19.7: Effect of Temperature

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Carbon monoxide is often thought of as nothing more than 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.

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

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 toward the reactants means that the \(K_\text{eq}\) value decreases. When the temperature is decreased, the shift in equilibrium toward the products means that the \(K_\text{eq}\) value increases.

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

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

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.