Question

Why the enthalpy of formation of carbon monoxide cannot be calculated directly?

Answer 1

Because when Carbon is combusted I excess of oxygen carbon dioxide is formed. Carbon monoxide is the intermediate compound .

The standard enthalpy of formation of carbon monoxide is -99 kJ/mol. To solve this problem, we use Hess's Law. Our target equation has CO(g) on the right hand side, so we reverse equation 2 and divide by 2. That means that we also change the sign of ΔH and divide by 2.

Answer 2

Do you mean why is it not possible to measure the heat of formation of CO directly in the laboratory?

It's because it is impossible to burn carbon in O2 and get it to stop at CO since the natural tendency is to oxidize to CO2. Even with a limited supply of O2 there will be a mixture of CO and CO2.

It is possible to measure the heat of formation of CO, but it requires a bit more work. First determine the heat of formation of CO2 by burning carbon in pure oxygen.

C + O2 --> CO2

Then burn CO in O2 to get CO2 (See below)

CO +1/2 O2 --> CO2

Then we can use Hess's law to determine the heat of formation of CO.

C+ O2 --> CO +1/2 O2 --> CO2

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CO can be prepared by one of these methods.

1. dehydration of formic acid, HCOOH, by H2SO4(conc)

HCOOH --H2SO4--> CO + H2O

2. heat a mixture of powdered zinc and powdered calcium carbonate

Zn + CaCO3 --> ZnO + CaO + CO