To calculate the standard enthalpy change for the reaction of gaseous hydrogen and gaseous iodine to form hydrogen iodide, we can use Hess's Law and the standard enthalpies of formation for the substances involved. The reaction can be represented as:H2 g + I2 g 2HI g Hess's Law states that the enthalpy change of a reaction is the sum of the enthalpy changes of its individual steps. In this case, we can break the reaction into two steps:1. Formation of H2 g and I2 g from their elements in their standard states.2. Formation of 2HI g from H2 g and I2 g .Step 1: Formation of H2 g and I2 g from their elements in their standard statesH2 g and I2 g are both in their standard states, so the enthalpy change for this step is 0.Step 2: Formation of 2HI g from H2 g and I2 g We can use the standard enthalpies of formation Hf for the substances involved in this step. The standard enthalpy of formation for a substance is the enthalpy change when one mole of the substance is formed from its elements in their standard states.The standard enthalpy of formation for H2 g and I2 g is 0, as they are both in their standard states. The standard enthalpy of formation for HI g is -26.5 kJ/mol.Using the standard enthalpies of formation, we can calculate the enthalpy change for this step:H = [2 Hf HI ] - [Hf H2 + Hf I2 ]H = [2 -26.5 kJ/mol ] - [0 + 0]H = -53.0 kJ/molNow, we can use Hess's Law to find the overall enthalpy change for the reaction:H reaction = H step 1 + H step 2 H reaction = 0 + -53.0 kJ/mol H reaction = -53.0 kJ/molSo, the standard enthalpy change for the reaction of gaseous hydrogen and gaseous iodine to form hydrogen iodide at standard temperature and pressure is -53.0 kJ/mol.