Question

The enthalpy of combustion of carbon and carbon monoxide are $-393.5$ and $-283kJ/mol$ respectively. The enthalpy of formation of carbon monoxide per mole is:

• A. $110.5kJ$
• B. $676.5kJ$
• C. $-676.5kJ$
• D. $-110.5kJ$

Answer 1

The correct option is D $-110.5kJ$

I: $C\left(s\right)+O_2\left(g\right)\to C{O}_2;\Delta H_1=-393.5kJ/mol$

II: $CO\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to C{O}_2\left(g\right);\Delta H_2=-283.0kJ/mol$

On substracting II from I we get the formation reaction of carbon monoxide.

III: $C\left(s\right)+\frac{1}{2}{O}_2\left(g\right)\to CO\left(g\right);\Delta H=\Delta H_1-\Delta H_2=-110.5kJ/mol$

This equation III represents the formation of $1$ mole of $CO$ and thus enthalpy of formation of $CO$(g) is $-110.5kJ/mol$.