Changing the temperature of an exothermic reaction can affect the reaction in two ways. First, the reaction's spontaneity may change. Chemical reactions have a measurable inclination toward spontaneity. If an exothermic reaction is spontaneous, raising the temperature may cause that reaction to become non-spontaneous. Second, an exothermic reaction may tend toward its endothermic reverse reaction when temperature is increased.
At constant pressure, a reaction's change in enthalpy describes its change in heat energy. If a reaction has a negative change in enthalpy, this means that it is exothermic; it gives off heat. If the reaction has a positive change in enthalpy, it is endothermic; it absorbs heat. The change in enthalpy for the reverse of a reaction is the same as the forward reaction, with an opposite sign. For example, if the change in enthalpy for a reaction is -50 kilojoules, the reverse reaction would have a change in enthalpy of +50 kilojoules.
Another important quantity in reaction energies is the change in entropy. Entropy describes the change in orderliness of the atoms in a reaction. If a reaction involves an increase in order, its change in entropy is negative. An example of this kind of change is when water freezes into ice; the molecules slow down and become more ordered. If a reaction involves an increase in disorder, its change in entropy is positive. An example of this kind of change is when water vaporizes into gas; the molecules become more animated and disorder increases.
Gibbs Free Energy
The spontaneity of a reaction is defined by a change in Gibbs free energy. The change in Gibbs free energy of a reaction equals the reaction's change in enthalpy minus the product of the temperature and the change in entropy. If a reaction has a negative change in Gibbs free energy, it is spontaneous. Therefore, an exothermic reaction will always be spontaneous when the change in entropy is positive; a negative enthalpy minus the positive product of entropy and temperature equals a negative number. On the other hand, if the change in entropy is negative and the temperature is high enough, the reaction will not be spontaneous. Therefore, if you increase the temperature of an exothermic reaction that has a negative change in entropy, the reaction will eventually become non-spontaneous.
Le Châtelier's Principle and Temperature
Reversible reactions have an equilibrium point at which the forward and reverse reactions happen at the same rate. Le Châtelier's Principle states that a system will maintain equilibrium if an outside force disrupts the system. In the case of an increase in temperature, the reaction rates will adjust to account for this change. If the forward reaction is exothermic and the temperature is increased, the reaction will tend toward the endothermic reverse reaction to offset this temperature increase.
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