(283236740) hanuman electrochemistry

8/16/2019 (283236740) Hanuman Electrochemistry

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1

Introduction

LESSON

ELECTROCHEMISTRY

Electrochemistry deals with the study of equilibrium and kinetic properties of ions in solution, the

production of electrical energy from the energy released during spontaneous chemical reactions or

the processes where the electrical energy is used to bring about non-spontaneous chemical reactions.

Conductance of Electrolytic Solution

Ba s i c Quanti t ies of electrochemistry

• Resistance The resistance, ‘R’, of a sample of an electrolytic solution solution that gi!es

ions

lwhen dissol!ed in water" as in case of metallic conductors is defined by the e#pression$ R%=

&

, where l is the length of a sample of electrolyte and & is the cross sectional area.

• Resisti!ity or S"ecific Resistance The symbol % , is the proportionality constant. 't is

a

property of an electrolytic solution. This property is called resisti!ity or s"ecific resistance.• Conducti!ity or S"ecific conductance ( The reciprocal of resisti!ity is called conducti!ity

or

( = ) = l

s"ecific conductance, kappa" ( where% R&• Conductance#$%- The in!erse of resistance, R, is called conductance, * and can be e#pressed

)as * = .

R

S&I& units

The +' units of abo!e quantities are$'& nit of ResistanceR" is ‘ Ω

’

(& nit of Resisti!ity % " is ‘ohm meter Ω

m"’)& nit of onducti!ity ( " is ‘siemens per meter or Ω

-1m

-1or

+m-1

"

4. Unit of Conductance(G) is ‘siemens or S’ or Ω

-1*act ors aff ect in+ the conducti!ity of electrolyt ic solutions,

The conducti!ity of the electrolytic solutions depends upon$

#i% the nature of the electrolyte added

#ii% the sie of the ions produced and their hydration

#iii% the nature of the sol!ent and its !iscosity#i!% the concentration of the electrolyte

#!% temperature affects the kinetic energy of the ions"

*ac t ors af f e c t in+ the conducti!ity of m e tals

Electrical conductance through metals is called metallic or electronic conductance and is due to the

mo!ement of electrons. This is different from electrolytic conductance. The electronic conductance

depends on$

#i% the nature and structure of the metal

#ii% the number of !alence electrons per atom

#iii% temperature it decreases with increase of temperature".&s the electrons enter at one end and go out through the other end, the composition of the metallic

conductor remains unchanged. The mechanism of conductance through semiconductors is more

comple#.


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Measurement of Conducti!ity of Ionic Solutions

Conduc t i!ity of ionic solutions

&s in case of our metallic conductors we cannot use the /heatstone bridge for finding the resistanceof ionic solution. This is because of the following reasons$

#i% 0irstly, passing direct current 1" changes the composition of the solution.

#ii% +econdly, a solution cannot be connected to the bridge like a metallic wire or other solid

conductor.

The first difficulty is resol!ed by using an alternating current &" source of power. The second problem is sol!ed by using a specially designed !essel called conducti!ity cell& 't is a!ailable in

se!eral designs and two simple ones are shown in the figure$

Conduc t i!ity Cell

onducti!ity cell is used for measuring

conducti!ity of solutions. The solution thatis to be measured for its conducti!ity is

taken in a special cell called conducti!itycell. The conducti!ity cell has two

platinum electrodes coated with platinum black of surface area & and are separated

by a distance l. The solution confined

between these electrodes is a column of

length l and area of cross section &. The resistance of such a column of solution is then gi!en by the

l lequation$ R = ρ =

The ratio lis constant, called the cell constant and is denoted by *

2.

& κ& &

Measurement of conduc t i !ity usin+ Conducti!ity Cell

onducti!ity can be measured in the following way$• The solution for which the conducti!ity is to be known is

ltaken in a conducti!ity cell of known cell constant ".

&

• /e can know the resistance of the solution using a/heatstone bridge as shown in the figure" with two known

resistances R ) and R 3, and a !ariable resistance R 4, as shown

in the figure. The resistance of the solution unknown

resistanceR 5 " is gi!en by R = R) R

3

R 4

when the bridge is

balanced using the !ariable resistance R 4.

• onductance conducti!ity" can be estimated by substituting

( = )

= l

the !alue of R 5 for R in the e#pression % R&.

Note, The /heatstone 6ridge is said to be balanced when there is no current flow through the &..

detector headphone or other electronic de!ice

Molar Conduct i !ity

7olar conducti!ity is defined as the conducti!ity of a solution per unit concentration Thus we ha!e$

κ7olar conducti!ity 8 Λm =

c

5


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Molar cond u c t i !ity for Stron+ E lectrolytes

0or strong electrolytes, :m increases slowly with dilution.

Dohlrausch determined the empirical relationship for molar conducti!ity :m and the concentration of the

solution c, as Λ =

ECC ;

− & c

where the molar conducti!ity at infinite dilution, ECC ;

, is

arri!ed at by plotting the molar conducti!ity at !arious

concentrations Λ

m

!=s concentration and e#trapolating

it to ero concentration. The !alue of the constant & for a

gi!en sol!ent and temperature depends upon the type of electrolyte e.g.- )-) for al, 4-) al4 or 4-4 7g+F3,

etc.", rather than its specific chemical identity.

0o h l rausc h 1 s La/ of the Inde"endent M i +ration of Ions

E#perimental !alues of & ha!e been carefully determined for many salts as a function of

concentration in !ery dilute solutions, and the intercept !alues of the resulting plots obtained. 'n

e#amining such data, it was disco!ered that EC

C ;for any electrolyte can be e#pressed as the sum of

independent contributions from the constituent cations and anions present. This is known as

0ohlrausch1s La/ of the Inde"endent Mi+ration of Ions /hich states that limiting molar

conducti!ity of an electrolyte can be represented as the sum of the indi!idual contributions of theanion and cation of the electrolyte.

7athematically it may be e#pressed as$; ; ; ; ;ECC

m8G

H I

HHG

−

I −

where GH and G- refer to the cations and anions respecti!ely"& Aere I H and I− are

limiting molar conducti!ities of the cation and anion respecti!ely.

2ea3 Electrolytes

/eak electrolytes ha!e lower degree of dissociation at higher concentration. Therefore in case of

weak electrolytes EC

C ;cannot be obtained by simple e#trapolation of Λ m on the plot of Λ m !=s c .

The weak electrolytes dissociate to a greater e#tent with increasing dilution.

0or dissociation of ethanoic acid we ha!e we ha!e A FFA ƒ A FF-

+ AH

5 5

'f J is the degree of dissociation, the concentrations of the dissociated species e#pressed as and

that of the un-dissociated acid as )-J". The dissociation constant can be e#pressed as$

KA FF-LKA

+ L

D 8 5 =α2

[CHC!!H" 1 −#

&rrehenius showed that α =Λ=

m @ where the limiting molar conducti!ity is Λ

;.

