Wednesday, 25 March 2009

S205: CMA-2

So - there is another computer marked assignment to do. There are 35 question to be done, and they vary a lot in format. Some you have to work out a number and type it in. Others require ticking boxes, and so on. These have their strengths and weaknesses. Firstly its a computer marking your efforts, although the questions are reviewed by the course team. This does mean you can sometimes get what you think is the right answer only to be caught out on a technicality, like typing 2.0 when it is expecting 2. Mostly though its pretty good at taking reasonable answers - but of course can't apply the same common sense a human can.

You generally get 3 attempts at an answer, unless its a true/false type one. A couple of them I got completely wrong. One was on the symmetry of SeOCl4 using VSEPR - which even after 3 attempts I still got wrong, and couldn't work out why. My tutor suggested some possibilities given I knew the answer (it tells you once you get it wrong). I also checked it out with a chemistry lecturer friend and she came to the same conclusion as I had. After taking it up with the course team they explained (after the deadline) why their answer was right, but eventually they zero'd out the question as it was rather ambiguous. Even given the answer I couldn't see how you could come up with a definitive structure based on just the course notes - there simply wasn't enough data to cover these weird edge cases. The trouble with issues like this is that I find it suddenly undermines my confidence. Its not a silly mistake, its something I've thought about and still got wrong. I suppose whatever doesn't fail makes me stronger, but in reality it tends to lead to a certain fear of similar questions.

The other question I got wrong I could eventually see I'd just made a silly mistake. Its a bit weird doing these things, you tend to get CMA blindness. For instance I'd take all the data given, do all the calculations and come up with an answer - I'd probably check it too - and then look at the screen and see if my answer was there. If it was, I'd tick it, if not then I clearly need to try again. However often you get one of these calculations where you end up doing a load of steps and then the final one is to multiply by 2 -- or is that divide by 2... after due consideration you decide its multiply by 2, see the answer is there, tick and go. Wrong answer. My immediate reaction is not to do any more work (because I'm lazy by nature) but to assume it was divide by 2, and the answer is there to, so tick that and resubmit immediately without rechecking things. Wrong again! This is where you're 3rd chance comes in, and where you should sit down and rework the problem again - rather than phoning a friend or asking the audience.

Anyway - that was the last of the two CMAs in this course.

Tuesday, 10 March 2009

S205: Book 9 - Elements of the p-block

Oh no -its another vast book - 242 pages about lots of stuff - its information overload, and I'm not really sure what you are suppose to take away from this book. There seems far too much to learn, so in the end I just end up reading the book and hoping some of the concepts sink in ... hummm.

It starts off of, by looking at oxidation states and how you can work them out from various rules. That's not too bad.
The next chapter starts to look at acids and bases, and in particular extends the definition to Brønsted acids - which are compounds that can donate a proton or hydrogen ion.
So having just got the idea of a new type of acid, we have another new type in the next chapter - the Lewis acid. This is a substance that can donate a pair of electrons. Okkkayyy - just about getting that.

Next chapter is all about the chemistry of hydrogen - what it can do and how it can form hydrides and things like that. I'm sure there is some general message here but it seems like just a large number of different reactions.
Next we look at halides, and all manner of ways that the halogens can form compounds. Again a confusing number of different reactions.
After that - a real walk on the wild side as we look at those most unreactive of elements, the noble gases and the various compounds they can form. Its a bit like being told there is no Santa Claus that compounds of the noble gases can form compounds. I mean - the one thing you generally learn in chemistry is that compounds struggle to form bonds to make noble gas like electron shells. Then along comes xenon, and starts disrupting these well rehearsed truths.

After a brief foray into some trends in second and third row elements (which is also covered - better for me - in a dvd accompanying the book), we launch into group 3. Boron, aluminium and so on. Boron is plain weird, it eschews the eightfold way and is happy to make compounds with only a shell of 6 electrons. Its all rather disconcerting.

Then its time to look at group 4. Carbon, silicon and their ilk. It seems we've been doing precious little other than looking at carbon for most of this course, but here it is again.
Follow this up with a look at group 5 nitrogen, and phosphorous, and all they can do. Then its on to oxgen, sulphur and its pals.
Finally another look at trends and the book is done, except for a case study look at acid rain.

So much information, so many reactions - my head is spinning. How can anyone possibly remember a 10th of this stuff.