Effect of Temperature

Learning Objectives

  • Describe how temperature affects the direction of an equilibrium reaction.

Don’t inhale

A mix of carbon dioxide and carbon generates carbon monoxide under high temperaturesWe 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 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, CO 2 formation is favored while CO is the predominant product above 800°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.

text{N}_2(g)+3text{H}_2(g) rightleftarrows 2text{NH}_3(g)+91 text{ kJ}

The forward reaction is the exothermic direction: the formation of NH 3 releases heat. The reverse reaction is the endothermic direction: as NH 3 decomposes to N 2 and 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 NH 3 in the system decreases, while the concentrations of N 2 and 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 NH 3 in the system increases, while the concentrations of N 2 and H 2 decrease.

For changes in concentration, the system responds in such a way that the value of the equilibrium constant, K_{eq} , is unchanged. However, a change in temperature shifts the equilibrium and the  K_{eq} value either increases or decreases. As discussed in the previous section, values of  K_{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_{eq} value decreases. When the temperature is decreased, the shift in equilibrium towards the products means that the  K_{eq} value increases.

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

text{N}_2text{O}_4(g)+ text{heat} rightleftarrows 2text{NO}_2(g)

Dinitrogen tetroxide (N 2 O 4 ) is colorless, while nitrogen dioxide (NO 2 ) is dark brown in color. When N 2 O 4 breaks down into 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.

Watch a video demonstrating the effect of temperature on the equilibrium between NO 2 and N 2 O 4 .

Summary

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

Practice

Read the material at the ChemWiki and do the problems

  1. Which is the exothermic direction in the Haber process?
  2. If we put more heat energy into the system, which way will the equilibrium shift?
  3. How can we pull the reaction more strongly to the right?