1. Hess’s Law Conservation of Energy
The overall enthalpy change in converting reactants to products is the same regardless of the route taken.
Example
A —>B (ROUTE 1) or
A —>C —>B (ROUTE 2)
A —>D —>E —>B ( ROUTE 3)

2. ΔH1 = route 1 ΔH2 + ΔH3 = route 2 ΔH4 + ΔH5 = route 3
Hess’s Law states the overall enthalpy change will be the same – therefore:
ΔH1 = ΔH2 + ΔH3 or
= ΔH4 + ΔH5

3. Uses of Hess’s Law
We can use Hess’s Law to calculate enthalpy changes which are very difficult or impossible to do by experiment.
Example
The neutralisation of KOH using HCl.
We can do this by adding KOH(s) directly to HCl(aq) – ΔH1 or
Dissolving KOH(s) in water – ΔH2
then
Adding KOH(aq) to HCl(aq) – ΔH3

4. Pathway 1. KOH(s) + HCl (aq) —> KCl (aq) + H2O(l) ΔH1
2. KOH(s) + H2O (aq) —> KOH (aq) ΔH2
3. KOH(aq) + HCl(aq) —>KCl (aq) + H2O ΔH3
According to Hess’s Law
ΔH1 = ΔH2 + ΔH3

5. Examples Calculate the energy change for the reaction :
RbCl(s) —> Rb+(g) + Cl-(g)
Use the following enthalpy changes:
RbCl(s) —> Rb+ (aq) + Cl- (aq) kJ
Rb+ (g) —> Rb+ (aq) kJ
Cl- (g) —> Cl- (aq) kJ

6. The Steps!
1. Write balanced eq for the reaction you wish to find the enthalpy change.
2. Write balanced eq for the reactions that have been given to you.
3. Label each step with an appropriate ΔH. Make sure you have identified more than one route for the reaction to follow.
4. Write down the enthalpy change for the direct route
( Hess’s Law)
5. Put in the values for each reaction.

7. 1. RbCl (s) —> Rb+ (g) + Cl- (g) Δ H = ?
2. RbCl (s) —> Rb+ (aq) + Cl- (aq) Δ Ha
Rb+ (g) —> Rb+(aq) Δ Hb
Cl- (g) —> Cl- (aq) Δ H c
3. Route 1 = Δ H ?
Route 2 = Δ H a + ( - Δ H b) + (- Δ H c)
4. Hess’s Law
Δ H =Δ H a + ( - Δ H b) + (- Δ H c)
5. Δ H = +17 +(+301) + (+364)
= kJ mol/l