To calculate the standard enthalpy of formation Hf of carbon dioxide CO2 , we can use the given reactions and their respective enthalpies. We want to find the enthalpy change for the reaction:C s + O2 g CO2 g We can use the given reactions to create a reaction pathway that leads to the formation of CO2 from its elements in their standard states. The first reaction is:CO g + 1/2 O2 g CO2 g H = -283.3 kJ/molWe can reverse this reaction to get:CO2 g CO g + 1/2 O2 g H = +283.3 kJ/molNow, we can add the second given reaction:C s + O2 g CO2 g H = -393.5 kJ/molAdding the two reactions together, we get:CO2 g CO g + 1/2 O2 g H = +283.3 kJ/molC s + O2 g CO2 g H = -393.5 kJ/mol---------------------------------------------C s + 1/2 O2 g CO g H = -110.2 kJ/molNow, we can reverse this reaction to get the formation of CO2 from its elements:CO g + 1/2 O2 g C s + O2 g H = +110.2 kJ/molSince the enthalpy of formation of C s and O2 g are both 0 kJ/mol, the standard enthalpy of formation Hf of carbon dioxide CO2 at 25C is:Hf CO2 = -393.5 kJ/mol