To calculate the enthalpy change for the reaction, we can use the balanced chemical equation and the standard enthalpies of formation for each compound involved. The balanced chemical equation for the reaction is:2 H g + O g 2 HO g Now, we need the standard enthalpies of formation Hf for each compound:Hf H = 0 kJ/mol since it is an element in its standard state Hf O = 0 kJ/mol since it is an element in its standard state Hf HO = -241.8 kJ/mol for water vapor Using the equation H reaction = Hf products - Hf reactants , we can calculate the enthalpy change for the reaction:H reaction = [2 -241.8 kJ/mol ] - [2 0 kJ/mol + 0 kJ/mol]H reaction = -483.6 kJ - 0H reaction = -483.6 kJSo, the enthalpy change for the reaction when 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of water vapor is -483.6 kJ.