A-Level Chemistry tips 21-30
This is the third in a series of posts to help you learn how to approach problems in Chemistry, show you easy ways to do the hard stuff and help you avoid common mistakes. If you missed A-level Chemistry tips 1-10 you’ll find them here, and if you missed tips 11-21 they’re here. If you want tips on how to actually study for A-level Chemistry, go here.
This set of tips has a heavy focus on organic chemistry. If this is something you struggle with, you’ll also find my framework for learning the reactions helpful.
When you see a cyclic compound, don’t automatically assume that it contains benzene. There are plenty of cyclic aliphatic compounds which may look like they contain benzene at first glance, but they don’t and they behave very differently. You might think ‘Oh, I’d never do that’. But most students do and you probably will – as soon as you’ve learned about Benzene, you’ll start ‘seeing’ it everywhere.
Remember that organic chemistry is all about the functional groups. if you know what functional group a compound contains, you know what reactions it does. So don’t get confused when you see an unfamiliar compound – just identify it’s functional groups and take it from there.
When you add a reagent to a compound with more than one functional group, bear in mind that more than one of them may react. E.g. if you add XS acidified potassium dichromate to a compound that has aldehyde group and a primary alcohol group and reflux it, both will be oxidised to carboxyl groups.
When you’re doing percentage yield calculations, stop and think really carefully about what you’re doing. These go wrong far more often than they go right. The main reason is that students use the mass of the reactant as the theoretical yield. The reason you’re given the mass of the reactant is so that you can use it to work out the theoretical yield, so make sure this is how you use it. Another common mistake is forgetting to factor in the stoichiometry – the moles of reactant is not necessarily the same as the moles of product.
When you draw an organic compound, make sure you count how many bonds each carbon has. Each carbon should form 4 bonds, so if it doesn’t you’ve missed something. This is just one of those brain-fail things that we all do, but you need to make a conscious effort to check your work so that you catch it – silly mistakes like this can add up to costing you a lot of marks.
If you see a carbonyl in the middle of a chain, you might automatically make the assumption that it’s a ketone. But before you jump the gun, take a look at what it’s bonded to. If the carbonyl is bonded to an oxygen, the compound is an ester, not a ketone. If it’s bonded to an N, the compound is an amide, not a ketone. This might not cause you any difficulties in simple compounds, but if you’re presented with an unfamiliar compound in the exam, you might get caught out by this – I still have to remember to double-check that the compound is what I think it is myself.
When you’re writing equations for enthalpy and entropy questions make sure you include state symbols. You will be marked on this – the state symbols are really important here as the enthalpy/entropy changes differ depending on the states. For other types of questions the state symbols are less important, unless you’re specifically asked to include them in the question instructions.
Make sure you learn the organic reactions forwards and backwards. What I mean by this is learn what compounds can take part in a reaction, and learn what reactions are required to make a product. For example, nucleophilic addition-elimination is a reaction of the acyl chloride. If I showed you an acyl chloride and asked you what reaction it would do, I bet you’d be able to suggest nucleophilic addition-elimination. But if I showed you an amide, would you be able to suggest a reaction that could be used to make it? I bet you’d be far less likely to get this right. But it’s the same reaction – if you do a nucleophilic addition-elimination with an acyl chloride and ammonia you’ll make an amide. So the reaction forwards is that acyl chlorides can undergo nucleophilic addition-elimination reactions. The reaction backwards, is that nucleophilic addition elimination reactions can be used to make amides.
A really simple tip, but make sure you know that the solubility of gases decreases as the temperature increases. This idea sometimes comes up as part of exam questions and I’m often surprised to find that students don’t know this. This explains why fizzy drinks get flat when you leave them out of the fridge – the carbon dioxide evaporates.
When you’re working out the shape of a molecule, count a double or triple bond as one bonding pair of electrons. The explanation for this is beyond what you learn for A-level. This explains why alkenes have a bond angle of 120° around the double-bonded carbons – 1 double bond and 2 single bonds = 3 bonding pairs.