Lecture 1.107 – Concentration Cells

“Its Natural to distribute equally “

just like in the very first lecture 1.101 (reference )We discussed the working principle of the electrode, we have a tendency which is different from metal to metal that actually get to decide what amount of electron will be collected and that is why we consider the standard electrode potential of a electrode in the Emf or nerst equation.

If I say that the temperature reaction or any other physical condition and the chemical tendency are same for the two electrode we can assume that we have connected the identical electrode which will not give any net Emf.

But just If I change the concentration in any of the electrode, I you can see through the picture we are having more or less number of electrons as we disturbed the equilibrium, So this is the case here

imgres

Same electrodes but of different electrolyte concentration, the above reaction, shows that there is no net chemical reaction but involves only transfer of Zn2+ ions from solution of higher concentration c2 to solution of lower concentration c1.

Concentration cell formed by the combination of same electrode dipped into same electrolyte of different concentration. The reaction, shows that there is no net chemical reaction but involves only transfer of Zn2+ ions from solution of higher concentration c2 to solution of lower concentration c1.

 conc

E1 = Eº + 0.059/n *  log 10c1  

E2 = Eº + 0.059/n *  log10 c2

Net reaction- Emf = E2 –E1 = 0.059/n * log10   [ c2 /c1 ]

Here we should analyse that there is no need of knowing the standard electrode potential as they will be cancelled out later in the final reaction. So At net we don’t need anything but the concentration of the ions in the two electrodes.

The Cell is represented as   M|M+n (C1) || M+n (C2)|M

Now there is another way of making a concentration cell without using the salt bridge, which is using a semi-permeable membrane.

Electrode concentration cell with Transference

In the electrolyte cell, the two electrode are separated by a diffusion membrane. Diffusion occurs from higher to lower concentration of solution. After diffusion say we have a HCl, concentration cell the high mobile H+ ions will be moving, then on side which had higher concentration of HCl will now have a negative charge because the hydrogen ion moves to other and so, the other side will have a positive charge layer on the junction of the membrane, which is known as Liquid Junction Layer.

Now this kind of phenomenon is prevented because this complicates the calculation of Emf in a cell, and this is why we use a salt bridge (1.102)which can reduce this charge density to a negligible amount.

 

With this topic done, we have completed our building of concepts in Electrochemistry, but Here is the trick, engineers aren’t suppose to just understand the concepts and switch to next but to go out and apply and the similarly interesting fact is that when we all are thinking of producing more and more energy, there has be comparatively no growth in storing capacity, so the next topic is the application of Electrochemistry in Battery Technology.

 

Abhishek kumar jha

(Chemistry at Utkarshini)

 

 

 

 

 

 

 

downloadable file1-107-electrochemistry-concentration-cell

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