Can you approximate the change in entropy of a reaction?

Question

I found a problem in which I am given a hypothetical reaction in which from 2 liquids (reactants) I get 4 moles of gas and 1 mole of a solid substance (products):

$$\ce{A (l) + B (l) -> 4C (g) + D (s)}$$

I am asked about the change in entropy (an approximation of it), so I am curious if there is some kind of approximation for the change in entropy of a reaction based on the change in the amount of substance of gaseous compounds (since I know that the highest influence on entropy is that of gases).

Answer 1

The molar standard entropy of formation ($S_{f}^{\ominus}$) for many common substances is documented in tables. You can approximate the overall change in entropy of the reaction ($\Delta S^{\ominus}_{reaction}$) with with Hess's law:

$\Delta S^{\ominus}_{reaction} = \sum S^{\ominus}_{f,molar,products} - \sum S^{\ominus}_{f,molar,reactants}$

You notice that there is no $\Delta$ in front of $S^{\ominus}_{f,molar}$, since entropy is an absolute physical quantity. Every system has an absolute entropy.

Don't forget to multiply every $S^{\ominus}_{f,molar}$ with the number of moles that you have. This is only an approximation because the molar entropy of a substance depends on Temperature as well, and the reaction likely doesn't take place at standard temperature.

As a sidenote please be aware that the molar entropy of formation for elements is not zero, although the molar enthalpy of formation is!

Answer 2

In such a case, you may simply count the number of gaseous molecules on each side of the arrow. In your reaction there are no gas molecules in the reactants, and 4 in the products. The number of moles of gaseous substances increases. So the qualitative entropy increases.