Lacture №10. Electrochemistry.ppt

8/10/2019 Lacture 10. Electrochemistry.ppt

1/68

Electrochemistry

Plan

1. Electrode processes. Electrode potential.

2. Different types of electrodes.

3. Cell potential.

4. Galvanic cells. Cell potential or EMF.

5. The inetics of electroche!istry

processes.

"ssistant #o$acho %.% prepared


2/68

The changes in which electrical energy is produced as a result

of chemical change. The devices used to produce electrical

energy from chemical reactions are calledelectrical cells&

'alvanic or voltic cells.In these cells, oxidation and reduction reaction reactions occur

in separate containers called half cellsand the red-ox reaction

is spontaneous.

The arrangement consists of two beakers, one of with contains

1,0 M solution of inc sulphate and the other 1,0 M solution ofcopper sulphate. ! inc rod is dipped into "n#$%solution

while a copper rod is dipped into &u#$% solution. These

metallic rods are known as electrodes.


3/68


4/68

The metallic rods in the beaker are connected to theammeter by means of an insulated wire through akey.Ammeter is used to know the passage of current

which moves in opposite direction to the flow ofelectrons.The solution in the two beakers areconnected by an in'erted (-tube containing

saturated solution of some electrolyte such as )*l,)+$, +%$ which does not undergo a chemical

change during the process.

The two openings of the (-tube are plugged with

some porous material such as glass wood or cotton.The (-tube which connects the two glass beakers iscalled a salt()rid'e.


5/68

hen the circuit is completed by inserting the key inthe circuit, it is obser'ed that electric current flowsthrough external circuit as indicated by the ammeter.

The following obser'ations are made/

Therefore, the current flows from copper to inc

+.. The flow of electric current is taken opposite to

the flows of electrons


6/68

There obser'ation can be explained as/

uring the reaction, inc is oxidied to "n23 ions

which go into the solution. Therefore,

the inc rod gradually loses its weight. The electrons

released at the inc electrode mo'e towards the other

electrode through outer circuit. ere, these areaccepted by &u23ions of &u#$%solution which are

reduced to copper.


7/68

The inc electrode where electrons are released or

oxidation occurs s called anodewhile the copper

electrode where electrons are accepted or reduction

occurs is called cathode.

The two containers in'ol'ing oxidation and reduction

half reactions are called half cells. The inc roddipping into a "n#$%solution is oxidation half cell

and the copper electrode dipping into a &u#$%

solution is reduction half cell


8/68

*.+.The gal'anic cells which consists of the inc rod dippinginto a "n#$%solution and the copper electrode dipping

into a &u#$%solution isDaniell cell.

Its formula is/

Salt bridge and its function. It4s usually an in'erted (-tubefilled with concentrated solution of inert electrolyte. The

essential re5uirements of electrolyte are/

a6 The mobility of the anion and cation of the electrolyteshould be almost same.

b6 The ions of the electrolyte are not in'ol'ed inelectrochemical change.

c6 The ions do not react chemically with the species of thecell.


9/68

7enerally, salts like )&l, )+$, etc. are used. The seturated

solutions of these electrolytes are prepared in agar agar8elly or gelatin. The 8elly keeps the electrolyte in semi-

solid phase and thus pre'ents mixing.The important functionsof the salt bridge are/

a6 #alt bridge completes the electrical circuit.

b6 #alt bridge maintains electrical neutrality of two half cell

solution.

The accumulation of charges in the two half cells9accumulation of extra positi'e charge in the solutionaround the anode according to the realiing of "n23inexcess and accumulation of extra negati'e charge in thesolution around the catode due to excess of #$%2-6 is

pre'ented by using salt bridge, which pro'ides a passagefor the flow of the charge in the internal circuit.


10/68


11/68

,EP,E%E*T"T-* F "* E/ECT,C0EM-C"/ CE//

!n electrochemical cells or gal'anic cell consists of twoelectrodes/ anode and cathode. The electrolyte solutioncontaining these electrodes are called half cells.

