Formation
Formation reactions are chemical reactions that form one mole of a substance from its constituent elements in their standard states. By standard states we mean as a diatomic molecule if that is how the element exists and the proper phase at normal temperatures (typically room temperature). The product is one mole of substance, which may require that coefficients on the reactant side be fractional (a change from our normal insistence that all coefficients be whole numbers).
Examples:
Hydrogen and oxygen combine to form water by the formula:
2 H2 + O2 → 2 H2O
The formation reaction of this process is:
H2 + ½ O2 → H2O
Heat of formation
It is also called standard heat of formation, enthalpy of formation or standard enthalpy of formation. In this formation the amount of heat absorbed or evolved when one mole of a compound is formed from its constituent elements each substance being in its normal physical state like gas, liquid or solid.
Formation reaction of water
The water molecule contains the hydrogen (H) and oxygen (O) elements. The standard heat of formation of liquid water would be defined as the enthalpy change when the elements hydrogen and oxygen in their standard states to produce liquid water.