Seriously, I repeat, don't try and take shit from your major. I had the opportunity and just didn't because it makes everything too murky. Just take electives!
It doesn't really work like that for the majority of Science majors... Because there's such a set sequence for most of them, especially chemistry, and the subjects are only offered in one semester each year, to not do those subjects while on exchange necessarily sets you back an entire year...
Not always, I'm taking subjects from second year in the first semester of 2013 to make up for what I missed. You are right though - chemistry has more of a set sequence than many other majors. But for the Biological Science majors, the sequencing isn't as strict because most subjects are offered in both semesters or can be taken as corequisites...so for the 'majority' of Science majors, that's not the case.
Science is notoriously difficult to exchange in for anybody. It takes a lot of preparation to ensure your course structure allows you to go overseas but there comes a point where you have to choose what is most important to you. And by that I mean, whether it's more important to complete your major or go on exchange. You can try and find equivalents to your prereqs but even if you do find them, there's no guarantee you will be able to take them. If you're trying to go to the US I would direct your attention to the massive red warning at the top of the list of partner institutions, which tells you that even if you find a subject it doesn't mean you will be able to take it. Most universities give their home students preference, and all the classes will have set quotas. Even if you're going somewhere in the UK or another country, the same thing is bound to happen - two friends of mine attending Unis in Britain got to take none of the subjects on their study planning sheet. They had to request further subjects for approval via email once they had already arrived in the UK. Luckily they did Arts so the hassle wasn't too great. I'm even luckier in that every subject I chose to do, I was accepted into. But the same cannot be said for everyone, and I'd be willing to bet for most other institutions aside from Penn who actually do give exchange students the same level of preference as their local students.
I might be telling you what you already know but I'm just advising you - if you do happen to find somewhere with a subject deemed equivalent by the Chemistry approval advisor then make sure you get a guarantee from your exchange institution that you will be permitted to enrol in that subject. The hunt isn't over as soon as you come across a subject that fits the description of what is required at home, unfortunately. If they can't guarantee you'll be let in, then I would advise against going there if it is absolutely necessary for you to take a prerequisite overseas or you will get set back a year as you said. And you might not know it's going to happen until you've already arrived on your new campus.
Your other options are to:
a) request the home institution to take a prereq as a coreq
b) take the prereq a year early with permission to take a first year subject as a coreq...which I'm assuming you haven't done so that's probably out of the question.
I see what you're saying, and I wish you luck!