How to unpack what you know and answer the question

Students sometimes tell me they know the material but still can’t answer certain types of questions. They often say that they can’t apply the Chemistry they know because they either don’t know where to begin or because there are many potential answers. Open ended questions and unstructured questions pose particular problems because they often have many arguments and ideas that need to be considered. Here is a brief guide to a general approach to solving these problems that I go through with in my online and in person Chemistry tutorials where the focus is on getting the key ideas in front of you and then matching the ideas to the question.

Contents

An example question on ionisation energies

Take the following A-level question on ionisation energies as an example.

Which element in the periodic table has the highest first ionisation energy? Justify your answer. (3 marks)

Define the problem

The first part of this problem is simple, find the element with the highest ionisation energy. This could be achieved, for example, by using a chart to look up the first ionisation energies of all the elements. Googling for answers is a common study method used by GCSE students and it mostly works well for one or two mark questions. It would probably also work for the first part of this question, too.

You might want to think carefully if this is an appropriate strategy for A-level though, as it is likely that next time around the question could be about elements in a particular group or period, or perhaps it will ask about the second ionisation energies. Hopefully you realise that learning all these answers off by heart will be far too much of a challenge with the large number of variations possible at A-level, although a large number of students persist with this for a considerable time in sixth form!

The second part of the question, justifying why that element has the highest first ionisation energy, is the key to understanding what you need to learn so that you can answer a wide variety of similar questions. I refer to this as focusing on the big ideas or key ideas and seeing which apply to the question using a best-fit approach.

Get the key information in front of you

Mind maps

There are a few big ideas associated with ionisation energies and it will be a worthwhile to try jotting them down in a list or perhaps with a concept map or mind map. A concept map is the same as a mind map (or spider diagram) but the connections between the ideas are made explicit on the diagram. Please have a go at doing this yourself before reviewing the points I have made below. Putting all of this information in front of you helps to get a wider view of the possible explanations and also lets you see where there are gaps are in your knowledge.

A mind map of a few of the key ideas could look like this (Created using Mindmup)

Make a list

An alternative to a mind map is to get the information in front of you in a list. A bullet-point list of a few of the big ideas would look like this:

  • For the 1st IE, one electron is removed from one mole the element in the gaseous state to produce one mole of gaseous ions
  • The electrons are removed from the valance shell because it is highest in energy
  • More protons increases the energy required for ionisation
  • Ionisation energies are endothermic
  • More closed electron shells increase the shielding of the outermost electrons from the nuclear charge and reduce the energy required to remove the electron
  • The energies of the valance shell change with the principal quantum number or period (E is proportional to 1/n^2)
  • The effects of shielding and more protons effectively cancel each other out going down a group
  • Electrons in higher electron shells are on average less close to the nucleus and therefore experience less force

Of course, you may wish to add or remove detail or change the order of this list, but having all the options in front of you will mean you can review all of them and select which ideas are essential to answer the question. Some of the points, although correct, are not appropriate or necessary to answer this question. Practice recalling these key ideas is still important because the ideas need to be familiar enough for you to consider them without having to resort to lists under exam conditions. Experts rifle through a series of possible options before they choose the most appropriate solution, this is why they make better decisions! Take time to think each idea over and practice recalling them in your weekly study reviews.

Answer the question

So, going back to the original question, which element has the highest first ionisation energy? From the list of key ideas, it seems that elements have higher ionisation energies when they have fewer completed electron shells and more protons. The elements with the smallest number of electron shells are hydrogen and helium. Helium has more protons, so a good guess would be helium. In this question, there were probably two marks for justification and only one for getting the correct answer, so be sure to back up the effect of fewer electron shells with some of the points made above.

If you would like some additional help with your studies, please email for my current availability.