Solutions and electrochemistry

Physical Chemistry VI (Solutions & Electrochemistry) Dr Fateh Eltaboni
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SOLUTIONS
CONTENTS
WAYS OF EXPRESSING CONCENTRATION: Molarity Molality Normality
SOLUTIONS OF GASES IN GASES: Henry‟s Law
SOLUTIONS OF LIQUIDS IN LIQUIDS:
SOLUBILITY OF COMPLETELY MISCIBLE LIQUIDS
SOLUBILITY OF PARTIALLY MISCIBLE LIQUIDS
VAPOUR PRESSURES OF LIQUID LIQUID SOLUTIONS
DISTILLATION: STEAM DISTILLATION
SOLUTIONS OF SOLIDS IN LIQUIDS
SOLUBILITY–EQUILIBRIUM CONCEPT
DETERMINATION OF SOLUBILITY

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SOLUTION
Solution: It is a homogeneous mixture of two or more substances.
Solution consists of solute and solvent.
The constituent of the mixture present in a smaller amount is called the
Solute and the one present in a larger amount is called the Solvent.
For example, when a smaller amount of sugar (solute) is mixed with
water (solvent).
SOLUTION CONCENTRATION:
The concentration of a solution is defined as : the amount of solute
present in a given amount of solution.
Concentration is generally expressed as:
A solution containing low concentration of solute is called DILUTE
SOLUTION. A solution of high concentration is called CONCENTRATED
SOLUTION.

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TYPES OF SOLUTIONS:
WAYS OF EXPRESSING CONCENTRATION:
(1) Per cent by weight (wt%)
(2) Mole fraction (x)
(3) Molarity (M)
(4) Molality (m)

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(5) Normality (N)
(1) Per cent by weight (wt%)
For example: if a solution of HCl contains 36 per cent HCl by weight, it
has 36 g of HCl for 100 g of solution.
(2) Mole fraction (x)

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(3) Molarity (M)
For 1 mole of solute dissolved in one litre of solution: M = 1 i . e.,
molarity is one. Such a solution is called 1 M (read “1 molar”). A
solution containing 2 moles of solute in one litre is 2M (“two molar”);
and so on. Unit of molarity is mol litre–1

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.
(4) Molality (m)
A solution obtained by dissolving one mole of the solute in 1000 g of
solvent is called one molal or 1 m solution.

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(5) Normality (N)
Thus, if 40 g of NaOH (eq. wt. = 40) be dissolved in one litre of solution,
normality of the solution is one and the solution is called 1 N (one-
normal). A solution containing 4.0 g of NaOH is 0.1 N.

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SOLUTIONS OF GASES IN GASES:
When a gas is mixed with another gas a completely homogeneous
solution results.
PROPERTIES OF GASEOUS SOLUTIONS:
(1) Complete miscibility.
When one gas is dissolved in another gas they form a
homogeneous solution.

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(2) Dalton‟s law of Partial pressures.
The total pressure exerted by a gaseous mixture is the sum of the
partial pressures of the component gases.
If p1, p2 ,p3 are the partial pressures of the constituents, the total
pressure PT of the mixture is given by:
PT = p1 + p2 + p3
HENRY‟S LAW:
The solubility of a gas in a solvent depends on the pressure and the
temperature.
When a gas is in saturated solution, the following equilibrium exists:
gas  gas in solution
If pressure is increased on the system, the equilibrium will move in the
direction which will reduce the pressure (Le Chatelier Principle).
The pressure can be reduced by more gas dissolving in solvent. Thus
solubility or concentration of a gas in a given solvent is increased with
increase of pressure.

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Henry‟s Law: for a gas in contact with a solvent at constant
temperature, concentration of the gas that
dissolves in the solvent is directly proportional to the pressure of the
gas.
Mathematically, Henry‟s Law may be expressed as:
Cα P
C = k P
Where k is Henry‟s Law constant depends on the nature of the gas
and solvent, and the units of P and C used.

