in this video we're going to focus on the electrolysis of water so what exactly is electrolysis it's the process by which we use electricity to break down a compound into its component molecules an example of water we can use electricity to break down this compound into hydrogen gas and oxygen gas now let's talk about how we can actually do this a quick and simple way is to take a beaker fill it with water and we need to use two electrodes one of them is going to be the anode and one will be the cathode and i'm going to use two carbon based graphite electrodes and i'm going to connect the battery to it so the long side is the positive terminal of the battery and the short side is the negative terminal of the battery i'm going to use a 9 volt battery so we're going to add water and i'm going to dissolve sodium hydroxide into the solution so what's going to happen once i connect the battery electrons will flow towards the positive terminal of the battery and away from the negative terminal of the battery now in an electrochemical cell electrons flow from the anode to the cathode which means that the electrode on the right represents the anode and the electrode on the left represents the cathode now you need to know that oxidation always occurs at the anode and reduction always occurs at the cathode so what is the oxidation half reaction that occurs at the anode at the anode hydroxide is oxidized into water and oxygen gas and it's going to give off four electrons in this process well technically it's oxidized to oxygen gas but what is a byproduct of that half reaction now at the cathode water is reduced to hydrogen so it's going to pick up some electrons and it's going to turn into hydrogen gas and it's going to generate hydroxide ions so let me put a line to separate these two half reactions so at the anode you have the evolution of oxygen gas oxygen gas is going to come out of the anode and at the cathode hydrogen gas will be produced and so that's the basic setup of an electrochemical cell so if you add electricity to an aqueous solution of sodium hydroxide you're going to get hydrogen gas at the cathode which is the electrode attached to the negative terminal of the battery and you're going to get oxygen gas at the anode which is the electrode attached to the positive terminal of the battery now let's talk about what happens in between these are the two electrodes that we're talking about earlier and we have a battery attached to it now the anode is attached to the positive terminal of the battery so therefore it's going to have a positive charge on it and the cathode is attached to the negative terminal of the battery so there's going to be an electric field that flows from the positive plate to the negative electrode or the negative plate now inside an electric field a positive charge will fill a force that will accelerate it in the direction of the electric field a negative charge will fill a force that will accelerate it opposite to the direction of the electric field so the sodium ions they carry a positive charge and so they will fill a force that will accelerate them towards the negatively charged plate or the cathode since the cathode bears a negative charge the hydroxide ions will fill a force that will accelerate them towards the positively charged anode now once the hydroxide ions make contact with the anode they will give up their electrons causing the electrons to flow in the circuit and as a result the hydroxide ions are oxidized into oxygen gas as they give up those electrons and what was the reaction that i wrote before it was o2 plus 2h2o plus 4 electrons now sodium is not going to be reduced at the cathode and let's talk about why if sodium picks up an electron and turns into sodium metal the cell potential for this process is not favorable it's a negative 2.71 whereas if water picks up the electrons and is reduced to hydrogen gas the cell potential is only negative 0.83 so comparing these two values we could see that it's a lot easier to reduce water at the cathode than sodium at the cathode so for the most part this reaction doesn't happen and if by some miracle it did happen notice what's going to happen so let's say if this if some sodium metal was actually reduced in the cathode sodium will immediately react with water to create hydrogen gas and sodium hydroxide because it's known that if you put sodium in water it reacts basically explodes in water and so hydrogen gas will be produced so therefore even if sodium did form it would immediately react with the water molecule surrounding it turning back into the sodium plus anion i mean but sodium plus cation so because this cell potential is highly unfavorable it's not going to form on the cathode and if it did it's immediately going to go back into the na plus state because that state is a lot more stable now water in this case is going to be oriented in this direction the oxygen part of water bears a partial negative charge and the hydrogen i need to put a 2 in front of it let me redraw that the hydrogen bears a partial positive charge so notice that the hydrogen of water faces the cathode the oxygen faces the anode the oxygen is attracted to the anode because they're oppositely charged and the partial positive charge of the hydrogen is attracted to the negatively charged cathode and so the water molecules that are close to the cathode they can easily be reduced into hydrogen gas those water molecules can pick up the electrons and some of it will turn to hydrogen and hydroxide will be produced now let's go back to this reaction now let's understand how sodium reacts with water if by some miracle is produced at the cathode so immediately sodium will give up its electrons to turn into this ion and then water is going to pick up those electrons and be reduced to hydrogen gas and hydroxide so if we combine these two reactions we could cancel the electrons and so we're going to get on the left side sodium water hydrogen gas and we can combine these two into sodium hydroxide so that's why sodium will not form at that at the cathode if it did it's going to immediately react with water producing this stuff now let's put it together so at the anode where oxidation occurs this is the half reaction that we have so we said that hydroxide is going to be oxidized into oxygen gas and water is going to be a byproduct of this reaction and the cell potential for this is negative 0.4 volts now reduction is going to occur at the cathode and so at the cathode water is going to be reduced now i'm going to multiply that reaction by two so it's going to be four h2o molecules and it's going to pick up four electrons to produce two hydrogen gas molecules and four hydroxide ions and so the cell potential for this is negative point eight three volts let me write that better now let's add these two reactions now you need to make sure that the number of electrons are equal so that when you add them they cancel notice that the hydroxide ions cancel as well and so we have four water molecules on the left two on the right so if we subtract both sides by two h2o molecules this will disappear and this will be reduced to four minus two which is two h2o so on the left side we have two water molecules and on the right side we have an oxygen gas molecule and two hydrogen gas molecules and the overall cell potential for this when we have a one molar sodium hydroxide solution so that's the standard concentration if you change the concentration the cell potentials will change but at a standard concentration of one molar sodium hydroxide it's going to be negative point four and negative point eight three together if you add those two the cell potential is negative one point twenty three so this is the minimum voltage that you need to drive this reaction forward now as you can see the cell potential is negative so this process is non-spontaneous the way it's written which means that water doesn't naturally just i gotta fix something this is supposed to be two hydrogen gas molecules not two h2o molecules so water doesn't naturally decompose into oxygen and hydrogen you got to put energy in order to break it down into those component elements so what is very stable and so that's electrolysis the process of using electrochemical energy or electric energy rather to decompose water into its component elements now the reverse reaction is spontaneous so hydrogen gas mixed with oxygen gas will decompose or rather will react to form water and the cell potential for that is going to be positive 1.23 as opposed to negative 1.23 so this will happen spontaneously now you may need a spark to speed up the reaction to get it started so even though this reaction is thermodynamically favored it's not very fast unless you increase its temperature or ignite it with a spark it can still happen but it happens slowly without meeting the activation energy needed to get this reaction started so that's it for this video thanks for watching and have a good day you