Enthalpy of reaction
When a system undergoes a chemical reaction, the enthalpy of reaction is the difference between the enthalpy of the products of the reaction and the enthalpy of the reagents necessary to their production.
It is equal to the heat exchanged between the system and its environment when the reaction is carried out at constant pressure.
Free enthalpy
of a reaction
The variation of free enthalpy of a system due to a chemical reaction.
It is measured by the difference between the free enthalpy of the products of the reaction and the free enthalpy of the reagents necessary to their production.
Free enthalpy
(denoted A or F)
This is defined by the equation
A = U - T S
where S is the entropy of the system and T is the temperature on the Kelvin scale.