Thus we ha!e

α4

; mm

Λ 4D 8 = ma) −

α"Λ; Λ; − Λ "

m m m

4""l i c a t ion of 0ohl r ausch la/

Dohlrausch law of independent migration of ions is useful in calculating;

indi!idual ions.

from the I ; of

'n case of weak electrolytes it is possible to determine the !alue of its degree of dissociation α

once $e %no$ t&e ' and ' m at a i*en concentration c.

m m

m

m

m

a

Λ

Λ

m

m


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α4

1issociation constant D 8 asα 8 Λ

m

a ) − α" ;

Electrochemical Cells

&ny electrochemical cell uses an o#idation-reduction reaction to generate an electric current0or e.g.- onsider the cell with the following reaction

Mns" H u2+⎯⎯→ Mn

2+H

us".

This can be seen as two half reactions,

u4H

aq" H 4e-⎯⎯→ us" Mns"⎯⎯→ Mn4Haq"H 4e-

Electrochemical cells can be classified into two types$i" which con!ert chemical energy of a spontaneous reaction to electrical energy *al!anic or

?oltaic cell".

ii" that make use of electric energy to carry non-spontaneous chemical changes Electrolytic cell".

.olta i c Cells

?oltaic cell makes use of *ibbs energy of spontaneous chemical reaction into electrical work.

Half cell

Reactions in cells can be seen as o5idation at anode and reduction at cathode. These o#idation and

reduction reactions indi!idually are known as half cell reactions. The electrode at which o#idation

takes place in an electrochemical cell is called the anode. The electrode at which reduction occurs iscalled the cathode. The identity of the cathode and anode can be remembered by recogniing that

positi!e ions, or cations, flow toward the cathode, while negati!e ions, or anions, flow toward theanode.

Notation for .oltaic Cells

?oltaic cells can be described by a line notation based on the following con!entions$ & single !ertical line indicates a change in state or

phase. /ithin a half-cell, the reactants are listed before the

products. oncentrations of aqueous solutions are written in parentheses after the symbol for the ion or

molecule. & double !ertical line is used to indicate the Nunction between the half-

cells.

Λm


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equi!alent weights.

Modern !ersion of *araday1s La/s 9 The amount of charge required for o#idation or reductiondepends on the stoichiometry of the electrode reaction.

Note:

• harge on one electron is ).Q;4)×);−1

.

Therefore charge on one mole of electron is 9

×).Q;4)×);−)

8 Q.;4 ×);45

mol-

)

×).Q;4)×);−)

8 Q3S ≅ QU;;

• The charge on the one mole of electrons is called the *araday1s constant and itsappro#imate !alue can be taken as QU;;.

4 study of ty"ical electrolytic "rocess,

The products of electrolysis depend upon the state of material being electrolyed and the type of

electrode used.

#i% Electrolysis of molten al

The electrode reactions can be summaried as$athode$ a+ H e-

→ a&node$ l

-→ )=4l4 H e

-#ii% Electrolysis of aqueous al

The electrode reactions may be summaried as$

alaq"⎯

A⎯

4F⎯→ aH aq" H l-aq"

athode$ A4Fl" H e- → V A4g" H FA- aq"&node$ l- aq" → V l4g" H e-F!erall reaction$ alaq" H A4Fl" → a+

aq" H FA-

aq" H V A4g" H V l4g"

#iii% Electrolysis of A4+F31uring the electrolysis of sulphuric acid, the following processes are possible at the anode$

0or dilute A4+F3

4A4Fl "W F4g" H 3AHaq" H 3e

9

0or concentrated A4+F3

Θcell " 8 H).45 ? )"

4− 4− − Θ4+F3

aq" → +4F

Saq" + 4e Ecell " 8 ).Q ? 4"

0or dilute sulphuric acid, reaction )" is preferred but at higher concentrations of A 4+F3 process,reaction 4" is preferred.

The Nernst E8uation

ernst showed that the potential for an electrochemical reaction is described by the following

equation.

E = EΘ

−R T

ln

Pn0

c

@ where E is the cell potential at some moment, E,is the cell potential when the

reaction is at standard-state conditions, R is the ideal gas constant in units of Noules per kel!in per

mole, T is the temperature in kel!in, n is the number of moles of electrons transferred in the

balanced equation for the reaction, 0 is the charge on a mole of electrons, and Pc

quotient at that moment in time.

is the reaction

• The standard electrode potentials are replaced by electrode potentials gi!en by ernst

equation

to take into account the concentration effects.

&

E


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Note,

• The symbol ln indicates a natural logarithm, which is the log to the base e, where e is anirrational number equal to 4.)S4S...• 0or standard condition three terms in the equation are constants$ R, T, and 0. The ideal

gas

constant is S.5)3 X=mol-D. The temperature is taken as 4SD. The charge on a mole of electrons

can be taken to be QU;; .

• +ubstituting this information into the ernst equation gi!es the following equation.

E = EΘ

−;.;4UQ

ln

P

0or base ); equation becomes E = EΘ

−;.;U

log P "

nc

n); c

• Three of the remaining terms in this equation are characteristics of a particular reaction$ n,

E,

,

and Pc.

• The standard-state potential for the 1aniell cell is ).); ?. Two moles of electrons

are

transferred from inc metal to u4H ions in the balanced equation for this reaction, so n is 4 for

this cell. &s we ne!er include the concentrations of solids in either reaction quotient or

equilibrium constant e#pressions, Pc for this reaction is equal to the concentration of the Mn4H

ion di!ided by the concentration of the u4H

ion.

KMn 4H

LPc

=4HKu L

+ubstituting what we know about the 1aniell cell into the equation gi!es the following result,

which represents the cell potential for the 1aniell cell at 4SD 4Uo" at any moment in time.

;.;4UQ KMn 4H

LE = E

,

−

ln4 Ku

4HL

- s i n+ the Nern s t E 8 uation to Mea s ure E8uil i 9rium Co n s tants

The ernst equation can be used to measure the equilibrium constant for a reaction. To understand

how this is done, we ha!e to recognie what happens to the cell potential as an o#idation-reduction

reaction comes to equilibrium. &s the reaction approaches equilibrium, the dri!ing force behind thereaction decreases, and hence the cell potential approaches ero.

Thus at equilibrium$ ; =

E,

−R T

ln D n0

c

Rearranging this equation and substituting for E, from ernst equation we ha!e

Θ

&t equilibrium$ n0E = RT ln D c

&ccording to this equation, we can calculate the equilibrium constant for any o#idation-reductionreaction from its standard-state cell potential$

n0EΘ

D c

= e RT or taking natural log we ha!e

ln D n 0 E

Θ

= or 4.5;5 logD n 0 E

Θ

=c

RT); c

RT

Electrochemical cells a n d $i9 9 s Ener+y of t h e react i on

The re!ersible work done by a gal!anic cell is equal to decrease in its *ibbs energy and therefore, if

the emf it is the potential of the cell when no current is drawn" of the cell is E and n0 is the amountof charge passed and ∆r * is the *ibbs energy of the reaction then ∆r * 8 -n0E, whereElectrical work done in one second is equal to electrical potential multiplied by total charge passed.


