19.7: Effect of Temperature

We tend to think of carbon monoxide only as a hazardous gas produced from incomplete combustion of carbon products. However, there is a large market for industrially-manufactured carbon monoxide that is used to synthesize most of the acetic acid produced in the world. One reaction that leads to $$\ce{CO}$$ formation involves its formation by passing air over excess carbon at high temperatures. The initial product (carbon dioxide) equilibrates with the remaining hot carbon, forming carbon monoxide. At lower temperatures, $$\ce{CO_2}$$ formation is favored while $$\ce{CO}$$ is the predominant product above $$800^\text{o} \text{C}$$.

Effect of Temperature

Increasing or decreasing the temperature of a system at equilibrium is also a stress to the system. The equation for the Haber-Bosch process is written again below, as a thermochemical equation.

$\ce{N_2} \left( g \right) + 3 \ce{H_2} \left( g \right) \rightleftharpoons 2 \ce{NH_3} \left( g \right) + 91 \: \text{kJ}$

The forward reaction is the exothermic direction: the formation of $$\ce{NH_3}$$ releases heat. The reverse reaction is the endothermic direction: as $$\ce{NH_3}$$ decomposes to $$\ce{N_2}$$ and $$\ce{H_2}$$, heat is absorbed. An increase in the temperature of a system favors the direction of the reaction that absorbs heat, the endothermic direction. Absorption of heat in this case is a relief of the stress provided by the temperature increase. For the Haber-Bosch process, an increase in temperature favors the reverse reaction. The concentration of $$\ce{NH_3}$$ in the system decreases, while the concentrations of $$\ce{N_2}$$ and $$\ce{H_2}$$ increase.

A decrease in the temperature of a system favors the direction of the reaction that releases heat, the exothermic direction. For the Haber-Bosch process, a decrease in temperature favors the forward reaction. The concentration of $$\ce{NH_3}$$ in the system increases, while the concentrations of $$\ce{N_2}$$ and $$\ce{H_2}$$ decrease.

For changes in concentration, the system responds in such a way that the value of the equilibrium constant, $$K_\text{eq}$$, is unchanged. However, a change in temperature shifts the equilibrium and the $$K_\text{eq}$$ value either increases or decreases. As discussed in the previous section, values of $$K_\text{eq}$$ are dependent on the temperature. When the temperature of the system for the Haber-Bosch process is increased, the resultant shift in equilibrium towards the reactants means that the $$K_\text{eq}$$ value decreases. When the temperature is decreased, the shift in equilibrium towards the products means that the $$K_\text{eq}$$ value increases.

Le Châtelier's principle as related to temperature changes can be illustrated easily by the reaction in which dinitrogen tetroxide is in equilibrium with nitrogen dioxide.

$\ce{N_2O_4} \left( g \right) + \text{heat} \rightleftharpoons 2 \ce{NO_2} \left( g \right)$

Dinitrogen tetroxide $$\left( \ce{N_2O_4} \right)$$ is colorless, while nitrogen dioxide $$\left( \ce{NO_2} \right)$$ is dark brown in color. When $$\ce{N_2O_4}$$ breaks down into $$\ce{NO_2}$$, heat is absorbed according to the forward reaction above. Therefore, an increase in temperature of the system will favor the forward reaction. Conversely, a decrease in temperature will favor the reverse reaction.

Summary

• The effect of temperature on the direction of an equilibrium reaction is described.

Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.