Carbon burns to CO with an enthalpy change of -111 kJ, and CO burns to CO2 with -283 kJ. What is the value for carbon burning directly to CO2?

1. Step 1 makes CO from carbon and half an O2, releasing 111 kJ. 2. Step 2 burns that CO with another half O2 to give CO2, releasing 283 kJ. 3. The CO made in step 1 is used up in step 2, so it cancels from the sum. 4. The two half O2 amounts add to one full O2, giving the direct reaction. 5. By Hess's law add the enthalpy changes: minus 111 plus minus 283. 6. That total is minus 394 kJ, matching the direct measurement. 7. Subtracting instead of adding gives minus 172 kJ, the usual sign slip. _Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_