1. Standard Enthalpies of
Formation
The standard enthalpy of formation (∆Hof) for a compound is defined as the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in the standard state.
A degree symbol on a thermodynamic function, for example, ∆Ho, indicates that the corresponding process has been carried out under standard conditions.
The standard state for a element is when it is at 25 oC and 1 atmosphere and in its most stable form and physical state (solid, liquid, or gas).
All values of ∆Hof for elements (in their standard states) are zero (cannot form an element from itself).

2. The enthalpy change for a given reaction can be calculated by subtracting the enthalpies of formation of the reactants from the enthalpies of formation of the products.
Mathematically:
∆Horeaction = ∑np ∆H f (products) - ∑nr∆Hof (reactants)
where the symbol ∑ (sigma) means “to take the sum of the terms”, and np and nr represent the moles of each product or reactant, respectively.
Elements are not included in the calculation because elements require no change in form.