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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