure 9.9 Reaction diagrams for an exothermic (a) and an endothermic reaction (b)

9.12

LE CHÂTELIER’S PRINCIPLE Reactions are usually run so as to maximize the ratio of the product concentration to the concentrations of one of the reactants (usually

the most expensive). For example, in the

synthesis of methanol (CH

OH) from methyl iodide (CH 3

I) with the following reaction, 3

CH

I + OH 3

1-^

U

CH

OH + I 3

1-, the conditions would be adjusted to maximize the

[CH

OH]/[CH 3

I] ratio in the final mixture. Th 3

e equilibrium constant expression is

1-

3

1-

3

[CH OH][I ]

K =

[CH I][OH ]

which can be solved for the [CH

OH]/[CH 3

I] ratio as follows: 3

1-

3

1-

3

[CH OH]

K[OH ]

=

[CH I]

[I ]

Thus, one way to increase the ratio is to increase K. The reaction is exothermic, so K can be increased by decreasing the temperature, but decreasing the temperature also decreases the rate of reaction. Consequently, the ratio

is usually changed by either adding more

hydroxide ion (increasing the numerator) or removing iodide ion (decreasing the denominator) as it is formed. Additional hydroxide ion reacts with more of the CH

I to 3

produce CH

OH. Adding a cation, such at Tl 3

1+, that precipitates iodide but not hydroxide

ion, would remove iodide ion from solution. In the absence of iodide ion, more CH

I and 3

OH

1- would have to react to produce more products and re-establish K. These

considerations are generalized for any e

quilibrium by Le Châtelier’s principle:

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