The quantity of energy associated with the formation of one mole of a substance from its elements (in their standard states).
The standard states (at SATP) of most elements is solid, except for the gaseous diatomic molecules, H2, O2, N2, F2, Cl2, and the two liquid elements Hg and Br2.
These energies can be looked up in standard tables and have the units, KJ/mol.
Writing Formation Reactions:
- Write one mole of the product in the state that has been specified on the product side.
- Write the reactant elements in their standard states
- Balance the equation for only one mole of product.
Write the formation reaction for glucose, C6H12O6(s).
C(s) + H2(g) + O2(g) -> C6H12O6(s) ΔH°f = -1273.1KJ/mol
Now, balance the equation for one mole of glucose produced.
6C(s) + 6H2(g) + 3O2(g) -> C6H12O6(s)
The standard state for elements in their standard states is zero.
Br2(l) → Br2(l) ΔH°f = 0
The enthalpy change for any equation (target equation) equals the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants.
ΔH = ∑nΔH°f(products) – ∑nΔH°f(reactants)
Note: Each of the product and reactants is multiplied by the number of moles of each in the equation (target equation).