Question

Given the equations below, what is the heat of reaction for the process in which carbon monoxide is formed from its elements?

C(s, graphite) + O2(g) → CO2(g), ∆H = –393.5 kJ

CO2(g) → CO(g) + ½O2(g), ∆H = 283.0 kJ

Answer 1

676.5 kJ
–676.5 kJ
110.5 kJ
–110.5 kJ

Answer 2

The nice thing about heat of reaction is that it is a state function. In other words it doesn't matter how you arrive at the final step, you get the same result.

So if the full reaction were A --> B, this is equivalent to doing
A --> C followed by
C --> B

Thus adding heats of AC and CB is the same as heat AB.