General question about reaction formula Watch

Badges: 2
Report Thread starter 3 years ago
Firstly, feel free to tell me this is a bone-headed question! (if it is, obviously...)

I'm picking up on my A-Level chemistry again after a long gap (got a C in 1991). because my son will soon be studying science at secondary school and I want to be able to help him, and I'm considering an OU degree at some point for a second career - I get to retire from my current job at 54, and my first degree was distinctly average (2:2). Feel free to tell me I'm insane, you won't be the first...

My question is, having to learn a lot of equations for various reactions at present, I can't honestly remember from actually studying in the classroom, as A-Level chemistry is currently taught, are you expected to learn the majority of the equations by rote, or expected to be able to explain WHY certain things happen, to the extent of considering electronegativity etc for every equation?

The revision guide I'm starting with seems to present most equations as simply given facts. For example I'm reading through halide reactions today, NaBr and H2SO4 going to NaHSO4 and HBr let's say, but then some of the HBr product reduces the H2SO4 to S)2, BR2 and H2O in a second reaction - are you expected to spot this yourself, or presented with it as an established fact, to be learnt by rote?

Any thoughts?
Badges: 19
Report 3 years ago
It probably depends on the teacher (and the ability of the class).

There is the odd bit of understanding required at A-level, but to what level can an A-level student be expected to understand stuff.

Using your halide example, a student will be expected to know that NaF and NaCl will react with c.H2SO4 to make HX and NaHSO4, which is not redox. They are also expected to know that this is because the F- and Cl- ions are poor oxidising agents and that is because they are small and there is little shielding so they are good at hanging on to e-.

Br- is bigger and has more shielding, which is more important than the extra nuclear charge means that it is a better oxidising agent and hence H2SO4 can be converted to SO2, with the creation of Br2.

I- can make H2SO4 go all the way to H2S (via SO2 then S), since it is a much better oxidising agent.

That's about the level required for A-level, in terms of oxidising power. The natural question is why can Br- only make it go to SO2 and not S? This is beyond the A-level expectation of understanding.

Some reactions are expected as rote learning. Some come with a limited amount of understanding as to why. Some teachers will teach a level of understanding, even if the spec. doesn't require it.

The OCR A spec I teach (I'm guessing you're looking at AQA) effectively expects me to now mention the inductive effect. One of these years I'm tempted to try to teach the course without mentioning it. I don't see it happening any time soon.

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