UNIT - III

​

CHEMICAL EFFECT OF ELECTRIC CURRENT

Introduction

• The passage of an electric current through a conducting liquid causes chemical reactions.
• The resulting effects are called chemical effects of currents.
• Luigi Galvani , an Italian professor accidently found out that an electric current flowed across two dissimilar metals. It was Michael Faraday who studied extensively the passage of electricity through liquids.

Electrolysis

• The process by which a liquid is decomposed into ions is called electrolysis.
• The liquid which conducts electricity and undergoes decomposition is called the electrolyte.
• The two plates dipped in liquid and connected in battery are called electrodes.
• The electrode connected to the positive terminal of the battery is called anode while that connected to negative terminal is called cathode.
• The vessel containing the electrolyte and electrodes is called the voltameter.
• When electrodes are connected to battery, the positive ions and negative ions move towards cathode and anode respectively.
• Positive ions are called cations and negative ions are called anions.

Electroplating

• The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.

• When electric current is passed through the gold chloride solution, gold chloride dissociates into gold and chlorine.
• The free gold gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it.
• But what about the loss of gold from the solution? From the other electrode, a gold plate, an equal amount of gold gets dissolved in the solution.
• Thus, the loss of gold from the solution is restored and the process continues.
• This means that gold gets transferred from one electrode to the other.
• The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.
• It is one of the most common applications of chemical effects of electric current.

Faraday’s laws of electrolysis

First law:

The mass of substance liberated by electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

If m kg of substance is liberated in electrolysis by Q coulomb of electricity passed through the electrolyte, m α Q. Or m = zQ

Here z is a constant called the electrochemical equivalent of the substance.

If Q = 1C, then z = m.

The electrochemical equivalent of an element is defined as the mass of the element in kilogram liberated by passing one coulomb of electricity through its electrolyte.

It is expressed in kilogram per coulomb.

Second law

• The masses of different substances liberated by passing the same quantity of electricity through their electrolytes are directly proportional to their equivalent weights.

​

Electrical conductivity of electrolyte

• This means that in order to liberate a gram equivalent of an element by electrolysis, the quantity of electricity needed is 96,500 C.
• This quantity of electricity is called one faraday.
• Thus 1 faraday = 96,500 C
• Thus a Faraday may be defined as the quantity of charge necessary to liberate one gram equivalent of any element.

​

​

​

• Conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is Siemens per meter.
• Electrical conductivity is also known as specific conductance. Conductivity is an intrinsic property of a material.

​

Kohlrausch investigated the relation between the emf and the current in the electrolytes.

The current passing through an electrolyte is given by

I = (E-e)/R

Where E is the applied emf, e is the back emf due to polarisation and R is the resistance of electrolyte.

The variation of current with the applied emf is shown in graph.

If l is length of the electrolyte through which the current passes and a is the area of cross-section of the electrodes then