1. Electrolytes are substances that dissociate into ions when dissolved in water, allowing them to conduct electricity. They can be classified as strong, weak, or non-electrolytes based on their conductivity. 2. Electrodes are materials inserted into electrolytic cells, and are classified as anodes or cathodes depending on whether oxidation or reduction occurs. During electrolysis, ions migrate to electrodes and undergo chemical reactions. 3. Faraday's laws of electrolysis describe the relationships between electrical charge passed, mass of substance deposited, current over time, and equivalent weights of elements involved in electrolysis reactions.

1. Easin Uddin Syed

2.  Electrolytes
 A substance which dissociates into ions as a result of
passageof electriccurrent through its aqueous solution is
known as electrolyte. Example:
Sodium chloride, NaCl
 Copper sulfate,CuSO4
 Types of electrolytes
Depending upon the capacity of conducting electricity
electrolytes are divided into 5 groups.
 Strong electrolyte (Strong acids, strong bases, saltsof
strong acid and base)
 Weak electrolyte (Weak acid, weak base, HgCl2)
 Moderately strong electrolytes (Na2CO3, CH3COONa,etc)
 Very weak electrolytes (CH3‐CO‐CH3)
 Non‐electrolytes/insulator (Sugar, alcohol)

3.  Electrodes
 The materials, usually small sheets, plates or wires of
metal or mayalso be of non‐metal thatare inserted in
the electrolytic solution of an electrolytic cell to pass
an electriccurrent through the electrolytesare termed
as electrodes. Example‐
 Copperelectrode
 Zincelectrode
 Classification
 According to the attraction of ions electrodesare
classified into twoclasses‐
 Anode
 Cathode

4.  Anode: Theelectrodeconnected to the positive terminal of
the battery, attracts the negative anions (anions) and
through which the electricity supposed to enter the
solution is termed as ananode.
 In otherwords, anode is theelectrodewhereoxidation
takes place. Some typical anodic reactionsare‐
Cu ‐ 2e‐ =
 Fe+2 ‐ e‐ =
Cu+2
Fe+3
 Cathode: The electrode connected to the negative
terminal of the battery, attracts the positive anions
(cations) and through which theelectricity supposed to
leave the solution is termed as acathode.
 In otherwords, cathode is theelectrodewhere reduction
takes place. Some typical cathodic reactionsare‐


Cu+2 + 2e‐
2H+ +2e‐
=
=
Cu
H2

6.  Mechanismof
electrolysis
 Due to the passage of electricity
through an electrolyte solution,the
electrolyte is dissociated into two
ions‐ cations and anions. The
cations migrate to the cathode and
form a neutral atom by accepting
electrons from it. The anions
migrate to the anode and yield a
neutral particle by transfer of
electrons to it. As a result of the
loss of electrons by anionsand gain
of electrons by cations at their
respective electrodes, chemical
reaction takes place.

7. Electrochemistry
 Electrochemistry is the branch of physical pharmacy
which elaborately deals with the phenomena of
interconversion of electrical and chemical energy, the
mechanism of such transformation and other
phenomena associated withthis.
 Electrochemistry indicates both the followingpoints:
 Conversion of electrical energy into chemicalenergy
 Conversion of chemical energy into electricalenergy

8. Faraday’s law of electrolysis
 In 1984, Michael Faraday announced his famous laws
of electricity which describes the relationshipbetween
the amounts of products liberated at the electrodes
and quantity ofelectricity.
 First law
 During electrolysis, the amount of an element
deposited ordissolved in an electrode is proportional
to the productof theamountof currentand the time
of its flow (i.e. the amount of electricity passed
through the electrolytesolution).

9.  Mathematical expression: If W is the mass ofsubstance
deposited on electrode by passing Q coulombs of
electricity, then according to the firstlaw‐
 W α QWhere,
 or, W α It I= the strength of current inampere
 or, W = ZIt t= time insecond
 Z= constant, known as electrochemical equivalentof
electrolyte.
 Electrochemical equivalent (Z): Electrochemical
equivalent is aconstant fora particularelement. When I= 1
amp and t= 1sec, thenZ=W.
 Therefore Z is equal to theamount of substancedeposited
due to the passageof 1 ampcurrent through 1 second.

10.  Importance of first law
 By using first law weare able tocalculate‐
 The value of electrochemical equivalents ofdifferent
substances.
 The masses of different substances produced bypassing
a known quantityof electricity through theirsolutions.