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SOLUTIONS OF LIQUIDS IN LIQUIDS:
1. SOLUBILITY OF COMPLETELY MISCIBLE LIQUIDS:
Liquids like alcohol and water mix in all proportions and they could be
compared to gases.
2. SOLUBILITY OF PARTIALLY MISCIBLE LIQUIDS:
A large number of liquids are known which dissolve in one another
only to a limited extent e . g ., ether and water. Ether dissolves about
1.2% water; and water also dissolves about 6.5% ether.
When equal volumes of ether and water are shaken together, two
layers are formed, one of a saturated solution of ether in water and
the other of a saturated solution of water in ether.
STUDY THE EFFECT OF TEMPERATURE ON THE COMPOSITION OF
SUCH MIXTURES WITH REFERENCE TO THREE TYPICAL SYSTEMS:
(1) Phenol-Water system
(2) Triethylamine-Water system
(3) Nicotine-Water system

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The temperature at which the two solutions layers merge into one
another to from one layer, is called the Critical Solution Temperature
(CST)
VAPOUR PRESSURES OF LIQUID–LIQUID SOLUTIONS:
The study of the vapour pressures of mixtures of completely miscible
liquids at constant temperature help in the separation of the liquids by
fractional distillation.

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By plotting the vapour pressure against composition it has been
revealed that mixtures of the miscible liquids are of three types:
1. First Type of Mixtures of Miscible Liquids: complete separation of
this type of solutions into components is impossible.
2. Second Type of Mixtures of Miscible Liquids: it is not possible to
effect a complete separation by fractional distillation
3. Third Type of Mixtures of Miscible Liquids: we can completely
separate the components by fractional distillation.
THEORY OF FRACTIONAL DISTILLATION:
Fractional distillation is the separation of a mixture into its
component parts, or fractions, such as in separating chemical
compounds by their boiling point by heating them to
atemperature at which one or more fractions of the compound
will vaporize. It is a special type of distillation. Generally the

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component parts boil at less than 25 °C from each other under a
pressure of one atmosphere. If the difference in boiling points is
greater than 25 °C, a simple distillation is used.
The efficiency of fractional distillation is considerably enhanced by the
use of the Fractionating columns.

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STEAM DISTILLATION:
Distillation carried in a current of steam is called steam distillation.
Steam distillation is used for purification of organic liquids which are
steam volatile and immiscible with water ( e. g., aniline).
THEORY OF STEAM DISTILLATION:
The vapour pressure of a liquid increases with increase of
temperature. When the vapour pressure equals the atmospheric
pressure, the temperature recorded is the boiling point of the given
liquid. In case of a mixture of two immiscible liquids, each component
exert its own vapour pressure as if it were alone. The total vapour
pressure over the mixture (P) is equal to the sum of the individual
vapour pressures (p1, p2) at that temperature.
P = p1 + p2
Hence the mixture will boil at a temperature when the combined
vapour pressure P, equals the atmospheric pressure. Since P > p1 or
p2, the boiling point of the mixture of two liquids will be lower than
either of the pure components.

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SOLUTIONS OF SOLIDS IN LIQUIDS:
The process of solution of a solid substance in a solvent is explained by
the electrical forces operating between the molecules or ions of the
solute and the molecules of the solvent.
Polar solutes dissolve easily in polar solvents while they remain
insoluble in non-polar solvents. For example, sodium chloride (an
electrolyte) is soluble in water which is highly polar solvent, while it is
insoluble in a non-polar solvent like chloroform.
The sodium chloride (NaCl) dissolves in water to give Na+
and Cl–
ions.
The Na+
ion is solvated (hydrated) to have around a layer of water
molecules.

