1.   The change in SSP causes no change in standard enthalpy changes and the entropy values of solids and liquids.

2.   The enthalpy change, H, of a system is thus the heat absorbed by the system when the reaction occurs at constant pressure.

3.   The enthalpy changes that occur during a chemical reaction or phase change vary depending on temperature, pressure and the physical states of the substances involved.

4.   The enthalpy changes also depend on the amount of substances involved in the process.

5.   It is therefore normal to quote the standard molar enthalpy change for a process.

6.   The standard molar enthalpy of a reaction is the enthalpy change under standard conditions per mole of the reaction as specified by a balanced chemical equation.

7.   However, since the definition given above specifies the enthalpy change per mole we must choose.

8.   Enthalpy changes also occur with other types of change.