You could possibly get a B in Chem with no retakes, but it would be quite difficult to pull off. Overall from both years, you need at least 420 ums to get a B grade.
Assuming you got 150 ums at AS (the bare minimum for a D grade), and you get full UMS from both exams and the coursework in the second year. You can come out with 450 UMS. You can afford to lose 30 UMS overall in the second year in order to get a B grade.
Obviously getting straight A grades in the second year with high UMS is no small feat. So I would recommend retaking unless you're happy with a C.
Tell me how much UMS you got in AS in Chem so I can give you a more accurate estimation of the grades you'll need to achieve.