Question

How can the standard enthalpy of formation of CO be calculated?

Answer 1

You can calculate it by constructing an energy cycle:

Enthalpies of formation are often difficult to measure directly. It is a lot easier to measure the enthalpy of combustion of the compound and the elements from which it is made.

Hess' Law states that the total enthalpy change of a reaction is independent of the route taken.

From the energy cycle you can see that the enthalpy change of the `color(red)("RED")` route is equal to the enthalpy change of the `color(green)("GREEN")` route.

This is because their arrows start and finish in the same place.

We need to find `color(red)(DeltaH_f)` for:

`C_((s))+1/2O_(2(g))rarrCO_((g))`

You use :

`C_((s))+O_(2(g))rarrCO_(2(g))` `color(green)(DeltaH_(c2)=-394"kJ"`

`CO_((g))+1/2O_(2(g))rarrCO_(2(g))` `color(red)(DeltaH_(c1)=-283"kJ"`

Applying Hess' Law : `color(red)("Red route")=color(green)("Green route")` `rArr`

`color(red)(DeltaH_f-283)`= `color(green)(-394)`

`color(red)(DeltaH_f)=-111.0"kJ/mol"`