Skip to 0 minutes and 10 secondsElectrolysis is an important chemical process. It's used to generate an awful lot of products that are used in everyday life. We're going to have a look today at the electrolysis of brine, which is a sodium chloride solution. This is a very important industrial process as one of the products is chlorine. And this is used in the manufacture of a large number of plastics. But most importantly, it's used also in the disinfection of water. And this is one of the most major advances in public health treatment. What we're going to do today is look at the practical that most people will come across in schools, which is often find in textbooks.

Skip to 0 minutes and 47 secondsAnd that's using a solution of sodium chloride to which we add some universal indicator, and then we look at the changes of the electrodes as we pass electrical current through it. When we add the universal indicator to our solution we should get a pH of about 7. So depending on your universal indicator, that will give you a green colour. However, be warned that if you use distilled water to make this up then you may find that the colour indicates that it's an acid and it goes ruddy orange colour. This is because distilled water itself is a weak solution of carbonic acid, which absorbs carbon dioxide from the air.

Skip to 1 minute and 22 secondsSo we've added two or three spatula fulls of sodium chloride into 75ml of water in a 100ml beaker. And what we're going to add is a couple of drops of universal indicator solution. Enough to give us a decent colour, but not so much that we can't actually see through the solution. And we're going to give that a quick stir.

Skip to 1 minute and 45 secondsOK, so now we've got our solution of brine with our universal indicator. I'm going to pop my safety glasses on. And then we're going to put electrodes into the solution. We're then going to have this at around 10 volts and we're going to turn the power pack on. What we're looking for is the production of gas at the two electrodes, and we're also looking for a potential colour change in the solution as well. So in the solution you can see that we've got a blue colour around the negative electrode, and we can trace that back to the negative terminal on here. Are we're getting a red colour, which is starting to also go colourless in solution around the positive terminal.

Skip to 2 minutes and 25 secondsWe're producing two gases in here. We producing hydrogen gas, which is coming out at the negative electrode. And we're getting a chlorine gas being produced at the positive electrode. Students find this quite difficult. This is a difficult area of chemistry because students assume that when you have a sodium chloride solution, you have sodium ions and chloride ions. But actually we also have to take into account the water, which can disassociate into hydrogen ions and hydroxide ions. So the red colour we have around the positive electrode is where chloride ions are giving up one electron and becoming chlorine molecules. And they cause the water to become acidic around it as some of the chlorine dissolves in water.

Skip to 3 minutes and 11 secondsAnd eventually, you get some bleaching occur around the electrode as well because it will react with the indicator. So effectively, we're producing a solution of sodium hydroxide in here. And this is a third use of this process. As well as just generating chlorine for use in disinfection or plastics, or hydrogen as a potential fuel, we also have sodium hydroxide left in solution which is then used as a further industrial chemical. What we're going to do now is look at the electrolysis of copper sulphate solution with carbon electrodes. So we're using a very similar setup to the one that we used with sodium chloride with brine.

Skip to 3 minutes and 47 secondsBut this time we're going to use a glass U tube, which sometimes can help us see the products of electrolysis a bit easier. We've got a solution here of copper sulphate solution, about 0.1 molar solution. And we're going to our U tube and then connect up our electrodes. So we're using about 75 millilitres or so of copper sulphate solution.

Skip to 4 minutes and 9 secondsAnd remember that when you're finished with this solution, don't pour it down the sink because it's not good for the environment. And then again to lower our electrodes in. And again, using our power pack turn on. And then we should start seeing some results at our electrodes. This time what we're looking for is at our electrodes we should see gas being produced at one electrode and nothing appearing to happen at the other electrode to start off with. However, if we wait a couple of minutes, then we should be able to see something produced at both electrodes. So what we're seeing here now is we're seeing bubbles at the positive electrode.

Skip to 4 minutes and 45 secondsBut initially, we can't really see much at the negative electrode until we lift the electrodes out of solution. At the positive electrode we're producing oxygen. And that will actually start to react sometimes with the carbon electrodes to give us carbon dioxide. But that's the approach that's best left for further study rather than trying to deal with that now. So we'll turn the power pack off, and then we can lift the electrodes out. And now if we look at the electrodes, we will see that on the negative electrode we've got some red deposits, which are pure copper being deposited on the carbon electrode. This process is used for refining copper.

Skip to 5 minutes and 20 secondsAnd for that, we have a pure copper rod on the negative electrode and we have impure lump of copper on the positive electrode. And as the reaction happens, the impure gets smaller and pure copper gets deposited on the pure copper wire. And that's used for commercial refining of copper.

Introducing electrolysis

The video above is the standard way that many of you will have carried out electrolysis at school. The first experiment starts with a chemical familiar to all students, common salt (sodium chloride) dissolved in water to make a salt solution (brine). One of the products of this reaction is chlorine, which is a very important industrial chemical. The second experiment uses copper (II) sulfate to produce copper metal.

Context

Electrolysis is an important industrial process to produce products that are used in everyday life.

Chlorine, produced in the electrolysis of brine (sodium chloride) solution is used for drinking water purification. The production of chlorine to form compounds to sterilise water has been one of the most important contributions to public health, giving millions of people clean drinking water, free from microbes.

Without electrolysis of other solutions the purification of metals such as copper would not be able to take place. Copper is the most vital metal in electrical systems, without electrolysis, the development of electricity supply would have been very limited. Similarly, electrolysis is used for electroplating (the coating of objects with a fine metal coating). Uses range from car wheels to manufacturing jewellery, as electrolysis can let us deposit a controlled amount of one metal onto the surface of another.

In the next steps we consider how we might engage students with this common practical.

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This video is from the free online course:

Teaching Practical Science: Chemistry

National STEM Learning Centre