Calculate the standard heat of formation of carbon monoxide (CO) from given combustion data.

Introduction / Context:
Hess’s law enables computation of heats of formation from tabulated reaction enthalpies. Here, we deduce the formation enthalpy of CO using two combustion reactions leading to CO2.

Given Data / Assumptions:

• C + O2 → CO2, ΔH = −394 kJ/kg·mole.
• CO + 1/2 O2 → CO2, ΔH = −284.5 kJ/kg·mole.
• Target: C + 1/2 O2 → CO (standard heat of formation of CO).

Concept / Approach:
Use Hess’s law by subtracting the CO combustion step from the carbon combustion step to isolate the formation of CO from its elements.

Step-by-Step Solution:
Write target: C + 1/2 O2 → CO (ΔH_f,CO = ?).Subtract (CO + 1/2 O2 → CO2, −284.5) from (C + O2 → CO2, −394).Resulting reaction: C + 1/2 O2 → CO with ΔH = −394 − (−284.5) = −109.5 kJ/kg·mole.

Verification / Alternative check:
Alternatively, use ΔH_f(CO2) = −394 and ΔH_rxn(CO→CO2) = ΔH_f(CO2) − ΔH_f(CO) to solve for ΔH_f(CO) = −394 − (−284.5) = −109.5.

Why Other Options Are Wrong:
+109.5 has wrong sign; +180 and +100 are inconsistent with given data; −180 does not follow from the provided enthalpies.

Common Pitfalls:
Forgetting to reverse/sign-change when subtracting reactions; mixing per-mole bases (ensure kg·mole basis is consistent).

Final Answer:
-109.5