Hess's law: ΔfH of carbon monoxide template

Hess cycle and enthalpy-level diagram for C(s) + ½ O₂ → CO from ΔcH(C) = −393.5 and ΔcH(CO) = −283.0 kJ mol⁻¹, giving ΔfH°(CO) = −110.5 kJ mol⁻¹.

What the template shows

• An enthalpy change that cannot be measured directly, from two combustion enthalpies that can
• Hess cycle
• Enthalpy levels (to scale)
• Hess's law: the enthalpy change of a reaction depends only on the start and end states, not the route (enthalpy is a state function).
• Burning carbon in limited oxygen always gives some CO₂, so ΔfH(CO) is found from combustion data instead.
• Route 1 (direct to CO₂) = route 2 (via CO): ΔH₂ = ΔH₁ + ΔH₃, so ΔH₁ = −393.5 − (−283.0) = −110.5 kJ mol⁻¹.
• Reversing an equation changes the sign of ΔH; multiplying it scales ΔH. Values: NIST, 298 K, 1 bar.
• Every part of this figure is editable · sources on the template card

Drawn with 27 shapes – every one editable in MolDraw.

Sources

• Atkins, de Paula & Keeler, Atkins' Physical Chemistry, 11th ed., OUP 2018, ch. 2 Textbook
• NIST Chemistry WebBook: carbon monoxide Database
• NIST Chemistry WebBook: carbon dioxide Database