Question

Calculate delta H for C+ 1/2 O2 --> CO

given:

C+ O2, delta H = -22kj

CO + 1/2 O2 --> Co2, delta H = -15kj

Answer 1

Our target equation is

C(s) + (1/2)O₂(g) → CO(g); ΔH=?

We have the following information:

1. C(s) + O₂(g) → CO₂(g); ΔH= -22 kJ
2. 2CO(g) + O₂ → 2CO₂(g); ΔH= -15 kJ

To solve this problem, we use Hess's Law.

Our target equation has C(s) on the left hand side, so we re-write equation 1:

1. C(s) + O₂(g) → CO₂(g); ΔH= -22 kJ

Our target equation has CO(g) on the right hand side, so we reverse

equation 2. (2CO(g) + O₂ → 2CO₂(g); ΔH= -15 kJ) and divide by 2.

3. CO₂(g) → CO(g) + (1/2)O₂; ΔH=+7.5 kJ

It will the change the sign of ΔH and value as it also divide by 2 as shown above

Then we add equations 1 and 3 and their ΔH values.

This gives

C(s) +(1/2)O₂(g) → CO(g); ΔH=-14.5 kJ

Using your numbers, the standard enthalpy of formation of carbon monoxide is
-14.5 kJ/mol.