The Foundations of Chemistry

an increase in temperature at constant pressure favors the reaction to the right. A decrease

in temperature favors the reaction to the left.

The value of any equilibrium constant changesas the temperature changes. Changing the

temperature of a reaction at equilibrium thus causes Qto differ from Kc, but this is now

because Kchas changed. The reaction then proceeds in the direction that moves Qtoward

the new value of Kc. As we will see in Section 17-13,

the Kcvalues of exothermic reactions decrease with increasing T,and the Kcvalues

of endothermic reactions increase with increasing T.No other stresses affect the

value of Kc.

Effect of temperature changes on
the equilibrium of the endothermic
reaction

[Co(OH 2 ) 6 ]^2 4Clheat 34

[CoCl 4 ]^2 6H 2 O

We begin with a purple equilibrium
mixture of the pink and blue
complexes at room temperature (not
shown). In hot water the forward
reaction (endothermic) is favored
and Kcis higher, so the solution is
blue (right). At 0°C, the reverse
reaction (exothermic) is favored and
Kcis lower, so the solution is pink
(left). Each insert shows the structure
of the cobalt complex species present
in highest concentration; other ions
and solvent molecules are not
shown.

Reaction proceeds more to right at higher temperature

Kc

at lower temperature

Kc

at higher temperature

For an endothermic reaction

Reaction proceeds more to left at higher temperature

Kc

at higher temperature

Kc

at lower temperature

For an exothermic reaction