standard enthalpy of formation for CO?

Question

Why does the standard enthalpy of formation for CO cannot be obtained directly by measuring the enthaply of this reaction
C+1/2O2 -> CO?

And instead have to be calculated from Hess law:
2CO2 -> 2CO + 02 Enthalpy =566.0 kJ
C(graphite) + O2 -> CO2 Enthalpy = -393.5 kJ
2CO -> C(graphite) + CO2 Enthalpy = -172.5 kJ

By Hess Law, enthalpy of formation of CO = 1/2(2(-172.5) + 566.0) = -110.5 kJ

Answer 1

Two problems with direct measurement: First, if you use an excess of oxygen, burning carbon gives CO2, not CO. Second, If you use an exact amount of Oxygen, you still have CO2 formed: what would really happen is that you'd form mostly CO2 first, when the atmosphere is oxygen-rich. Then, when the carbon was half burned, the atmosphere would be mostly CO2, and you'd have to wait until the reaction of CO2 + C -> 2 CO was complete - and that's going to be slow. VERY slow if the calorimeter is busy absorbing all the heat the first phase of combustion is done. (like a million years, say).

Answer 2

Lancenigo di Villorba (TV), Italy

Compliments! You made right!
However, this time I agree not the HCBIOCHEM's opinion.

How do he do directly measure the "Enthalpy's Change" related to a LONELY reaction

2 C(s) + O2(g) ---> 2 CO(g)

I repeat :"how do he do it?"
He does not, I am sure!

HCBIOCHEM desire charcoal may burn in a gas mixture defecting in oxygen's containt.
HCBIOCHEM said about a theoretical experiment where a defined mass of charcoal reacts in an "Air's Defecting Burner".
Several metalurgical textbooks reported the FUNDAMENTAL CHEMICAL EQUILIBRIUM on "Ore's Reduction", that is the following chemical equilibrium :

C(s) + CO2(g) <---> 2 CO(g)

Oxygen's defecting gas cannot avoid the carbon dioxide, thus the measures give wrong data.

I hope this helps you.

Answer 3

I'm sure that if done carefully, it could be accurately measured. But when you burn carbon in an oxygen-limited atmosphere, you generally form a variety of things rather than getting ALL CO.