Hello!
I was working through the chemistry topic test book and got confused on how to answer this question about combustion reactions. The question is
"Combustion reactions are typically considered to be irreversible reaction. To what extent is this conception a genuine description of the system, as opposed to simply a useful way of thinking about the reactions?"
I understood why they aren't reversible, but the answer at the back said something about the increase in entropy, which i didn't get. Someone please explain how they are related.
Thank you
Recall what entropy means. Entropy is the degree of disorder in a system and generally refers to the number of possible arrangements. Entropy tends to increase in reactions where:
- number of particles increase
- number of gaseous particles increase
Combustion reactions are always spontaneous, meaning they do not require a continuing supply of energy for the reaction to occur. Recall also that the symbol used to represent this fact is ΔG < 0, or ΔH - TΔS < 0. Since combustion reactions are exothermic, there is a release of energy and therefore, ΔS of the surroundings increase (release of gaseous particles increases the number of possible arrangements). Putting this into the equation:
ΔH = negative
ΔS = positive
ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.
The question asks you to discuss whether the fact that knowing combustion reactions are irreversible is a sufficient description of the nature of the system, and in short, it is. The high activation energy of the reverse reaction
(since combustion reactions are exothermic, therefore, Ea of reverse is greater than the forward reaction) reduces the chance that the products will recombine to form reactants. And since they occur in an open system, the products cannot recombine and therefore an equilibrium is impossible. (Here you can talk about entropy)
I hope this helps
(I'm also answering you to help myself as well!)