WHS AP Chemistry 6 • Thermochemistry HEATS OF FORMATION Indirect Method: Hess’s Law 1. Calculate the standard enthalpy of formation of carbon disulfide (CS2) from its elements, given that Hint: first identify the target equation! C !"#$!!"# + O ! (!) → CO ! (!) ∆° !"# = −393.5 kJ/mol S !!!"#$% + O ! (!) → SO ! (!) ∆° !"# = −296.4 kJ/mol CS ! ! + 3O ! (!) → CO ! (!) + 2SO ! (!) ∆° !"# = −1073.6 kJ/mol 2. From the following data, calculate the enthalpy change for the reaction 2 C(graphite) + 3H2(g) → C2H6(g). C !"#$!!"# + O ! (!) → CO ! (!) ∆° !"# = −393.5 kJ/mol H ! (!) + ! ! O ! (!) → H ! O (! ) ∆° !!" = −285.8 kJ/mol 2C ! H ! ! + 7O ! (!) → 4CO ! (!) + 6H ! O (! ) ∆° !"# = −3119.6 kJ/mol 3. Use the thermochemical equations shown below to determine the enthalpy for the reaction: H2CO + O2 H2CO3 ∆° = ??? H2CO3 H2O + CO2 ∆° !"# = 77.5 kJ H2CO + O2 H2O + CO2 ∆° !"# = 62.5 kJ 4. Use the thermochemical equations shown below to determine the enthalpy for the reaction: N2(g) + 2O2(g) N2O4(g) ∆° =??? 2NO2(g) N2(g) + 2O2(g) ∆° !"# =101.7 kJ N2O4(g) 2NO2(g) ∆° !"# = 87.3 kJ 5. Use the thermochemical equations shown below to determine the enthalpy for the reaction: PCl5(g) PCl3(g) + Cl2(g) ∆° = ??? 4PCl3(g) P4(s) + 6Cl2(g) ∆° !"# =2168 kJ 4PCl5(g) P4(s) + 10Cl2(g) ∆° !"# =3056 kJ 6. Use the thermochemical equations shown below to determine the enthalpy for the reaction: NO2(g) + 7/2H2(g) 2H2O(l) + NH3(g) ∆° =??? N2(g) + 3H2(g) 2NH3(g) ∆° !"# = 46 kJ NO2(g) + 2H2(g) 1/2N2(g) + 2H20(l) ∆° !"# = 85 kJ