Standard enthalpy of formation for CO?

Why does the standard enthalpy of formation for CO cannot be obtained directly by measuring the enthaply of this reaction


C+1/2O2 -%26gt; CO?





And instead have to be calculated from Hess law:


2CO2 -%26gt; 2CO + 02 Enthalpy =566.0 kJ


C(graphite) + O2 -%26gt; CO2 Enthalpy = -393.5 kJ


2CO -%26gt; 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

Standard enthalpy of formation for CO?
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 -%26gt; 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).
Reply: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.
Reply: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) ---%26gt; 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) %26lt;---%26gt; 2 CO(g)





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





I hope this helps you.