ELECTROCHEMISTRY

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 1

References:

1.

Engg.Chemistry by Jain and Jain 2.

3.

Engg.Chemistry by Dr. R.V.Gadag and Dr. A.Nithyananda Shetty Principles of Physical Chemistry by Puri and Sharma

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 2



Electrochemistry is a branch of chemistry which deals with the properties and behavior of electrolytes in solution and inter-conversion of chemical and electrical energies.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 3

 An

electrochemical cell

can be defined as a single arrangement of two electrodes in one or two electrolytes which converts chemical energy into electrical energy or electrical energy into chemical energy.

   It can be classified into two types:

Galvanic Cells.

Electrolytic Cells.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 4

Galvanic Cells:

A galvanic cell is an electrochemical cell that produces electricity as a result of the spontaneous reaction occurring inside it. Galvanic cell generally consists of two electrodes dipped in two electrolyte solutions which are separated by a porous diaphragm or connected through a salt bridge

. To illustrate a typical galvanic cell, we can take the example of

Daniel cell.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 5

Daniel Cell.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 6

At the anode: Zn → Zn 2+ + 2e At the cathode: Cu 2+ + 2e → Cu Net reaction: Zn(s)+Cu 2+ (aq)→ Zn 2+ (aq)+ Cu(s ) www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 7

 ELECTROLYTIC CELL An electrolytic cell is an electro –chemical cell in which a

non- spontaneous reaction

is driven by

an external source of current

although the cathode is still the site of reduction, it is now the negative electrode whereas the anode, the site of oxidation is positive . www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 8

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 9

Representation of galvanic cell.

   Anode Representation: Zn │ Zn 2+ or Zn ; Zn 2+ Zn │ ZnSO 4 (1M) or Zn ; ZnSO 4 (1M) Cathode Representation: Cu 2+ /Cu or Cu 2+ ;Cu Cu 2+ (1M) ; Cu or CuSO 4(1M) /Cu Cell Representation: Zn │ ZnSO 4 (1M) ║ CuSO 4(1M) /Cu www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 10

Liquid Junction Potential.

 Difference between the electric potentials developed in the two solutions across their interface .

E j = Ø soln, R - Ø soln,L

Eg: *Contact between two different electrolytes (ZnSO 4 / CuSO 4 ).

*Contact between same electrolyte of HCl / different concentrations(0.1M 1.0 M HCl).

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 11

Salt Bridge.

 The liquid junction potential can be reduced (to about 1 to 2 mV) by joining the electrolyte compartments through a salt bridge. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 12

Function Of Salt Bridge.

 It provides electrolytic contact between the two electrolyte solutions of a cell.

 It avoids or at least reduces junction potential in galvanic cells containing two electrolyte solutions in contact.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 13

Emf of a cell.

   The difference of potential, which causes a current to flow from the electrode of higher potential to one of lower potential.

E cell = E cathode - E anode

The E Cell depends on:

the nature of the electrodes.



temperature.



concentration of the electrolyte solutions.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 14



Standard emf of a cell(E o cell )

is defined as the emf of a cell when the reactants & products of the cell reaction are at unit concentration or unit activity, at 298 K and at 1 atmospheric pressure.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 15

The emf cannot be measured accurately using a voltmeter : 

As a part of the cell current is drawn,thereby causing a change in the emf.



As a part of the emf is used to overcome the internal resistance of the cell.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 16

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 17

 The emf of the cell E x

E x α

is proportional to the length AD.

AD

 The emf of the standard cell E s

E s α AD 1

is proportional to the length AD 1 .

E x E s ═ AD AD 1 E x = AD x E s AD 1 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 18

Standard Cell.



It is one which is capable of giving constant and reproducible emf.



It has a negligible temperature coefficient of the emf.



The cell reaction should be reversible.



It should have no liquid junction potential.

Eg: Weston Cadmium Cell. The emf of the cell is 1.0183 V at 293 K and 1.0181 V at 298 K.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 19

 Weston Cadmium Cell Sealed wax Cork CdSO4.8/3H 2 O crystals Cd-Hg 12-14% Cd Soturated solution of CdSO 4 .8/3H 2 O Paste of Hg 2 SO 4 Mercury, Hg www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 20

 Cell representation: Cd-Hg/Cd 2+ // Hg 2 SO 4 /Hg At the anode: Cd (s) → Cd 2+ + 2e At the cathode: Hg 2 SO 4(s) + 2e → 2 Hg (l) + SO 4 2 (aq) Cell reaction: Cd + Hg 2 2+ → Cd 2+ + 2Hg www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 21

Origin of single electrode potential.