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'f the acti!ity of all the reacting species is unity, then E 8 E , and we

ha!e

∆r *,

8

-n0E,

0rom the standard *ibbs energy we can calculate equilibrium constant using equation ∆r *,

8

-RT

ln D c.

Note$'t may be remembered that Ecell" is an intensi!e parameter but ∆r * is an e#tensi!e

thermodynamic

property and the !alue depends on n. Thus, if we write the reactionMns" H u2+

aq" → Mn2+

aq" H us"

∆r * 8 - 40Ecell" but when we write the reaction

4 Mns" H 4 u2+

aq" → 4 Mn2+

aq" H 4 u s"

rG / -40(ce)Batteries

&ny battery it may ha!e one or more than one cell connected in series" or cell that we use as a

source of electrical energy is basically a gal!anic cell where the chemical energy of the redo#reaction is con!erted into electrical energy. Aowe!er, for a battery to be of practical use it should be

reasonably light, compact and its !oltage should not !ary appreciably during its use. There aremainly two types of batteries.

'& 7ri m ary B a t teries

'n the primary batteries, the reaction occurs only once and battery then

becomes dead after use o!er a period of time and cannot be reusedagain.

#i% :ry cell

The most familiar e#ample of this type is the dry cell known as


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MnAg"H AgFs" → MnFs" H Agl"The cell potential is appro#imately ).5U ? and remains constant during its life as the o!erall

reaction does not in!ol!e any ion in solution whose concentration can change during its lifetime.

(& Secondary Batteries

& good secondary cell can undergo a large number of discharging and charging cycles.

#i% Lead Stora+e Battery, The most important secondary cell is the lead

storage battery commonly used in automobiles and in!ertors. 'tconsists of a lead anode and a grid of lead packed with lead dio#ide

BbF4" as cathode. & 5SY solution of sulphuric acid is used as an

electrolyte.

The cell reactions when the battery is in use are gi!en below$

&node $ Bbs"H +F32-aq" → Bb+F3s" H4e-athode $ BbF4s" H +F32-

aq" H 3A+

aq" H 4e-

→ Bb+F3s" H 4A4Fl"i.e., o!erall cell reaction consisting of cathode and anode reactions is$Bbs" H BbF4s" H 4A4+F3aq" → 4Bb+F3s" H 4A4Fl"Fn charging the battery the reaction is re!ersed and Bb+F 3s" on anodeand cathode is con!erted into Bb and BbF4, respecti!ely. #ii% Nic3el Cadmium Cell,

&nother important

secondary cell is the

nickel - cadmium cell,which has longer life

than the lead storage

cell but moree#pensi!e tomanufacture. The o!erall

reaction during discharge

is$ d s" H 4iFA"5 s"W dF s" H 4iFA"4s" H A4Fl "


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Lead stora+e 9attery

*uel Cells

*al!anic cells that are designed to con!ert the energy of combustion of fuels like hydrogen,

methane, methanol etc. directly into electrical energy arecalled fuel cells. Fne of the most successful fuel cells uses

the reaction of hydrogen with o#ygen to form water. 'n the

cell, hydrogen and o#ygen are bubbled through porous carbon

electrodes into concentrated aqueous sodium hydro#idesolution. atalysts like finely di!ided platinum or palladium

metal are incorporated into the electrodes for increasing therate of electrode reactions. The electrode reactions are gi!en below$

&node $ 4A4g" H 3FA-aq" → 3A4Fl" H

3e-


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athode $ F4g" H 4A4Fl" H 3e-→ 3FA

-

aq"

F!erall reaction being$

2H2() + !2() 3

2H2!()Corrosion

orrosion slowly coats the surfaces of metallic obNects with o#ides or other salts of the metal. Therusting of iron, tarnishing of sil!er, de!elopment of green coating on copper and brone are some of

the e#amples of corrosion.

Ru s t in+

orrosion of iron commonly known as rusting" occurs in presence of water and air. orrosion may be considered essentially as an electrochemical phenomenon though the chemistry of corrosion is

quite comple#".

&t a particular spot of an obNect made of iron, o#idation takes place and that spot beha!es as ananode and we can write the reaction$&node $ 40es" → 40e

2+H

3e-

E,

0e2+

, 0e" 8 -;.33?

Electrons released at anodic spot mo!ethrough the metal and go to another spot on

the metal and reduce o#ygen in presence of

AH

which is belie!ed to be a!ailable fromA4F5 formed due to dissolution of carbon

dio#ide from air into water. Aydrogen ion in

water may also be a!ailable due todissolution of other acidic o#ides from theatmosphere". This spot beha!es as a cathode with the reaction$

athode$ F4g" H 3A+

aq" H 3e-

→ 4A4Fl" K E,

8 ).45

?LThe o!er all reaction being$

40es" H F4g" H 3A+

aq" → 40e2+

aq" H 4A4Fl" K E,

cell" 8

).Q?L

The ferrous ions are further o#idied by atmospheric o#ygen to ferric ions which come out as rust in

the form of hydrated ferric o#ide 0e4F5.#A4F" and with further production of hydrogen ions.

20e2+ + 2H2! + !2() 3 0e2! +

4H+

7re!e nti on of CorrosionFne of the simplest methods of pre!enting corrosion is to pre!ent the surface of the metallic obNect

to come in contact with atmosphere. This can be done by$

). o!ering the surface by paint or by some chemicals e.g. bisphenol".4. Fther simple method is to co!er the surface by other metals +n, Mn etc." that are either inert or

react to sa!e the obNect.

5. &n electrochemical method is to pro!ide a sacrificial electrode of another metal like 7g, Mnetc." which corrodes itself but sa!es the obNect.


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SOL.E: 7ROBLEMS

;E5am"le ', Resistance of a conducti!ity cell filled with ;.)7 Dl solution is );; Ω . 'f theresistance of the same cell when filled with ;.;4 7 Dl solution is U4; Ω , calculate theconducti!ity and molar conducti!ity of ;.;4 7 Dl solution. The conducti!ity of ;.) 7 Dlsolution is ).4 +=m.

Solution, The cell constant is gi!en by the equation$ell constant 8 *

58 conducti!ity × resistance 8 ).4 +=m × );; Ω 8 )4 m

-18 ).4 cm

-1onducti!ity of ;.;4 7 Dl solution 8 cell constant = resistance

8 *2

= R 8 )4 m-)

= U4; Ω 8 ;.43S + m-)

oncentration 8 ;.;4 mol = <

8 );;; × ;.;4 mol = m

8 4; mol = m5

7olar conducti!ity 8 Λ m = κ = c

8 43S × );- + m-1 = 4; mol m-8 )43 × );

-4+ m

2mol

-1&lternati!ely, κ = ).4 cm

−1= U4;Ω

8 ;.43S × );-2

+ cm-1

and Λ = κ ×);;; cm

<−1

= molarity

8 ;.43S × );-2

+ cm-1

× );;; cm

<-1

"= ;.;4 mol <-1

8 )43 + cm4

mol-)

;E5am"le (, The electrical resistance of a column of ;.;U 7 aFA solution of diameter ) cm and

length U; cm is U.UU × );

ohm. alculate its resisti!ity, conducti!ity and molar conducti!ity.

