This is the fourth 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, if you missed tips 11-21 they’re here and tips 31-40 are here. If you want tips on how to actually study for A-level Chemistry, go here.
When doing bond enthalpy calculations it’s better to do Σreactants – Σproducts, not Σproducts – Σreactants as shown in some text books (see caption for an explanation)
If you do Σreactants – Σproducts and the reaction is exothermic, the answer will be negative (which is what it should be for an exothermic reaction) and if the reaction is endothermic, the answer will be positive (as it should be).
But if you do Σproducts – Σreactants, as it says in some text books, it will be the other way around -endothermic reactions will come out negative and exothermic reactions will come out positive. This means you’ll have to remember to add a negative sign for an exothermic reaction, and remove the negative sign for an endothermic reaction. Which is more confusing and more work than you want in the exam. So yeah, just make life easy for yourself and do it the first way.
When you’re calculating molar enthalpy change starting with q = mcΔT (then q in kJ/moles), remember to add a ‘-’ to the final answer if it’s an exothermic reaction. This often gets forgotten. It’s a shame to lose a mark for something so simple. You can tell whether the reaction is endothermic or exothermic by looking at what If the temperature increases or decreases: if the temperature increases, it’s an exothermic reaction, if the temperature decreases it’s an endothermic reaction.
When you’re doing calculations that involve ΔT (which means the temperature change) it doesn’t matter whether you’re given the temperature in ℃ or K, as a change of 1℃ is equal to a change of 1 K. This means that the temperature change will be the same, no matter which of the units its measured in. For example, an increase of 10℃ = increase of 10 K, so either way, ΔT would be 10. But if you’re doing a calculation that involves the actual temperature (if for example, you were using the ideal gas equation), it’s important to make sure you use the correct units for temperature.
Make sure you know what ethers are. They are functional group isomers of alcohols and though you don’t learn about their reactions or nomenclature, you will see them in this context. They also come up in analytical chemistry where you may be asked to work out the structure of a compound that turns out to be an ether. The ether functional group is an oxygen bonded to 2 alkyl groups.
For example, methoxymethane which is a functional group isomer of ethanol:
If you’re confused by the way compound is represented in organic chemistry questions (for example if they’ve given you the structural formula or the skeletal formula) and this is making it hard for you to get your head around the question, start by working out the displayed formula of the compound. Most students find it much easier to see what’s going on when they have the displayed formula to work with. Having to do this each time you have an organic question isn’t ideal as it’s time consuming (and remember, more often than not, it’s the functional group that’s important rather than the compound as a whole so as long as you can recognise this you’ll usually be fine) but if you’ve got to do it to answer the question, just do it.
Enthalpy of combustion of an element = enthalpy of formation of it’s oxide.
For example, this equation represents the enthalpy of formation of water and the enthalpy of combustion of hydrogen:
H2(g) + 1/2O2(g) → H2O(g)
You need to see this in context to fully understand why knowing this is important, and that’s a topic for another post, but it will help you with questions where you have to draw a Hess’s cycle and it’s not immediately obvious how you should put it together.
Make sure you know how to interpret Kc. This is pretty basic, but seems to get missed. Kc tells you the relative proportions of reactants and products, so the position of equilibrium. If Kc is less than 1, it means the proportion of reactants is greater than the proportion of products (so the equilibrium position is on the left). If Kc is greater than 1, the proportion of products is greater than the proportion of reactants (so the equilibrium position is on the right). The larger Kc, the further to the right the equilibrium position, and the smaller Kc, the further to the left it is.
Another simple one, but worth mentioning while we’re on the subject of Kc and Kp, as it often gets forgotten: the values of both Kc and Kp are only affected by changes to temperature. If you change the pressure/concentration/whatever they won’t change.
If you’re doing an amount of substance calculation that involved mass = moles/Mr at any point and you get the answer wrong, the first thing you should check is that you calculated the Mr correctly. Mistakes with the Mr are responsible for a ridiculous number of incorrect answers so there’s a good chance that this is what you’ve done wrong. Plus it’s probably the easiest mistake to spot and even if it’s not the mistake you’ve made, it’s best to rule it out before you start redoing the whole calculation. Another common mistake is not factoring in the stoichiometry (see this video lesson).
When you’re answering mass spectrometry questions and you need to give the formula of the molecular ion (and the fragment ions if this is relevant to your specification), don’t forget to include the positive charge. For example, if you were analysing propane (C3H8) in the mass spectrometer, the molecular ion would be [C3H8]+ . You won’t get the mark without the charge.