What effect does mace have on snakes, if any?

Not a lot, immediately. Capsaicin spray (rather than CN tear gas) is irritating to them, but doesn't last long enough to become a repellant. If you get it inside the mouth it will incapacitate them for a good long while, but you may get bitten if you wait that long.

A long stick is better in an immediate confrontation, and sulfur powder is a cheaper long term repellant.