The following con'entions are used in representing anelectrochemical cell:

1. ! gal'anic cell is represented by writing the anode 9whereoxidation occurs6 on the left hand side and cathode 9wherereduction occurs6 on the right hand side.

2. The anode of the cell is represented by writing metal orsolid phase first and then the electrolyte 9or the cation ofthe electrolyte6 while the cathode is represented by writing

the electrolyte first and then metal or solid phase.. The salt bridge which separates the two half cells is

indicated by two 'ertical lines.


12/68


13/68


14/68

Electrode Potential and E.M.F. of a 'alvanic cell

Electrode Potential

The flow of electric current in an electrochemical cell

indicates that a potential difference exists between twoelectrodes.


15/68

If the metal has relati'ely high tendency to get oxidised, itsatom will lose electrons readily and form &u23ions, whichgo into the solution. The electrons lost on the electrodewould be accumulated on the metal electrode and theelectrode ac5uires a slight negati'e charge with respect tothe solution. #ome of the &u23ions from the solution willtake up electrons and become &u atoms. !fter some time, ane5uilibrium will be established as/

hen such an e5uilibrium is attained, it results in separation

of charges 9negati'e on the electrode with respect to thesolution6.


16/68

Forming the double layer

1- metal

2 - solutionC


17/68

#imilarly, if the metal ions ha'e relati'ely greatertendency to get reduced, they will take electronsfrom the electrode. !s a result, a net positi'e

charge will be de'eloped on the electrode withrespect to the solution. This will also result intoseparation of charges 9positi'e on the electrodewith respect to the solution6.

Due to separation of charges between the electrodeand the solution, an electrical potential is set upbetween metal electrode and its solution.

The electrical potential difference set up between themetal and its solution is known aselectrodepotential.


18/68

The !echanis! of the do)le layer for!in'

:or example of the copper electrode is dipped into &u#$%solution.

The chemical potential of copper4s ions in the metal and inthe solution is not e5ual. The chemical potential ofcopper4s ions in the metal at the gi'en temperature isstable 'alue, the chemical potential of copper4s ions in thesolution depends on the solution4s concentration.

If the at the gi'en concentration of solution the chemicalpotential of copper4s ions in the solution is greater thanthe chemical potential of these ions in the metal . Then atthe dipping of the metal in the solution some 5uantity of

&u23ions are hydrated and transferred on the metalaccording to that positi'e charge forming on the metal4ssurface. #ulfate4s anions are attracted to metal4s surface,they courses the negati'e charge. These processes causethe double electrical layer and related with it the potentialdifference.


19/68

C

C%4

(+

++

+

+

+

- - -

- -----

-

-

--

-

-

For!in' the do)le layer


20/68

2.Reduction potential. The tendency of an

electrode to gain electrons or to get reduced.

The electrode potential may be of two types:

1. Oxidation potential:

The tendency of an electrolyte to lose electronsor to get oxidised


21/68

E.M.F. or ell !otential o" a ell

The difference between the electrode potentials of the twoelectrodes constituting an electrochemical cell is known as

electro!otive force e.!.f. or cell potential of a cell. This

acts as a dri'ing force for the cell reaction. The potential

difference is expressed in olts.

Th f th ll t ti l f i f th diff


22/68

Therefore, the cell potential or e.m.f. arises from the differencein the tendencies of the two ions to get reduced.

!t is e"ual to the reduction potential for the substance that

actually undergoes reduction minus the reductionpotential of the substance that undergoes oxidation.

Thus, e.m.f. of a cell may be defined as the potentialdifference between two electrodes of the cell when either noor negligible current is allowed to flow in the circuit.


23/68

%t d d l t d t ti l


24/68

%tandard electrode potential

#ince a half cell in an electrochemical cell can work only incombination with the other half cell and does not work

independently, it is not possible to determine the absoluteelectrode potential of an electrode. e can, therefore, findonly the relati'e electrode potential.

