To solve this problem using Hess's Law, we need to find a series of reactions that add up to the overall reaction we are interested in. The overall reaction is:2CO g + O2 g 2CO2 g We are given two reactions:1 Decomposition of 2 moles of carbon dioxide to 2 moles of carbon monoxide and 1 mole of oxygen gas:2CO2 g 2CO g + O2 g H1 = -566 kJ/mol2 Combustion of 1 mole of carbon to form carbon dioxide gas:C s + O2 g CO2 g H2 = -394 kJ/molWe can manipulate these reactions to add up to the overall reaction. First, we need to reverse reaction 1:2CO g + O2 g 2CO2 g H1' = 566 kJ/molNow, we need to multiply reaction 2 by 2 to match the number of moles of CO2 in the overall reaction:2C s + 2O2 g 2CO2 g H2' = -788 kJ/molHowever, we need to get rid of the 2 moles of carbon in reaction 2. We can do this by adding the following reaction:2C s 2CO g H3 = 2 * H_formation_of_CONow, we can add reactions 1', 2', and 3 to get the overall reaction:2CO g + O2 g 2CO2 g H_total = H1' + H2' + H3To find H3, we can use the fact that H_formation_of_CO = H_formation_of_CO2 - H_combustion_of_C:H_formation_of_CO = -394 kJ/mol - -566 kJ/mol = 172 kJ/molSo, H3 = 2 * H_formation_of_CO = 2 * 172 kJ/mol = 344 kJ/molNow, we can find H_total:H_total = 566 kJ/mol - 788 kJ/mol + 344 kJ/mol = 122 kJ/molTherefore, the standard enthalpy change of the reaction of 2 moles of carbon monoxide and 1 mole of oxygen gas to form 2 moles of carbon dioxide gas is 122 kJ/mol.