Chapter 1:-Â Concept of Electrode Potential

B.sc 2nd year Book
(Page 8)

# Concentration Cell

Concentration cells are electrochemical cells and a limited form of galvanic cells. They are made up of two half-cells in which electrodes are the same, varying only in concentration.Â  In the process of reaching equilibrium for the cell as a whole, the more concentrated half-cell is diluted and the concentration of the lower concentration gets increased through the electrons transfer between the two. It means on reaching a state of chemical equilibrium, a difference is created. When the cell reaches equilibrium, it creates a voltage. This difference can be found by using the Nernst Equation with the following formula:-

$$E_{cell\:}=\:E^o_{cell}\:-\frac{0.0592}{n}\left(log\:Q\right)$$

In simple language, Concentration cells are voltaic cells that have the same components in both compartments, but at different concentrations.

For example, a Copper ion concentration cell is represented as:

Cu(s), CuSO4(C1) || CuSO4(C2), Cu(s); where C1 â‰  C2

###### The following is the diagram explaining the Concentration Cell:-

Here also electrolysis of different concentrations is in communication through a salt bridge. When the two electrodes are connected, the left-hand electrode acts as an anode due to the occurrence of the reaction,

$$Cu_{\left(s\right)}\frac{oxidation\:}{ }Cu^{2+}_{\left(C1\right)}_{\:\:}+\:\:2e^-$$

and the right-hand electrode acts as a cathode due to the occurrence of the reaction,

$$Cu^{2+}_{\left(C2\right)}\:+\:2e^-\:\frac{reduction}{ }\:Cu_s$$

The overall chemical reaction is, therefore, represented as

Cu(s) Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â > Cu2+(C1)Â  +Â  2e (oxidation)
Cu2+(C2)Â  +Â  2eÂ Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â > Cu(s)Â  (reduction)
Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â

Similarly, in zinc ion concentration cell, the electrode reactions are

(i) At anode: Zn Â  Â  Â  Â  Â  Â > Zn2+(C1) + 2e (Oxidation)
(ii)At cathode: Zn2+(C2) + 2eÂ Â  Â  Â  Â  Â  Â > Zn (Reduction)
Overall reaction: Zn2+(C2) Â  Â  Â  Â  Â  Â > Zn2+(C1)

Thus, zinc ion concentration cell is represented as:-

Zn(s), ZnSO4(C1)||ZnSO4(C2), Zn(s); Here also C1 â‰  C2

For such cells, the EMF of the cell is calculated by the following expression-

$$e^0cell^{ }\frac{2.303RT}{nF}log\frac{C^2}{C^1}……….\left(8\right)$$

$$e^0cell^{ }\frac{0.0591RT}{n}log\frac{C^2}{C^1}……….\left(9\right)$$

Thus, if C2 > C1, the EMF of the cell will be positive. In such cells, oxidation occurs on the electrode with a lower concentration. Hence, this electrode acts as an anode or negative terminal. While the reduction takes place on the electrode with higher concentration, therefore acting as a cathode or positive terminal. As the cell reaction proceeds, C2 decreases with a simultaneous increase of C1. After sometimes a situation will come when C1 becomes equal to C2 i.e. C1=C2. At this stage, E0cell = 0 which shows the cell stops working. This stage is called equilibrium. Some other examples are-

(a) A Hydrogen ion concentration cell: It is represented as

H2, H+(C1) || H+(C2), H2

Example: Calculate the EMF of the following concentration cell at 25o C

Zn, ZnSO4(C1=0.05 V) || ZnSO4(C2=0.5 V), Zn

Solution: For the above cell reaction

$$E_{cell}\:=\:\frac{0.0591}{n}log\frac{C_2}{C_1}$$

$$=\:\frac{0.0591}{2}log\frac{0.5}{0.05}\:=\:0.0296\:V\:Answer$$

##### Concentration cells are of two types:

1- Electrode Concentration Cells
2- Electrolyte Concentration Cells

• Electrode Concentration Cells
In these cells, identical solutions are used as electrolytes in each half-cell. The concentration of the electrode is different in the two half-cells.
• Electrolyte Concentration Cells
The electrodes in these cells are identical and are submerged in solutions of the same electrolytes but at different concentrations. The electrolyte tends to diffuse in these cells from solutions with greater concentrations to solutions with lower concentrations. Thus, at the start the emf of the cell is maximum and it gradually falls to zero.
##### Components of Concentration Cell

A concentration cell is built with three components. Those are theÂ salt bridge, electrode, and voltmeter. They are explained in brief:-

Salt Bridge
A salt bridge is the most important part of a concentration cell that creates an electrolytic connection between two half-cells allowing the flow of ions between the cells to maintain the equilibrium. It is a piece of permeable material, typically a gel or tubing filled with an electrolyte solution, and helps to stabilize the potential difference across the half-cells and allows the concentration cell to function effectively.

Electrode
There are two electrodes utilized in this kind of reaction. The left and right sides, respectively, are referred to as the anode and the cathode. The anode is the side that loses electrons (oxidizing) and the cathode is the side that is receiving electrons (reduction).

Voltmeter
A voltmeter is used to determine the cell potential (electromotive force) that flows between the two sides. Typically, it is situated between the two cells.

###### Difference between Concentration cells and Chemical cells:

The compositions of two half-cells in concentration cells are similar, whereas, in chemical cells, the compositions may or may not be similar. This is the main distinction between concentration cells and chemical cells.