Solution, & 8 π r 2 8 5.)3 × ;.U2 cm2 8 ;.SU cm2 or ;.SU × );-4

m2

and l 8 U; cm or ;.U mR = ρl = & or ρ = R& = l = U.UU × );

Ω × ;.SU cm

2= U; cm

8 S.)5U Ω cmonducti!ity 8 κ = ) = ρ = )= S.)5U"+ cm

−18 ;.;))3S + cm-)

7olar conducti!ity, Λ = κ ×);;; cm5

<−)

= molarity

8 ;.;))3S + cm-1

× );;; cm<

-1=;.;U mol <

-18 44. Q + cm4 mol-)

'f we want to calculate the !alues of different quantities in terms of ‘m’ instead of ‘cm’,ρ = R& = l

8 U.UU × );

Ω × ;.SU × );-4

m2

= ;.U m

8 S.)5U × );-2

Ω mκ = ) = ρ = );; = S.)5U Ωm = ).)3S+ m

−1and Λ = κ = c = ).)3S+ m

−1" = U; mol m

−8 44.Q × );

-3+ m

4mol

-).

;E5am"le ), alculate Λ; for al4 and 7g+F3. Λ;

for !arious cations are a4H

8 )).;, 7g4H

8

);Q.;, l-

8 Q.5, +F

2−8 )Q;.;"

m

m

m m

3


15/34

Solution, /e know from Dohlrausch law that Λ

al " = λo

a 2+

" + 4λo

l−

"

8 )).; + cm4 mol-) H 4Q.5" + cm4 mol-)

8 )).; H )U4.Q" + cm4 mol-)

8 4).Q + cm4

mol-)

; o 4+ o 4−Λm 7g+F3 " =

λ7g " +

λ+F

3"

8 );Q.; + cm4

mol-)

H )Q;.; + cm4

mol-)

8 4QQ + cm4

mol-).

;E5am"le


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Λ 3S.)U+ cm4 mol−)

α =m

= = ;.)4555;.U+ cm

2mol

−14 4

D =cα

=;.;;);4S × ;.)455"

= ).S ×);−Ua) − α" ) − ;.)455

;E5am"le >, & solution of u+F3 is electrolysed for ); minutes with a current of ).U amperes.

/hat is the mass of copper deposited at the cathodeZ

Solution, t 8 Q;; sharge 8 current × time

8 ).U & × Q;; s 8 ;;

&ccording to the reaction$

u4Haq" H 4e- 8 us"/e require 40 or 4 × Q3S to deposit ) mol or Q5 g of u.

0or ;; , the mass of u deposited

8 Q5 g mol-1

× ;; " = 4 × Q3S mol-1

"

8 ;.45S g.

;E5am"le ?, Represent the cell in which the following reaction takes place

7gs" + 4&g+

;.;;;)7" → 7g4+ ;.)5;7" + 4&gs"alculate its E if EΘ = 5.) ?.Solution, The cell can be written as7g O 7g

2+;.)5;7" OO &g

+;.;;;)7" O &g.

RT K&g+

L4

E = E, cell"

+

ln40 K7g4+ L

= 5.) ? +

8 4.Q ?.

;.;U? ;.;;;)"4

log4 ;.)5;

/ .167 − .217

;E5am"le @, alculate the equilibrium constant of the reaction$

us" + 4&g+

aq" → u 2+aq" + 4&gs"

EΘ

= ;.3Q ?

Solution, E,cell"

=

;.;U ?

4

log D

or

log D -

=;.3Q ? × 4

= )U.Q;.;U?

)UD

= 5.4 × ); .

;E5am"le A, The standard electrode potential for 1aniell cell is ).)?. alculate the standard *ibbs

energy for the reaction$

Mns" + u 2+

aq" → Mn 2+

aq" + us"

ΘΘ

Solution, ∆r * = − n0E

r G, / −2 8 1.17 8 946 C mo−1

Λm


17/34


18/34

)

A+

&g

+ƒ

A

++

&g4 4 g" aq" &q" s "

Aere n 8 );.;U4

⎡A+

⎤

;.;U4⎡A+ ⎤

Θcell cell

log⎣ ⎦

n⎡⎣&g+ ⎤⎦

@).;3 = ;.S;

−

log⎣ ⎦

) [;.;)]

∴).;3 − ;.S; = ;.;U4log A+

− log ;.;)"

∴;.43

= − log ⎡A+ ⎤ +−4.;;;;"

∴ 3.U; + 4.;; = − log ⎡A+ ⎤

;.;U4 ⎣ ⎦ ⎣ ⎦

∴ − log ⎡⎣A+ ⎤⎦ = pA = Q.U;

E5am"le '), The potential of the following gi!en cell is ;.U !olt. alculate ionic product of water

D w".

Bt O A4 l atm "

O DFA ;.;)7 "

OO Al;.;;)A" O A4 l atm "

O Bt

Solution , Cell reaction,

)A < A

++ e

−

o#idation"&node$4

4l atm " #7 "

A+

+

e−

<)

A reduction"athode$ ;.;) 7 " 4

4 l atm "

+ +ell reaction$ A ;.;)7 " ƒ A #7 "

The AH

ion concentration change only in the cell∴ EΘ in the cell will be ;.;; !olt

The concentration of FA-in solution of DFA is equal to ;.;) 7. 'n aqueous solution, the e#istence

of A

H

and FA

-

ions is always there and their concentration product becomes equal to D w. Aence, for DFA solution,+ − −A

⎡⎣FA= D

w, but

FA

= ;.;)7

∴ A+ ⎤ 'n solution DFA 8D

w

;.;)+

Θcell cell −

;.;U4log

con cent ration of A in DFA so lution

) concentration of A+

in Al solution

∴ ;.U = ;.;; −;.;U4

logD

w=

;.;)

) ;.;)

∴;.U

= − logD

w= −

log D

+ log;.;)"4

;.;U4 ⎡⎣;.;)"2 ⎤⎦∴ .QS = − log D

w− 3.;;

∴ log D

w

= −.QS − 3.;; = −)5.QS

−)3

∴ D w

= &nti log)3.;54 = ).;Q × );E5am"le '


19/34

will be obtainedZ &t. wt.$ u 8 Q5.U, i 8 US., &g 8 );S"

/eight of copper (in gram ) ).;

Solution, 7oles of ) gram of copper 8&t.wt.of u (gram = mole)

=Q5.U

8 ;.;)UU mole

The same quantity of electricity is passed through each cell.

The chemical reactions at cathode in each cell$

Cu( ) +2e

3 Cu( )

(2 moes e =1 moe Cu

)4+ − −aq s

;i(a> ) +2e

3 ;i(s )

(2 moes e =1 moe ;i

)2+ − −

2?( a> ) +2e

3 2?(s )

( 2 moes e = 2 moe ?

