The size of the jump mostly depends whether your teacher taught you only enough chemistry to pass the exams you were going to be sitting, or whether they got to teach you chemistry.
Most schools/teachers plump/only have time for the first option, so the jump largely depends on which GCSE spec. you were studying. Best of luck if you did core + additional OCR A.
Oh, and how good you are at chemistry & how willing you are to work hard might have an influence.