ELECTROCHEMICAL REACTION

Here, each of the reactant is taken in a separate container in contact with a rod/sheet of a metallic-conductor (electronic conductor) called an electrode. Electrical contact between the two reactants is established by placing a conducting salt bridge in-between.

Fig: 9.2 - Experimental set up for an indirect redox reaction

The species capable of losing electrons transfers them to the electrode placed in it. This makes the metallic conductor or electrode negatively charged due to the accumulation of electrons. On the other hand, the species capable of gaining electrons accepts them from the metallic conductor (electrode) placed in it. As a result this metallic conductor electrode) gets positively charged due to the deficiency of electrons. When a metallic wire connects the two electrodes, the electrons flow from the negatively charged electrode to the positively charged electrode. Thus, in an indirect redox reaction, electrons flow in a particular direction through an external conducting wire connecting the two electrodes.

This flow of electrons generates electricity, which can be used for doing some useful work.

Thus, in an indirect redox reaction, the decrease in the chemical energy is liberated in the form of electrical energy.

An electrochemical reaction differs from a chemical reaction in the following respects.

Chemical reaction	Electrochemical reaction
Electron transfer from one species to another takes place directly in the same medium.	Electron transfer from one species to another takes place indirectly through electrodes.
Energy is liberated in the form of heat, light and sound.	Energy is liberated in the form electrical energy.
The chemical reaction is instantaneous proceeding at a finite rate.	This reaction takes place only on the application of electricity.
Redox reactions take place in the same medium.	Redox reaction takes place separately at the Anode and cathode surface.

Electrochemical cell

Spontaneous redox reactions are the metal displacement reactions. Therefore such reactions have been used for producing electricity. This is done by carrying out these reactions in specially designed units called electrochemical cells.

An electrochemical cell is a device used to convert chemical energy of an indirect redox reaction into electrical energy. This is also called Voltaic cell or a Galvanic cell. It is set up by dipping two electrodes (conducting rods) into the same or two different electrolytes. No reaction takes place inside the cell until a conducting wire joins the two electrodes.

Fig: 9.3 - An electrochemical cell (a) one electrolyte (b) two electrolytes

A typical metal displacement reaction is the reaction between zinc metal and copper sulphate solution i.e.,

The electrochemical cell based on this reaction is set up as follows.

Zinc sulphate solution is taken in a beaker and a zinc rod is dipped in to it. Similarly copper sulphate solution is taken in another beaker and a copper strip is dipped in to it. An inverted U tube containing concentrated solutions of inert electrolytes such as KCl, KNO₃ etc., connects the two solutions. The two openings of the U tube are plugged with porous materials like glass wool or cotton. This U tube is called as salt bridge as it acts like a bridge connecting the solutions of the two beakers. In place of salt bridge, one can also use either a paper strip, unglazed porcelain or clay porous pot or asbestos fibre for developing electrical contact between the two half-cells. When a key is inserted to complete the outer circuit, the following observations are made.

• There is a flow of electrical current through the external circuit.

• The zinc rod loses weight, while the copper rod acquires weight.

• The concentration of ZnSO₄ solution increases, while that of CuSO₄ solution decreases.

• The two solutions in the beakers have electrical neutrality.

The above indications clearly show the overall reaction to be:

Zinc is oxidized to Zn²⁺ ions and go into the solution during the reaction.

The electrons released at the electrode move towards the other electrode through the outer circuit. These electrons are accepted by Cu²⁺ ions of CuSO₄ solution. Thus the Cu²⁺ ions are reduced to metallic copper, which get deposited on the copper electrode.

With the onset of the chemical reaction, the zinc plate begins to dissolve and loses weight, electrons get generated and move and finally, the copper plate gains weight. Therefore, this indirect redox reaction is accompanied by the liberation of energy in terms of electric charge, which is the electrical energy. In this way the chemical reaction leads to the production of electrical energy that further helps in doing useful work (deposition of copper metal).

Fig: 9.4 - Electrochemical cell based on redox reaction of zinc and copper sulphate

Salt Bridge and Its Function

A salt bridge is a low resistance device, which establishes an electrical contact between two electrolytes not in direct contact. This is used to overcome the direct liquid-liquid junction that leads to intermixing. The salt bridge consists of a glass U-tube filled with KCl containing Agar-Agar paste, which sets into a gel. The choice of the electrolyte added to the gel depends upon the nature of the electrolytes used in the cell. However these electrolytes are inert for they should not react chemically or undergo any electrochemical reactions with the electrolytes electrically connected with them. Commonly used salts are, KCl, NH₄NO₃, KNO₃, or K₂SO₄. The KCl-bridge cannot be used when any salt of lead, or silver is used in the cell because lead chloride and silver chloride are insoluble in water.

Functions of a Salt Bridge

• When in a galvanic cell two solutions are kept in separate containers, an electrical contact between the two is needed to complete the circuit. A salt bridge completes the circuit by allowing the migration of anions from one container into the salt bridge and from the salt bridge into the other container.

• The salt bridge prevents the physical transference/diffusion of the electrolytes from one container to the other.

• A salt bridge helps in maintaining the charge balance in the reactions taking place at the two containers by releasing counter ions into the solution. Other wise due to the accumulation of the respective charges in the two containers there will be no flow of electrons and the cell will stop functioning.

• A direct liquid-liquid junction is thermodynamically unstable state. The unequal rates of migration of the cations and anions across a liquid-liquid junction give rise to a potential difference across the junction. This potential difference across the liquid-liquid junction is called liquid junction potential. A salt bridge eliminates a direct contact between the two solutions, and thus minimizes the liquid junction potential.

