Electrolytes 1

8/3/2019 Electrolytes 1

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ELECTROLYTES


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Electrolyte is anysubstance that give

ions when dissolved inwater, that make thesubstance electricallyconductive.These ionic solutionsconduct electricitydue to the mobility of the positive andnegative ions, which

are called cationsand anionsrespectively.


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ARRH ENIUS T H EO RY OFARRH ENIUS T H EO RY OFELECTOL Y TICELECTOL Y TICDISSOCI A TIONDISSOCI A TION1 . An electrolyte, when dissolved in water, breaks up into

two types of charged particles, one carrying a positive

charge and the other a negative charge. These chargedparticles are called ions. Positively charged ions aretermed cations and negatively charged as anions.

AB --> A+

+ B-

NaCl --> Na + + Cl -

K2SO4 --> 2K ++ SO 42-


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2. The process of splitting of the molecules into ions of anelectrolyte is called ionization. The fraction of the totalnumber of molecules present in solution as ions is known asdegree of ionization or degree of dissociation. It is denotedby

Degree of ionization ( )

nearly 1 = strong electrolytes(0 < < 1 ) = weak electrolytes0 = non-electrolytes

the degree of dissociation decreases with the increase of concentration

%001ondissociati beforeeelectrolytof moleculesof number Total

moleculesddissociateof Number % v!E

ondissociati beforeeelectrolytof moleculesof number Totalmoleculesddissociateof Number

!E


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V antV ant H off factor ( )H off factor ( )))1(1 E! ni

n = number of ions

= degree of dissociation= the factor which is defined as theratio of the observed colligativeproperty produced by the givenconcentration of electrolytic solutionto the colligative property produced inthe solution of non-electrolytes as the

same concentration.

i

ii


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3 . for weak electrolytes : Ions present insolution constantly re-unite to form neutralmolecules and, thus, there is a state of dynamic equilibrium between the ionized andthe non-ionized molecules, i.e.,

AB A+ + B -Applying the law of mass action to aboveequilibrium

[A+ ][B- ] /[AB] =K


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4. When an electric current is passed throughthe electrolytic solution, the positive ions(cations) move towards cathode and thenegative ions (anions) move towards anodeand get discharged, i.e., electrolysis occurs.

R eaction at Catode R eaction at A node1 . Ions from group IA, IIA, Al 3 +

,Mn2+

2H 2O + 2e 2O H - + H 22. Ions from other metalsM n + + ne M3 . Ion H+ (acid)2H + + 2e H 2

1 . Ions like SO 42-, NO3 -

2H 2O 4 H + + 4 e +O 22. Ions F - , Cl - , Br- , I-

2X - X 2 + 2e

3 . Ion OH-

(base)4 O H - 2H 2O + 4 e + O 2


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Electrolysis of sodium chloride solution:NaCl Na + + Cl -

H2O H+

+ OH-

At catode At AnodeH+ + e - --> H Cl - --> Cl + e -

2H --> H 2 2Cl --> Cl 2Electrolysis of copper sulphate solution using

platinum electrodes:CuSO 4 Cu 2+ + SO 42-

H2O H + + OH -

At catode At AnodeCu 2+ + 2e - --> Cu 2OH - --> H 2O + O + 2e -


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6 . The properties of electrolytes insolution are the properties of ionspresent in solution. For example, acidicsolution always contains H + ions whilebasic solution contains OH - ions andcharacteristic properties of solutions are

those of H-

ions and OH-

ionsrespectively.


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7 . The ions act like molecules towardsdepressing the freezing point, elevating theboiling point, lowering the vapour pressureand establishing the osmotic pressure.

8. The conductively of the electrolytic solutiondepends on the nature and number of ions asthe current is carried through solution by themovement of ions.


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DEBYE HUCKEL THEORY OFDEBYE HUCKEL THEORY OF

INTERIONIC ATTRACTIONINTERIONIC ATTRACTIONI. The strong electrolytes is completely ionized at all dilutionI. Anions and cations are not uniformly distributed in a solution of an electrolyte but that

the cations tend to be found in the vicinity of anions and vice versa.

I. Decrease in equivalent conductance with increase in concentration is due to fall inmobilities of the ions due to greater interionic effect and vice versa .

I. the ratio of equivalent conductance at given concentration to theequivalent conductance at infinite dilution does not correctly give the

degree of dissociation of electrolytes, but only the conductance or conductancecoefficient

I. Inspite of almost complete ionization is much less than

gPP

v

vP

gP


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Faradays Laws of ElectrolysisFaradays Laws of Electrolysis

Faraday's First Lawthe weight of substance liberated or deposited at electrodeduring electrolysis is directly proportional to the quantityof electricity passed through the electrolytic cell

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 QA coulomb is the quantity of charge when a current of one ampere is

passed for one second. Thus, amount of charge in coulombs,Q = current in amperes time in seconds

= I t

So W I tor W = z I t


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Faraday's Second LawWhen the same quantity of charge is passed through

different electrolytes, then the masses of differentsubstances deposited at the respective electrodes will bein the ratio of their equivalent masses

The law can be illustrated by passing same quantity of electric

current through three voltametres containing solutions of H 2SO 4,CuSO 4 and AgNO 3 respectively as shown in Fig. 1 2.1 . In the firstvoltameter, hydrogen and oxygen will be liberated; in the second,copper will be deposited and in the third, silver will be deposited.


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ExampleExampleHow long has a current of

3ampere to be applied through a solution of silver nitrateto coat a metal surface of 80 cm2 with 0.00 5 cm thick layer? Density of silver is

1 0.5 g/cm3 .Solution: Mass of silver to be deposited

= Volume density= Area thickness density

Given: Area = 80 cm 2, thickness = 0.000 5 cm and density = 1 0.5 g/cm 3

Mass of silver to be deposited = 80 0.000 5 1 0.5= 0.42 g

Applying to silver E = Z 9 65 00Z = 1 08/9 65 00 g

Let the current be passed for r seconds.

We know thatW = Z I t

So, 0.42 = 1 08/9 65 00 3 tor t = (0.42 9 65 00)/( 1 08 3 ) = 1 25 .09 second

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Electrolytes 1