Colour

Topic 13 is one of my favourites – how do you find it?

In this topic, we start to explore complex ions the reasons why these ions are coloured. How do you deal with this topic.

I like the fact that it is very visual. It can make a nice transition from topic 3 which is very theoretical –after a couple of weeks of topic 3 theory, the students are usually ready for some lab work!

Do you carry out lab work?

I like to show the students a few reactions – some of the reactions are off topic / syllabus but it stimulates them.

This may involve adding HCl and NaOH to random transition metal solutions – cobalt and nickel are good ones to use. Precipitates from on addition of the alkali but the reaction can be reversed by adding some acid. This is also a good practical to show with regards to introducing reversible reactions. The colour gets diluted it captures the students attention all the same.

A good class experiment For example, showing some potassium dichromate (IV) solution (orange) and then adding some alkali (eg, NaOH) and watching the colour changing to yellow (chromate (IV). Addition of sulfuric acid drives the equilibria the other direction. The RSC have a good practical on this that you may want to try. You can find out more about it here:

http://www.rsc.org/learn-chemistry/resource/res00001710/an-equilibrium-involving-chromate-vi-and-dichromate-vi-ions?cmpid=CMP00005224

I also like to do some colorimtery with my students. Using a colorimeter I ask them to make up some standard solutions of CuSO4 (from 01. mol dm-3 to 1.0 mol dm-3 – good quantitative chemistry revision)  and then provide them with a solution of Cu SO4 of unknown concentration and ask them to determine its actual value. It’s also a good lab to do with respect to plotting graphs, lines of best fit, error analysis and can be carried out in less than an hour if the students pool results (good for your ATL collaboration!)

coloured-transition-metal-solutions

Source: https://commons.wikimedia.org/w/index.php?curid=2009079 – if there is a problem with this image, please contact me and I will remove it.

Another good free resource comes from the ‘chemsheets’ website (http://www.chemsheets.co.uk/Chemsheets%20A2%20038%20(Transition%20metals%20introduction).pdf)

It is designed for the UK ‘A-Level’ course but there is plenty of crossover to IB Chemistry.

When it comes to resources I also use quite a few free resources from ‘compound chem’ – this website has been mentioned before but it is updated weekly so it is worth bringing  your attention to it again.

With respect to transition metals there is a particularly good section on coloured compounds (http://www.compoundchem.com/category/colourful-chemistry/).

In this section you will find articles such as ‘Chemistry of the Colours of Blood’ as well as a colourful ‘Transition Metal Ion’ download.

So, how do you teach this topic? As ever, I’d love to hear your ideas and the URL’s to any good website you may have, so please do share them below.

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