Electrochemistry part 17

Ajouté : 2026-05-12

[00:00:00]next we'll do concentration cell now what is the concentration cell write the definition first concentration cell is a cell in which the two electrodes are of the same material in which the two electrodes are of the same material and the concentration of the electrolyte is different in which the two electrodes are of the same material and the concentration of the electrolyte is different let us say you are having uh what you can say a cell which contains two zinc electrodes same material and the concentration of the zinc sulphate solution in the electrolytic electrolytic solution is different let us say one ah is one molar another is two molar all right the definition can be in this way also radar concentration cell is a cell in which in which the electrodes are of different concentration but the electrolyte is the same in which the electrodes are of different concentration but the electrolyte is the same all right the first one we generally prefer that is electrodes are of the same material and the concentration of the electrolyte is different that means here you will be taking that is let us say i have taken this particular cell which contains a zinc electrode and this is your zn so4 solution which is one molar we can say that and we have taken another one another one another cell we have taken it it also contains znso4 solution this also contains zna support solution but the concentration of the electrolyte is two molar we can say that and here the electrode is what that is the same thing it is a zinc electrode it is being connected ah what you can say by the help and here we are having a voltmeter attached to it and here this is your salt bridge that is your salt bridge here we are having a salt bridge here all right just a rough diagram i did it so this is known as a concentration cell we can say that fine now how to calculate the now the emf of the concentration cell now you got it what is the concentration now we will see how to calculate the emf now here it can be in the same way also suppose we have used two hydrogen electrodes all right two hydrogen electrodes she have used it and i have taken hcl solution of different concentration i have taken hcl solution of different concentration and here i have used the shg the hydrogen gas electrode and used it that also can be taken okay understood yeah so now you see so what i said we need to calculate the formula for concentration cell we need to calculate the formula so same thing e cell is equal to now let me write first the equation zl slash zn 2 plus double slash zn 2 plus slash zn so this is your solid this is aqueous this is again aqueous and this is again your solid we can say that so i can write it in this way let us say the concentration of this is c y let us say and the concentration of this is nothing but c 2 all right c 2 here it is considering that c 2 is greater than c 1 now generally this is your oxidation electrode and this is a reduction isn't it this oxidation potential this is reduction potential generally where the oxidation takes place the concentration is less and generally where reduction takes place the concentration is this more why because always the current flows from higher potential to that of the lower potential this is higher and this is lower clearer this particular thing fine so zn minus 2e minus this one we are writing first this one see if zn2 plus all right so what is this concentration c1 next in the second one what is happening zn2 plus what is its concentration c2 z2 plus plus 2e minus will give you what z now i am going to add it up so what will happen this zinc and this zinc also is going to cancel if i am going to add it about will i get it here see zl plus that into plus gives z n plus n two plus give z n two plus plus does that zinc zinc will get cancelled for the solid no both the solid of the same electrode they will get cancelled and the concentration solid is how much it's once it will get cancelled so what i am having it here zl2 plus the change c2 gives zn2 plus concentration is c1 so this is your product and this is your reactant all right fine now i put the value here all right and put the value so ah i am rubbing it at the top the first one i'll rub it and to calculate the value of the e cell because i don't have the space here so now see here e cell will be nothing but e naught cell so minus 0.0591 by n log concentration of product so what is the concentration of product it is nothing but c one and what is the concentration of the reactant it is nothing but c two concentration of product where reactant nurse equation now e naught cell is what zero why is it not cell is zero why because here the cathode is also zinc and it is also zero so both will have the same electrode potential zinc will have the same electron potential so if you subtract it cathode minus sine it will be what zero did you get my point the cathode is also synthetic electrode and it is also same electrode the zinc electrode will have the same potential or not see if you subtract it the value of e naught cell will be r zero so hence each cell is equal to it is how much minus zero point zero five nine one by n log c one by c two you'll say no sir i want plus then what will it do zero point zero five nine one by n let us write one properly log c 2 by c 1 log c 2 by c 1 in this way okay e cell is equal to this is the formula to calculate the e cell of a concentration so result is equal to 0.0591 by n log c2 by c1 if it is positive suppose if you want to keep it negative then the same thing is h if it is going in terms of pressure all right so whatever what it will be all right it's a gaseous electrode e cell is equal to 0.059 by n log p 1 by p 2 considering that p 1 is greater than p 2 p 1 is a partial pressure p 1 by p 2 p 1 is the partial pressure is greater than p2 since it will be positive if p2 is greater than p1 then it will be negative all right clear so in this way if it is pressure then this is a formula if it is concentration that this is a formula for a