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CH4(g) + 2O2(g) ⇒ CO2(g) + 2H2O(l) DH = - 890.3 kJ mol-1 |
|
Substance |
Entropy (J mol-1 K-1) |
|
CH4(s) |
186.2 |
|
O2(g) |
205.0 |
|
CO2(g) |
213.6 |
|
H2O(l) |
69.9 |
Inspection of the equation shows that there is no change in the number of particles, but three moles of very disordered gases give only one mole of gas as a product. We would predict a decrease in DSsystem.
DSsystem = 213.6 + 2 × 69.9 - 186.2 - 2 × 205.0 = - 242.8 J mol-1 K-1
As the reaction is exothermic, the surroundings are getting hotter and the entropy must increase:
DSsurroundings = - - 890.3 × 1000/298 = + 2988 J mol-1 K-1
This gives DStotal = - 242.8 + 2988 = + 2745 J mol-1 K-1
This reaction is clearly feasible. Perhaps this is not surprising as this reaction represents the combustion of methane, a reaction which takes place every time you use a Bunsen burner. However, it does illustrate another important point. Whilst this reaction is feasible at 298K, it is incredibly slow (methane and oxygen will co-exist in a container together for a very long time at 298K). It is important to remember that this technique tells us whether a reaction is feasible, but says nothing about the rate of reaction.