At `25^(@)C`, the standard reaction enthalpy for the reaction, 2C(graphite, s)`+O_(2)(g)to2CO(g)` is -221.0kJ. Does this enthalpy change indicate the standard enthalpy of formation of CO(g) ? If not, then what would be the value of the enthalpy of formation of CO(g) at `25^(@)C`?

Explanation

In the given reaction, 2 mol CO(g) is formed from its stable constituent elements. So, by definition, `DeltaH^(0)` of this reaction does not represent standard enthalpy of formation of CO(g).
2C(graphite, s)`+O_(2)(g)to2CO(g),DeltaH^(0)=-221.0kJ`
Dividing both sides by 2, we get,
C(graphite, s)`+(1)/(2)O_(2)(g)toCO(g),DeltaH^(0)=-110.5kJ` . . . [1]
In reaction [1], 1 mol of CO(g) is formed from its stable constituent elements. thus, in this reaction `DeltaH^(0)`=standard enthalpy of formation of CO(g).
So, at `25^(@)C` the standard enthalpy of formation of CO(g)`=-110.5kJ*mol^(-1)`.