Hess's law

1. Lesson
Hess’s Law
IB Chemistry Power Points
Topic 05
Energetics
www.pedagogics.ca

2. Great thanks to
JONATHAN HOPTON & KNOCKHARDY PUBLISHING
www.knockhardy.org.uk/sci.htm
Some taken from
ENTHALPY
CHANGES

3. “The overall enthalpy change of a chemical process is
independent of the path taken”
The enthalpy change going from A to B can be found by
adding the values of the enthalpy changes for the reactions
A to X, X to Y and Y to B.
ΔHr = ΔH1 + ΔH2 + ΔH3
HESS’S LAW

4. Dissolving solid sodium
hydroxide in water
This process produces sodium and
hydroxide ions ie. NaOH (aq)
solution.
1. NaOH(s) + H2O  NaOH (aq) + H2O ΔH1
Consider three reactions

5. Reacting the sodium hydroxide
solution with a hydrochloric acid
solution = neutralization
Na+ OH-
2. NaOH (aq) +HCl (aq)  NaCl (aq) + H2O ΔH2

6. Alternatively - add solid
sodium hydroxide directly to
hydrochloric acid solution.
H3O+
Cl-
3. NaOH(s) + HCl (aq)  NaCl (aq) + H2O ΔH3

7. Recap:
1. NaOH(s) + H2O  NaOH (aq) + H2O ΔH1
2. NaOH (aq) +HCl (aq)  NaCl (aq) + H2O ΔH2
3. NaOH(s) + HCl (aq)  NaCl (aq) + H2O ΔH3
Show that equation 1 plus equation 2 is the same as equation 3
What conclusion about enthalpy can be made?

8. Equation 1 + 2
1. NaOH(s) + H2O  NaOH (aq) + H2O ΔH1
2. NaOH (aq) +HCl (aq)  NaCl (aq) + H2O ΔH2
3. NaOH(s) + HCl (aq)  NaCl (aq) + H2O ΔH3
Combined – equations 1 and 2 give equation 3
Since the equation 1 then 2 sequence starts and
finishes with the same reactants and products as
equation 3, Hess’s Law says ΔH3 = ΔH1 + ΔH2

9. Represented as an enthalpy level diagram
NaOH (aq) +HCl (aq)
ΔH3
NaOH (s) reactants + HCl (aq)
products NaCl (aq) + H2O
ΔH1
ΔH2
+ H2O - 42 kJ/mol
- 57 kJ/mol
- 99 kJ/mol ΔH3
ΔH1
ΔH2

10. Represented as an enthalpy cycle
+ H2O
NaOH (aq)
ΔH3
NaOH (s) + HCl NaCl (aq) + H2O
ΔH1
+ HCl
ΔH2