To calculate the change in internal energy U of a system, we can use the equation:U = q + wwhere q is the heat absorbed or released by the system, and w is the work done on or by the system.In this case, we have a constant temperature and pressure reaction of hydrogen gas H2 and oxygen gas O2 forming water vapor H2O :2 H2 g + O2 g 2 H2O g Since the reaction is occurring at constant temperature and pressure, we can use the enthalpy change H as a proxy for the heat absorbed or released q in the reaction. The enthalpy change for this reaction can be found using the standard enthalpies of formation Hf of the products and reactants:H = Hf products - Hf reactants The standard enthalpy of formation for H2 g and O2 g is 0 kJ/mol, as they are in their standard states. The standard enthalpy of formation for H2O g is -241.8 kJ/mol. Using these values, we can calculate the enthalpy change for the reaction:H = [2 -241.8 kJ/mol ] - [2 0 kJ/mol + 1 0 kJ/mol] = -483.6 kJNow, we need to consider the work done on or by the system. Since the reaction is occurring at constant pressure, we can use the equation:w = -PVwhere P is the pressure and V is the change in volume. However, since we are given no information about the pressure or volume changes, we cannot calculate the work done on or by the system.Without the work w value, we cannot accurately calculate the change in internal energy U of the system. However, we can still conclude that the reaction is exothermic, as the enthalpy change H is negative, meaning heat is released during the reaction.