Physical chemistry -- Calculations of enthalpy of reaction
2) Calculation based on Hess's Law
Hess's Law enables you to calculate enthalpies of reactions based on data from other reactions involving the same substances you are dealing with. It was discovered by Hess (German chemist) that these equations can be manipulated as in algebra. That means that you can add them together, rearrange, etc...
It is usually used representing steps towards making a particular molecule, usually 2 or 3 reactions added together.
If more steps are involved (a more complex route) the method is called Born-Haber (after other 2 german chemists- Fritz Haber and Max Born), and it is used normally in calculations involving ionic compounds (solids), in particular the calculation of lattice enthalpy.
-Example:
Given the 2 equations below:
1) N2(g) + O2(g) --> 2 NO(g)
ΔH= 180 kJ / mol
2) 2 NO(g) + O2(g) --> 2 NO2(g)
ΔH = - 114 kJ /mol
Calculate the following reaction enthalpy:
3) 2 NO2(g) --> N2(g) + 2 O2(g)
Answer:
Firstly we invert the second equation (2), to get NO2(g) on the left side of the equation as needed:
2 NO2(g) --> 2 NO(g) + O2(g)
ΔH = 114 kJ /mol
Observe that when an equation is inverted, its enthalpy is also reversed (goes from positive to negative or vice versa).
Next, we subtract the second equation(2) from the first equation (1):
N2(g) + O2(g) --> 2 NO(g)
ΔH= 180 kJ / mol
-
2 NO2(g) --> 2 NO(g) + O2(g)
ΔH = 114 kJ /mol
_________________________
N2(g) + O2(g) - 2 NO2(g) --> - O2(g)
ΔH= 66 kJ /mol
Rearranging:
- 2 NO2(g) --> - 2 O2(g) - N2(g)
Multiplying all terms by -1:
2 NO2(g) --> 2 O2(g)+N2(g)
Which is the reactions sought; answer:
ΔH= 66 kJ /mol
This example illustrates various of the manipulations that can be performed on these equations, similarly to what we do in algebra.
Other ways of calculating enthalpy of reaction:
3) Using enthalpies of formation