EXERCISES Standard heat of reaction and Hess's law 12.1 Using Hess's law, evaluate the heat of formation of solid CaCO3. The following data are available: Ca (s) + 1/2 O2(g) → CaO (s) 2 C(s) + O2(g) → CO2(g) CO(g) → CaCO3(s) ΔH 298 = -635.77 kJ ΔH 298 = -393.51 kJ ΔH 298 = -178.15 kJ