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?
The simplest way is to calculate  $\Delta$H and $\Delta$G 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.