Question

If combustion reactions are spontaneous, why is there an increase in entropy for the system and surrounding?

Answer 1

When ethane and oxygen are the system, the combustion will release heat from them while forming `"CO"_2(g)` and `"H"_2"O"(g)`.

`"C"_2"H"_6(g) + 7/2"O"_2(g) -> 2"CO"_2(g) + 3"H"_2"O"(g)`

The change in mols of gas is `2 + 3 - 7/2 - 1 = 0.5`, so the entropy of the system (ethane + oxygen) increases due to an increased volume of gases inside the reaction vessel.

The combustion releases heat out to the surroundings (outside the reaction vessel), so the entropy of the surroundings also increases due to thermal energy release.