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Current time:0:00Total duration:11:55

Video transcript

metal ear cells are a relatively new type of portable energy source consisting of a metal anode an alkaline electrolyte paste that contains water and a porous cathode membrane that lets in oxygen from the air a schematic of the cell is shown above and so we see the different parts right over here we see the metal anode it's an anode so this would be the negative terminal of our power source it'll be the source of electrons we have an alkaline electrolyte paste that contains water let me underline that there's a lot of interesting words there alkaline electrolyte paste that contains water that's this right over here in the middle they when they say it's alkaline that means that it's going to be basic versus acidic it's going to have a pH higher than 7 electrolyte paste an electrolyte is something that if you dissolve it in a polar substance a polar substance like water well then the solution is going to be good at conducting electricity so what they tell us it this electrolyte paste is going to be something that's good at conducting electricity and they tell us it's alkaline so it's going to be basic it's going to have a pH higher than 7 and a porous cathode membrane that lets in oxygen from the air so this is the cathode this is going to be the positive terminal this is where the electrons are going to be attracted to and it's porous cathode membrane that lets in oxygen from the air and you so you can see that oxygen is available in this membrane it sits so it somehow is allowed to seep in because it is porous reduction potentials for the cathode and three possible metal anodes are given in the table below so remember reduction potentials you can view these are reduction is is is the gaining of electrons so this is the potential to gain electrons is one way to think about a reaction that somehow involves electrons being incorporated in some way and right over here you have molecular oxygen reacting with water which for every molecule of molecular oxygen you have two molecules of water four electrons and then they react to form four hydroxide anions so once again this is a reduction reaction because those electrons are being incorporated into into the molecule and so you're left with the hydroxide right over here and if you look at our our metal Aircel up here where is that occurring well you need oxygen you need water and you need electrons well if you look at the cathode right over here you got your electrons coming in you got your oxygen coming in and water we can assume is available in this area the electrolyte paste has water in it this the cathode is porous it's a porous membrane type substance so they're all available over here so you can assume that that reaction could happen right over here in the cathode so let me just write it o to in as a gas plus so for every molecule of o2 you're going to have two molecules of h2o in a liquid form and then you got your electrons coming in on the wire I guess you could say and that is going to yield four hydroxide anions for hydroxide anions and they're in a solution they're part of that electrolyte paste that's in water they're an aqueous solution a water-based solution and so that's going to happen right over there so the hydroxide anions I can even say for hydroxide anions are going to be produced every time this reaction is happening and they tell us the reduction potential this has a positive potential has a positive voltage and notice they say the reduction potential at pH 11 so that's consistent with a alkaline alkaline conditions because this paste might seep in through here a little bit and obviously you have all this hydroxide being produced so you're going to have a higher pH a more basic pH and so the fact that this is a positive voltage so plus 0.34 volts means that that the the potential is going in this direction the electric potential favors this reaction to happen going from left to right now they also said they gave us the reduction potentials for the cathode which we just talked about this is the reduction potential for the cathode and three possible metal anodes are given in the table below so here there are three possible metal anodes so we're going to zinc we could have sodium we could have we could have calcium and so just go with zinc since it's the first one listed here so if we assumed that the metal here was zinc what's going to be going on we're going to have the zinc reacting with these hydroxides that are being produced over in the cathode and then you're going to use that to produce zinc oxide water and electrons so we're actually going to have the reverse of this reaction you're going to have let me write the reverse you're going to have zinc in the solid state this whole animal anode is made out of metal zinc and then for every molecule of that you're going to have two hydroxide anions that are dissolved in water that's what makes the electrolyte paste alkaline and you are going to then go to your they're going to react and you're going to form zinc oxide zinc oxide in the solid state plus water in the liquid state plus two plus two electrons and so this reaction that I just described this is going to be happening right over here so you could think of it as the hydroxide anions get formed at the cathode and then they move their way over to the left to the anode where they react with the zinc and they turn they or the zinc reacts and it forms a zinc oxide so you have more and more zinc oxide being formed water which can then see if it's way back into the electrolyte