This difficulty can be sol'ed by selecting one of the electrodes

as a reference electrode and arbitrarily fixing the potentialof this electrode as#ero. :or this purpose, re'ersiblehydrogen electrode has been uni'ersally accepted as areference electrode. It is called standard hydrogen

electrode $S.%.E& or normal hydrogen electrode $'.%.E.&

%t d d h d l t d It i t f l ti


25/68

%tandard hydro'en electrode. It consists of platinumwire sealed in a glass tube and has a platinum foilattached to it. The foil is coated with finely di'ided

platinum and acts as platinum electrode. It is dippedinto an acid solution containing 3ions in 1 Mconcentration 91M &l6. ;ure hydrogen gas at 1atmospheric pressure is constantly bubbled intosolution at constant temperature of 2


26/68

Th l t d t ti l f l t d b


27/68

The electrode potential of an electrode can bedetermined by connecting this half cell with astandard hydrogen electrode.The electrode

potential of the standard hydrogen electrode is takenas ero.

The electrode potential of a metal electrode as

determined with respect to a standard or normalhydrogen electrode is called standard electrodepotential E5. #tandard electrode potentials arealways associated with the reduction occurring at

the electrodes.


28/68

In this case the electrons flow from inc electrode to


29/68

In this case, the electrons flow from inc electrode tohydrogen electrode and therefore, the inc electrode acts asanode and #..>. acts as a cathode. The cell may berepresented as/

2 Measre!ent of Electrode Potential of C246C


30/68

2. Measre!ent of Electrode Potential of C246CElectrode

In this case, the hydrogen has greater tendency to loseelectrons. Therefore, oxidation occurs at hydrogen electrodeand reduction occurs at copper electrode.


31/68

It may be noted that it is not always con'enient to usestandard hydrogen electrode as reference electrodebecause of experimental difficulties in itspreparation and use.


32/68

#.$.The standard electrode potentialsgiven in the

following table are measured in their standard stateswhen the concentration of the electrolyte solutions arefixed as 1M and temperature is 298 .


33/68

%pplication o" the Electrochemical


34/68

%pplication o" the Electrochemical

&electromoti'e( series:

1. ,elative stren'ths of o7idisin' and redcin'

a'ents.

The substances which ha'e lower reduction potentials

are stronger reducing agents while ha'e higher

reduction potentials are stronger oxidations agent.

2. Calclation of the E.M.F. of the cell.

>0cell

? >09cathode6 - >09anode6

3 Predictin' feasi)ility of the reaction


35/68

3. Predictin' feasi)ility of the reaction.

!n general, a red(ox reaction is feasi)leonly if the

species to release electrons must hae lower

reduction potential as compared to the species

which is to accept electrolytes.

4. To predict 8hether a !etal can li)erate

hydro'en fro! acid or not.

In general, only those metals can liberate hydrogen

from the acid which ha'e negati'e 'alues of

reduction potentials , - >0'alues.

Dependence of electrode and cell potentials on


36/68

Dependence of electrode and cell potentials on

concentration9 *ernst e:ation

The electrode potentials depend on the concentration of the

electrolyte solutions.


37/68

or @9a56MnAlogn0.0B0 +

+=E


38/68

#erst e)uation#erst e)uation

a

nF

RT

dOxdOxln

0

CeDCeD +=

an

dOxdOx

lg0B


39/68

dc!"#aA ne ++

In general, for an electrochemical cell reaction/

The +erst e5uation may be written as/

The alue of a, b, c, d and n are obtained from the balanced


40/68

The alue of a, b, c, d and n are obtained from the balancedcell reactions.

*.+. It must be remembered that while writing the +erst

e5uation for the o'erall cell reaction, the log term is thesame as the expression for the e5uilibrium constant for thereaction. owe'er, some books use the expression in there'erse form as the expression for the e5uilibrium constant

but, sign after >0

is changed.

E:ili)ri! constant fro! *ernst e:ation


41/68

E:ili)ri! constant fro! *ernst e:ation

The e.m.f. of the cell may be used to calculate thee5uilibrium constant for the cell reaction. !t

e5uilibrium, the electrode potentials of the twoelectrodes become e5ual so that e.m.f. of the cell isero. &onsider the following redox reaction/


42/68

%i'nificance of #c. The 'alue of )cgi'es the extent of

the cell reaction. If the 'alue of )c is large, the reaction

proceeds to large extent.

