Temperature and the Size of K

Temperature is the only factor for a reaction that can change the numerical value of K.  Earlier we showed that you could use le Châtelier's principle to predict the effect of temperature on a reaction.  This same idea can be used to understand how temperature will change the value of K.

As the concentration of reactants increases, K decreases.

Suppose that we have an imaginary reaction that is exothermic:

R equilsymbol.gif (73 bytes) P + energy

le Châtelier's principle allows us to predict that if the temperature increases, then the reaction will shift to the left – in other words, the concentration of R will increase, while P will decrease.  This means that K will decrease.

For an exothermic reaction, an increase in temperature will cause a decrease in the value of K

 

As the concentration of reactants decreases, K increases.

Now consider an endothermic reaction:

R + energy equilsymbol.gif (73 bytes) P

This time, for a temperature increase, le Châtelier's principle predicts the reactants will decrease, while the products will increase.  This will increase the value of K.

For an endothermic reaction, an increase in temperature will cause an increase in the value of K

Here are some values of Kp for the Sabatier reaction CO2 (g) + 4 H2 (g) CH4 (g) + 2H2O (g)

Temperature (oC) Kp
400 1.44 x 103

Notice that these are equilibrium constants expressed in terms of gas pressure.  Therefore, they are Kp values.  Click here if you would like to get some more information about the difference between Kc and Kp.
500 20.6
600 0.738

Is this reaction:   a) endothermic     b) exothermic?

The Clausius - Clapeyron equation gives the quantitative relationship between temperature and the value of K.