Standard states and standard enthalpies of formation
The standard state for a substance is the pure form at a pressure of 100 kPa and the temperature specified. At 25°C (298 K) the standard state for
The standard enthalpy of formation (ΔfH°) for a compound is the enthalpy change for the reaction in which one mole of the compound in its standard state is formed from its elements in their standard state.
At 25°C
Note that this definition implies that the standard enthalpy of formation of elements is zero because H° for the reaction in which the elements are formed from themselves is zero.
- carbon is C(s)
- oxygen is O2(g)
- calcium carbonate is CaCO3(s)
The standard enthalpy of formation (ΔfH°) for a compound is the enthalpy change for the reaction in which one mole of the compound in its standard state is formed from its elements in their standard state.
At 25°C
- H2(g) + ½O2(g)
H2O(l)
ΔH° = ΔfH°(H2O, l)
- C(s) + ½O2(g) CO(g)
ΔH° = ΔfH°(CO,g)
Note that this definition implies that the standard enthalpy of formation of elements is zero because H° for the reaction in which the elements are formed from themselves is zero.