FARADAY'S LAW

Faraday's laws of electrolysis:

Michael Faraday, a pioneer in the properties of electric currents, formulated two basic laws of electrolysis:

FARADAY'S FIRST LAW may be stated as follows:

Faraday's First Law of Electrolysis
"The amount of any substance deposited, evolved, or dissolved at an electrode is directly proportional to the amount of electrical charge passing through the circuit."

The amount of electricity passing through the circuit in a given time is the number of moles of electrons passing through the circuit in that time, and the charge Q is related to the current I by

The charge on the electron is 1.602 x 10^-19 C, and Avogadro's number is 6.023 x 10²³. It follows that one mole of electrons has a charge of 9.65 x 10⁴ C. This quantity is known as the FARADAY or FARADAY'S CONSTANT (F).

FARADAY'S SECOND LAW may be stated as follows:

Faraday's Second Law of Electrolysis
"The mass of different substances produced by the same quantity of electricity are directly proportional to the molar masses of the substances concerned, and inversely proportional to the number of electrons in the relevant half-reaction."

This means that z moles of electrons are needed to discharge an ion X^z+ or X^z-.

In the apparatus below, 1 Faraday will discharge 9 g Al (1/3 mole), 20 g Ca (1/2 mole) and 23 g Na (1 mole). The relevant half reactions are: Al3+ + 3e- Al Ca2+ +2e- Ca Na+ + e- Na

APPLICATION

Extraction of aluminium:

Aluminium is obtained by the electrolytic reduction of its molten oxide, alumina (Al₂O₃). Because alumina has a very high melting point (2045 ºC), the mineral cryolite (Na₃AlF₆) is added to lower the melting point in order that the electrolysis may be carried out at about 950 ºC. The electrolytic cell has carbon anodes and a carbon cathode (which forms the lining of the tank in which the electrolysis takes place). Carbon dioxide is formed at the anodes, and aluminium at the cathode. It is heavier than the molten alumina/cryolite mixture, and sinks to the bottom of the cell, where it is tapped off. The procedure is known as the Hall-Héroult process.

Aluminium extraction is very demanding on electrical current (typically, 3-5 V and 100 000 A), and is economical only where power is cheap.

Electrorefining of copper:

When copper is first obtained by reduction of its ores, it is cast as impure slabs or ingots, called blister copper. In the electrorefining process, the blister ingots are used as anodes in an electrolytic cell, where an acid solution of copper (II) sulphate is used as electrolyte. Initially, the cathodes consist of thin sheets of pure copper.

During electrolysis, copper passes into solution from the anodes, (leaving the impurities, normally containing silver, gold and platinum) as an anode slime, which sinks to the bottom of the cell. The anode reaction is

At the cathode, copper (II) ions are discharged and the pure copper sheet becomes coated with an increasingly thick layer of very pure copper:

Electroplating:

Electroplating consists of depositing a thin layer of a metal on another, either for protection or for the sake of appearance. Typically, a brass or nickel object is coated with a layer of silver by making use of electrolysis of a silver solution, using the object to be coated as the cathode:

The anode consist of pure silver, and the cathode is the object to be plated. The electrolyte is a mixure of silver nitrate with potassium cyanide. The reactions are: At the anode: Ag → Ag+ + e- At the cathode: Ag+ + e- → Ag The cyanide ensures a low concentration of silver ions, a condition for providing the best plating results.

During the process, the concentration of silver in the electrolyte remains constant, as the rate of reduction at the cathode (which is the rate of deposition of silver on the object) is the same as the rate of reduction at the anode (which is the rate of rate of dissolution of the silver anode).

ELECTROLYTIC CELL VS GALVANIC CELL

Electrolytic Cell V/s Galvanic Cell

The main points of difference between an electrolytic cell and a galvanic cell (electrochemical cell) are:

Difference between an electrolytic cell and a galvanic cell

Electrochemical cell (Galvanic Cell)	Electrolytic cell
A Galvanic cell converts chemical energy into electrical energy.	An electrolytic cell converts electrical energy into chemical energy.
Here, the redox reaction is spontaneous and is responsible for the production of electrical energy.	The redox reaction is not spontaneous and electrical energy has to be supplied to initiate the reaction.
The two half-cells are set up in different containers, being connected through the salt bridge or porous partition.	Both the electrodes are placed in a same container in the solution of molten electrolyte.
Here the anode is negative and cathode is the positive electrode. The reaction at the anode is oxidation and that at the cathode is reduction.	Here, the anode is positive and cathode is the negative electrode. The reaction at the anode is oxidation and that at the cathode is reduction.
The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit.	The external battery supplies the electrons. They enter through the cathode and come out through the anode.

ELECTROLYSIS OF WATER

Electrolysis of water:

Water may be electrolyzed in the apparatus shown below. Pure water is however a very poor conductor of electricity, and one has to add dilute sulphuric acid in order to have a significant current flow.

The electrodes consist of platinum foil. The electrolyte is dilute sulphuric acid. Hydrogen gas is evolved at the cathode, and oxygen at the anode. The ratio, by volume, of hydrogen to oxygen, is exactly 2:1. Remember that electron flow in the circuit is opposite to the conventional current flow. Thus, while the conventional current flows from the positive pole through the electrolyte and ends up at the negative pole, electrons flow from the negative pole in the reverse direction. The positive pole of a battery accepta electrons from the electrolyte by means of the anode of the electrolytic cell. The reaction at the cathode (tube A) is the reduction of protons:

Oxidation takes place at the anode (tube B). There are two anions competing to give up their electrons, namely sulphate (SO₄^2-), and hydroxide (OH^-) from the ionization of water. The oxidation of OH^- according to the reaction

has a standard electrode potential of -0.40V, compared to the oxidation of sulphate (-0.17V), and consequently, OH^- will be oxidized preferentially. The overall reaction is therefore

and the electrons are reurned to the battery, thus completing the circuit.