Example: Combustion of Hydrogen
Consider the combustion of hydrogen in air. In a flame of pure hydrogen gas burning in the air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy.
Energetically, the process can require the energy to dissociate the H2 and O2, but then the bonding of the H2O returns the system to a bound state with negative potential. It is more negative than the bound states of the reactants, and the formation of the two water molecules is, therefore, an exothermic reaction, which releases 5.7 eV of energy.
2H2(g) + O2(g) → 2H2O(g)
The balance of energy before and after the reaction can be illustrated schematically with the state in which all atoms are free taken as the reference for energy.