Question

What would cause entropy to decrease in a reaction?

Answer 1

If entropy is decreasing, the reactants are reacting to produce products which have less disorder.

Explanation:

Entropy is disorder.

Solids have the lowest amount of entropy, since atoms are tightly packed together and have little movement. Gases have the highest amount of entropy, because they are widespread and can move around all over the place in many different ways. Liquids are in the middle.

So, if moles of solid are reacting to produce gas moles, entropy is increasing.

If entropy is decreasing, the reactants are reacting to produce products which have less disorder. Ex. gases + liquids --> liquids

This also applies to moles. If, for example, there are only gases on both sides of the reaction, but the products have less moles of gas than the reactants, entropy is decreasing.
Ex. `2SO_2 + O_2` --> `2SO_3`
The reaction is going from 3 moles of gas to 2 moles of gas. So there is a decrease in entropy.