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DISSOLUTION DUE TO HYDROGEN BONDING:
SOLUBILITY–ITS EQUILIBRIUM CONCEPT:
(1) Dissolution – the particles of the solute leaving the solid and
passing into solution.
( 2) Recrystallisation – the particles of the solute returning from the
solution and precipitating on the solid.
(dynamic equilibrium)

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THE SOLUBILITY is defined as the concentration of the solute in
solution when it is in equilibrium with the solid substance at a
particular temperature.
THE SATURATED SOLUTION is defined as one which is in equilibrium
with the excess of solid at a particular temperature.
THE SUPERSATURATED SOLUTION is a solution which would contain
more solute than the saturated solution at that temperature.
DETERMINATION OF SOLUBILITY:
The solubility of a substance is determined by preparing its saturated
solution and then finding the concentration by evaporation or a
suitable chemical method.
SOLUBILITY CURVES:
A curve drawn between solubility and temperature is termed Solubility
Curve.
THE SOLUBILITY CURVES ARE TWO TYPES:
(1) Continuous solubility curves (2) Discontinuous solubility curves

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THEORY OF DILUTE SOLUTIONS
CONTENTS
COLLIGATIVE PROPERTIES: LOWERING OR VAPOUR PRESSURE: RAOULT‟S LAW
MEASUREMENT OF LOWERING OF VAPOUR PRESSURE: (1) Barometric Method
(2) Manometric Method
(3) Ostwald and Walker‟s Dynamic Method
BOILING POINT ELEVATION: Relation between Boiling-point elevation and Vapour-
pressure lowering
MEASUREMENT OF BOILING POINT ELEVATION: (1) Landsberger-Walker Method
(2) Cottrell‟s Method
FREEZING-POINT DEPRESSION
MEASUREMENT OF FREEZING-POINT DEPRESSION: (1) Beckmann‟s Method
(2) Rast‟s Camphor Method
COLLIGATIVE PROPERTIES OF ELECTROLYTES

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COLLIGATIVE PROPERTIES
Dilute solutions containing non-volatile solute exhibit the following
properties:
(1) Lowering of the Vapour Pressure
(2) Elevation of the Boiling Point
(3) Depression of the Freezing Point
(4) Osmotic Pressure
Colligative Properties: (Greek colligatus = Collected together).
Its defined as property which depends on the number of particles in
solution and not on the size or chemical nature of the particles
 Each colligative property is related to any other.
 The colligative properties are used to measure the molecular
weights of the dissolved substances.
LOWERING OF VAPOUR PRESSURE: RAOULT‟S LAW

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P is the vapour pressure of the pure solvent and P is the pressure of
the solution. (P-Ps) is the lowering of vapour pressure.
Raoult‟s Law: the relative lowering of the vapour pressure of a dilute
solution is equal to the mole fraction of the solute present in dilute
solution.
IDEAL SOLUTIONS AND DEVIATIONS FROM RAOULT‟S LAW:
 A solution which obeys Raoult‟s law strictly is called an Ideal
solution.
 A solution which shows deviations from Raoult‟s law is called a
Nonideal or Real solution.

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Determination of Molecular Mass from Vapour Pressure Lowering:
If in a determination w grams of solute are dissolved in W grams of the
solvent, m and M are molecular masses of the solute and solvent
respectively, we have:

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MEASUREMENT OF LOWERING OF VAPOUR PRESSURE:
(1) Barometric Method
 Measuring the individual vapour pressure
of a liquid and then the solution
 This method is not accurate as the lowering
of vapour pressure is too small.
(2) Manometric Method
 The air in the connecting tube removed by a vacuum pump.
 The manometric liquid can be mercury or n-butyl phthalate which
has low density and lowvolatility .

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ELEVATION OF BOILING POINT:
Relation between Elevation of Boiling Point and Lowering of Vapour-
pressure:
 When a liquid is heated, its vapour pressure rises and when it
equals the atmospheric pressure, the liquid boils.
 The addition of a non volatile solute lowers the vapour pressure
and consequently elevates the boiling point as the solution has to
be heated to a higher temperature to make its vapour pressure
become equal to atmospheric pressure.
 If Tb is the boiling point of the solvent and T is the boiling point of
the solution, the difference in the boiling points (ΔT) is called the
elevation of boiling point.
Determination of Molecular Mass from Elevation of Boiling Point:

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where Kb is a constant called Boiling point constant or Ebulioscopic
constant of molal elevation constant. If w/ m = 1, W = 1, Kb = ΔT .
and the units of Kb are :

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MEASUREMENT OF BOILING–POINT ELEVATION:
(1) Landsberger-Walker Method
(2) Cottrell‟s Method

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FREEZING–POINT DEPRESSION
The difference of the freezing point of the pure solvent and the solution
is referred to as the Depression of freezing point. It is represented by
the symbol ΔT or ΔTf.
Determination of Molecular Weight from Depression of Freezing point:
Where Kf is a constant called Freezing-point constant or Cryoscopic
constant or Molal depression constant. If w /m = 1 and W = 1, Kf =
ΔT.

