While at ACE Chemicals, you'll have to track down five workers. All you have to do is return the surviving workers to Gordon on the bridge outside the factory.
If you get stuck, keep in mind that a lot of this requires you to work in conjunction with the Batmobile. At one point, you actually have to fire through a small window past Batman's head to disrupt some pipe's for example. Once you have the final worker, take on the chopper (use the mini-gun on the rockets) and return the workers to Jim outside.