To calculate the standard enthalpy change for the given reaction, we need to use the following formula:H = Hf products - Hf reactants where H is the standard enthalpy change, and Hf is the standard enthalpy of formation for each substance.First, we need to find the standard enthalpy of formation for each substance involved in the reaction:Mg s : Hf = 0 kJ/mol since it is in its standard state HCl aq : Hf = -167.2 kJ/molMgCl2 aq : Hf = -641.3 kJ/molH2 g : Hf = 0 kJ/mol since it is in its standard state Now, we can plug these values into the formula:H = [ -641.3 kJ/mol + 0 kJ/mol ] - [ 0 kJ/mol + 2 -167.2 kJ/mol ]H = -641.3 kJ/mol - -334.4 kJ/mol H = -306.9 kJ/molSo, the standard enthalpy change for the given reaction at 298 K is -306.9 kJ/mol.