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MEASUREMENT OF FREEZING–POINT
DEPRESSION
(1) Beckmann‟s Method (1903):
(2) Rast‟s Camphor Method

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COLLIGATIVE PROPERTIES OF ELECTROLYTES
 The electrolytes ionize and yield more than one particle per
formula unit in solution. Therefore, the colligative effect of an
electrolyte solution is always greater than that of a nonelectrolyte
of the same molal concentration.
Van‟t Hoff factor (i): its the ratio of the colligative effect produced
by an electrolyte solution to the effect for the same concentration
of a nonelectrolyte solution.
is the freezing-point depression for the electrolyte. is the
value of depression of freezing-point of the electrolyte solution
assuming no ionization (nonelectrolyte).

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ABNORMAL MOLECULAR MASSES OF ELECTROLYTES:
Since the value of (i) is always greater than 1 the experimental
molecular weight will always be less than the theoretical value
calculated from the formula.
Relation Between van‟t Hoff Factor (i) and Degree of Dissociation (α):
v is number of ions produced

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CONCEPT OF ACTIVITY AND ACTIVITY COEFFICIENT:
 The experimentally determined value of concentration whether of
is less than the actual concentration.
The „activity‟: its the effective concentration of a molecule or ion in a
solution. Activity coefficient (γ) is:

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OSMOSIS AND OSMOTIC PRESSURE
CONTENTS
WHAT IS OSMOSIS? The Egg Experiment Silica Garden
SEMIPERMEABLE MEMBRANES Preparation of Cupric ferrocyanide membrane
OSMOTIC PRESSURE Pfeffer‟s Method Berkeley and Hartley‟s Method A Modern
Osmometer
ISOTONIC SOLUTIONS THEORIES OF OSMOSIS Molecular Sieve Theory Membrane
Solution Theory Vapour Pressure Theory Membrane Bombardment Theory
REVERSE OSMOSIS Desalination of Sea Water
LAWS OF OSMOTIC PRESSURE Boyle-van‟t Hoff Law for Solutions
Charles‟-van‟t Hoff Law for Solutions Van‟t Hoff Equation for Solutions
Avogadro-van‟t Hoff Law for Solutions VAN‟T HOFF THEORY OF DILUTE SOLUTIONS
CALCULATION OF OSMOTIC PRESSURE DETERMINATION OF MOLECULAR WEIGHT
RELATION BETWEEN VAPOUR PRESSURE AND OSMOTIC PRESSURE
OSMOTIC PRESSURE OF ELECTROLYTES

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WHAT IS OSMOSIS?
 The flow of the solvent through a semipermeable membrane from
pure solvent to solution, or from a dilute solution to concentrated
solution, is termed Osmosis (Greek Osmos = to push).
COMPARISON BETWEEN DIFFUSION AND OSMOSIS

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OSMOSIS EXPERIMENTS
1. Thistle Funnel Experiment:
 The phenomenon of osmosis can be demonstrated by fastening a
piece of cellophane over a thistle funnel as shown in Fig. 16.3. A
concentrated aqueous sugar solution is placed inside the thistle
funnel which is then immersed in water. The osmosis takes place
through the semipermeable membrane from water to the sugar
solution. The flow of water into the funnel shows up as the
solution is seen rising in the tube remarkably.
2. The Egg Experiment:

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 The outer hard shell of two eggs of the same size is removed by
dissolving in dilute HCl. One of these is placed in distilled water
and the other in saturated salt solution. After a few hours it will be
noticed that the egg placed in water swells and the one in salt
solution shrinks. In the first case, water diffuses through the skin
(a semipermeable membrane) into the egg material which swells.
In the second case, the concentration of the salt solution being
higher than the egg material, the egg shrinks.
WHAT IS OSMOTIC PRESSURE?