11.  Second law
 If the same amount of electricity is passed through
different electrolyte solutions during electrolysis, the
amountof differentelements deposited ordissolved at
the electrode are proportional to their respective
equivalentweight.
 Mathematical expression:

12.  Let theweight of the substancedeposited = W gm
 theequivalentweight of the substance = E gm
 According to the second law,
 W α E when Q isconstant
 If the weights of two substances dissolved bypassing
the same quantity of electricity is W1 and W2 having
the respective chemical equivalent E1 and E2, the
second law can be mathematicallyexpressed‐
 W1/W2 = E1/E2……………..(1)
 From the first law weget,
 W = ZIt, W1 = Z1It and W2 = Z2 It

13.  So, W1/W2 = Z1/Z2…………………………..(2)
 From equation (1) and (2)
 E1/E2 = Z1/Z2
 So, Z α E
 That is electrochemical equivalent is proportionalto
thechemical equivalentweight of a substance.
 Importance of second law
 The second law helps todetermine‐
 The equivalent weights of metals
 The unit of electriccharge
 The Avogadro’s number

14.  Conductance may be defined as the ability of the
electrolytes in electrolytic solution to conduct electric
current. Mathematically conductance of anelectrolytic
solution is the reciprocal of its resistance. Unit of
conductance is Siemens, S.
 Mathematical expression: The conductance of the
electrolytic solution may be measured by Ohm’slaw.
According to this law‐
 I = E / R
 Where, I= the current flowing through the solutionin
ampere


R= resistance of the solution
E= potential difference involt
Conductance

15.  The resistance R of a conductor is directly proportional
to its length in cm and inversely proportional to its
cross sectional area A incm2.

16. Transport number
 During electrolysis thecurrent is carried by theanions
and the cations. The fraction of the total current
carried by the cation or the anion is termed its
transport or Hittorf’snumber.

17. Measurement of transport number
 Transport number can be measured by thefollowing
ways‐
 Hittorf’s method
 Moving boundary method
 The moving boundary method
 The moving boundary method is based on the direct
observation of migration of ions under the influence
of applied potential.

18. Conductometric titration
 The titration in which conductance measurementsare
employed to determine the end point of acid‐alkali
reactions, some displacement reactions or
precipitation reactions are called conductometric
titration.
 For this purpose, the titrant is added from a burette
into a measured volume of the solution to be titrated
which is taken in a conductance cell and the
conductance readings corresponding to the various
additionsare plotted against thevolumeof the titrant.
In thisway two linearcurves areobtained, the pointof
intersection of which is the endpoint.

19. 1. Titration of a strong acid against a
strong base
 For example, the reaction in which HCl is titrated
againsta solutionof NaOH. This can be represented
by‐
 (H+ + Cl‐) + (Na+ + OH‐) → Na+ +Cl‐ +H2O
 The highly conducting hydrogen ions present in the
solution are replaced by sodium ions having ions
having much smaller conductance. The conductance
of Cl‐ remains constant and removed H+ combine with
OH‐ to form less ionized water. As a result the
conductance of solution decreases with subsequent
addition of alkali till the end point is reached.

21. 2. Titration of a weak acid against a
strong base
 For example, the reaction in which CH3COOHis
titrated against a solution of NaOH. This can be
represented by‐
 (H+ + CH3COO‐) + (Na+ + OH‐) → Na+ +CH3COO‐ +H2O
 The initial conductance of the solution is low because
of the poor dissociation of the weak acid. On adding
alkali, highly ionized sodium acetate is formed. The
acetate ions at first tend to suppress the ionization of
acetic acid further due to common ion effect. But after
a while the conductance begins to increase because the
conducting power of highly ionizes salt exceeds that of
the weak acid.

23. 3. Titration of a strong acid against a
weak base
 For example, the reaction in which HCl is titrated
againsta solutionof NH4OH. This can be represented
by‐

(H+ + Cl‐) + (NH4 + OH ) → NH4 +Cl +H2O+ ‐ + ‐
 The highly conducting hydrogen ions present in the
solution are replaced by ammonium ions havingions
having much smaller conductance. As a result the
conductance of solution decreases with subsequent
addition of alkali till the end point is reached.

25. 4. Titration of a weak acid against a
weak base
 The conductometric method is particularlysuitableas
such titrations do not give a sharp end‐point with
indicators. For example, the reaction in which
CH3COOH is titrated against a solution of NH4OH.
This can be representedby‐
 (H+ + CH3COO‐) + (NH4
+ + OH‐) → CH3COO NH4 +H2O
 The initial conductance of the solution is low because
of the poor dissociation of the weak acid. But it starts
increasing as the salt CH3COO NH4 is formed.

27. Advantages of Conductometric
titrations
 No indicator isrequired
 Colored/turbid solution: Where ordinaryindicator
give poorresultscan be successfully titrated
 Moreaccurate resultsareobtained as the end point is
determined graphically
 The method is useful in caseof verydilutesolution
 The method is useful forthe titration of weak acids
against weak bases
 Itcan be used forquantitativeestimation of cations
and anions in a varietyof reaction.