To calculate the standard enthalpy change H for the reaction, we need to use the following equation:H = Hf products - Hf reactants First, we need to find the Hf for D g . We can use the given S values and the equation:G = H - TSSince the reaction is at standard conditions, the temperature T is 298 K. We know that at equilibrium, G = 0. Therefore, we can rearrange the equation to find H for D g :H = TSH for D g = 298 K 200 J/Kmol = 59600 J/mol = 59.6 kJ/molNow we can calculate the standard enthalpy change for the reaction:H = [3Hf C + Hf D ] - [Hf A + 2Hf B ]Since we don't have the Hf for B g , we can't directly calculate the enthalpy change for the reaction. However, we can express the enthalpy change in terms of Hf B :H = [3 -50 kJ/mol + 59.6 kJ/mol] - [-100 kJ/mol + 2Hf B ]H = [-150 kJ/mol + 59.6 kJ/mol] - [-100 kJ/mol + 2Hf B ]H = -90.4 kJ/mol + 100 kJ/mol - 2Hf B H = 9.6 kJ/mol - 2Hf B Without the Hf for B g , we cannot provide a numerical value for the standard enthalpy change of the reaction. The answer is:H = 9.6 kJ/mol - 2Hf B