In Antigone, what is the purpose of the exchange between the Chorus and Chorus leader?
I am afraid we are going to need more information than this to successfully answer your question. I have just quickly skimmed through the play, and although the Leader of the Chorus does have many lines and interacts with other characters, mostly with Creon, there are no occasions when this character interacts with the Chorus as a whole. The Leader of the Chorus seems to have the role of openly expressing and voicing the thoughts, feelings and questions of the Chorus as a whole, which serves to make characters such as Creon think about what they are doing and their motives behind their actions. Consider the following example, which is said to Creon:
ever since he began I've been debating in my mind,
could this possible be the work of the gods?
Note how this subtly questions the actions of Creon and tries to suggest that something bigger than mere human agency is at play. However, I can find no examples of the Chorus Leader talking with the Chorus as a whole. Rather the Leader is the representative of the Chorus and gives voice to their thoughts and ideas.