Maybe do more for this subject than your others.
Bar Physics AS I would say that Chem is the 2nd hardest AS, with this being reversed at A2.
Make sure you max out on all the units.
Make notes on everything, do legacy papers, chemguide.
Maybe even do more Chemistry - iGCSE chem if you didn't get A Star, Olympiad, SAT 2 Chem.
Watch Khan Academy videos, maybe get a tutor too.
Aim to get all the content complete by the end of february on your own.
There is your A, now study and get it.
If you are doing OCR A maybe I can help more.