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Current time:0:00Total duration:7:17

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alright Part B a fresh zinc air cell is weighed on an analytical balance before being placed in a hearing aid for use as the cell operates does the mass of the cell increase decrease or remain the same justify your answer to Part B 1 or B I have B 1 in terms of the equation for the overall cell reaction so let's just think about it a little bit does the mass of the cell increase decrease or remain the same let's go to our original reaction over here this is our total reaction the zinc is part of the metal air cell so that would contribute to its its it's weight before hand and then it reacts with the oxygen well where is this oxygen coming from well as oxygen is coming from the air and essentially it gets incorporated into the zinc so before the oxygen is in the air but then after the reaction after the total reaction it gets incorporated with the zinc I guess I should say not into the zinc or it gets incorporated with the zinc it as zinc oxide and so now the oxygen is part of the cell part or contributes to weight of the cell so contributes contributes to the weight the weight of cell it doesn't contribute before their action but contributes after the reaction so our the mass and I guess you could say the weight is going to increase so I'll just write increase and part two we need to justify our answer in terms of the equation for the overall cell reaction so let me just write the overall cell reaction so to zinc molecules react with one molecular oxygen 102 to yield to zinc oxide two zinc oxide molecules well what we could say is is that this was not part of cell before reaction not part of cell before reaction is coming from the air before reaction and then you could say part of cell after reaction part of cell after reaction and we could write it out in words if we like after reaction and so I could say oxygen oxygen comes from air from from air air to be incorporated into the zinc oxide to be incorporated into zinc oxide which contributes to weight a cell or to the mass of cell which then contributes to mass of cell to mass of cell alright I feel pretty good about that now let's do Part C the zinc air cell is taken to the top of a mountain where the air pressure is lower will the cell potential be higher lower or the same as the cell potential at the lower elevation justify your answer to the first part based on the equation for the overall cell reaction and the information above well let me just write the overall cell reaction again so let me the overall cell reaction I'll rewrite it again so we have two zinc molecules we write that a little bit neater to have rotor Messier to zinc molecules one molecular oxygen were going to have this ingrained in our brain by the time this is done I'm writing it so frequently we get to zinc oxides to zinc oxides and so what's going to happen if if we go to the top of the mountain where the air pressure is lower we have just fewer air molecules in general that means we're also going to have lower oxygen molecules so we could say that the partial pressure of oxygen is going to be lower so we could say partial pressure of reactant will be lower partial pressure there's just going to be fewer oxygen molecules from the air being able to bump and react with the zinc molecules well actually it's not reacting with the zinc molecules as we saw in the original reactions when we look at the half reactions fewer oxygens from the air just bumping in the right way with the with the with the water molecules and actually the electrons that are coming in through the circuit there's going to have fewer of them bouncing around being able to do this reaction so you could say partial pressure partial pressure of reactant or maybe I say partial pressure or you could also do the concentration of the reaction there's going to be just reactant there's going to be less of it around partial pressure or we could say concentration concentration lower lower so lower so lower cell potential lowers cell cell potential so actually I just did in Reverse I answered part two first and so part one is lower so we're going to have a lower cell potential lower cell potential and it's really important to get the conceptual understanding of why it's happening remember when we look at the half reaction what's happening what is happening at our cathode we have oxygen coming in from the air and it gets into the kind of the pores of this porous substance and when the oxygen the the o2 molecules bump in just the right way with the water and the electrons coming in they just kind of go together and react you get your hydroxide for they react to form the hydroxide now if you have less you know at a lower elevation you're going to have a lot of oxygen going to have a high partial pressure of the molecular oxygen so there a lot of them are any moment in time we're going to be available to bounce around and react but if you go to a higher relevation you're just going to have of these characters so you're going to have fewer of these characters so you're going to have a lower partial pressure of oxygen or you can think of it you have a lower concentration of oxygen inside the pores so it's going to be harder for this reaction to move forward and you can think about the extreme situation but if you kept going to a super high elevation if you're eventually going into space where you have very little oxygen or no oxygen at all well then then the reaction is going to stop all together so hopefully hopefully that that makes let me just go back to Part C here I'm in trouble my well this go back to Part C and there you go
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