)+ − −

Fbser!ing the abo!e reactions, ) mole u, ) mole i and 4 moles &g are deposited by 4 moles of e-

&ccording to the second law of 0araday, proportion of moles of metals.

u$ i$ &g 8 )$ )$ 4

'n e#periment ;.;)UU mole from )g. of u are obtained. &s a result, the proportion of the moles

of the metals produced are

Cu@ ;i@ ? / .1A6A@ .1A6A@ (2 8 .1A6A)./eight of nickel metal 8 moles of i × at. wt. gram=mole" of i

/ .1A6A 8 AB.6 / .24A ram nic%e $i e otained./eight of &g metal 8 moles of &g × at. wt. gram=mole" of &g.

8 4 × ;.;)UU" × );S 8 5.3;4 gram &g will beobtained.

E5am"le '=, &n acidic solution of uH4

salt containing ;.3 g of uH4

is electrolysed until all the u

is deposited. The electrolysis is continued for se!en more minute with the !olume of solution kept at

);; m< and the current at ).4 ampere. alculate !olume of gases e!ol!ed at .T.B. during entireelectrolysis. &t. /t. of u 8 Q5.Q.

Solution& *or I "art of electrolysis4node, 4A4F → 3A+ H F4 H 3e−Cathode, u

+2H 4e → u

∴ Eq. of F2

formed 8 Eq. of u

=;.3 × 4

= )4.US × );−Q5.Q

*or II "art of electrolysis, +ince uH

ions are discharged completely and thus further passage of

current through solution will lead the following changes.4node, 4A4F → 3A+ H F4 H 3eCathode, 4A4F H 4e → A4 H 4FA−

Eq. of A4 8 Eq. of F4 8 '.tQU;;

).4 × ×

Q;8

QU;;

8 U.44 × );−

∴ Total Eq. of F4 Eq. of A4 8 U.44 × );−

8 U.44 × );−

H )4.US × );−

P 4 Eq. of A4 at TB

/ 16.B 8 1− / 22.4 itre


20/34

P 3 Eq. F4 at TB 8 44.3 litre ∴ U.44 × );−

Eq. at TB

44.3 × U.44 × );−5

16.B 8 1− >. !2 at∴

;DE44.3 × ).S × );−5

= litre4

= litre 8 US.3Q m<3

8 .QS m<∴ Total !olume of F4 H A4 8 .QS H US.3Q 8 )US.)3 m<

E5am"le '>, 1issociation constant for &g ( A5

)

the half reaction.+

into &gH

and

;H is 9 8 1−14

. alculate

E,

for

&g ( A5)

H e → &g H 4A5

*i!en, &g+

H e → &g has E,

8 ;. ?.

Solution, se

&g → &g+ + e@ EΘ = ;. ?

&g ( A)

+

+ e → &g + 4A @ EΘ = Z

4

+ +&gA5"

4ƒ &g + 4A

⎡&g ( A5 ) ⎤

E = EΘ

+;.;U

log ⎣ 4 ⎦

andcell cell

)); &g

+

[ A ]2

&lsoΘ

cell

= ; and EΘ

Θ

Θ

+ RB

+

∴

EΘ =

;.;Ulog D &g = &g &g A5 "4 = &g

=;.;U

log Q × );−)3 = −;.S;?cell

)Θ

); )

);

∴ E +&g( A5 )4 =

&g

/ − .6B + .68 H ;.;) ?

E5am"le '?, alculate the equilibrium constant for the reaction,

0e H u+F3 ƒ 0e+F3 H u at 4Uo .

*i!en Eo 8 ;.33 ?@ E, 8 − ;.55 ?Solution& 'n the change 0e is o#idied and u

4His reduced

∴ E = EE + EFE

cell F0e = 0e

= E0e = 0e

4 +

-u2 += -u

−;.;U

log

4

⎡⎣0e

4+ ⎤ ⎦

+ E

Cu2+ Cu +

;.;Ulog

4

⎡⎣u

4+ ⎤⎦

FB ); RB );

E = E

E

+ EFE

+;.;U

4

log1 ⎡⎣u

2+⎤⎦

⎡0e4+

⎤

)"

FB

5 RB

5

Ecell

E E

FB0e FBu

4 +

+

4

4

+

=

FB+

+


21/34

cell F


22/34

0e = 0e4+


23/34

-u4

= -u

0or 0e H u+F3 ƒ 0e+F3 H u

⎡⎣ 0e4+⎤⎦

D c 8⎡⎣u

4+ ⎤⎦

a

nd at equilibrium Ecell 8 ;

IJ >uation (1)


24/34

;.;Ulog

); 8 ;.33 H ;.55 H

4

∴ D c 8 4.)S × );4Q);

c

E5am"le '@, alculate the equilibrium constant for the reaction$

0e4H

H e3H

ƒ 0e5H

H e5H

Θ Θ*i!en E

e3 +

= e5+ 8 ).33 ? and E

0e5+

= 0e4 + 8 ;.QS ?

Solution& Θ

cell

;.;U

)

log);

D c

Θ Θ ΘEcell

= EFB

0e2+

0e+

+ ERB

Ce4+

Ce+

= −;.QS + ).33

8 ;.Q ?

∴ log);

D c

=;.Q

8)4.SS)3;.;U

∴ D c 8 .Q × );)4

E5am"le 'A, 0or the cell 7gs" O +4(aq ) O O &g

+ O &g , calculate the equilibrium constant at 4U

o

and the ma#imum work that can be obtained during operation of cell. *i!en

E,

= +4.5? and E,

= +;.S;? , R 8 S.5)3 X

Solution& 0or the gi!en cell, at equilibrium, the reaction is

7g H 4&gH ƒ 7g

H4H 4&g

Ecell = ; = EFB

7g = 7g+4

+ ERB

&g+

= &g

; = EΘ

K K+2 −

;.;Ulog

4

⎡⎣7g+4 ⎤⎦ +

EΘ

?+ ? +

;.;Ulog

4

⎡⎣&g+ ⎤⎦

FB ); RB );

, ,

/ ;.;U

⎡⎣&g+ ⎤⎦

; = EFB

+ ERB

+4

log);⎡7g+4 ⎤7g = 7g

4&g = &g

;.;U

⎣ ⎦ )

; 8 4.5 H ;.S; H4 log);

D c

∴

log1

)

D c

8 − );.3U

Θ

or log); D c 8 );.3U and Ecell 8 4.5 H ;.S; 8 5.) ? ow ma#imum work that can be obtained by cell is gi!en by

−∆*,

8

/ma#∴ /

maL8 −∆*

,8 n E

,0 8 4 × QU;; × 5.)

