When learning about Entropy in my introductory lecture, I learnt that in basic terms, entropy can be spoken about as
$$dS = \frac{dq_{rev}}{T}$$
and the lecturer mentioned that as Entropy $S$ and $T$ are state functions and $dq_{rev}$ is a path function, this can't hold true all the time for the surroundings.
Hence, to solve this problem, the reaction must be a reversible process, so that the heat is delivered very slowly. As a result, the heat flow will be very very slow. He didn't make any further explanation to this statement.
I'm slightly confused by this statement that he made.
I can't understand the purpose of the reaction having to be reversible in order to calculate the entropy of a reversible reaction.
Must the reaction be reversible so the temperature of the system and the surroundings be very close, so that the heat flow rate can be made very slow, or is there another reason?