 Consider Zn (s) / ZnSO 4 Anodic process: Zn (s) → Zn 2+ (aq) Cathodic process: Zn 2+ (aq) → Zn (s)  At equilibrium: Zn (s) ↔ Zn 2+ (aq) Metal has net negative charge and solution has equal positive charge leading to the formation of an Helmholtz electrical layer.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 22

Single electrode potential.

 Electric layer on the metal has a potential Ø (M) .

Electric layer on the solution has a potential Ø (aq) Electric potential difference between the electric double layer existing across the electrode /electrolyte interface of a single electrode or half cell.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 23

De-electronation Electronation Helmholtz double layer www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 24

MEASUREMENT OF ELECTRODE POTENTIAL.



It is not possible to determine experimentally the potential of a single electrode.



It is only the difference of potentials between two electrodes that we can measure by combining them to give a complete cell.



By arbitrarily fixing the potential of reversible hydrogen electrode as zero it is possible to assign numerical values to potentials of the various other electrodes.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 25

Sign Of Electrode Potential.

 The electrode potential of an electrode:

Is positive

electrode : If the electrode reaction is reduction when coupled with the standard hydrogen

Is negative:

If the electrode reaction is oxidation when coupled with standard hydrogen electrode. According to latest accepted conventions,

all single electrode potential values represent reduction tendency of electrodes.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 26

 when copper electrode is combined with SHE, copper electrode acts as cathode and undergoes reduction hydrogen electrode acts as anode.

H 2 (g) → 2H + +2e (oxidation)

Cu 2+ +2e → Cu (reduction)

Hence electrode potential of copper is assigned a positive sign. Its standard electrode potential is 0.34 V. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 27

 When zinc is coupled with S.H.E. zinc electrode acts as anode and hydrogen electrode acts as cathode.

Zn → Zn 2+ +2e 2H + + 2e → H 2 . Hence, electrode potential of zinc is negative. The standard electrode potential of zinc electrode is -0.74 V.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 28

Nernst Equation.

  It is a quantitative relationship between electrode potential and concentration of the electrolyte species.

Consider a general redox reaction: M n+ (aq) + ne → M(s) ----(1) We know that, Δ G =-nFE ----- (2) Δ G = Δ G o Δ G o =-nFE o -----(3) +RT ln K www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 29

Δ G = Δ G o +RT ln K Δ G = Δ G o +RT ln[M]/[M n+ ]-----(4) -nFE= -nFE o + RT ln [M]/[M n+ ]----(5) E= E o – RT/nF ln 1/[M n+ ]------(6) E=E o - 2.303 RT/nF log 1/[M n+ ]---(7) At 298K, E= E o -0.0592/n log 1/[M n+ ]-------(8) www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 30

1.

A galvanic cell consists of copper plate immersed in 10 M solution of CuSO 4 and iron plate immersed in 1M FeSO 4 at 298K. If E 0 cell =0.78 V, write the cell reaction and calculate E.M.F. of the cell.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 31

Solution:

Cell reaction: Fe + Cu 2+ ↔ Fe 2+ + Cu E Cell = E 0 Cell -0.0592/2 log [Fe 2+ ]/[Cu 2+ ] E Cell = 0.78 + 0.0296 log 10/1 =0.8096V

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 32

Calculate E.M.F. of the zinc – silver cell at 25˚C when [Zn

2+

] = 1.0 M and [Ag + ] = 10 M (E 0 cell =1.56V at 25˚C).