8 Q.))S × );A Noule

8 Q.))S × );4

kX

E5am"le (, The potential of a standard electrochemical cell is ).); !olt at 4U° temperature.alculate the equilibrium constant and free energy change of the following gi!en reaction$

4+

(s )

(a> )

4+

(a> ) (s )

E =

7g aq

7g = 7g+4

&g+

=

4

Mn + u ƒ Mn +

D

+

4

+


25/34

Solution, 0or this reaction n 8 4

Θ Θ ∆* = −n0Ecell = −RT ln

D

8 - 4.5;5 RT log D

∴ log D = n0E

cell

4.5;5RT( R = S.5)3 XD −1 mol−1 )

=4 × QU;; × ).)

= 5.4;34.5;5 × S.5)3 × 4S

∴ D = ).Q)×);

6

ΘΘG / −n0ce / - 2 8 9A 8 1.1 couom-*ot or Moue8 - 4)45;;.; Noule3.)S3 Noule 8 ) calorie

∴ −4)45;;

3.)S3

8 - U;3;. calorie 8 - U;.3; kcal


26/34

7ROBLEMS

E5ercise I

Theoretical Questions

Q&' &rrange the following metals in the order in which they displace each other from the solution

of their salts.

&l, u, 0e, 7g and Mn.

Q&( *i!en the standard electrode potentials,

D H=D 8 94.5?, &g

H=&g 8 ;.S;?,

Ag4H

=Ag 8 ;.?7g4H=7g 8 94.5 ?, r 5H=r 8 9 ;.3?

&rrange these metals in their increasing order of reducing power.

Q&) 1epict the gal!anic cell in which the reaction Mns" H 4&gHaq" W Mn

4Haq" H 4&gs" takes

place. 0urther show$

i" /hich of the electrode is negati!ely chargedZ

ii" The carriers of the current in the cell.iii" 'ndi!idual reaction at each electrode.

Q&< 1efine conducti!ity and molar conducti!ity for the solution of an electrolyte. 1iscuss their !ariation with concentration.

Q&= Bredict the products of electrolysis in each of the following$i" &n aqueous solution of &gF5 with sil!er electrodes.

ii" &n aqueous solution of &gF5with platinum electrodes.

iii" & dilute solution of A4+F3with platinum electrodes.

i!" &n aqueous solution of ul4 with platinum electrodes.

Numerical 7ro9lems

Q&> alculate the standard cell potentials of gal!anic cell in which the following reactions take place$i" 4rs" H 5d4Haq" W 4r 5Haq" H 5dii" 0e4Haq" H &gHaq" W 0e5Haq" H &gs"

alculate the \r *,

and equilibrium constant of the reactions.

Q&? /rite the ernst equation and emf of the following cells at 4S D$i" 7gs"O7g

4H;.;;)7"OOu

4H;.;;;) 7"Ous"

ii" 0es"O0e4H;.;;)7"OOAH)7"OA4g")bar"O Bts"

iii" +ns"O+n4H;.;U; 7"OOAH;.;4; 7"OA4g" ) bar"OBts"

i!" Bts"O6r 4l"O6r 9 ;.;); 7"OOAH;.;5; 7"O A4g" ) bar"OBts".

Q&@ 'n the button cells widely used in watches and other de!ices the following reaction takes place$Mns" H &g4Fs" H A4Fl" W Mn

4Haq" H 4&gs" H 4FA 9 aq"

1etermine \r *,

and E,

for the reaction.


27/34

Q&A The conducti!ity of ;.4; 7 solution of Dl at 4S D is ;.;43S + cm 9)

. alculate its molar

conducti!ity.

Q&' The resistance of a conducti!ity cell containing ;.;;)7 Dl solution at 4S D is )U;; ]./hat is the cell constant if conducti!ity of ;.;;)7 Dl solution at 4S D is ;.)3Q > ); 95 +

cm 9).

Q&'' The conducti!ity of sodium chloride at 4S D has been determined at differentconcentrations and the results are gi!en below$

oncentration=7 ;.;;) ;.;); ;.;4; ;.;U; ;.);;

);4

> (=+ m 9)

).45 )).SU 45.)U UU.U5 );Q.3

Cacuate ' for a concentrations and dra$ a Not et$een ' and c. 0ind t&e *aue of ' .Q&'( onducti!ity of ;.;;43) 7 acetic acid is .SQ > );

9U+ cm

9). alculate its molar

conducti!ity. 'f Λ; for acetic acid is 5;.U + cm4 mol 9), what is its dissociation constantZ

Q&') Aow much charge is required for the following reductions$

i" ) mol of &l5H

to &l.ii" ) mol of u4H to u.iii" ) mol of 7nF3

9 to 7n4H.

Q&'< Aow much electricity in terms of 0araday is required to produce

i" 4;.; g of a from molten al4.ii" 3;.; g of &l from molten &l4F5.

Q&'= Aow much electricity is required in coulomb for the o#idation of i" ) mol of A4F to F4. ii" ) mol of 0eF to 0e4F5.

Q&'> & solution of iF5"4 is electrolysed between platinum electrodes using a current of Uamperes for 4; minutes. /hat mass of i is deposited at the cathodeZ

Q&'? Three electrolytic cells &,6, containing solutions of Mn+F3, &gF5 and u+F3,

respecti!ely are connected in series. & steady current of ).U amperes was passed throughthem until ).3U g of sil!er deposited at the cathode of cell 6. Aow long did the current

flowZ /hat mass of copper and inc were depositedZ

Q&'@ sing the standard electrode potentials, predict if the reaction between the following is

feasible$

i" 0e5H

aq" and ' 9 aq"

ii" &gH

aq" and us"

iii" 0e5H

aq" and 6r 9

aq"

i!" &gs" and 0e5H

aq"!" 6r 4 aq" and 0e

4H aq".

E5ercise II


Q&' /hat is meant by 0araday constantZ

m m m

m


28/34

Q&( E#plain with e#amples the terms weak and strong electrolytes. Aow can these bedistinguishedZ

Q&) /hy do we get different products at cathode during the electrolysis of molten al andaqueous sodium chlorideZ

Q&< i" +tate Dohlrausch’s law of independent migration of ions.

ii" Aow can the law be applied for calculating degree of dissociation of weak electrolytesZ

Q&= 1efine conducti!ity and molar conducti!ity for the solution of an electrolyte. 1iscuss their

!ariation with concentration. /hat is the effect of temperature on the molar conducti!ityZ

Q&> /hat are the factors effecting the conducti!ity of electrolytic solutionsZ

O.6 P&J is it not Nossie to determine 'o for a $ea% eectroJteQ LNain.Q&@ /hat is corrosionZ

Q&A /hat are primary cellsZ Aow does a dry cell functionZ

Q&' *i!ing suitable e#amples e#plain the difference between gal!anic cell and electrolytic cell.

Q&'' /rite down the e#pression for molar conductance. +tate the meaning of symbols used. /hatare its unitsZ

Q&'( an we store copper sulphate in iron !esselZ /hyZ

E,

$ u2+

O u 8 ;.53 ?, E,

$0e2+

O 0e 8 - ;.33 ?.