pace and then it can eventually react again at the cathode and then you have these two electrons so these electrons this turns into an electron source right over here and then the electrons would migrate and then they can go to the positive side the cathode to react again and so you can start to see how this will be an energy source that you can tap into this current that'll be dental form to to do some to do some useful work that's why you have an energy source all right so let's read the questions now that we have a decent understanding of what's going on early forms of metal air cells use zinc as the anode all as the example we just thought about zinc oxide is produced as the cell operates according to the overall equation below two to four every two molecules of zinc and one molecule of molecular oxygen you produce two molecules of zinc oxide use the data in the table above calculate the cell potential for the zinc-air cell so let's think about let's break down this reaction we actually we can just break it down into into these two steps here because notice this is this is you can't see them both at the same time but this the the the things that are reacting you have zinc let me underline the things that are reacting you have zinc you have zinc you have oxygen you have oxygen and so if you return this if you return the second reaction around like we did before and let me rewrite it right let me rewrite both of them actually a little bit lower right over here so you have this top reaction so you have o to gaseous state plus 2 h2o liquid plus 4 electrons oops 4 electrons yields 4 hydroxide anions and aqueous solution and then this one we said we're going to go in the other direction so you're going to have zinc you're going to have zinc in the solid state plus 2 hydroxide anions and then that's going to yield zinc zinc oxide in the solid state plus liquid water plus 2 electrons so when you see this you see that we are reacting we have the oxygen we have the zinc which are the two things we were on interact here and if you want to make them equivalent in terms of the number of molecules we could say alright with one molecule of molecular oxygen and we need to zinc's so let me multiply this whole this whole reaction by 2 so it's 2 zinc's react with 4 hydroxides react to produce two zinc oxides 4 oh sorry 2 molecules of water 2 molecules of water and then 4 electrons I just said oh if this reaction happened twice you would have just twice as many of the reactants and and the products here and then notice now you have a molecule of molecular oxygen 2 molecules of zinc molecule of molecular oxygen two molecules of zinc if all of this were to happen you are going to produce two molecules of zinc oxide and what about everything else that I that I put that we're in these reactions well the water is here but then water also gets two molecules of water two molecules of water so it's I guess if you look at the total reaction we're neutral there we haven't produced or lost any water you have four electrons you have four electrons so that's why they didn't write it in this total reaction here and these four hydroxide molecules iron ions well they're going to be used up in this second part of the reaction so we're in neutral there they get produced but then they get used so if you look at the total reaction you're using the molecular oxygen and the zinc to produce zinc oxide and so they say use the data in the table above calculate the cell potential for the zinc air cell well the potential for this top reaction is zero is positive 0.34 volts and now what's the reaction what's the potential for this bottom reaction well it's the reverse of this reaction so this reaction had this reaction has a negative potential but since we reverse it it's going to be a positive potential so it's going to be positive 1.3 1 volts now some of you might be saying well not only did we reverse it we multiplied it by two we said two of those reactions when we multiplied the voltage by two well if we were just thinking about the energy released from a reaction well they absolutely if you're gonna have the reaction twice or twice as much of the reaction you're gonna have twice as much of the energy but voltage you have to remember that's you can view it as potential energy per unit charge so since we're not this isn't kind of your absolute energy this is your energy per unit charge is one way to think about it when you're doing it more of it it doesn't change the actual voltage so that's an important thing thing to think about if this was talking about total energy or enthalpy or something like that then if you're if you were to multiply both sides by two you would multiply the energy released or taken by two but since we're talking about voltage voltage isn't a quantity that depends on the number of charges or the number of molecules you're doing - it's a per its what is the potential per unit charge is one way to think about it and so for this whole reaction we would just add the combined voltage of both of these and then you would get one point six five one point six five volts now let's do part two the electrolyte paste contains hydroxide and hydroxide ions on the diagram of the cell above draw an arrow to indicate the direction of migration of hydroxide ions through the electrolyte as a cell operates well we've already thought about that the hydroxide gets produced at the cathode with this reaction and then it gets used up in the anode so the hydroxide is going to be moving the hydroxide is going to be moving in that direction
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