Chapter 5, Problem 99AP

From the enthalpy of formation for CO, and the following information, calculate the standard enthalpy of formation for carbon monoxide (CO).

$\begin{array}{l}\text{CO(g) + }\frac{1}{2}{\text{O}}_{\text{2}}\text{(}g\text{) }\stackrel{}{\to }{\text{ CO}}_{\text{2}}\text{(}g\text{)}\\ \Delta H°\text{ = -283}\text{.0 kJ/mol}\end{array}$

Why can't we obtain the standard enthalpy of formation directly by measuring the enthalpy of the following reaction?

$\text{C}\left(\text{graphite}\right)+\frac{1}{2}{\text{O}}_{\text{2}}\text{(}g\text{) }\stackrel{}{\to }\text{ CO(}g\text{)}$