concentration cell i should say that okay now i hope you understood the concentration cell next is a very important question to you you know that a cell dies off means a cell stops working after a certain time isn't it which yourself dry cell galvanic cell like land cell mercury cell any particular cell suppose the cell which you are buying it from the market battery so it is it lasting forever no after some time it stops working why what is the reason why we are not able to draw current from that particular cell what is the reason behind it see now when the cell is working on it when the cell is working slowly and slowly the potential difference is what decreasing the voltage is decreasing definitely because when you are when you are buying a new battery all right so you put it into a torch you will see whatever the light you are getting it so as you are going on using and using and using the uh intensity of the light remains the same or not it doesn't remain the same the intensity of light no intensity of light decreases that means what the potential difference decreases that means what the voltage decreases so very simple thing whenever you're using a particular cell whenever you're using a particular battery the voltage goes on decreasing and you know that always a current flows from higher potential to lower potential that is from cathode to another already we have studied this cathode to annette because cathode is higher potential and it is lower potential the difference in potential gives the earth so higher is the difference between the two electrodes and scattered and higher is your ef higher is the difference between the two electrodes higher is the emf all right you know what all these things now i'll come to it when you're using a particular battery when you're using a battery the concentration of the ions changes all right listen carefully when you're using a particular cell or a battery the concentration of the iron changes how see here now i'll show you graphically this one so here i am taking in the exact x axis that is time and in the y axis i am taking that is nothing but reduction potential i am taking it yeah and here that is i am taking that is here uh e cell to be as zero means the value of e the reduction potential to be a zero this one is zero taken into bs all right the value of e zero or i can write it simply zero or here so now listen carefully here what i said as the time progresses the voltage goes on decreasing and the concentration of the ions and the electrolyte also changes so what we find is that as the time progresses that is the concentration that is of the electrons in the oxidation electrode increases isn't it let us say i am taking itself this is a zinc electrode you have taken it all right and i have taken this is your zinc sulfate solution so what will happen oxidation takes place you know that in zinc oxidation takes place so in oxidation what happens loss of electrons zn minus two e minus it gives zn2 plus so this ions this electron goes and sticks here so the concentration of the electrons in the what you can say goes on increasing concentration of the electrons goes on increasing the concentration of the ions electrons means the ions goes on increasing so hence what we find is that the concentration of the anode goes on increasing as the time progresses the concentration the ionic it is increasing all right now at the same time in case of copper what will happen suppose i am taking a galvanic circuit suppose here i am taking that is copper all right instead of zinc now it is copper all right i am taking cathode i understood for the iron and what happens the electrons goes on sticking the particular what you can say as a result the concentration of the ions increases but in at the same time in copper what will happen the concentration io is going on decreasing why because cu 2 plus plus 2 e minus will give you what cu so copper gets precipitated here copper gets precipitated the concentration of the ion decreases so the concentration of the ion is decreasing in reduction potential the concentration decreases this is for copper and this is for zinc and in case of zinc it increases now in this way what will happen is that when zinc is increasing the oxidation potential is increasing reduction potential is decreasing alright how it is here that concentration ion decrease increases in case of zinc and in case of copper it decreases a point will come where both will meet together all right a point will come when both is going to meet together that is concentration of zinc is going to increase concentration of copper is going to decrease the oxidation potential increases reduction potential decreases and what is this point called here the value of e cell is zero i'll be zero because both in this particular point both are having the same potential at this point both are having the same potential you can see here the zinc is increasing copper is decreasing so when it reaches a particular point what happens as you are going on using the battery the voltage is decreasing how is decreasing see here because the potential here it increases and here it decreases so in this when they will meet the difference in potential is what zero e cell is to e category both are same cathode is also same energy is also same so hence the value of e cell becomes zero due to which when the value of e cell becomes zero the cell attains equilibrium and when the cell attains equilibrium we are not able to draw any current from the cell and hence the cell stops working this is how the cell stops working after some time audit when you are using a particular battery why is the voltage decreases why does the cell stops working after some time the reason i have said it to you graphically i have explained it all right why did i take reduction potential because i'm saying that all the potential should be converted into reduction potential when i say e cell is equal to e cathode minus enough both are at reduction potential the cathode you know it i've told you earlier all right thank you very much