Enthalpy is the heat energy exchange that takes place during chemical reactions. It has the symbol H and is measured in kJ/mol, or kilojoules per mole. The energy exchanged with the surrounding environment at constant pressure is called the enthalpy change of a reaction. To measure the change, standard conditions are used, including a pressure of 1 atmosphere and a temperature of 298 Kelvin (77°F or 25°C).
When the reaction is giving off heat, it is said to be exothermic. In this situation, the enthalpy change is negative, since the reaction is going from high energy to low energy due to the loss of heat energy to its surroundings. If energy flows from the surrounding environment into a system, or heat is being taken in, it is said to be endothermic. In this instance, the change is positive as the system is gaining energy in the form of heat.
Energy level diagrams are the most common way of showing such changes in heat. There are two general diagrams used, one for exothermic reactions and one for endothermic reactions. For an exothermic reaction, the energy of the reactants of the reaction would be higher than the energy of the products resulting. The opposite is true for endothermic reactions. The change is clearly marked on both diagrams.
Two ways of measuring a change in enthalpy are a simple calorimeter or a simple flame calorimeter. A simple calorimeter is made of a container of water, with heat transferred into the water. The change in temperature of the water that is recorded will be due to the energy exchange of the reaction. When a simple calorimeter is used, the reaction takes place within the water in the calorimeter itself.
With a simple flame calorimeter, the reaction takes place outside of the device. During the reaction, the heat has to be transferred to the water inside of the calorimeter. This type of calorimeter is used for measuring the change of combustion reactions.
In order to calculate the change in enthalpy, three things must be known. The first is a constant, which is the specific heat capacity of the water, which is 4.2 J/g/°C. The mass of the water and the amount of the temperature change of the water must be measured. The formula for calculating this change is 4.2 x mass of water x temperature change.