Electrochemistry is a branch of chemistry which deals with inter-conversion of chemical energies and electrical energy. We’ll discuss electrolytic reactions (reactions that occur when electricity passes through solutions) as well as electromagnetic reaction (reactions that produce electric energy...
1. INTRODUCTION
Electrochemistry is a branch of chemistry which deals with inter-conversion of chemical energies and electrical
energy. We’ll discuss electrolytic reactions (reactions that occur when electricity passes through solutions) as well
as electromagnetic reaction (reactions that produce electric energy). Some examples of electrolytic reactions are
electrolysis, electroplating, electro refining of metals, etc. Some examples of electro genetic reactions are reactions
occurring in cells or batteries.
2. TERMINOLOGIES IN ELECTROCHEMISTY
Some important terms used in Electrochemistry are as follows:
(a) Electrical Conductors: Materials that allow flow of electrons are known as conductors. There are broadly two
types of conductors-Electronic/Metallic and Electrolytic/Solution.
Table 17.1: Difference between Electronic Conductor and Electrolytic Conductors
Electronic Conductors or Metallic Conductors Electrolytic Conductors or Solution Conductors
1. Passage of current by movement of electrons in the Passage of current by ions in molten state or in aqueous solution
metallic lattice, e.g., Cu, Ag, etc. of electrolytes, e.g., NaCl (aq) or NaCl (fused).
2. Passage of current brings in only physical changes. Passage of current brings in physical as well as chemical changes.
3. It generally shows no transfer of matter. It involves transfer of matter in the form of ions.
4. It generally shows an increase in resistance during the It generally shows a decrease in resistance due to decrease in
passage of current due to increase in temperature. viscosity of the medium and degree of hydration of ions with
Thermal motion of metal ions hindering the flow of increase in temperature.
electrons increases with increase in temperature.
5. The conducting power of metals is usually high. The conducting power of electrolytic conductors is relatively low.
(b) Insulators: Those materials which don’t allow the passage of electrons are known as Insulators. For e.g. wood,
wool, plastic, silk, etc.
(c) Electrolytes: The substance that in solution or in the molten state, conducts electric current and is
simultaneously decomposed is called an electrolyte.The extent or degree of dissociation of different electrolytes
in solution is different. Electrolytes can be broadly categorized into two: Strong and Weak Electrolytes.
(d) Strong Electrolytes: Substances which are largely dissociated and form a highly conducting liquid in water
are strong electrolytes, e.g., All salts (except CdBr2, HgCl2), mineral acids like HCl, H2SO4, HNO3, etc. and
bases like NaOH, KOH, etc. are strong electrolytes. The strong electrolytes are almost 100% ionized at
normal dilution.
1
,(e) Weak Electrolytes: Substances which dissociate only to a small extent in aqueous solution forming low
conducting liquid are weak electrolytes, e.g., All organic acids (except sulphonic acids), inorganic acids
like HCN, H3BO3, etc. and bases like NH3, amines, etc. are weak electrolytes.
(f) Electrodes: In order to pass the current through an electrolytic conductor, two rods or plates are always
needed which are connected with the terminals of a battery. These rods/plates are called Electrodes. The
electrode where oxidation reaction takes place is anode and electrode where reduction takes place is cathode.
3. ELECTROLYSIS
The phenomenon in which passage of current through an electrolyte (molten or solution) brings in chemical
changes involving electronation (reduction) as well as de-electronation (oxidation) of ions is known as electrolysis.
3.1 Preferential Discharge Theory
If an electrolytic solution consists of more than two ions and the electrolysis is done, it is observed that all the
ions are not discharged from the electrodes simultaneously but certain ions are liberated from the electrodes in
preference to others. This is explained by preferential discharge theory. It states that if more than one type of
ions are attracted towards a particular electrode, then the one discharged is the ion which requires least energy.
The potential at which the ion is discharged or deposited on the appropriate electrode is termed the discharge
potential or deposition potential. The values of discharge potential are different for different ions.
Table 17.2: Examples of preferential discharge theory
Silver nitrate solution Pt 1
Ag e Ag 2OH O H O 2e
2 2 2
Sodium nitrate Pt 1
solution
2H 2e H2 2OH O H O 2e
2 2 2
Illustration 1: Find the charge in coulomb on 1 g-ion of N3 . (JEE MAIN)
Sol: First determine charge on one ions of this can be calculated as product of number of electron and charge of
electron. According to Avogadro’s law one g of ion contains 6.02 1023 ions. So, charge on one g-ion of N3 can be
calculated by multiplying charge.
Charge on one ions of N3 into Avogadro number.
3
Charge on one ions of N 3 1.6 10 coulomb One g-ion = 6.02 1023 ions
19
Thus, charge on one g-ion of N3 3 1.6 1019 6.02 1023 = 2.89 105 coulomb
Misconception: Electrolysis does not mean breaking up of an ionic compound into ions. An ionic
compound even on dissolution in water furnishes ions.
Note: During electrolysis, oxidation-reduction occurs simultaneously. Oxidation occurs at anode whereas
reduction occurs at cathode.
3.2 Faraday’s Law of Electrolysis
The relationship between the quantity of electric charge passed through an electrolyte and the amount of the
substance deposited at the electrodes was presented as the ‘laws of electrolysis’ by Faraday in 1834.
3.2.1 Faraday’s First Law
When an electric current is passed through an electrolyte, the amount of substance deposited is proportional to
the quantity of electric charge passed through the electrolyte.
If W be the mass of the substance deposited by passing Q coulomb of charge, then according to the law, we have
the relation: W Q
Q = current in amperes × time in seconds I t
So, W I t or W Z I t
Where Z is a constant, known as electrochemical equivalent and is characteristic of the substance deposited.
When a current of one ampere is passed for one second, i.e., one coulomb (Q = 1), then W = Z.
Definition of electrochemical equivalent: Mass of the substance deposited by one coulomb of charge or one
ampere current for one second.
3.2.2 Faraday’s Second Law
When the same quantity of charge is passed through different electrolytes, then the masses of different substances
deposited at the respective electrodes will be in the ratio of their equivalent masses. Again according to first law,
W ZQ
When, Q 96500 coulomb, W becomes gram equivalent mass (E).
E
Thus, E Z 96500 or Z = ; Z 1 E1
96500 Z2 E2
3.2.3 Faraday’s Law for Gaseous Electrolytic Product
ItVe
For the gases, we use V
96500
Where, V = Volume of gas evolved at STP at an electrode
Ve = Equivalent volume = Volume of gas evolved at an electrode at STP by 1 faraday charge
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