Reactions can be classified as either exothermic (heat releasing) or endothermic (heat absorbing). A mnemonic for remembering which type of reaction is defined as what is that heat exits from exothermic reactions and goes in to endothermic reactions. If the pressure in a system remains constant, as with a system exposed to ordinary atmosphere, the change in the heat of the system is known as the change in enthalpy. If the volume is constant (the container is sealed), the change in heat is equal to the change in energy. Usually only the change in enthalpy is of concern. Enthalpy is defined absolutely as
where E is the internal energy of the system. If q is the heat output of the system, the change in enthalpy is given by
Equivalently, at constant pressure equation (1) gives the change in enthalpy as
