To calculate the heat released or absorbed in this reaction, we can use the following equation:q = n * Hwhere q is the heat released or absorbed, n is the number of moles of the substance, and H is the enthalpy change per mole.In this case, the balanced chemical equation for the reaction is:2 H g + O g 2 HO g The enthalpy change H for this reaction is given as -483.7 kJ/mol. Since 2 moles of hydrogen gas H react with 1 mole of oxygen gas O to form 2 moles of water vapor HO , we can calculate the heat released or absorbed q as follows:q = n * Hq = 2 moles * -483.7 kJ/mol q = -967.4 kJThe negative sign indicates that the reaction is exothermic, meaning that heat is released during the reaction. Therefore, the heat released when 2 moles of hydrogen gas reacts with 1 mole of oxygen gas to form 2 moles of water vapor at constant volume is 967.4 kJ.