Dear Aspirant 

Yes definitely you are eligible for iit jam from chemistry even if you have done your b. Sc from zoologand I would like to tell you would only be able to do so if you have chemistry as another subject along with zoology as your subject, so if it is there then you are eligible otherwise you will not be able to change to a completely new stream. 

Good luck for your future :) 

Feel free to ask if you have any further queries.