The enthalpy of formation is carbon monoxide is difficult to determine experimentally because it is difficult to burn carbon without producing carbon dioxide as well as carbon monoxide. However, the heat released by burning CO to form CO2 can easily be measured. ?Hf° for CO2 is -94.05 kcal and ?HR° is -67.64 for the reaction CO + 1/2 O2 -> CO2. What is the enthalpy of formation of CO?