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 The hydrostatic pressure built up on the solution which just stops
the osmosis of pure solvent into the solution through a
semipermeable membrane, is called Osmotic Pressure.
Another definition:
 Osmotic pressure may be defined as the external pressure
applied to the solution in order to stop the osmosis of solvent into
solution separated by a semipermeable membrane.

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DETERMINATION OF OSMOTIC PRESSURE
 The osmotic pressure of a given solution can be determined
experimentally by the methods detailed below. The apparatus
used for the purpose is often referred to as osmometer.
(1) Pfeffer‟s Method:
(2) Berkeley and Hartley‟s Method:

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(3) A Modern Osmometer:
ISOTONIC (ISO-OSMOTIC) SOLUTIONS

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 When two solutions are separated by a semipermeable
membrane and there is no flow of water across the membrane,
the solutions are said to be Isotonic.
THEORIES OF OSMOSIS
(1) The Molecular Sieve Theory:
 Smaller solvent molecules can pass through the pores but the
larger molecules cannot.

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(2) Membrane Solution Theory
 Membrane proteins bearing functional groups such as – COOH, –
OH, – NH2, etc., dissolve water molecules by hydrogen bonding or
chemical interaction. Thus membrane dissolves water from the
pure water (solvent) forming what may be called „membrane
solution‟.
(3) Vapour Pressure Theory:
REVERSE OSMOSIS
Desalination of Sea Water by Hollow-fibre Reverse Osmosis:

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LAWS OF OSMOTIC PRESSURE
(1) Boyle-van‟t Hoff Law for Solutions:
(2) Charles-van‟t Hoff Law for Solutions:
at constant pressure
(3) Van‟t Hoff Equation for Solutions:

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(4) Avogadro-van‟t Hoff Law for Solutions:
CALCULATION OF OSMOTIC PRESSURE
 All gas laws may be considered to apply to dilute solutions rigidly.
This gives an easy solution to problems on osmotic pressure.

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DETERMINATION OF MOLECULAR WEIGHT FROM OSMOTIC
PRESSURE

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RELATION BETWEEN VAPOUR PRESSURE AND OSMOTIC PRESSURE

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OSMOTIC PRESSURE OF ELECTROLYTES
 Like other colligative properties, observed osmotic pressure of
electrolytes (π ) in aqueous solutions is higher than the value
calculated using van‟t Hoff equation, π0V = nRT . Expressing in
terms of van‟t Hoff factor (i):
Van‟t Hoff Equation for Solutions of Electrolytes:
 For example, if NaCl is ionized completely as at infinite dilution, 1
mole of NaCl will give 2 moles of particles (Na+
and Cl–
, one mole
each). Therefore, the observed osmotic pressure will be twice the
calculated value for no ionization. That is,
 Similarly, the value of i for CaCl2 at infinite dilution will be 3; for
FeCl3 it will be 4.
 If electrolytes are partially ionized in aqueous solution. α be the
degree of ionization for a salt AB,

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ELECTROLYSIS AND ELECTRICAL CONDUCTANCE
CONTENTS
Electrolysis and Electrical Conductance MECHANISM OF ELECTROLYSIS ELECTRICAL UNITS
FARADAY’S LAWS OF ELECTROLYSIS FARADAY’S FIRST LAW FARADAY’S SECOND LAW
IMPORTANCE OF THE FIRST LAW OF ELECTROLYSIS IMPORTANCE OF THE SECOND LAW OF ELECTROLYSIS
CONDUCTANCE OF ELECTROLYTES SPECIFIC CONDUCTANCE EQUIVALENT CONDUCTANCE
SUMMARY OF ELECTROCHEMICAL QUANTITIES STRONG ELECTROLYTES WEAK ELECTROLYTES
MEASUREMENT OF ELECTROLYTIC CONDUCTANCE DETERMINATION OF THE CELL CONSTANT