Write the cell representation and cell reaction www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 33

    Solution: Cell representation Zn/ Zn 2+ ((1M)//Ag + (10M) /Ag Cell reaction: Zn + 2Ag + ↔ Zn 2+ E Cell = E 0 Cell -0.0592/2 log [Zn 2+ + 2Ag ]/[Ag + ] 2 E Cell = 1.56 + 0.0592 log 10/1.0

=1.6192 V www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 34

The emf of the cell Mg │ Mg 2+ (0.01M) ║ Cu 2+ /Cu is measured to be 2.78 V at 298K. The standard eletrode potential of magnesium electrode is -2.37 V. Calculate the electrode potential of copper electrode www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 35

Cell reaction: Mg + Cu 2+ ↔ Mg 2+ E= E o -0.0592/n log 1/[M n+ ] E Mg = E o Mg -0.0592/2 log 1/[Mg 2+ ] =-2.4291V

E cell =E Cu -E Mg 2.78 = E Cu -[-2.429] E Cu =2.78-2.429

=0.3509 V + Cu www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 36

The emf of the cell Cu │ Cu 2+ (0.02M) ║ Ag + /Ag is measured to be 0.46 V at 298K. The standard eletrode potential of copper electrode is 0.34 V. Calculate the electrode potential of silver. electrode www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 37

Energetics of Cell Reactions.

 Net electrical work performed by the cell reaction of a galvanic cell: W= QE ------(1) Charge on 1mol electrons is F(96,500)Coulombs.

When n electrons are involved in the cell reaction, the charge on n mole of electrons = nF www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 38

Q = nF Substituting for Q in eqn (1) W = nFE ----------(2) The cell does net work at the expense of Δ G accompanying.

Δ G = -nFE

Δ G = nFE www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 39

From Gibbs – Helmholtz equation.

Δ G = Δ H + T [ δ ( Δ G)/ δ T] P -nFE = Δ H – nFT ( δ E/ δ T) P ------- (2) Δ H = nFT ( δ

Δ H = nF[T( δ

E/ δ T) P

E/ δ T) P

– nFE

–E]

 We know that, [ δ (∆G)/ δ T] P =

Δ S = nF ( δ E/ δ T) P

Δ S www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 40

Problem: Emf of Weston Cadmium cell is 1.0183 V at 293 K and 1.0l81 V at 298 K.

Calculate ∆G, Δ H and Δ S of the cell reaction at 298 K.

Solution:- ∆G: ∆G = - n FE n = 2 for the cell reaction; F = 96,500 C E= 1.0181 V at 298 K ∆G = -2 x 96,500 x 1.0181 J = -196.5 KJ www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 41

        ∆H: ∆H = nF [ T ( δ E / δ T)P – E] ( δ E/ δ T)p = 1.0181 – 1.0183 / 298-293 = 0.0002 / 5 = -0.00004VK-1 T = 298 K ∆H = 2 x 96,500 { 298 x (-0.00004) – 1.0181) = -198. 8 KJ Δ S: Δ S = nF ( δ E / δ T) P = 2 x 96,500 x (0-00004) = -7.72JK-1 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 42

Classification of Electrodes.



Gas electrode ( Hydrogen electrode).



Metal-metal insoluble salt (Calomel electrode).



Ion selective electrode.(Glass electrode).

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 43

Gas electrode.

 

It consists of gas bubbling over an inert metal wire or foil immersed in a solution containing ions of the gas.

Standard hydrogen electrode is the primary reference electrode, whose electrode potential at all temperature is taken as zero arbitrarily .

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 44

Construction.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 45

 Representation: Pt,H 2(g) / H +  Electrode reaction: H + + e  1/2 H 2(g) The electrode reaction is reversible as it can undergo either oxidation or reduction depending on the other half cell.

 If the concentration of the H + ions is 1M, pressure of H 2 is 1atm at 298K it is called as

standard hydrogen electrode (SHE).

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 46

Applications.

  To determine electrode potential of other unknown electrodes.

To determine the pH of a solution.

E=E o - 2.303 RT/nF log [H 2 ] 1/2 /[H + ] = 0 -0.0591 log 1/[H + ] = -0.0591pH.

Cell Scheme: Pt,H 2 ,H + (x) // SHE www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 47

 The emf of the cell is determined.

 E (cell) = E (c) – E (A) = 0 – (- 0.0592 pH) E (cell) = 0.0592 pH

pH = E (cell) / 0.0592

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 48

Limitations.

   Constuction and working is difficult.

Pt is susceptible for poisoning.

Cannot be used in the presence of oxidising agents.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 49

Metal –metal salt ion electrode.

 These electrodes consist of a metal and a sparingly soluble salt of the same metal dipping in a solution of a soluble salt having the same anion.

