A: The department's guidance for supervisors recommends setting around 3 hours of work per subject, per week for IA. Scaling this up to IB, this would be around 4 hours per week. For Parts II and III, you should probably expect it to take a bit longer.
The guidance is difficult to adhere to, because the same number of questions takes everyone a different amount of time. If you find that you're spending much longer than 3 or 4 hours (for IA/IB) on the work that I set, let me know and don't try to force yourself to do all of it.
A: When I set work, I instruct you to "attempt" a given set of questions. This means that I am not expecting you to produce a perfect set of answers. I am just expecting you to have a good go at them, by looking at the relevant section of the lecture notes and trying to apply the concepts to the question. If you can't do a question, that's fine! We will be able to discuss it in the supervision. Just try and make sure that you've given yourself the best chance at being able to do it by looking through the notes and having a think about it.
If you don't have time to do all the work, this is also fine. As your supervisor, I have precisely no impact on your final mark, so you don't have to worry about it being used against you. At the end of the day, the work and the supervisions are there to help you get the most out of the lectures, and more generally out of the course and your time as an undergraduate. I am not saying that it's a good idea to get into the habit of regularly not handing in work, or only doing a subset of the questions; I am saying that you are the best judge of what you are capable of in a specific week. I am therefore not going to judge you if you don't get everything done.
A: In IA and IB, in general the work quite closely follows the material in the lectures. The goal of the problems that are set is to help you understand the material, by making you go back through the lecture notes and get your head around the concepts by applying them to some questions. To this end, it is a brilliant idea to do the questions with the lecture notes open next to you. The more you are looking through the notes while dong your supervision work, the more you are learning and the better you will retain the information.
In Parts II and III, the questions tend to be slightly more subtly linked to the lecture material (some courses are worse than others in this regard). It may feel as though the lecture notes are not as directly useful as they were in IA/IB. In particular, the organic courses begin to adopt a style more akin to examples classes, where the lecturer talks through some syntheses or reactions and then the supervision work consists of drawing mechanisms and designing syntheses that may or may not be related to what the lecturer talked about. My advice is to not get too disheartened, and focus on the basics of drawing reasonable mechanisms that don't include any obviously impossible steps. Organic chemistry is not a subject that can be examined by repeating exactly the same material as from the lectures; you are being taught the concepts of reactivity and synthesis, and you will need to learn to apply these concepts to new situations and unseen molecules.
Having said this, some of the supervision problems and Tripos questions are just really hard. If you find yourself stuck on a question without much idea of what's going on, chances are that everyone else is too. Stick to the same principle as always: try and come up with something reasonable, that has a decent stab at explaining the reaction/making the requested molecule. In the exam, you don't need to get a mechanism or synthesis exactly right to get a good mark, as long as what you draw makes some kind of chemical sense.
Finally, if it's supervision work that you're stuck on, for Parts II and III, try using Reaxys to find the reaction that is the subject of the question. Draw the starting material in ChemDraw, and then search for it in Reaxys. Most of the time, you will be able to find the research paper that originally reported the reaction. Sometimes, the authors of the paper will have discussed the reaction and its mechanism; other times, they won't have mentioned it at all. But this is certainly worth a try, as long as you've had a good go at the question yourself first. Note: do not try this for A2 in Part II: a large number of the molecules you are asked to write syntheses for are not reported in the literature, having just been made up by Bill.
A: The first thing to say is that you should keep your DoS and supervisor (me?) updated with any concerns that you have about your work. It's much better to make us aware of the situation early, than to let it build up and get to the point where you're approaching the exam and are getting more and more worried.
The second thing to say is that the exams, by design, are not easy. I don't think there are many students at Cambridge who have never had trouble with at least some of the material/exam questions. Coming from A levels, it is very easy to have misaligned expectations of what represents a good exam performance. The exams are not like A levels (for more detail, see the answer to the next question). In general, the examiners are looking to discriminate between those who have a decent understanding, and those who have a really deep understanding of the material. What this means is that if your goal is simply to pass, it is often less difficult than you might think to get a passing mark.
