Chemical Energetics and Change
He teaches the hardest class I've ever taken, but also the best class. The material he teaches is well above the standard general chemistry level (more on the order of graduate level), but he makes it all very understandable. Furthermore, he writes you a fantastic textbook of lecture notes (and does NOT charge you for it). Perhaps the best thing about his class is his teaching style, there are no hand-wavey explanations. Everything is derived, so it makes sense. He is very nice in office hours and answers all questions very fully. tl;dr: if you can put in a lot of work it's the best class available.
I'm going to be honest about my experience in this course. Dr. Liu definitely knows her stuff, but as far as this course, I was on the edge of having a heart attack after every exam. The course is quite simple as far as content, but if you do not catch on quickly then your exams will show it. I did very poorly in the beginning of the class; most did. Fears of dropping, changing majors. I heard it all. In the end, you learn that Dr. Liu is extremely concerned with our learning and will work with you to improve your understanding. I haven't pushed myself as hard as I ever did at UCLA. I recommend this course over the 20B course if you have a strong grasp of kinetics and thermodynamics beforehand. Otherwise, be prepared to study hard. It's a rollercoaster as far as expecting how well you'll do. It isn't bluntly easy and enjoyable as others make it sound.
I thought Mason was quite effective. He covered what needed to be covered, and I left 20BH with a solid knowledge of thermodynamics and equilibria. This is only his second year teaching 20BH, but I think he has improved a lot, compared to last year's rating and comments. He only keeps one midterm this year, and makes it 30% of the grade. The final was 40%, and the homework 30%, which I thought was really generous. He offers extra credit on every homework assignment, so that helped a lot. These problems might sound intimidating at first, but keep searching (Internet, thermodynamics textbooks, etc) and you'll improve your homework grade a lot. The first midterm in my opinion was not hard, but it was really time constrained, and you really need to do many practice problems before hand. Don't rely on the cheat sheet too much; within the time limit (50 mins, 4 problems, average 3 parts per problem) you need to know what you're doing really well. My advices: watch out for sig. figs on the homework (he's strict with them, you might lose up to 1 point - each hw assignment is 10 pts - just for these), do lots and lots of practice problems before exams, and you should be fine. Really try to include every single equation that you think is important on the cheat sheet. One equation can make a 20 pts difference on the exam. Darcy was a TA for my year (Winter 2007); she's an amazing TA who never gets tired helping people. And really funny. No matter who you have for TA, just go to her session for reviews and such. About the book Statistical thermodynamics by Engel and Reid, you don't really need it. It helps for many extra credit problems, but they have it at college library and Young, so go check those out.
I had Tolbert for the 2nd half of 20BH during Winter 2003. Surprisingly, I actually learned a lot from taking her class. Her lectures are usually interesting and well-organized but her homework and midterms are nightmarish. Half of the homework problems are written by her and these are the one's you MUST KNOW HOW TO DO -- she writes problems that force you to think so it's no longer the simple process of plug and chug. If you can't do these problems, you're screwed -- don't bother showing up for the midterms/final. Overall, I would still say that Professor Tolbert is a decent professor. You will learn a lot from taking her class, but that may not necessarily be reflected in the grade you receive in her class.
Winter 2019 - Paul is definitely a GREAT professor for Chem 20BH! First of all, I believe all of you who are viewing this page must have noticed the difference between Paul's rating for Chem 20B and Chem 20BH. This arises from the different nature of these two: If you just want to learn some chemistry and pass 20B(H) as easily as possible, stay away from Paul's class; if you want to really learn SCIENCE, including chemistry and involving all aspects of science (research skills, analytic skills, etc.), you can hardly find a better professor than Paul. Second, I acknowledge that the workload is heavy, but it is well manageable. In the quarter I took this Paul's class (winter quarter of my first year ---- as most of you who will take this course will most probably take at this time of the year), I took 5 courses including 2 maths and a cluster and I survived. Other comments have talked about what to expect, and they are all precise. However, if you are very academic, you will definitely handle all this. Last but not least, Paul is pretty chill. Just think about how many 60-year-old professor would like their students to call them by their first name instead of Prof. X or Dr. X. His class is extremely interesting, where Paul would occasionally tell jokes and personal experience. And again, if you are a very science guy and want to get into a lab, Paul is extremely helpful. He knows who to turn to regarding every aspect of chemistry or materials science if he can't answer himself. The only minor problem with this course is that the exams are very comprehensive. As other comments have said, many of the materials are complementary, which results in a heavy workload. It will be great if you command them all, but since most of us can't, you should be wise at telling the most important points from others (as I did). Other than that, Paul's course is very amazing.