Eg: Calomel electrode.

Ag/AgCl electrode.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 50

Construction.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 51

 Representation: Hg; Hg 2 Cl 2 / KCl

It can act as anode or cathode depending on the nature of the other electrode.

 As anode: 2Hg + 2Cl → Hg 2 Cl 2 + 2e  As Cathode: Hg 2 Cl 2 + 2e → 2Hg + 2 Cl www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 52

E = E o – 2.303 RT/2F log [Cl-) 2 = E o -0.0591 log [Cl ] at 298 K Its electrode potential depends on the concentration of KCl.

Conc. of Cl Electrode potential 0.1M 0.3335 V 1.0 M 0.2810 V Saturated 0.2422 V www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 53

Applications.

   Since the electrode potential is a constant it can be used as a secondary reference

electrode.

To determine electrode potential of other unknown electrodes.

To determine the pH of a solution.

Pt,H 2 /H + (X) // KCl,Hg 2 Cl 2 ,Hg

pH = E (cell) – 0.2422/ 0.0592

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 54

Ion Selective Electrode.

   It is sensitive to a specific ion present in an electrolyte.

The potential of this depends upon the activity of this ion in the electrolyte.

Magnitude of potential of this electrode is an indicator of the activity of the specific ion in the electrolyte.

*This type of electrode is called indicator electrode. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 55

 Glass Electrode:` www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 56

Scheme of typical pH glass electrode

1. a sensing part of electrode, 2. a bulb made from a specific glass sometimes electrode contain small amount of AgCl precipitate inside the glass electrode 3 internal solution, usually 0.1M HCl for pH electrodes 4.internal electrode, usually silver chloride electrode 5.body of electrode, made from non conductive glass or plastics. 6.reference electrode, usually the same type as 4 or 7.junction with studied solution, usually made from calomel electrode ceramics or capillary with asbestos or quartz fiber.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 57

 The hydration of a pH sensitive glass membrane involves an ion-exchange reaction between singly charged cations in the interstices of the glass lattice and protons from the solution. H + + Na + Na + + H + Soln. glass soln. glass E g = E o g – 0.0592 pH www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 58

Electrode Potential of glass electrode.

The overall potential of the glass electrode has three components:  

The boundary potential E b , Internal reference electrode potential E ref.



Asymetric potential E

bulb to changes in [H + ].

asy

.

- due to the difference in response of the inner and outer surface of the glass

E g = E b + E ref.

+ E asy.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 59



E g

E E b E b g = E = RT/nF ln C = L + RT/nF ln C 1 depends upon [H = E b 1 – E 2 + E Ag/AgCl 1 – RT/nF ln C 2 + + E asy.

= L + RT/nF ln C 1 ] = E = E o o

= E o g

g g + RT/nF ln C 1 + E Ag/AgCl + 0.0592 log [H + ]

– 0.0592 pH.

+ E asy.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 60

 1.

2.

3.

4.

Advantages:

It can be used without interference in solutions containing strong oxidants, strong reductants, proteins, viscos fluids and gases as the glass is chemically robust.

It can be used for solutions having pH values 2 to 10. With some special glass (by incorporation of Al 2 O 3 or B 2 O 3 ) measurements can be extended to pH values up to 12.

It is immune to poisoning and is simple to operate The equilibrium is reached quickly & the response is rapid www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 61

5. It can be used for very small quantities of the solutions. Small electrodes can be used for pH measurement in one drop of solution in a tooth cavity or in the sweat of the skin (micro determinations using microelectrodes) 6. If recently calibrated, the glass electrode gives an accurate response.

7. The glass electrode is much more convenient to handle than the inconvenient hydrogen gas electrode.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 62

Disadvantages: The bulb of this electrode is very fragile and has to be used with great care. The alkaline error arises when a glass electrode is employed to measure the pH of solutions having pH values in the 10-12 range or greater. In the presence of alkali ions, the glass surface becomes responsive to both hydrogen and alkali ions. Low pH values arise as a consequence and thus the glass pH electrode gives erroneous results in highly alkaline solutions.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 63

The acid error results in highly acidic solutions (pH less than zero)Measured pH values are high.