Q&') Aow is electrical energy obtained in fuel cellsZ

Q&'< Aow does the molar conductance of the electrolyte !ary upon dilutionZ

Numerical 7ro9lems

Q&'= Aow much charge is required for the following reductions$i" ) mol of u

4Hto u.

ii" ) mol of 7nF3-

to 7n4H

Q&'> Aow many faradays of charge would be required to reduce 4).; g of a 4Kdl3L to metallic

cadmiumZ 'f current strength is .U ampere, how much time will be neededZ&t. /t. of d 8))4.3"

Q&'? 'n an electrolytic cell, how many moles of copper will be deposited from a solution of u+F 3 by 43)4U of electricityZ

Q&'@ 'n a simple electrochemical cell, the half cell reaction with their standard electrode potentials

are$

Bbs" - 4e- → Bb2+aq" @ E, 8 - ;.)5 ? &gs" 9 e- → &g+aq" @ E, 8 H;.S; ?

/hich of the following reactions take placeZ &lso calculate the e.m.f. of cell$


29/34

i" Bb4H

aq" H 4&gs" → 4&gHaq" H Bbs"ii" Bb4Haq" H &gs" → &gHaq" H Bbs"iii" &g+

aq" H Bbs" → &gs" H Bb2+

aq"

(i*) 2?+(a>) + E(s) 3 2?(s) + E2+(a>)Q&'A /rite the ernst equation and e.m.f. of the following cells at 4S D$

i" 7gs" O 7g4H ;.;;)7" OO u4H ;.;;;) 7" O us"

ii" 0es" O 0e4H ;.;;) 7" OO AH)7" O A4g" ) bar" O Bt s"

iii" +ns" O +n4H

;.;U; 7" OO AH;.;4; 7" O A4g" ) bar" O Bts".

*i!en data-

7gs"= 7g4H

aq" @ E Θ 8 -4.5S ? u4H

aq"=us" @ E Θ 8 ;.53 ?

0es"= 0e4H

aq" @ EΘ 8 ;.3)? A

Haq"=A4g" @ E

Θ 8

+ns"=+n4H @ E , 8 ;.)3?

Q&( alculate the emf of a cell operating with the following reaction at 4U°, in which K7nF3-L 8;.;); 7, K6r

-L 8 ;.;); 7, K7n

4HL 8 ;.)U 7, and KA

HL 8 ).; 7.

47nF3-aq" H );6r

-aq" H )QA

+aq" → 47n

2+aq" H U6r 4l" H SA4Fl"

*i!en data-

7n F−

→7n

4H @ E Θ 8 ).3? 6r -aq" → 6r 4l" @ EΘ

8 ).;?

Q&(' 'n the commercial preparation of aluminum, aluminum o#ide, &l4F5, is electrolyed at

);;;°. The mineral cryolite is added as a sol!ent." &ssume that the cathode reaction is&l

+

H 5e

-

→ &lAow many coulombs of electricity are required to gi!e U.)4 kg of aluminumZ

Q&(( onsider the gi!en standard potential in !olts" in ) 7 solution$

0e4H → 0e @ E Θ 8 - ;.3 ?, 0e5H → 0e4H @ E Θ 8 H;.S? 7n4H → 7n @ E Θ 8 - ).4?, 7n5H → 7n4H @ E Θ 8 H ).U?i" omment on the relati!e stabilities of H 4 and H 5 o#idation states of iron and manganese.

ii" /hich of the two metals may be more easily o#idied to H 4 stateZ

Q&() alculate the emf of the following cell at 4U°.rs"Or

+).; × );

-27" OO i

2+4.; 7" O is"

*i!en data 9

rs"=r 5H

aq" @ E Θ 8 ;.3 ?@ i4H

aq"= is" @ E Θ 8 ;.45?@

*lash9ac3

CBSE ((

Q&' Aow does a fuel cell operateZ #' out of ?%

Q&( The measured resistance of a conductance cell containing .U # );-

7 solution of Dl at 4U° was );;U ohms. alculate i" specific conductance ii" molar conductance of the solution.

ell constant 8 ).4U cm-).

#) out of ?%

CBSE ()

3


30/34

Q&' /hy does the molar conductance increase on diluting the solution of a weak electrolyteZElectrolytic conducti!ity of ;.5; 7 solution of Dl at 4S D is 5.4 × );

-2+ cm

-1.

alculate

its molar conducti!ity.#( out of ?%

Q&( i" +tate the factors that influence the !alue of cell potential of the following cell $7g s" O 7g4H aq" OO &gH aq" O &g s"

ii" /rite ernst equation to calculate the cell potential of the abo!e cell.

CBSE (<

#( out of ?%

Q&' Aow does molar conducti!ity !ary with concentration for i" weak electrolyte and for ii"

strong electrolyteZ *i!e reasons for these !ariations.

Q&( /rite the ernst equation and calculate the e.m.f. of the following cell at 4S D$

u s" O u4H

;.)5; 7" OO &gH

).;; × );-3

7" O &g s"

#( out of ?%

^

u 4H

=u

CBSE (=

8 H;.53 ? and E&gH =&g 8 H;.S; ?. #) out of ?%

Q&' Bredict the products of electrolysis obtained at the electrodes in each case when the

electrodes used are of platinum$

i" &n aqueous solution of &gF5ii" &n aqueous solution of A4+F3.

#( out of ?%

Q&( The E, !alues at 4S D corresponding to the following two reduction electrode

processes

are$

i" uH=u 8 H ;.U4 ?

ii" u4H

=uH

8 H ;.)Q ?

0ormulate the gal!anic cell for their combination. /hat will be the cell potentialZ alculate

the

r Gfor the cell reaction. 0 8 QU;; mol-)

" #) out of ?%

Q&) The half-reactions are

i" 0e5H

H e- → 0e4H, EΘ 8 ;.Q ?ii" &gH H e- → &g,

E

Θ 8 ;.S; ?alculate D c for the following reaction at4U

o$

&g+

H 0e2+

→ 0e+

H &g

0 8 QU;; mol-)"

CBSE (>

Q&' a" +tate two ad!antages of A4 9F4 fuel cell o!er ordinary cell.

#= out of ?%

#( out of ?%

b" +il!er is electrodeposited on a metallic !essel of total surface area ;; cm4 by passing a

current of ;.U amp for two hours. alculate the thickness of sil!er deposited.

K*i!en$ 1ensity of sil!er 8 );.U g cm 95

, &tomic mass of sil!er 8 );S amu, 0 8 Q,U;; mol

9)L

CBSE (?#) out of ?%

*i!en$ _^

Θ


31/34

Q&' Fn the basis of the standard electrode potential !alues stated for acid solution, predict

whether Ti3H

species may be used to o#idied 0e''

to 0e'''

.Reaction E

o= ?

TiR7

H e

→ Ti+

$ H ;.;)

0e5H H e 9 → 0e4H $ H ;.

Q&( 1efine conducti!ity and molar conducti!ity for the solution of an electrolyte.

#( out of ?%

#( out of ?%

Q&) alculate the standard cell potential of the gal!anic cell in which the following reaction takes

place$

4 r s" H 5 d2+

aq."⎯⎯→ 4 r +

aq." H 5 d s"

Θ

&lso calculate the ∆r *

!alue of the reaction.

