An exothermic process is a process that releases energy as heat into its surroundings.
All combustion reactions are exothermic. An endothermic process is a process
in which energy is acquired from its surroundings as heat. An example of an endothermic
process is the vaporization of water. To avoid a lot of awkward language, we
say that in an exothermic process energy is transferred ‘as heat’ to the surroundings
and in an endothermic process energy is transferred ‘as heat’ from the surroundings
into the system. However, it must never be forgotten that heat is a process (the transfer
of energy as a result of a temperature difference), not an entity. An endothermic
process in a diathermic container results in energy flowing into the system as heat
to restore the temperature to that of the surroundings. An exothermic process in a
similar diathermic container results in a release of energy as heat into the surroundings.
When an endothermic process takes place in an adiabatic container, it results in
a lowering of temperature of the system; an exothermic process results in a rise of temperature.
These features are summarized in Fig. 2.2.