Dehydration of the working surface may cause erratic electrode performance. It is crucial that the pH electrode be sufficiently hydrated before being used. When not in use, the electrode should be stored in an aqueous solution because once it is dehydrated, several hours are required to rehydrate it fully.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 64

As the glass membrane has a very high electrical resistance (50 to 500 m Ω ), the ordinary potentiometer cannot be used for measurement of the potential of the glass electrode. Thus special electronic potentiometers are used which require practically no current for their operation.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 65

Standardization has to be carried out frequently because asymmetry potential changes gradually with time. Because of an asymmetry potential, not all glass electrodes in a particular assembly have the same value of E o G . For this reason, it is best to determine E o G for each electrode before use. The commercial verson is moderately expensive www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 66

Limitations.

The bulb is very fragile and has to be used with great care.

In the presence of alkali ions, the glass surface becomes responsive to both hydrogen and alkali ions. Measured pH values are low.

In highly acidic solutions (pH less than zero) measured pH values are high.

When not in use, the electrode should be stored in an aqueous solution. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 67

Applications.

 Determination of pH: Cell: SCE ║ Test solution / GE E cell E cell = E = E g o g – E cal.

– 0.0592 pH – 0.2422

pH = E o g -E cell – E cal.

/ 0.0592

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 68

Problems

The cell SCE ΙΙ (0.1M) HCl Ι AgCl(s) /Ag gave emf of 0.24 V and 0.26 V with buffer having pH value 2.8 and unknown pH value respectively. Calculate the pH value of unknown buffer solution. Given E SCE = 0.2422 V www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 69

E o g = 0.0592pH +E cell + E cal.

= 0.0592x2.8 +0.24 + 0.2422

=0.648 V pH = E o g -E cell – E cal.

/ 0.0592

= 0.648 -0.26-0.2422/0.0592

= 2.46

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 70

CONCENTRATION CELLS.



Two electrodes of the same metal are in contact with solutions of different concentrations.



Emf arises due to the difference in concentrations.



Cell Representation:

M/ M n+ [C1] ║ M n+ /M [C2] www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 71

Construction.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 72

 At anode: Zn → Zn 2+ (C1) + 2e  At cathode: Zn 2+ (C2) + 2e → Zn   E cell = E C -E A = E 0 + (2.303RT/ nF)logC 2 [E 0 +(2.303RT/nF)logC 1 ] E cell = (0.0592/n) log C 2 /C 1

E cell is positive only if C 2 > C 1

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 73

 Anode - electrode with lower electrolyte concentration.

 Cathode – electrode with higher electrolyte concentration.

 Higher the ratio [C 2 /C 1 ] higher is the emf.

 Emf becomes zero when [C 1 ] = [C 2 ].

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 74

Problems

 Zn/ZnSO 4 (0.001M)||ZnSO 4 (x)/Zn is 0.09V at 25˚C. Find the concentration of the unknown solution.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 75

E cell = 0.0592/n log C 2 /C 1 0.09 =(0.0592/2) log ( x / 0.001) x =1.097M

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 76

2. Calculate the valency of mercurous ions with the help of the following cell.

Hg/ Mercurous || Mercurous /Hg V nitrate (0.001N) nitrate (0.01N) when the emf observed at 18˚ C is 0.029 E cell =(2.303 RT/nF) log C 2 /C 1 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 77

E cell =(2.303 RT/nF) log C 2 /C 1 0.029 = 2.303RT/n) log (0.01/0.001) 0.029 =0.057 x 1/ n n = 0.057/0.029 = 2 Valency of mercurous ions is 2, Hg 2 2+ www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 78

Assignment

Answer the following questions: 1.Distinguish between electrolytic and galvanic cells.

2.Explain the origin of electrode potential. What are the sign conventions for electrode potential?

3.Give reasons for the following.

i) The glass electrode changes its emf over a period of time.

ii) KCl is preferred instead of NaCl as an electrolyte in the preparation of salt bridge 4. What is meant by a standard cell? Give an example

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 79

5. Quote any four limitations of glass electrode 6.Define liquid junction potential. How it can be eliminated or minimized?

7.Derive Nernst equation for the single electrode potential.

8.Describe potentiometric determination of emf of a cell.

9.Writ e construction and working of Calomel Electrode 10.What are concentration cells? Show that emf of concentration cell becomes zero at a certain point of its working.

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 80