[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe the reactions that take place during electrolysis of an aquous solution you should then be able to predict the products at the electrodes during electrolysis of aquous solutions and finally if you're a higher tier student you should be able to write half equations for the reactions at the cathode and the anode now I should point out that this topic covers some difficult ideas so I've split it over two videos in the last video we looked at electrolysis of molten compounds such as aluminium oxide we saw that positive ions move to the cathode which is the negative electrode here they gain electrons and they form atoms negative ions move to the anode which is the positive electrode and here they lose electrons to form atoms I'm showing you these reactions for aluminium oxide now the key fact about electrolysis of molten compounds is that we only have to think about two ions in this case the aluminium ion and the oxide ion now in this video we're looking at electrolysis of aquous solutions so let's get started the word aquous means dissolved in water so that means that we need to take a closer look at water molecules water molecules ionize or split forming hydrogen ions and hydroxide ions I'm shown you that here so when we look at the electrolysis of aquous solutions we need to consider what happens to these ions we're going to start by looking at the electrolysis of copper sulfate solution copper sulfate solution has the formula C so4 AQ this contains the copper ion cu2+ and the sulfate ion s so42 minus because it's dissolved in water we also need to consider the hydrogen ion H+ and the hydroxide ion o minus so let's see what happens when we carry out electrolysis on this solution we're going to start by looking at the cathode which is a negative electrode our solution contains two different positive ions that will be attracted to the cathode these are the copper ion cu2+ and the hydrogen ion H+ so the question is which of these two ions will be reduced at the cathode to answer that we need to look at the reactivity series here are the two ions that we're looking at and here is the rule that you need to learn hydrogen is produced at the cathode if the metal is more reactive than hydrogen so as you can see in this case copper is less reactive than hydrogen this means that copper will be produced at the cathode not hydrogen so here's the copper being produced at the cathode at the anode we make oxygen gas and I'm showing that here now I should point out that if we're carrying our electrolysis on an aquous solution then we usually get oxygen produced at the anode there is an exception to that rule which we'll see in the next video so it's really important that you watch that now it's important that the electrodes do not react with the chemicals that we're making an electrolysis scientists say that the electrodes are inert in other words they do not react Platinum is often used as an electrode as it's a very unreactive metal now if you're doing the higher paper then you need to know the half equations at the electrodes here's the half equation for the cathode the copper ions are gaining two electrons to form copper atoms that's a reduction reaction here's the half equation for the anode this is a bit more complicated but as you can see four hydroxide ions react to form oxygen gas and water losing a total of four electrons in the process that's an oxidation reaction the reaction of the anode can be written like this instead but both these equations show exactly the same reaction so please don't get confused by this remember you'll find plenty of questions on the electrolysis of aquous Solutions in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe the reactions that take place during electrolysis of an aquous solution you should then be able to predict the product at the electrodes during electrolysis of aquous solutions and finally if you're a higher tier student you should be able to write half equations for the reactions at the cathode and the [Music] anode