A-Level Chemistry tips 1-10. This is the first 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.
When balancing equations, leave the Hydrogens and Oxygens until last. You’ll often find that they automatically balance once you’ve sorted everything else, which saves you a job. But if you go in and tackle them too early you may find that it messes up the rest of the equation to the point where you can’t fix it without starting from scratch.
When writing ionic equations, make sure you include species that change oxidation state and/or change physical state. So when you’re figuring out what the equation should be, make sure you actually work out what the oxidation states and physical states are.
Make sure you know how to write half equations for the redox reactions of group 2, group 7 and period 3. These aren’t in most of the textbooks, but they do come up in the exams.
If formula triangles help you learn the equations, just use them. Some teachers get a bit snotty about this, and I get this to some extent as I wouldn’t be happy if you had to rely on them to help you rearrange the equation. But students who are visual learners often find them really helpful as a memory aid for learning the equation, so if that includes you just go ahead and use them. Do whatever works for you.
When writing ionic equations, do a final check to make sure the total charge of the reactants is the same as the total charge on the products. This is really important. With ionic equations it’s perfectly possible to come up with something that’s balanced in terms of the atoms, but still incorrect. The only way to check that you have the correct equation is to make sure it’s balanced for both atoms and charge.
Revise expanding brackets and simplifying equations from GCSE maths. You’ll need to be able to do this for some of the calculation questions for A-level Chemistry. Equilibrium (kp and Kc) and mass spectrometry are topic areas where this is likely.
When drawing displayed formulae make sure you include every atom and EVERY bond. Forgetting to include the bond between the O and the H in the OH group is a really common mistake (I bet you’ve done this more than once).
For calculation questions, give your final answer to the same number of significant figures as the lowest number of significant figures in the data. For example, if you have 3 pieces of data, 2 of which are given to 3 significant figures and the other to 2 significant figures, you need to give your final answer to 2 significant figures. This is because your answer can only be as precise as the least precise piece of data.
(Following on from tip 8)… but don’t round too early. You’re less likely to end up with a rounding error if you keep in one extra significant figure than you need for your final answer then round up at the end. If you often find that you almost get the correct answer, but not quite, you’re probably rounding too early.
Practice unit conversions and make sure you have a solid thought process to help you decide whether to multiply or divide. It’s suprisingly easy to get this wrong. In particular, make sure you know how to convert between m3, and cm3 and dm3. This is the one that seems to go wrong most often.
A-level chemistry tips 11-20
See the next post in this series for more A-level chemistry tips.