Electrochemical Cell and Its Construction
It is the apparatus or setup used for the study of electrochemical runs. “Or” It is a system utilizing redox reactions to produce or consume electrical energy to initiate a reaction. An electrochemical cell consists of two electrodes, electrolytic tank, electrolyte and circuits. As EC reactions are of two types, thus electrochemical cells are also of two types.
Voltaic/ galvanic cells | Electrolytic cell | |
1 | It involves spontaneous electrochemical reactions. | It involves non-spontaneous electrochemical reactions. |
2 | Gibbs free energy: ∆G is negative  | ∆G is positive  |
3 | Electromotive force: Ecell is positive | Ecell is negative |
4 | Chemical energy converts to electrical energy | Electrical energy converts to chemical energy |
5 | Flow of electron is from anode to cathode | Flow of electron is from cathode to anode |
6 | Anode has negative charge Cathode has positive charge | Anode has positive charge Cathode has negative charge |
7 | Two half cells are connected by a salt bridge. | A simple container is a complete cell |
Voltaic cell
It is used for Spontaneous EC reactions to generate electrical energy. High potential reactants are converted to low potential products with the generation of electricity.
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Construction of a voltaic cell:
Voltaic cell consists of two half cells which are connected by circuits. Each electrochemical (EC) reaction is a redox reaction consisting of oxidation and reduction reaction. Each half cell consists of an electrode immerse an electrolyte. On one half cell oxidation takes place, on other half cell reduction takes place.
The electrode of oxidation half-cell is called anode and electrode of reduction half-cell is called cathode. On anode, always oxidation takes place. On cathode, always reduction takes place. Both cells are connected by two types of circuits
1) External circuit: It consists of wire with a voltmeter (may or may not) and a switch to open or close the circuit. External circuit is used for the flow of electrons from one half cell (oxidation half-cell) to another half-cell (reduction half-cell). Its function is to connect two electrodes.
2) Salt Bridge: It is an inverted U-shaped tube which contains a non-interacting electrolyte in gel. It maintains the electrical neutrality of two half cells. It prevents the mixing of two electrolytes.
By convention, oxidation half-cell is always written at left side and right side and reduction half-cell at right side.
Process in voltaic cell:
A + Bn+ → An+ + B ( e.q.2.1)
- A is oxidized so, [A Ã An+ + ne] is oxidation half reaction and [Bn+ +ne → B] is reduction half reaction. after The combination of both reactions is the redox reaction (e.q.2.1).
- The oxidation of A at anode causes the formation of ions and electrons in the solution. The cations enter in solution while the electrons released flow to the reduction half-cell through external circuit.
- This electron is taken up by the cathode in reduction half-cell and gets reduced.
- At start, both electrolytes are neutral. With time, the concentration of cations (positive ions) in oxidation half-cell increases. After sometime the concentration of anions (negative ions) increases in red half-cell. At some stage, there occurs an imbalance of charge which stops the reaction and hence current. Thus salt bridge is used to balance the charges and for the reaction to continue.
Oxidation and reduction cells are separated to get current. If both cells are placed together then there is no current gain.
Zn + Cu+2 → Zn+2 + Cu
Zn → Zn+2 + 2e- [oxidation]
Cu+2 + 2e- → Cu [reduction]
Relative charge on electrode:
In the absence of a battery, there is no apparent charge on an electrode. However, one of the electrode is more positive (cathode) than the other (anode).
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1. Reaction between Cr and Ag
Draw diagram and write balance equation for a voltaic cell that consists of Cr bar in Cr (NO3)3 solution, Ag bar in AgNO3 sol and KNO3 sol as salt bridge. Measurements indicate that Cr is relatively negative as compared to Ag.
Balance Equation
Cell diagram
2. Cell diagram for a reaction between Zn and Cu
Types of Electrodes
1. Active electrode: They are the components of half-cell reaction by itself. These electrodes act as reactants or products of the half cell. Active electrodes participate in redox reactions. For example, oxi-red half- cell. An oxidation electrode also acts as reactant. For example, Zn. A reductive electrode also acts as products e.g. Cu.
2. Inactive electrode: The certain reactions in which electrodes do not acts as reactant or product in half-cell are called inactive electrodes. In such runs, inert electrodes are used (Pt, graphite) to remove from oxidation half-cell to enter in reduction half-cell. They are actually the sites of oxidation and reduction reaction.
Electrodes can be anode or cathode.
- Anode: Anode has excess of negative charge and acts as electron donor. At this state it is an active electrode. After donating all of its electrons, it will have positive charge and will act as inactive electrode.
- Cathode: Cathode has excess of positive charge and acts as electron acceptor. At this state it is in active form. After acceptance all of the electrons, it will have negative charge and will act as inactive electrode.
