When experimentally
determining enthalpy changes that accompany reactions, it is important to
specify the conditions under which the reaction was done. This is because it is very useful to compare enthalpy changes for various processes.
Enthalpy changes and energy changes are very closely related, and these quantities both
depend on the
state of the substances involved in the reaction.
For example if H2O is present as a gas, it has a higher energy (enthalpy) than if it is present as a liquid, and the enthalpy change for a reaction involving water would be different.
With a view to enabling scientists all over the world to make meaningful
comparisons between
enthalpy changes,
standard conditions have been defined as well as a
standard state for
substances, and measurements are referred to these..
This is similar to defining the metre as the standard length.
The
standard state for a substance is the pure form at the standard pressure of 100 kPa and the temperature specified.
This definition refers to both ithe physical state (gas, liquid, volume) and chemical state (formula).
Thus at 25°C (298 K) the standard state for
- carbon is C(s)
- nitrogen is N2(g)
- calcium carbonate is CaCO3(s)
- methane is CH4(g)