To calculate the standard enthalpy change for the reaction, we can use the following equation:H reaction = Hf products - Hf reactants In this case, the reactants are N2 g and H2 g , and the product is NH3 g . The standard enthalpy of formation for elements in their standard states N2 and H2 is zero. Therefore, the equation becomes:H reaction = [2 Hf NH3 ] - [Hf N2 + 3 Hf H2 ]Given that the standard enthalpy of formation of NH3 g is -46.1 kJ mol-1:H reaction = [2 -46.1 kJ mol-1 ] - [0 + 3 0]H reaction = -92.2 kJ mol-1So, the standard enthalpy change for the reaction N2 g + 3H2 g 2NH3 g at 298 K is -92.2 kJ mol-1.