The standard enthalpy change for the reaction between hydrogen gas and oxygen gas to produce water vapor at standard conditions can be determined using the balanced chemical equation and the standard enthalpies of formation for the reactants and products.The balanced chemical equation for the reaction is:2 H g + O g 2 HO g The standard enthalpies of formation Hf for the substances involved are:H g : 0 kJ/mol since it is an element in its standard state O g : 0 kJ/mol since it is an element in its standard state HO g : -241.8 kJ/molUsing the equation H = Hf products - Hf reactants , we can calculate the standard enthalpy change for the reaction:H = [2 -241.8 kJ/mol ] - [2 0 kJ/mol + 0 kJ/mol]H = -483.6 kJ/molSo, the standard enthalpy change for the reaction between hydrogen gas and oxygen gas to produce water vapor at standard conditions is -483.6 kJ/mol.