1. Hess's Law

2. Hess’s law
Hess’s Law states that the heat of a whole reaction is equivalent to the sum of it’s steps.
For example: C + O2  CO2
The book tells us that this can occur as 2 steps
C + ½O2  CO H = – kJ
CO + ½O2  CO2 H = – kJ
I.e. C + O2  CO2 H = – kJ
Hess’s law allows us to add equations.
We add all reactants, products, & H values.

3. Example using Hess’ Law
Determine the enthalpy of formation of pentane, C5H12, using the information provided.
Overall rxn
5C + 6H2 → C5H12
C5H12 + 8O2 →5CO2 + 6H2O kj
C + O2 → CO kj
H2 + ½ O2 → H2O kj

4. 1. FLIP Since C5H12 is a product … reverse the equation and the sign of the ∆H value
5CO2 + 6H2O → C5H O kj
MULTIPLY
Multiply
5 (C + O2 → CO2) (-393.5kj)
So C + 5O2 → 5CO2
Because 5CO2 and 5C overall)
Multiply
6(H2 + ½ O2 → H2O) 6(-285.8kj)
So H2 + 3O2 → 6H2O
Because need 6H2 and 6H2O overall

5. 5C + 6H2 → C5H12 -147.3 kj ADD Now Cancel like terms
5CO2 + 6H2O → C5H12 + 8O kj
5C + 5O2 → 5CO (-393.5kj)
6H2 + 3O2 → 6H2O (-285.8kj)
5C + 6H2 → C5H kj