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

Question

Henry Antrim · Accepted Answer

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"#

Noah Aldrich · Answer

undefined The standard enthalpy of formation of CO can be calculated using Hess's law, where the enthalpy change for the reaction producing CO from its constituent elements in their standard states is measured and summed up. This reaction can be represented as:

CO2(g) → C(s) + O2(g)

Then, using the known standard enthalpies of formation for carbon in its standard state (graphite) and oxygen gas, the standard enthalpy of formation of CO can be determined. It's important to ensure the reactions are balanced and to use the correct coefficients when calculating the enthalpy change.