Another type of energy that contributes to the internal energy of a system is chemical energy. This is energy associated with chemical bonds and intermolecular attractions.
If we think of a chemical reaction as a process in which some chemical bonds are broken and others are formed, then, in general, we expect the chemical energy of a system to change as a result of a reaction.
Furthermore, we might expect some of this energy change to appear as heat. A heat of reaction, is the quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature.
One of the most common reactions studied is the combustion reaction. This is such a common reaction that we often refer to the heat of combustion when describing the heat released by a combustion reaction.
If a reaction occurs in an isolated system, that is, one that exchanges no matter or energy with its surroundings, the reaction produces a change in the thermal energy of the system the temperature either increases or decreases.
Imagine that the previously isolated system is allowed to interact with its surroundings. The heat of reaction is the quantity of heat exchanged between the system and its surroundings as the system is restored to its initial temperature
In actual practice, we do not physically restore the system to its initial temperature. Instead, we calculate the quantity of heat that would be exchanged in this restoration. To do this, a probe (thermometer) is placed within the system to record the temperature change produced by the reaction.
Then, we use the temperature change and other system data to calculate the heat of reaction that would have occurred at constant temperature.
Two widely used terms related to heats of reaction are exothermic and endothermic reactions. An exothermic reaction is one that produces a temperature increase in an isolated system or, in a nonisolated system, gives off heat to the surroundings.
For an exothermic reaction, the heat of reaction is a negative quantity In an endothermic reaction, the corresponding situation is a temperature decrease in an isolated system or a gain of heat from the surroundings by a nonisolated system.
In this case, the heat of reaction is a positive quantity Heats of reaction are experimentally determined in a calorimeter, a device for measuring quantities of heat.
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