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)
or
• A —>D —>E —>B ( ROUTE 3)
•
•
•
•
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
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
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
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) + 17 kJ
Rb+ (g) —> Rb+ (aq)
- 301 kJ
Cl- (g) —> Cl- (aq)
- 364 kJ
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.
1. RbCl (s) —> Rb+ (g) + Cl- (g)
ΔH=?
2. RbCl (s) —> Rb+ (aq) + Cl- (aq) Δ Ha
Rb+ (g) —> Rb+(aq)
Δ Hb
Cl- (g) —> Cl- (aq)
ΔHc
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)
= + 682 kJ mol/l