# How is entropy change defined for a chemical reaction?

I know that:

ds = dq/T for reversible processes.

Can I write change in entropy of a system in which a chemical reaction is happening using this equation. Basically, my question is about whether a chemical reaction happening in an open vessel(isobarically and isothermally) be treated as a reversible process?

• Google van’t Hopf equilibrium box. Dec 9, 2019 at 18:52
• You can't treat it as reversible. Any spontaneous chemical reaction at constant $T$ and $P$ is necessarily irreversible. Dec 11, 2019 at 20:30
• @ChetMiller van't Hoff
– Karsten
Jan 29, 2022 at 17:23

$$dS = dq/T$$ is most useful for calculating the entropy change of a waterbath that is heated or cooled reversibly.
The simplest way is to calculate $$\Delta$$H and $$\Delta$$G = $$\pu{-RTln K}$$ (or $$\pu{\Delta G = -zEF}$$) of the reaction, then calculate the difference $$\Delta$$H - $$\Delta$$G. And you divide this difference by the temperature T. The result is $$\Delta$$S of the reaction.