|Table of contents|
|Metallic & Electrolytic Conductors|
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Electrochemistry is the subdiscipline of chemistry that deals with the study of the relationship between electrical energy and chemical changes.
Chemical reactions that involve the input or generation of electric currents are called electrochemical reactions. Such reactions are broadly classified into two categories:
An Electrochemical Cell
Solid and liquid substances, which are able to conduct the electric current, can be roughly divided into two categories i.e. metallic conductors and electrolytic conductors.
The Distinction between Metallic and Electrolytic conduction.
|S.No.||Metallic conduction||Electrolytic conduction|
|1||Electric current flows by movement of electrons.||Electric current flows by movement of ions.|
|2||No chemical change||Ions are oxidized or reduced at the electrodes.|
|3||It does not involve the transfer of any matter||It involves the transfer of matter in the form of ions.|
|4||Ohm's law is followed||Ohm's law is followed|
|5||Resistance increase with increase of temperature||Resistance decreases with increase of temperature|
|6||Faraday law is not followed||Faraday law is followed|
In electrochemistry, spontaneous reaction (redox reaction) results in the conversion of chemical energy into electrical energy. The reverse process is also possible where a non-spontaneous chemical reaction occurs by supplying electricity. These interconversions are carried out in equipment called electrochemical cell.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells: in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells: which produce electricity as a result of a spontaneous cell reaction
The electrolytic cell converts electrical energy to chemical energy. Here the electrodes are dipped in an electrolytic solution containing cations and anions.
On supplying current the ions move towards electrodes of opposite polarity and simultaneous reduction and oxidation take place.
For example, in the electrolysis of molten sodium chloride, sodium chloride is melted (above 801oC), two electrodes are inserted into the melt, and an electric current is passed through the molten salt. The chemical reaction that takes place at the electrodes are:
● Sodium-ion migrates to the cathode, where sodium ion gains one electron and reduces to sodium metal.
Na+ + e–→ Na
● Chloride ions migrate towards the anode where it loses one electron and gets oxidized to chlorine gas.
Cl–→1/2 Cl2 + e–
The overall reaction is the breakdown of sodium chloride into its elements
2NaCl→ 2Na(s) + Cl2(g)
Electrolysis is a process of chemical decomposition of the electrolyte by the passage of electric current.
Loss of electron
|Cathode||Negative||Gain of electron|
Faraday established the relationship between the quantity of electricity passed through an electrolyte and the amount of material liberated or deposited at the electrode.
(i) First Law of Electrolysis
When an electric current is passed through an electrolyte, the amount of substance deposited is proportional to the quantity of electricity passed through the electrolyte.
(ii) Second Law of Electrolysis
The amount of different substances deposited or dissolved by the same quantity of electricity are proportional to their respective chemical equivalent weights.
Charge and Potential Difference
As one g-equivalent of an ion is liberated by 96500 coulombs, it follows that the charge carried by one g-equivalent of an ion is 96500 coulomb. If the valency of an ion is ‘n’, the one mole of these ions will carry a charge of nF coulomb. One g-mole of an ion contains 6.02 × 1023 ions.
Then, the charge carried by anion coulomb
For n = 1, The fundamental unit of charge =
= 1.6 × 10–19 coulomb
or 1 coulomb* = 6.25 × 1018 electrons
The rate of flow of electric charge through a conductor is called the electric current.
1 coulomb = 1 ampere-second
Electric current =
1 ampere =
Volt is a unit of electrical potential difference, it is defined as potential energy per unit charge.
1 volt =
Electrical energy = potential difference × Quantity of charge
= V × Q
= V × I × t (I = ampere; t = second)
One faraday is the charge required to liberate or deposit one gm equivalent of a substance at the corresponding electrode.
Faraday’s Law for Gaseous Electrolytic Product We know W = ZQ = Z I t
W = ..(i)
where Z = E/96500
Equation (i) is used to calculate the mass of a solid substance dissolved or deposited at an electrode.
For the gases, we use
V = ..(ii)
where V = volume of gas evolved at S.T.P. at an electrode
Ve = Equivalent volume
= Volume of gas evolved at an electrode at S.T.P. by 1 Faraday charge.