electrolysis is easy well the basics are easy anyway the only tricky bit is remembering a few rules depending on what's being electrolyzed what is electrolysis well in simple terms it's just using electricity to cause a reaction you need a power supply or a battery it must be DC direct current this is connected to two electrodes two pieces of graphite that's carbon or a metal they're inserted into an electrolyte a solution that's water with an ionic compound disol in it we call the electrode attached to the negative terminal of the battery the cathode and the one attached to the positive terminal the anode now what's funny is that there are some contexts in which these names can be swapped legitimately it's a bit complicated but you can rest assured that for GCSE Science these are definitely the right way round the negative electrode is the cathode the positive electrode is the anode now it might look like we don't have a complete circuit but we need to remember that electricity is the flow of any charge not just electrons in wires and what do we know any solution contains ions atoms that have a charge and they're free to move in solution what happens once you connect the battery or turn on the power supply will depend on what the electrodes are made from and what electrolytes you have so let's look at the first classic example involving carbon electrodes and a salt solution let's say the salt is sodium chloride our electrolyte is sodium chloride solution carbon Rod electrodes are inert which means they won't change or react during the process that's why we use them in this case next let's look look at the solution any solution is a result of an ionic compound being dissolved in water the ions partially dissociate we say we now have a soup as it were a mixture of these ions yes na+ and cl minus CU those are the sodium chloride ions but it's also true for the water it dissociates into ions too H+ and O minus these ions are always present in any solution now you know that when it comes to electrical charge Opposites Attract positive ions will be attracted to the cathode which is negative that's why we call such ions catons na+ and H+ are the catons we have here negative ions are called anion as they're attracted to the anode which is positive you might find it helpful to remember which way around these are by thinking about cats which can be nice creatures sometimes cations are positive or positive catons are positive which means they're missing electrons so when they get to the cathod they're meeting an electrode that has electrons that it's willing to give away if cations gain electrons they become neutral and turn back into atoms if you remember oil rig oxidation is loss reduction is gain of electrons we can say the catons are always reduced at the cathode reduction always happens there the opposite is true at the anode anion have extra electrons which the anode which is positive can accept from them anions are therefore oxidized at the electrode they give their extra electrons to the anode these can result in a Metal Coating the electrode or bubbles of gas being produced as the ions turn back into atoms but there's an issue only one type of cation can be reduced to the cathode and similar for the anode this is potentially the trickiest bit about electrolysis remembering what the rules are that determine what will be produced at each electrode and it's all to do with reactivity here's the first rule that you need to remember at the cathode the less reactive cation will be reduced and the more reactive cation will stay in solution so let's look at our reactivity series we can see that hydrogen is less reactive than sodium which means that hydrogen will be reduced while the sodium ions stay in solution we can write a half or ionic equation for what's happening H+ plus e minus that's the electron it picks up from the cathode makes just H the issue is is that hydrogen atoms can't exist on their own so they pair up to make H2 hydrogen gas so all we have to do is double up the ions and electrons too as you can see there are lots of metals more reactive than hydrogen so we often get hydrogen gas produced when we electroly a salt solution so the rule can be simplified hydrogen is produced at the cathode if the metal is more reactive if not the metal is reduced instead which will coat the carbon electrode the cathode the second rule to do with what happens at the anode is a bit trickier again in short if the annion in solution is a halide ion that's fluoride chloride bromide or iodide they're oxidized at the anode they lose an electron each to turn into atoms the half or ionic equation looks like this note that we put the electron on the other side we should never really put take away subtract an electron in an equation similar to hydrogen hogen gases are diatomic two atoms so they go around in pairs this then leaves the O minus ions in solution if the anion is anything other than a halide ion say sulfate nitrate or whatever it stays in solution and oxygen gas is produced at the anode instead it's extremely unlikely that you'll be expected to know the half equation for What's Happening Here but here it is anyway 4 oh minus goes to O2 + 2 H2O plus 4 electrons in the case of our sodium chloride solution that leaves the na+ and O minus ions in solution so we've actually made sodium hydroxide solution and that's all there is of the electrolysis of solutions try and use these rules to predict what will happen in another classic example the electrolysis of copper sulfate solution pause the video and see if you can figure out what will be produced at each electrode the half equations what solution is produced as a result here's the answer let's use our rule about catons copper is less reactive than hydrogen so it's reduced to the cathode copper atoms don't go around in a certain number so it's just cu2+ gaining two electrons each leaving H+ in solution we should therefore see a coating of copper appear on the outside of the carbon cathode we don't have any halide ions in the solution so it's oxygen that will be oxidized at the anode so we should see bubbles leaving the sulfate ions in solution so we've got H+ and S so42 minus ions left in solution balancing the charges on this gives us h2so4 which you should hopefully know is the formula for sulfuric acid well done if you got that right what's interesting is that if you just electrolyze pure water the H+ and O minus ions aren't competing with anything else so they have to be reduced and oxidized making hydrogen gas at the cathode and oxygen gas at the anode this is the most common method of making these gases don't forget that you can test for what gas is produced from electrolysis using the standard gas tests watch my video on that if you don't know these yet ions are free to move when an ionic compound is dissolved in solution but they're also free to move if the ionic compound is molten too no water is needed this can be used to obtain pure metal from its compound like aluminium from aluminium oxide which is found in the OR boite similar to neutralizing just water the ions aren't competing with anything else so the cations are reduced at the cathode and the anions oxidized at the anode in this specific case that results in pure aluminium coating the cathode while oxygen is made at the anode this requires a lot of energy not least because you need to melt the compound first that's why sometimes another substance will be added to reduce the melting point in this case that's something called cryolite can you give the half equations for what's happening at each electrode here pause the video and have a go and here they are unfortunately in this case while fairly inert the carbon anodes will react with the oxygen which means they need to be replaced over time there's one more thing that electrolysis can be used for purifying impure Metals copper being the most common example we come across if you have a lump of copper which has impurities in it what can you do you make it the anode the cathode ideally will be a bit of pure copper you already have the electrolyte will be a Sol ution of copper sulfate so no other ions are involved and we know the H+ ions from the water are more reactive so they won't be reduced first the copper atoms in the anode lose electrons to become cu2 plus ions they're oxidized and they enter the solution this cannot be true for any impurities though so they just remain solid as the mass of the copper anode decreases impurities merely sink to the bottom of the vessel likely a Bea if you're doing this in a school lab copper ions are then attracted to the cathode where they're reduced and turn back into copper atoms so you see the mass of the cathode increase as pure copper is deposited onto it clever right so I hope you found this helpful leave a like and a comment if you did check out the channel for more including videos going through whole papers see you 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