SF Chemistry 1

I'm going to be writing down what I remember from each of my modules this year, and I hope to do it before exam results come out in two weeks so that I won't have a biased recollection of them.

I did one semester of Chemistry this year (two last year). This one had bits from all three branches of Chemistry, Physical, Inorganic and Organic.

We had a week on Chemical Kinetics (physical), which was crammed in extremely quickly. It covered reaction rate laws, integrated rate laws for zeroeth, first and second order reactions, activation energy, the Arrhenius equation, the Arrhenius equation for when the pre-exponential factor doesn't remain constant with temperature, sequential and bimolecular reactions, enzyme kinetics and catalysis e.g. Michaelis-Menten derivation, and some theory about what the rate laws actually mean.

We then had about 14 lectures (so 3.5 weeks) on Inorganic Chemistry, covering first Coordination Chemistry (the chemistry of Transition Metals and their complexes, colours, magnetic properties...) and then Molecular Orbital Theory. I quite liked Coordination Chemistry apart from the first lectures which were just about shapes and I found quite boring. I really liked Crystal Field Theory/Ligand Field Theory and doing spin-only magnetic moments. Here's an example of a field splitting diagram you'd use to get a Crystal Field or Ligand Field Stabilisation Energy (for an octahedral complex):

Source: https://socratic.org/questions/56573035581e2a07d81c00ac

I really liked those questions -- they were sort of like a puzzle. You might be told just 'Find the CFSE of high-spin [Mn(H₂O)₆]^3+' 
So you'd work out the oxidation state of the metal (so this one has to be +3 because water is neutral) and compare that to its place in the periodic table to work out how many electrons the metal currently has, so if it was a, say,  d⁹ metal originally it has 6 electrons to fill in the diagram with. It's ambiguous whether it would be high spin or low spin because water has middling strength as a ligand, Mn is in the first row, and it has a relatively high oxidation state, so you're given that in the question and you start adding electrons (as arrows) to the slots, ending up with (for 6 electrons) one in each spot plus an extra one in dxy. Then you'd use the fact that electrons on the upper level are each +0.6 and on the lower are each - 0.4 to find that it's +1.2 - 1.6 so -0.4 dOct + 1 P (pairing energy) where dOct is the octahedral field splitting parameter. If you wanted to work out the spin-only magnetic moment it's sqrt(n(n + 2) where n is the number of unpaired electrons, so it'd be sqrt(24) Bohr magnetons.

In some you don't need to be given whether it's high spin (so electrons will go upstairs rather than pairing because there's a relatively low deltaOct) or low spin (so electrons will pair before going upstairs because deltaOct is relatively high and not worth it), because you might have a really weak (like a halogen) or strong (like cyanide) ligand, where a strong ligand will increase deltaOct and give low-spin complexes; if the metal is in row 3 it's probably low-spin; and if it has certain genometries it'll always be one or the other, like if a complex is tetrahedral it's always high spin because deltaTet is only 4/9 of deltaOct and so it's always smaller than the pairing energy. You can also have square planar complexes.

There was also a cool question one year that was very much a puzzle, where you'd be given a description (so for example, come up with a good metal (M) that fits with [M(H₂O)₆]³⁺] where M is a second-row transition metal and its LFSE is -2.4 deltaOct, or tetrahedral [M(tetrachloride)]- where M is a first-row transition metal the complex has a spin-only magnetic moment of 5.92 Bohr magnetons. I liked working backwards for those but tragically it didn't come up quite like that this year.

I learned this quite well during term but I also found that reading the two Oxford primers d-block Chemistry and Chemical Bonding was extremely helpful, and I also found some other random textbook that explained how the geometry gives rise to the energies in the field splitting diagram (so for example in octahedral the six ligands are coming in along the coordinate axes so that orbitals directly on those axes have higher energy due to repulsion (are 'destabilised') and that's the dz-squared and dxsquared-ysquared orbitals, so those are higher up in the diagram and the others are lower. It's the opposite way around in a tetrahedral complex where the ligands are coming in offset from the coordinate axes. It's also interesting when you get Jahn-Teller distortion, when electrons are going to unevenly fill degenerate orbitals; the system distorts so that the ones the electrons go into are lower energy.

Molecular Orbital Theory was mostly okay except the triatomic molecules but I struggled with Quantum Chemistry. It was fairly similar to Quantum Physics from last year but alas I was not a great student last year and didn't remember anything. The maths was just very ugly and it seemed to require lots of visualisation.

Organic Chemistry was actually ... less hellish than expected. I went in with a much better attitude this year, and also the lecturer was good, and we spent the first 6 lectures of about 14 on sterochemistry and enantiomers which was interesting and didn't have the awfulness of the mechanisms aspect of Organic Chemistry. Mechanisms were slightly improved by the fact that we'd inexplicably done our Organic Chemistry labs at the start of the semester but the lectures much later, so I'd been forced to familiarise myself with the stuff before lectures while writing lab reports. Made the lab reports a lot harder but the lectures a bit easier. The mechanisms sort of clicked with me the night before the exam, although unfortunately I don't think I remembered things very well on the day and may have made some silly mistakes. I also hope my stereochemistry answers were legible because it's a very messy question. It was also helpful that the lecturer took quite a biological approach to it, showing why stereochemistry was important by talking about things like thalidomide. Poor physicists in the class but hey they had  help with Quantum Chemistry and Thermodynamics from physics and we had help with Organic (especially Carbohydrate Chemistry) and Enzyme kinetics from biology. It was funny, our Kinetics lecturer told us in the pre-exam tutorial that he wouldn't really ask about Enzyme Kinetics in the summer exams, it's more of a Schols question, when it comes up all the time for the Biology people in the summer exams. I guess we had more time with it though.

Thermodynamics Grrrrr. I didn't pay a ton of attention to this module I will admit because it was in the last two weeks before college ended for Christmas and by that point I was deep in Schols study and didn't really have brain space for anything else. It was also super weird, in that the first week seemed mad easy (the lecturer basically drumrolled up to a topic and it turned out to just be deltaG = deltaH - TdeltaS, which we learned last year and I've done a bunch of in biology, and then it suddenly got really difficult. The first half of the course did seem like we should've been taught it in first year instead of what we were actually taught in first year.

I'm also mad about the course because I emailed the lecturer to double-check some things (while I wasn't incredibly engaged with the course due to Schols, I did go to lectures and pay enough attention to know what she had said) to ease my anxiety (specifically to check that a typo in her notes was indeed a typo, and that we definitely didn't need to know the bits in black boxes), she sent back an extremely snappy email. Like I'm sure it's annoying to hear people are just trying to get through the exam for your course but not everyone wants to be a chemist, and additionally some people have anxiety my dude, even though I heard something in class doesn't mean I'm confident that's what you actually said or meant, and it's safer to have it in writing.

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So there you go. Chemistry was my least favourite module this year but it was still better than any of last year's modules except maybe Maths semester 1; I even properly enjoyed some parts of it like field splitting diagrams for Inorganic Chemistry and stereochemistry for Organic.