1. Hess’s Law

2. What is Hess’s Law Hess’s Law states:
The total enthalpy change accompanying a chemical change is independent of the route by which the chemical change takes place

3. For example:
Suppose that you have determined ΔH for the displacement of copper by zinc:
Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq);
ΔH = -216 kJ mol-1

4. If the copper so displaced were used to displace silver ions, we could measure ΔH for the change in a similar manner:
2Ag+(aq) + Cu(s) Ag(s) + Cu2+(aq);
ΔH = -147 kJ mol-1

5. However, if zinc were used to displace the silver directly, ΔH should be the same for the reactions previously shown:
2Ag+(aq) + Zn Ag(s) + Zn2+(aq)
ΔH = -363 kJ mol-1

6. These changes can be illustrated grammatically as shown:
2Ag+(aq) + Zn(s)
-363 kJ
2Ag+(aq) + Cu(s) + Zn(s)
2Ag(s) + Cu(s) + Zn2+(aq)
2Ag+(aq) + Cu(s)
-147 kJ
Cu2+(aq) + Zn(s)
-216 kJ
2Ag(s) + Cu2+(aq) + Zn(s)

7. Advantages of Hess’s Law
It can be used to calculate enthalpy changes that cannot be determined by experiment.
e.g. The formation of methane from graphite and hydrogen because graphite and methane do not combine and nor does methane decompose readily to hydrogen and carbon.

8. How is it done? First write down the equation:
C(graphite) + 2H CH4(g)
Now write the equation for burning both the reactants and products in oxygen.
CH4 + 2O CO2 + 2H2O
C(graphite) + 2H2 + 2O CO2 + 2H2O
ΔHØf,298

9. This can be shown in diagrammatic form
ΔHØf,298
C(graphite) + 2H2(g)
CH4(g)
+ 2O2(g)
+ 2O2(g)
ΔH1
ΔH2
CO2(g) + 2H2O(l)

10. ΔH2 is the standard enthalpy change of combustion of methane
ΔH1is made up of the standard enthalpy change of combustion of graphite and twice the standard enthalpy change of hydrogen
ΔH2 is the standard enthalpy change of combustion of methane

11. Therefore the enthalpies are as follows:
= kJ
ΔH2 = kJ
Therefore
ΔHØf,298 = ΔH1 – ΔH2
= – (-890.3)
= kJ mol-1