In key stage 3 and 4 students perform experiments that show them that copper doesn’t appear to react with acid. Part of the point of introducing a reactivity
series is so that you can later on develop this into the electrochemical series.
So when it gets to ‘A’ level it makes sense to show students that copper will actually
react with some acids. In fact its reaction with nitric acid can be particularly impressive
as we’ll see here. There’s more to an acid than a bunch of dissolved protons after all. So this experiment is a twist on a classic.
You’ll need 5 grams of copper turnings and 40 ml of concentrated nitric acid in a 1 litre
borosilicate flask. Round bottomed is preferable as you can see
the reaction better but if you need to get a thick walled borosilicate flask a buchner
flask may be a suitable alternative. Immediately we begin to form NO2 gas above
the copper nitrate that’s formed. I’ve got a delivery tube here that’s feeding
into a 1 litre flask of water. The glass wool is there to minimise diffusion
of any of the toxic NO2 that doesn’t dissolve and as such you can perform this experiment
without the use of a fume cupboard. As the reaction cools the gases inside the
flask will contract drawing some water back from the conical flask.
This is where additional steps have been taken to minimise risk.
There’s a small chance that the reaction flask could be damaged by the rapidly changing temperatures which follow the addition of the water. As such, if this experiment is performed in
the open lab, both students and the demonstrators should be protected by safety screens.
A trough, or bowl, is placed underneath to contain the mixture in case of any leaks.
I’ve placed a small LED light in mine to help the camera pick up the colours. When the water reaches the reaction flask the NO2 dissolves in and reacts with the water.
The pressures drops, still further, leading a fountain effect.
The colour goes from a greenish colour to a blue one as the incoming water negates the
copper ions. Once complete, you can give your students
a relevant selection from the electrochemical series. You can get them to explain why the
copper would react with nitric acid but not with hydrochloric acid.
There’s also some relevant points to be drawn out about ligand effects in the spectrochemical
series, if you’ve covered this.