I personaly dont think A2 is any harder than AS, indeed I would go as far to say that in OCR salters I am actually finding the content easier to take in (maybe im thinking more like a chemist than i was last year). If you get a good A at AS it would be difficult for your teachers not to predict you an A at A level, infact you would only need to achieve the equivalent of a mid B to end up with an A overall.
Talk to your teachers, say that you will do anything to get an A, ask for extra work, do past exam papers, explain to them that your heart is set on Bristol and you need an A.
My offer from Bristol specificaly states that I would need an A grade in chemistry, andif your so confident as to say 'your definatly going to get an A at AS' You shouldnt have any problems.
The best of Luck to you.