Understanding Electrochemical Cell: Batteries are everywhere in modern societies. They provide electric current to start automobiles and power a host of products such as pocket calculators, digital watches, radio, tape recorder etc. The reactions carried out electrochemically cause less pollution. They are becoming more eco-friendly with the advancement of the technology. Electrochemistry is the branch of science that deals with the interconversion of chemical and electrical energy.
Electrochemical Cell Definition
|A device employed to convert the chemical energy of a reaction into electrical energy.|
What is Electrochemical Cell?
It is also commonly referred to as a Galvanic or Voltaic cell after the names of Galvani (1780) and Volta (1800) who were the first to work in this field.
The main requirements are:
- A reaction is carried out in two separate half-cells. The electrons are lost in one cell and gained in the other.
- The substance which loses the electrons and the one which accepts the electrons should not be in direct contact with each other. The electron transfer must take place through an external circuit.
Electrochemical Cell Diagram
In Electrochemical process heat energy is not produced. It is due to the fact that chemical energy gets transformed into electrical energy. The cell is made up of two half-cells, In each of which a half-reaction takes place. So the two partial reactions which occur simultaneously are known as half-cell reactions. The Experiment of this electrochemical will clear more. A video is given below.
Electrochemical Cell Experiment
Let us illustrate the principle by taking an example of Daniell cell (in which Zn-CuSO4. reaction takes place).
Zn(s) + CuSO4(aq) → ZnSO4 (aq) + Cu(s)
Ionic equation is:
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s)
It consists of two beakers one containing a solution of ZnSO4 (1M) and zinc plate is dipped it. The other beaker contains CuSO4 solution (1M) and a copper plate is dipped in it. The two solutions are connected by means of a salt bridge. A salt bridge is a U-shaped tube containing a concentrated solution of an inert electrolyte like KCl, KNO3, NH4NO3, etc., or solidified solution of an electrolyte in Agar-Agar and gelatin. The ends of the salt bridge are plugged with glass wool or cotton. The salt in the bridge is such that it does not react chemically with either of the two solutions connected by it.
Working of Electrochemical cell
When the two metallic plates are joined through a connecting wire, the reaction takes place and the following observations are recorded:
- Deflection of the needle in voltmeter takes place from zinc to the copper plate. It is due to the fact that plugs electrons flow from zinc to the copper plate. The flow of electrons constitutes an electric current, since electrons, negative electricity flows from zinc to the copper plate, the conventional current, i.e., positive electricity flows from copper to zinc plate. Thus zinc plate serves as a negative electrode and copper plate serves as a positive electrode.
- Zinc plate loses its weight. It is due to the fact that zinc gets oxidized to zinc ions which go into the solution
Zn(s) → Zn2+ (aq) + 2e–
Such electrode at the surface of which oxidation, i.e., loss of electrons takes place is known as an anode.
- Copper plate gains weight. It is due to the fact that copper ions get reduced to metallic copper which is deposited on the copper plate.
Cu2+(aq) + 2e– → Cu(s)
Such electrode at the surface of which reduction, i.e., the gain of electrons takes place is known as a cathode.
- The solution remains electrically neutral. It is due to the fact that salt bridge maintains electrical neutrality in both the solutions.
Must Read: Electrode Potential and Types
Function of Salt bridge in Electrochemical cell
The functions of the salt bridge are:
- To complete the circuit.
- Prevent intermixing of the solutions of both the half cells.
- To maintain electrical neutrality in the solutions of both half cell.
In the oxidation half cell, Zn will lose electrons and would change to Zn2+ ions. So positive charge may accumulate in this half-cell due to excess of Zn2+ ions. This prevents the release of electrons from the zinc plate. Similarly, in the reduction of half-cell, Cut2+ ions, would gain electrons and would be deposited as Cu on the electrode. So the negative charge would accumulate due to the excess of SO42- ions in this half-cell. This prevents the flow of electrons up to the copper plate.
These negative ions (anions) of the electrolyte in the salt bridge would move to the oxidation half cell in order to neutralize excess of zinc ions. Similarly, the positive ions (cations) move to the reduction half-cell, in order to neutralize the excess of sulfate ions so as to maintain the electrical neutrality in both the half-cells.
In the absence of salt bridge, the current would have stopped flowing in circuit due to accumulation of opposite charges in the two half-cells
Electrochemical Cell Notation
The galvanic cells are represented with the help of certain notations and conventions, which are given below:
- The anode is written on the left-hand side and is represented by writing metal or solid phase first and then the metal ions. The two are separated by a semicolon or a vertical line. For example,
Zn(s); Zn2+(aq) or Zn(s) | Zn2+(aq)
- The cathode is written on the right-hand and is represented by writing metal ions first and the metal. The two are separated by a semicolon or vertical line. For example,
Cu2+(aq); Cu (s) or Cu2+ (aq) Cu (s)
- When a salt bridge is used, it is indicated by two vertical lines separating the two half cells.
Anode || Cathode
- Now let us represent the view of the above convention
Zn(s); Zn2+(aq) (1M) || Cu2+(aq) (1M); Cu (s)
Electrochemical Cell vs Electrolytic Cell
|Chemical energy is converted into electrical energy in the cell.||Electrical energy is converted into chemical energy in the cell|
|Ions are discharged only on the cathode.||lons are discharged at both electrodes.|
|The anode is negative while Cathode is a positive electrode.||Anode is positive electrode and cathode is a negative electrode.|
|The electrodes are present in different compartment||Both the electrodes are present in the same Compartment.|
|A salt bridge or porous partition is needed between the two electrodes.||No salt bridge or porous partition is needed between the two electrodes.|
Electrochemical Cell Examples
Q- Suggest a cell notation for the net cell reaction.
Cd + Ni2+ → Cd2+ + Ni
Solution. The given reaction Cd + Ni2+ – Cd2+ + Ni can be split into two half-cell reactions as
Cd → Cd2+ +2e– (oxidation)
Ni2+ + 2e– → Ni (reduction)
- Since oxidation occurs at the anode of the cell, the negative electrode or the anode is represented as Cd | Cd2+.
- As the reduction takes place at the cathode of the cell, positive electrode or the cathode is denoted by Ni2+ | Ni.
- Combining the anode and the cathode electrodes, using salt bridge, the cell notation is
- Cd | Cd2+ || Ni2+/Ni
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