Concentration Cell

What is a Concentration Cell?

A concentration cell is formed when two half cells with the same electrodes and different concentrations are connected. In a concentration cell, the concentration of the more dilute solution increases, and the concentration of the concentrated solution decreases. The process of the concentration and dilution of the solutions continue to happen until a state of equilibrium of concentration is obtained. This means that the shift in concentration stops only after the concentration of solutions in both the half cells are equal. 

A Sneak Peek Into Concentration Cells 

Standard electrode potential which is commonly called Cell, in scientific terms is defined as the measures of individual potential of a reversible electrode at standard state with ions at a concentration of 1 mol/ dm³. The electrolyte concentration cell consists of 2 half cells or galvanic cells. 

What is Half Cell? 

A half cell is an electrochemical arrangement. It consists of two electrode rods made of any conductive material which is dipped in a conductive electrolyte. The electrodes are connected to a source of energy, most likely a 9 volt DC battery. This facilitates the motion of electrons between both the half cells until a state of equilibrium is obtained. Hence, using conductive electrolytes and electrode rods is essential, as this facilitates the movement of electrons and aids in obtaining a state of equilibrium.

More Interesting Facts About Concentration Cells

Half cells are used in concentration Cells. Concentration Cells, which is our topic today is a special case of electrolysis. Generally, the two electrode rods used and electrolytes used are different. However, in electrolytic concentration cells, both the electrodes and electrolytes used are the same but they differ in concentration. The method is used to make two solutions of equal concentration. 

[Image will be Uploaded Soon]

The diagram above illustrates a half cell. Now, keenly observe the elements that it consists of. It clearly shows the use of electrodes and electrolytes.  In this case, two different electrolytes are used. 

Representing an electrolytic concentration cell through half cell reactions. 

Pt|H₂(1 atm)|H⁺||H⁺|H₂(1 atm)|Pt

The above-mentioned reaction signifies a concentration cell reaction where the concentration of H+ ions differs in each half cell. The electrodes used in the reaction are made of platinum. Once the battery source is turned on the concentration of H+ will shift from high to low. The electrodes used don't have any role to play, instead of platinum, any rods made of conductive material could be used. 

Types of Concentration Cells

Electrolyte Concentration Cells: The method is used to obtain two same electrolyte solutions of the same concentration. In this method, two conducting electrodes are dipped into the electrolytes, and the current is passed through the cell. Once the current is passed, the excessive electrons steadily move from higher concentration to lower concentration. The process continues until the concentration of both solutions is equal. For instance ,Fe/Fe²⁺(0.1M)  is the cathode and Fe²⁺(0.01M)/Fe is the anode. In this case, we can notice that Fe²⁺ in the cathode is more concentrated whereas the anode is dilute. Hence, electrons would move the cathode to anode until the concentration of Fe²⁺ in both the half cells is equal, in this case, 0.55M. 

Electrode Concentration Cell: Electrode concentration cells are the opposite of electrolyte concentration cells. In this case, identical solutions are used in both the half cells, cathode, and anode respectively. The electrodes in electrode concentration cells differ in concentration. Once the circuit is electrocuted, the concentration effect is negated and electrons move from the concentrated electrode to the diluted electrode. 

An example of an Electrode Concentration Cell is a cell consisting of two hydrogen electrodes which are subjected to varied pressure. 

Both these types of concentration cells are concentration cells with transference as they include the transfer of electrons from one half to another. Example of concentration cells without transference include

Pt,H₂, HCl/ AgCl, Ag 

The two electrolytes don't come in direct contact with each other and hence are concentration cells without transference.

Solved Problems Concentration Cell

Let us look at a problem related to concentration cells, this will give us a better understanding of the topic. 

1. The Concentration Cell Below Determines the Flow of Electrons.

Zn|Zn²⁺(0.01 M)||Zn²⁺(0.1 M)|Zn

Ans. The question indicates the presence of a higher concentration of ions in the anode than the cathode. Hence, according to the concept, we learned today, Zn²⁺ ions from the anode would move towards the cathode until both the half cells have 0.55 M concentration. 

2. Calculate the Cell Potential for a Concentration Cell with Two Iron Electrodes with Concentrations 0.2 M and 3.0 M.

Ans:

Reaction:

Fe²⁺+2e− ⟶ Fe(s)

Cell Representation :

Fe(s)|Fe²⁺(0.2 M)||Fe²⁺(3.0 M)|Fe(s)(8)

Nernst Equation:

E=E⁰−0.05922log0.023.0(9)

**E⁰= 0 for concentration cells

E = 0.0644 V