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 Water-soluble substances are distinguished as electrolytes or
nonelectrolytes.
 Electrolytes are electrovalent substances that form ions in
solution which conduct an electric current. NaCl, CuSO4 and
KNO3 are examples of electrolytes.
 Nonelectrolytes are covalent substances which furnish neutral
molecules in solution. Their water-solutions do not conduct an
electric current. Sugar, alcohol and glycerol are typical
nonelectrolytes.
 Electrolysis The decomposition of an electrolyte by passing
electric current through its solution is termed Electrolysis ( lyo =
breaking).
 Electrolytic cell. The cell contains water-solution of an electrolyte
in which two metallic rods (electrodes) are dipped .These rods
are connected to the two terminals of a battery (source of
electricity). The electrode connected to the positive terminal of
the battery attracts the negative ions (anions) and is called
anode. The other electrode connected to the negative end of the
battery attracts the positive ions (cations) and is called cathode.
MECHANISM OF ELECTROLYSIS
 Example: Let us consider the electrolysis of hydrochloric acid as
an example. In solution, HCl is ionised

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 In the electrolytic cell Cl–
ions will move toward the anode and H+
ions will move toward the cathode. At the electrodes, the
following reactions will take place.
 At cathode:
 At anode:
 Overall reaction:
ELECTRICAL UNITS
Coulomb

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1. A coulomb is a unit of electricity. It is the amount of electricity
which will deposit 0.001118 gram of silver from a 15 per cent
solution of silver nitrate in a coulometer.
2. Ampere is a unit rate of flow of electricity. It is that current which
of silver in one second. In other words, an ampere is a current of
one coulomb per second.
3. Ohm is a unit of electrical resistance.
4. Volt is a unit of electromotive force. It is the difference in
electrical potential required to send a current of one ampere
through a resistance of one ohm.
FARADAY‟S LAWS OF ELECTROLYSIS
 If m is the mass of substance (in grams) deposited on electrode
by passing Q coulombs ofelectricity, then
(First law)
 We know that Q = I × t, where I is the strength of current in
amperes and t is the time in second. Therefore,
(Second law)
 Where Z is the constant known as the Electrochemical equivalent
of the substance (electrolyte).
 THE ELECTRICAL UNIT FARADY:

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VERIFICATION OF THE SECOND LAW OF ELECTROLYSIS
 According to this law when the same quantity of electricity is
passed through different electrolyte solutions, the masses of the
substances deposited on the electrodes are proportional to their
chemical equivalents.

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 To verify the law, let us take an arrangement of the type shown in
Fig. 24.2. Pass the same quantity of electricity through the three
coulometers (the term „coulometer‟ is now in practice replaced
by the older term „voltameter‟) containing solution of dilute
H2SO4, CuSO4 and AgNO3.

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CONDUCTANCE OF ELECTROLYTES
 Electrolyte solutions conduct electric current by movement of the
ions to the electrodes. The power of electrolytes to conduct
electric currents is termed conductivity or conductance.
 Like metallic conductors, electrolytes obey Ohm‟s law.
According to this law, the current I flowing through a metallic
conductor is given by the relation:

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 Where E is the potential difference at two ends (in volts); and R
is the resistance measured in ohms (or Ω). The resistance R of a
conductor is directly proportional to its length, l, and inversely
proportional to the area of its cross-section, A . That is,
 Where ρ “rho” is a constant of proportionality and is called
resistivity or specific resistance. Its value depends upon the
material of the conductor. From above we can write:
 Specific resistance: It is the resistance in ohms which one
centimetre cube of it offers to the passage of electricity
Specific conductance or Specific conductivity.
 It is defined as: the conductance of one centimetre cube (cc) of a
solution of an electrolyte.

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Units of Specific conductance:
or
 The specific conductance increases with : (i) ionic concentration,
and (ii) speeds of the ions concerned.
Equivalent Conductance ():
 It is defined as the conductance of an electrolyte obtained by
dissolving one gram-equivalent of it in V cc of water.

