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

1 Answer
Michael
Apr 17, 2015

You can calculate it by constructing an energy cycle:

www.4college.co.uk

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"#

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