Θ Θ

−)

*i!en $ E

CBSE (@

Cr+ Cr /

−.64 7T

Cd2+ Cd / −.4 7 and 0 / 9A C

mo )

#) out of ?%

Q&' E#press the relation between the conducti!ity and the molar conducti!ity of a solution.

#' out of ?%

Q&( 1epict the gal!anic cell in which the reactionMn s" H 4 &g+

aq" → Mn2+

aq" 4&g s"

takes place, 0urther indicate what are the carriers of the current inside and outside the cell.

+tate the reaction at each electrode.#( out of ?%

Q&) The resistance of a conducti!ity cell containing ;.;;) 7 Dl solution at 4S D is )U;;Ω./hat is the cell constant if the conducti!ity of ;.;;) 7 Dl solution at 4S D is ;.)3Q×);

-+

cm-)Z

CBSE (A

#( out of ?%

Q&' /hat type of cell is a lead storage battery Z /rite the anode and the cathode reactions and the

o!erall cell reaction occurring in the use of a lead storage battery.OR

Two half cell reactions of an electrochemical cell are gi!en below $

7nF−

aq" + SA+

aq" +

Ue−

→ 7n 4+ aq" + 3A Fl", E

o

= +).U)?

+n 2+

aq" → +n 4+

aq" + 4e−

, Eo

= +;.)U?

onstruct the redo# equation from the two half cell reactions and predict if this reaction

fa!ours formation of reactants or product shown in the equation.#( out of ?%

Q&( & copper-sil!er cell is set up. The copper ion concentration in it is ;.); 7. The concentration

of sil!er ion is not known. The cell potential measured ;.344 ?. 1etermine the concentrationof sil!er ion in the cell.

*i!en $ E&g+ = &g= +;.S; ?, E

-u 4+

= u= +;.53 ?

#) out of ?%

CBSE ('

3 4

o o


32/34

Q&' E#press the relation among the cell constant, the resistance of the solution in the cell and theconducti!ity of the solution. Aow is the conducti!ity of a solution related to its molar

conducti!ity Z

Q&( *i!en that the standard electrode potentials Eo" of metals are $

D H

= D 8 94.5 ?, &gH

= &g 8 ;.S; ?, u4H

= u 8 ;.53?, 7g4H

= 7g 8 94.5 ?,r

5H= r 8 9;.3 ?, 0e

4H= 0e 8 9;.33 ?.

&rrange these metals in an increasing order of their reducing power.

OR

Two half-reactions of an electrochemical cell are gi!en below $

#( out of ?%

7nF−

aq" + SA+

aq" +

Ue

−+n 2+

aq" → +n 4+

aq" +

4e−

,

Eo

8 H ;.)U ?

3 Kn 2+ (a>) + 4H !()=

o

8 H ).U)?,

onstruct the redo# reaction equation from the two half-reactions and calculate the cell

potential from the standard potentials and predict if the reaction is reactant or productfa!oured.

#( out of ?%

Q&) E#press the relation among the cell constant, the resistance of the solution in the cell and the

conducti!ity of the solution. Aow is the conducti!ity of a solution related to its molar

conducti!ityZ

CBSE (''

#( out of ?%

Q&' a" /hat type of a battery is lead storage batteryZ /rite the anode and cathode reactions

and the o!erall cell reaction occurring in the operation of a lead storage battery.

b" alculate the potential for half-cell containing

;.); 7 D 4r 4F aq", ;.4; 7 r 5H

aq" and ).; > ); 93

7 AH

aq"

The half-cell reaction is4− + − 5+r

4F

aq" + )3A aq" +Qe

→ 4r aq" + A4 Fl",

and the standard electrode potential is gi!en as Eo

8 ).55?.

OR a" Aow many moles of mercury will be produced by electrolysing ).;7 AgF5"4

solution with a current of 4.;; & for 5 hoursZ

KAg F−

" = 4;;.Q g mol−1

L

b" & !oltaic cell is set up at 4Uo with the following half-cells &l5H;.;;) 7" and i4H

;.U; 7". /rite an equation for the reaction that occurs when the cell generates an

electric current and determine the cell potential.

*i!en Eo = −;.4U?, Eo = −).QQ? "

#= out of ?%

4NS2ERS

3 4

5 4

i4+

= i &l5+

=


33/34

E5ercise I

Numerical 7ro9lems

E5ercise II


Q&' 't represents QU;; of charge(

Q&'' :8 where c, the concentration of the electrolyte is e#pressed in mol m -5.The units of ( c

thus take the form +m-4mol-).

Numerical 7ro9lems

Q&'= i" 40, ii" U0 Q&'> ;.)3; 0, 4.S min Q&'?

)mol

S

Q&'@ H;.5? Q&'A i" 4.QS? ii" ;.U5 ? iii" ;.;S? i!" -).5)U?Q&( ;.4? Q&(' U.3 # );

Q&(( i" 7n4H is more stable than 0e4H but 0e5H is more stable than 7n5H

ii" 7n will be o#idied to 7n4H more readily as compared to 0e to 0e4H

Q&() ;.UQ?

*lash9ac3

CBSE ((

Q&( i" ;.;;)435 ohm-) cm-) ii" )QU.5 ohm-) cm4 mol-)

CBSE ()

Q&' )43 + cm4

mol-)

Θ 4.5;5RT K7g4+

aq"LQ&( ii" E cell − log

40 K&g+

aq"L4

CBSE (<

Θ

Θ

;.;U Ku 4+

aq"L

Q&( Ecell = E &g+ = &g−

E

u + = u

−

log , ;.4U)?4 K&g

+ aq"L

4

CBSE (=

Q&(

,/ .97= G, / − 464 mo−1

Q&) D 8 3.3Q

Q&> i" EΘ

8 ;.53?, ∆r *Θ 8 9)Q.SQ kX mol 9), D 8 5.)Q > );53,ii" EΘ 8 ;.;5?, ∆r *Θ 8 9 4.SU kX mol 9), D 8 5.4

Q&?

Q&@

i" 4.QS ? ii" ;.U5 ? iii" ;.;S ? i!" 9).5)U ?).);U ? Q&A )43.; + cm4 mol 9) Q&' ;.4); cm

9)

Q&'( ).SU > ); 9U Q&') 50, 40, U0 Q&'< )0, 3.33 0

Q&'= 40, )0 Q&'> ).S;5 g

Q&'? )3.3; min, opper ;.34g, Minc ;.35 g

CBSE (>

Q&' b" 3.4 > );-3cm

CBSE (?

Q&' Ti3H would not o#idie 0eH4 Q&) )Q.SQ DNmol-)

cell r


34/34

CBSE (@

Q&) ;.4) cm-)

CBSE (A

P&( ;.;Q7

CBSE ('

Q&( Eo

= ).3Q?

CBSE (''

P&' b"

ce /

.6B7 !F (a) .112 moes of H () ce / 1.497

(283236740) hanuman electrochemistry

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