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Electrochemical Cell

An electrochemical cell converts chemical energy to electrical energy, and the reaction which occurs in an electrochemical cell is always spontaneous. These cells are very important because of their many practical applications.

An early example of electrochemical cell is the Daniell cell, which was invented by the British chemist John Daniell in 1836. Daniell cell was constructed on the basis of the following spontaneous redox reaction

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

 

Definition of Electrochemical Cell

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"The devices in which chemical energy released during a chemical reaction is converted into electrical energy are called Electrochemical Cells. "
They are also called Galvanic cell. In these cells, oxidation and reduction reactions occur in separate containers called half cells and the redox reaction is spontaneous.

Electrochemical Cell Diagram

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Let us explain the working of an electrochemical cell with the help of the following diagram.

Electro Chemical Cell

The arrangement consists of two beakers, one of which contains CuSO4 solution and the other one contains AgNO3 solution. A copper rod is dipped in the CuSO4 solution and a silver rod is dipped in the AgNO3 solution. These metallic rods are called Electrodes. The solutions in the two beakers are connected by an inverted U tube containing a saturated solution of KNO3, which does not undergo any chemical change during the process. This U tube is called a Salt Bridge. A volt meter is attached between the two electrodes to measure the potential difference between the two electrodes.

When the circuit is completed, it is observed that an electric current flows through the external circuit, which we can detect by placing an ammeter in the circuit.
The following observations are made
  1. The copper rod gradually loses its weight.
  2. The concentration of Cu2+ ions in the CuSO4 solution increases.
  3. The silver gets deposited on the silver electrode.
  4. The concentration of Ag+ ions decreases in the solution.
  5. There is a flow of electrons in the external circuit from the Copper rod to the   Silver rod. Therefore, the current flows from silver to copper. It may be noted that as a convention, the flow of electric current is taken opposite to the flow of electrons.
These observations can be explained as follows:

During the reaction, Cu is oxidized to Cu2+ ions, which go into the solution.

Cu(s) Cu2+(aq) + 2e- (OXIDATION)

Therefore, the copper rod gradually loses its weight. The electrons released at the electrodes move towards the other electrode through the outer electrode. These are accepted by Ag+ ions of AgNO3 solution which are reduced to silver metal and it gets deposited on the silver electrode.

2Ag+(aq) + 2e- 2Ag(s) (REDUCTION)

  • Anode : The electrode where electrons are released or oxidation occurs is called the Anode. It is the negative terminal of the cell.
  • Cathode : The electrode where electrons are accepted or reduction occurs is called Cathode. It is the positive terminal of the cell.
  • Half cells : The two containers involving oxidation and reduction half reactions are called Half cells.

Important Functions of Salt bridge

  • Salt bridge completes the electrical circuit.
  • Salt bridge maintains the electrical neutrality of the two half cell solutions.

Electrochemical Cell Notation

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An electrochemical cell consists of two electrodes Anode and Cathode. The electrolyte solution containing these electrodes are called Half cells. When the two half cells are combined, a Cell is formed. The following conventions are used to represent an electrochemical cell.
  1. A single vertical line will indicate a change in state or phase.
  2. Inside the half-cell, the reactants are listed before the products.
  3. Aqueous solutions concentrations are written in parentheses after the symbol for the ion or molecule.
  4. Double vertical line would be used to indicate the junction between the half-cells or the salt bridge.
  5. The line notation for the anode (oxidation) is written before the line notation for the cathode (reduction).

Example:- 1

The following cell reaction is given
Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
Cell Notation:- Zn | Zn2+(1.0 M) || Cu2+(1.0 M) | Cu

Example:- 2

Ni- AgNO3 cell
Oxidation half reaction Ni(s) → Ni2+(aq) + 2e-
Reduction half reaction 2Ag+(aq) + 2e- → 2Ag(s)
Overall cell reaction Ni(s) + 2Ag+(aq) → 2Ag(s) + Ni2+(aq)
Cell Notation :- Ni(s) | Ni2+(aq) || Ag+(aq) | Ag(s)

Example:- 3

Zn – HCl reaction
Oxidation half reaction Zn(s) → Zn2+(aq) + 2e-
Reduction half reaction 2H+ + 2e- → H2(g)
Overall cell reaction Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
Cell Notation :- Zn(s) | Zn2+(aq) || H+(aq) | H2(g)

Working of Electrochemical Cell

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When the circuit is completed, a deflection is observed in the (G) towards the zinc electrode indicating that the e- are flowing from the Zn electrode to Cu electrode.

At the Zn electrode, oxidation takes place.

Zn $\rightarrow$ Zn+2 + 2e- (oxidation) .... (1)

The electron 'e' is removed, lost or retained by the metal move through the material contained in the electrode, and reach the Copper electrode at which they are accepted by Copper ions of the solution to form the neutral Copper atoms.

Cu+2 + 2e- $\rightarrow$ Cu ..... (2)

In an electrochemical cell, each electrode constitutes one half of the cell and the reaction taking place at the electrode is called half-cell reaction. The overall cell reaction is obtained by adding the two half-cell reactions (1) and (2).

Zn + Cu+2 $\rightarrow$ Zn+2 + Cu (Overall Reaction)

From this, it is found that, when Zn is added to CuSO4 solution, Zn displaces Cu from CuSO4 with the liberation of heat. But, in the electrochemical cell, there is no direct contact between Zn and CuSO4. The heat energy that would have been liberated, appears in the form of electrical energy. Hence, the electrochemical cell acts as a source of current, although for a short interval.

The electrode at which oxidation takes place, or the metal rod that becomes negatively charged is called the negative electrode.

Accordingly, in the above constructed electrochemical cell, zinc electrode acts as negative electrode while Cu electrode acts as the positive electrode.

The salt bridge :

  • acts as a link between the two aqueous solution.
  • overcomes liquid junction potential.
  • maintains the electrical neutrality of the aqueous solution of the electrodes by releasing or sending oppositely charged ions into the solution.

In general, KNO3 salt bridge is used when silver electrode is involved as one of the electrodes.

Electrochemical Cell Equation

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In general, the electrode at which reduction takes place is written on the RHS of the salt bridge and the electrode at which oxidation takes place is written on the LHS of the salt bridge. The salt bridge linking the aqueous solutions is represented by two vertical parallel lines having ions on both sides.

Left hand side
Right hand side
Anode Cathode
(Oxidation)
(Reduction)

In the above system, Zn electrode is written on the LHS while the Cu electrode is written on the RHS of the salt bridge.
Electrochemical Cell Equation

In general, of the two electrodes, the electrode having higher SRP (standard reduction potential) acts as the positive electrode.

Example: Zn| Zn2+ (C1) | | Cu2+ (C2)| Cu

Metal/Metal ion (Conc.) || Metal ion (Conc.) | Metal,
C1 and C2 are the concentrations of Zinc and Copper ions respectively.
||-Represents the salt bridge.

Electrochemical Cell Examples

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Below you could see examples

Solved Example

Question: Depict the Galvanic cell in which the following reaction takes place
 Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)
Also show individual reactions at each electrode.
Solution:
The cell notation is Zn(s) | Zn2+(aq) || Ag+(aq) | Ag(s)
Reaction at Anode: Zn(s) → Zn2+(aq) + 2e-
Reaction at cathode: 2Ag+(aq) + 2e- → 2Ag(s) 


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