Chemical Effect of Current

The fact that chemical changes produce electrical effect was discovered accidentally in 1971, by Luigi Galvani, an Italian professor. He found that an electric current flowed across two dissimilar metals. Volta, professor of natural philosophy successfully reproduced some of Galvani's results using inanimate things. The basic reason for electrochemical effects became clear rather slowly. Scientific investigations in this field actually began with the converse phenomenon namely chemical effects of electrical currents.
Michael Faraday, the great British experimental physicist, began his experiments on the passage of electricity through liquids, in 1834.

Electrolysis

In 1834, Michael Faraday studied the passage of electricity through liquids. He called it electrolysis, as it was accompanied by the chemical decomposition of the electrolyte. The term ‘lysis’ in Greek means “setting free”. The metallic conductors through which the current enters and leaves the electrolyte are called electrodes. The electrode at high potential is called anode and the other at lower potential is called cathode. The passage of current through electrolytes was considered to take place through moving charged particles, which were called ‘ions’ by Faraday. The term ‘ion’ in Greek means a ‘wanderer’. The ion with negative charge is called ‘anion’ and the one with positive charge is called ‘cation’.

Faraday's Laws of Electrolysis

• First law states that the mass of a substance deposited or liberated on an electrode during electrolysis is proportional to the total quantity of electric charge passed through the electrolyte.

• Second law states that if same quantity of charge is passed through several electrolytes, the mass of substance deposited or liberated at electrodes is proportional to their chemical equivalent (equivalent weight)

            The Process of Electrolysis
The above statements are the conclusions made by Faraday after conducting a number of experiments on 'electrolysis'. The process of electrolysis is carried out in an apparatus called voltameter.
If the electrolyte is a solution of copper sulphate (CuSO₄) and the electrodes are copper plates, it is called a copper voltameter. On the other hand if the electrolyte is a solution of silver nitrate AgNO₃ and electrodes are silver plates it is called a silver voltameter. When appropriate direct potential difference is applied across the electrodes, the electrolyte starts conducting current. Faraday's second law is illustrated in the figure where silver and copper voltameters are connected in series. For a given time, the same charge will pass through each voltameter. It will be seen that the masses of silver (Ag) and copper (Cu) deposited on the respective cathodes are in the ratio of 108:32. These values of 108 and 32 are called the chemical equivalents of silver and copper respectively.

Electroplating

The following figure shows a copper voltameter with copper electrodes. Current starts flowing in the circuit when the key is closed. Copper is removed from the anode and gets deposited on the cathode. The passage of current through the electrolyte is causing a chemical change. These chemical changes take place as long as current flows through the electrolyte. This process of electrolysis is utilised in many industrial and commercial applications. One of them is electroplating. Electroplating is a process of depositing a thin layer of metal like gold or silver over an inferior material like iron. The object to be plated is used as a cathode. The metal to be deposited is used as an anode.

Electroplating of Silver

Illustration

In the above figure, the metal to be deposited is silver and is used as anode. The object to be plated is used as  cathode. When the current flows through the electrolyte, the silver rod continuously dissolves into the solution and gets deposited on the object (key).

Electrochemical Cells

A cell is a source of electricity in which chemical energy is converted into electrical energy. There are two types of cells called the primary and secondary cells A cell in which chemical reaction is not reversible is called a primary cell. E.g., Daniel cell, Leclanche cell
Secondary cell is a cell in which chemical action is reversible. (e.g., lead acid accumulator, alkali accumulator). Many chemical reactions take place and energy is released. If this happens in an electrolyte, with one or more of the ionic species in it as participants, it is then possible that the energy released directly and solely increases the electrical potential energy of the ions. The chemical reaction is thus a source of electrical energy. The system can be used as a source of electrical power if the chemical reaction proceeds at a steady rate. This is what one tries to achieve in an electrochemical cell.

Leclanche Cell

Construction

Leclanche cell of an outer glass vessel which is filled with saturated ammonium chloride (NH₄Cl) solution. In it there stands a zinc rod and porous pot 'P' containing a carbon rod 'c' which is packed round with a mixture of manganese dioxide (MnO₂) and powdered carbon.

Wet Leclanche Cell
Therefore the carbon rod forms the positive pole and the zinc rod the negative pole. When the carbon rod and zinc rod are connected by a wire, the current flows from carbon to zinc through the wire.

Working

Ammonium chloride, splits into ammonium and chloride ions. The chloride ions (Cl^-) migrate to the zinc rod and deposit their negative charge at the zinc rod. Hence zinc becomes negatively charged and the reactions takes place in which zinc is converted to zinc chloride. The ammonium ions migrate to the carbon rod and make it positively charged. The hydrogen is then oxidised by MnO₂ to form water and thus polarisation is prevented. Here Mn₂O₃ again changes to MnO₂, by taking oxygen from the air.

The Dry Cell

The dry cell is a modification of the wet Leclanche cell in which the ammonium chloride solution is replaced with a jelly composed of starch, flour and ammonium chloride. The positive electrode, namely carbon rod is surrounded by a mixture of manganese dioxide and carbon. This is placed inside a zinc can which serves as the negative electrode. The space between the central core and the zinc can is filled with the ammonium chloride jelly. The jelly is prevented from drying up by sealing the top of the cell with pitch. The carbon rod is prevented from coming in contact with the base of the zinc can, by a cardboard washer. The zinc can is also surrounded by an insulating thick paper covering. The working of the cell is similar to that of wet Leclanche cell.