The key to obtaining said passing mark is to do your best to keep up with the work. Attend the lectures, and do the supervision work, with your lecture notes open next to you, reading through them to find the section that is relevant for a given question. If you do this, you will find that at least some of the material is sinking in
I am afraid that you will probably find that you will need to do more than just doing the supervision work in order to make a decent attempt at the exam. Do not underestimate the pedagogical value of past Tripos papers. They are a brilliant way to learn the material. For most courses, which have a large bank of past papers, past Tripos questions represent (nearly) all of the core examinable concepts for a given course. Of course it's not an absolute guide, because anything in the lecture notes (and indeed maybe not in the lecture notes) is fair game, but Tripos papers give you a good idea of what you actually need to know and be able to do. When you first start revising (I am not going to tell you when to start revising, because that's different for everyone, but there are always people who start as early as the end of Michaelmas), do the past Tripos questions in exactly the same way as you'd do supervision work, i.e. with your lecture notes open next to you. As it gets closer to the exam, you can slowly start to work on timing and doing the questions closed book. As long as there are enough past Tripos questions available for a course, I would personally recommend that you don't feel scared of starting Tripos questions before you "know the material". Indeed, I repeat my thesis that you should use the Tripos questions as a way to learn the material.
Disclaimer: I have never marked any Cambridge exams. What I say here is based off my experience with exams when I was an undergraduate, reading the examiner's reports, some mark checking I've done on marked scripts, and things other people from around the department have told me.
A: This is a tough question to answer, because the exams are marked by lots of different people, and they all have different ideas of what they are looking for. However, if I were to generalise, I would say the following. The marking of the exams is nothing like for A level. In the overwhelming majority of cases, there is no "mark scheme". The examiners are generally not reading your answer looking for specific things you've mentioned and giving each "marking point" a tick.
Instead, the examiners are looking for a good answer. It is quite easy to tell whether an answer is good or not, just by having a quick look at it. In an organic question, a good answer would be one where the candidate has drawn their molecules in a nice way, the mechanism/synthesis is clear and broadly correct, and crucially where the answer doesn't contain any egregious chemical errors (e.g. forming a carbon with 5 bonds).
The other key thing that makes an answer good is detail. A level exams have a hard cap: full marks. Some of you probably reached this cap in some of your mocks or actual exams. No matter how good your answer is, it can never get more than full marks. Cambridge exams effectively do not have this cap. The examiners are aiming to award an average mark of around 65%, so this leaves a lot of room for excellent answers to be elevated. You can write a brilliant answer that gets everything right, and it still might only get 20 out of 25. The additional marks are reserved for answers that show a really deep understanding of the material. This is why it is so important to show your working in exam questions, and to include as much detail as you can think of and have time to include. Students have asked me in the past, "do I need to write about this aspect of the reaction?". The answer is that you probably don't need to (although of course this depends on what mark you are aiming for), but that you have a good chance of being suitably rewarded if you do.
Some additional advice: in IA and IB, the examiners are not the same as the lecturers. They are different academics from the department. This means that the style of questions changes a bit year to year, and it's not always easy to predict what will come up. Sorry!
However, in Parts II and III, the lecturer for a course is also responsible for examining it. Most courses have had the same lecturer for several years, so you can get a good feel for the style of their questions. Do not forget that you have access to suggested answers for Tripos papers on Moodle. I recommend that you make good use of these suggested answers when it comes to revising for the exam. Importantly, because these answers are written by the lecturer, they are often (NOT always, some lecturers write terrible suggested answers) a good guide to what the examiner is expecting in a good/excellent answer to their question. If you can produce in the exam an answer that closely matches the style and substance of the examiner's suggested answers, the chances are that you will do well.