As others have said, very little really. What I covered during AS Chem has been more than enough... topics that have come up:
Redox
Acid-base balance, pH, etc.
Activation energy I think might have at some point... maybe.
Certainly helps to have an intuitive understanding of reaction rates, and chemical equilibrium, etc but technical details have never come up.
If you remember the structure of some things that might come up during A-level organic e.g. glycerol, triglycerides, glucose, etc - then I guess that could be of some limited use.