Editing Le Chatelier's principle (section)

===Effect of change in temperature===
[[File:NO2-N2O4.jpg|thumb|The [[reversible reaction]] 2NO<sub>2</sub>(g) ⇌ N<sub>2</sub>O<sub>4</sub>(g) is exothermic, so the equilibrium position can be shifted by changing the temperature.<br/>When heat is removed and the temperature decreases, the reaction shifts to the right and the flask turns colorless due to an increase in N<sub>2</sub>O<sub>4</sub>. This demonstrates Le Chatelier's principle: the equilibrium shifts in the direction that releases energy.<br/>When heat is added and the temperature increases, the reaction shifts to the left and the flask turns reddish brown due to an increase in NO<sub>2</sub>, in accordance with Le Chatelier's principle.]]
The effect of changing the temperature in the equilibrium can be made clear by 1) incorporating heat as either a reactant or a product, and 2) assuming that an increase in temperature increases the heat content of a system. When the reaction is [[exothermic]] (Δ''H'' is negative and energy is released), heat is included as a product, and when the reaction is [[endothermic]] (Δ''H'' is positive and energy is consumed), heat is included as a reactant. Hence, whether increasing or decreasing the temperature would favor the forward or the reverse reaction can be determined by applying the same principle as with concentration changes.

Take, for example, the [[reversible reaction]] of [[nitrogen]] gas with [[hydrogen]] gas to form [[ammonia]]:

:N<sub>2</sub>(g) + 3 H<sub>2</sub>(g) ⇌ 2 NH<sub>3</sub>(g)&nbsp;&nbsp;&nbsp;&nbsp;Δ''H'' = −92 [[Kilojoule|kJ]] mol<sup>−1</sup>

Because this reaction is exothermic, it produces heat:

:N<sub>2</sub>(g) + 3 H<sub>2</sub>(g) ⇌ 2 NH<sub>3</sub>(g) + ''heat''

If the temperature were increased, the heat content of the system would increase, so the system would consume some of that heat by shifting the equilibrium to the left, thereby producing less ammonia. More ammonia would be produced if the reaction were run at a lower temperature, but a lower temperature also lowers the rate of the process, so, in practice (the [[Haber process]]) the temperature is set at a compromise value that allows [[ammonia]] to be made at a reasonable rate with an equilibrium concentration that is not too unfavorable.

In [[exothermic reaction]]s, an increase in temperature decreases the [[equilibrium constant]], ''K'', whereas in [[endothermic reaction]]s, an increase in temperature increases ''K''.

Le Chatelier's principle applied to changes in concentration or pressure can be understood by giving ''K'' a constant value. The effect of temperature on equilibria, however, involves a change in the equilibrium constant. The dependence of ''K'' on temperature is determined by the sign of Δ''H''. The theoretical basis of this dependence is given by the [[Van 't Hoff equation]].