The reaction between hydrogen gas and oxygen gas to form water is highly exothermic. At standard temperature and pressure, the reaction proceeds according to the equation: 2H2 + O2 → 2H2O. The bond energy of the hydrogen-hydrogen bond in H2 is 436 kJ/mol, while the bond energy of the oxygen-oxygen bond in O2 is 498 kJ/mol. The bond energy of the oxygen-hydrogen bond in H2O is 463 kJ/mol. As the reaction proceeds, the total energy of the system decreases by 484 kJ/mol.
