It depends on what you're aiming for in Chemistry, what the return is on study (relative to other subjects), and hence where your priorities lay.
ninwa's technique is great for learning stuff on express. I know I used this (going through past papers) effectively in Unit 4 to learn a lot of things from the course without having looked at the chapters first.
The responsible path, which is more time consuming but better for long-term knowledge, completeness and full flexibility in working with surprise questions on exams is to learn the theory first (from a good and engaging teacher, or textbook), then to practice them.
However, learning concepts can still be done by practice questions - in a very natural way, your brain pieces together 'how it works' after a while, but the patterns your brain picks up may be flawed without learning the basics at first.
So, it depends on how much time you are allocating to Chemistry, which boils down to where your priorities lay for Chemistry.