Electroche!ical cell and free ener'y


43/68

Electroche!ical cell and free ener'y

In electrochemical cells, the chemical energy is

con'erted into electrical energy. The cell potential is

related to free energy change. In an electrochemical

cell, the system does work by transferring electrical

energy through an electric circuit.

here is the standard free energy for the reaction.


44/68

here is the standard free energy for the reaction.

%i'nificance.The abo'e e5uation helps us to predict thefeasibility of the cell reaction. :or a cell reaction to be

spontaneous, must be negatie. This means thatE mustbe positiefor a spontaneous cell reaction.

*OME OMMER%, E,,*


45/68

*OME OMMER%, E,,*

$ne of the main uses of gal'anic cells is thegeneration of portable electrical energy. These

cells are also popularly known as batteries. Theterm battery is generally used for two or moregal'anic cells connected in series. Thus, a "attery

is an arrangement of electrochemical cells used asan energy source. The basis of an electrochemicalcell is an oxidation$reduction reaction.

Types of co!!ercial cells.

16 ;rimary cells

26 #econdary cells

Pri!ary cells. In these cells, the electrode reactions


46/68

Pri!ary cells. In these cells, the electrode reactionscannot be re'ersed by an external electric energysource. In these cells, reactions occur only once and

after use thaey become dead. Therefore, they are notchargeable. >xamples are dry cell, mercury cell.

%econdary cells stora'e cells or acc!lators.In

the secondary cells, the reaction can be re'ersed byan external electric source. Therefore, these cellscan be rechargedby passing electric current andused again and again. >xamples are lead storage

battery and nickel-cadmium storage cell.The most popular example is of lead storage cell

which is used in automobiles.


47/68

>ach battery consists of a number of 'oltaic cells connected in


48/68

yseries. Three to six such cells are generally combined to getE to 12 'olt battery. In each cell, the anode is a grind of lead

packed with di'ided spongy lead and the cathode is a grindof lead packed with ;b$2.

The electrolyte is a5ueous solution of sulfuric acid 9=F bymass6 ha'ing a density 1,0 g ml-1sulfuric acid. hen the

lead plates are kept for sometimes, a deposit of leadsulphate is formed on them.

!t the anode, lead is oxidised to ;b23ions and insoluble ;b#$%

is formed. !t the cathode ;b$2 is reduced to to ;b23ions and

;b#$%is formed.The following reactions take place in the lead storage cell/


49/68

uring the working of the cell, ;b#$%is formed at each


50/68

g g , %electrode and sulphuric acid is used up. !s a result, theconcentration of 2#$% decreases and the density of the

solution also decreases. hen the density of 2#$% falls

below 1.2 g ml-1, the battery needs recharge.

,echar'e the +attery

The cell can be charge by passing electric current of a suitable

'oltage in the opposite direction. The electrode reaction getsre'ersed. !s a result, the flow of electrons gets re'ersed andlead is deposited on anode and ;b$2on the cathode. The

density of sulphuric acid also increases. The reaction can be

written as/

The !ost i!portant types of electrodes are9


51/68

p yp

1. The first reference electrode/ Metal-metal ionelectrodes and gas-ion electrodes

2. The second reference electrode/ Metal-insolublesalt-anion electrodes

. The third reference electrode/ inert Goxidation-

reductionG electrodes%. Membrane electrodes

The !etal ( !etal ion electrodeconsists of % metal in contact with


52/68

its ions in solution.