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Variation of Equivalent conductance with Concentration (or Dilution):
 Strong electrolytes are completely ionised at all concentrations
(or dilutions). But weak electrolytes are partially ionised.
 The increase in equivalent conductance is not due to the increase
in the number of current carrying species. This is, in fact, due to
the decrease in forces of attraction between the ions of opposite
charges with the decrease in concentration (or increase in
dilution). At higher concentration, the forces of attraction
between the opposite ions increase
 Consequently, it affects the speed of the ions with which they
move towards oppositely charged electrodes. This phenomenon
is called ionic interference.
 As the solution becomes more and more dilute, the equivalent
conductance increases, till it reaches a limitary value. This value

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is known as equivalent conductance at infinite dilution (zero
concentration) and is denoted by 0.
 Weak electrolytes have low ionic concentrations and hence
interionic forces are negligible. Ionic speeds are not affected with
decrease in concentration (or increase in dilution). The increase
in equivalent conductance with increasing dilution is due to the
increase in the number of current-carrier species. In other
words, the degree of ionisation (α) increases.
 Thus increase in equivalent conductance (Λ) in case of a weak
electrolyte is due to the increase in the number of ions.
 In case of a weak electrolyte Λ∝ is the equivalent conductance
when ionisation is complete. So, the conductance ratio Λ / Λ∝ is
the degree of ionisation. That is,

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Molar Concentration (μ):
 It is defined as : the conductance of all ions produced by one
mole of an electrolyte when dissolved in a certain volume V cc.
 Upon dilution specific conductance decreases, while Equivalent
conductance and Molar conductance increases.

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Variation of Conductance with Temperature:
 The conductance of a solution of an electrolyte generally
increases with rise in temperature. For example, the
conductances of 0.1 M KCl at two different temperatures are

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 The conductance of a given electrolyte depends on two factors :
1. The number of ions present in unit volume of solution
2. The speed at which ions move towards the electrodes
 At a given temperature, the first factor remains the same for a
particular electrolyte. Thus the increase in conductance with rise
in temperature is due to the influence of factor (2). With rise in
temperature the viscosity of the solvent (water) decreases which
makes the ions to move freely toward the electrodes.
 For weak electrolytes, the influence of temperature on
conductance depends upon the value of ΔH accompanying the
process of ionisation. If the ionisation is exothermic (–ΔH), the
degree of ionisation is less at higher temperature (Le Chatelier‟s
principle) and conductance decreases.Conversely, if the
ionisation is endothermic (+ ΔH), the degree of ionisation is more
at higher temperature and conductance increases.
STRONG AND WEAK ELECTROLYTES
 Electrolytes may be divided into two classes :
1. Strong electrolytes
2. Weak electrolytes
1. Strong Electrolytes
 A strong electrolyte is a substance that gives a solution in which
almost all the molecules are ionised. The solution itself is called a

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strong electrolytic solution. Such solutions are good conductors
of electricity and have a high value of equivalent conductance
even at low concentrations. The strong electrolytes are :
2. Weak Electrolytes:
 A weak electrolyte is a substance that gives a solution in which
only a small proportion of the solute molecules are ionised. Such
a solution is called a weak electrolytic solution, that has low
value of equivalent conductance. The weak electrolytes are:
MEASUREMENT OF ELECTROLYTIC CONDUCTANCE
 The conductance is the reciprocal of resistance. Therefore it can
be determined by measuring the resistance of the electrolytic
solution. This can be done in the laboratory with the help of a
Wheatstone bridge.The solution whose conductance is to be
determined is placed in a special type of cell known as the
conductance cell.

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DETERMINATION OF THE CELL CONSTANT

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 To determine the cell constant, a standard solution of KCl whose
specific conductance at a given temperature is known, is used.
Then a solution of KCl of the same strength is prepared and its
conductance determined experimentally at the same
temperature. Substituting the two values in the above expression,
the cell constant can be calculated.
 For example, according to Kohlrausch the specific conductance
of N/50 solution at 25ºC is 0.002765 mho. Now, an N/50 solution of
KCl is prepared by dissolving 0.372 g pure KCl in 250 cc „extra-
pure‟ water (conductance water) and its conductance
determined at 25ºC. The cell constant is then calculated by
substituting the observed conductance in the expression.


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Solutions and electrochemistry

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Solutions and electrochemistry