14. Chemical Equilibrium   Previous PageNext Page
       Temperature and the Equilibrium Constant

A particularly important application of Le Chatelier's principle is the prediction of what happens when the temperature is raised or lowered. In this case, unlike the previous example, the actual numerical value of will change.

Chemical reactions arise from collisions between molecules, and molecules move faster and collide more often and more effectively at higher temperatures. Both the forward and reverse reactions will be speeded up by a rise in temperature, and slowed by a temperature drop, but not necessarily by the same amounts

In our crabapple analogy, a cold spell may cause both of the combatants to slow down, but may affect the old man more than the boy.

The effect can be predicted without any calculations. If heat is supplied to a chemical reaction to raise the temperature, then Le Chatelier's principle predicts that the equilibrium will be shifted in the direction in which heat is absorbed, since some of the applied heat is thereby removed. Conversely, a heat-emitting, or exothermic reaction, is favored by a drop in temperature, since the external removal of heat will be counteracted partially by heat from the reaction.

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