It's not the professor who's different, it's the students. Pre-med students were insufferable in Chem classes much the same way Civil Engineering students were

It's not the professor who's different, it's the students. Pre-med students were insufferable in Chem classes much the same way Civil Engineering students were insufferable in Circuits class, only instead of merely needing to pass the class, pre-med students NEED an A. It's basically accepted that pre-med students are doomed if they can't graduate with something very close to 4.0, so they are very sensitive to a tough class. Chem students who are planning to go to grad school or industry don't NEED straight A's, so they can be content with "Wow, that class was tough, glad I managed a B". Combine that with a class that is the first real hard step in a chemistry education, and thus will by necessity be tough, and you're in for a bunch of pissed of pre-med students.