!n example is H piece of sil'er metal immersed in H solution ofsil'er nitrate. The diagram for such an electrode ser'ing as H

cathode 9it would appear at the right in H cell diagram6 is/ !g39a56 |!g9s6

and the cathode half-reaction is/ !g39a56 3 e-!g9s6

in which the electrons JmK from the external circuit. hen this

electrode ser'es as an anode, it is diagramed as/ !g9s6 |!g39a569as it would appear at the left in H cell diagram6, and its half-

reaction e5uation is/

!g9s6 !g39a56 3 K-

!n general the first reference electrons can be represented as:M$4;M. The half reduction reaction is/

M$4 $e M

:ollowing con'ention the half reaction that occurs on the electrode

is written as a reduction reaction

*ernst e:ation for these type electrodes is9


53/68

@9a56MnAlog

n

0.0B 0 ++= &E

Mn

0 alogn

0.0B ++= &E

@9a56AMn@AMn9s6log

n0.0B

0

+=E

+

=

&

E

Mn

Mn9s60

a

a

logn

0.0B

The gas(ion electrode$Standard hydrogen electrode& d l d h k h f ll i di i


54/68

ydrogen electrode that works at the following conditions/? 1, p3 ? 101, k;a, T ? 2B0& ? 2lectrochemical potential of this electrode depends on the hydrogenions concentration. owe'er the standard potential of this electrodee5uals o and the 'alency of hydrogen e5uals 1 9n?16 +ernste5uation is/

>cell? 0,0B< log A3@ ? - 0,0B< p

Measrin' of p0to use potentiometric method of the determinationof hydrogen ions concentration.

This method is based on the measuring of e.m.f of the cell whichconsists of the reduction electrode $calomel& and the electrodewhich has dependence on the hydrogen ions concentration $gas(ion electrode, glass electrode, "uinonhydrone electrode&

+'a

2. -n the !etal(insol)le salt(anion electrode, H metal is in


55/68

contact with one of its insoluble salts and also with Hsolution containing the anion of the salt.

!n example is the so-called siler ( siler chloride electrode,written as H cathode as/

!l-"a#$ |%g!l"s$ |%g"s$

for which the cathode half-reaction is&

%g!l "s$ ' (-%g"s$ ' !l- "a#$

>!g,!g&l L &l-? >0!g3D!g&l - 0.0B< lg a&l->0!g3D!g&l? 0,222%

!

0 alogn

0.0B = &E


56/68

Siler ( siler chloride electrode

%g is co'ered by the layer o" nonsoluble %gl

l solution/l %gl0 %g

Calo!el electrode consists of the mercury and


57/68

ycalomel past that is dipped in potassium chloridesolution. It is often used as a reference electrode to

determine the standard electrode potential 9 moreoften than hydrogen electrode6. Its scheme is/

&l-Lg2&l2, g

The half reaction is/g2&l2 3 2e 2g 3 2&l-

>cell? >0- 0.0B< lg a&l-

!s a rule to use the calomel electrodes that contain0,1 M, 1 M and saturated solution of potassium

chloride. Their standard potential at 2


58/68

It consists of H strip, wire, or rod of an inert materiel 9;l, !u,

IrP6 in contact with H solution, which contains ions of H

substance is two different oxidation states 9oxidation andreduction form6. The difference between general metal

electrode and ox-red electrode is that oxDred electrode does

not take place in ox-red reaction which exist in solution but is

the electrons conductor. :or example/ ;tL #n23, #n%3or ;tL :e23,:e3

$x 3e Q Ced

There are two types of ox(red electrodes:1.#imple/ :e23, :e3 L ;t :e33 e Q :e23

$x

Ced0

a

alog

n

0.0B =E

+

+

=

2

:e

:e0

a

alog

n

0.0BE

**cheme o" ox-red electrodecheme o" ox-red electrode


59/68

*cheme o" ox red electrode

&&the third re"erence electrode(the third re"erence electrode(

!t

Fel+ Fel2

Fe+ Fe2+ 0 !t

Fe+ + e =Fe2+

+

+

++++ +

=

2

,

2,2, lg0B


60/68

the composition of the ions

Mn23, Mn$%-, 3 L ;t Mn$%-3 =33BeQMn233%2$

>xample is "uinonhydrone electrode.

It is prepared by the platinum strip or wire which is contained

in the glass tube. The electrode is dipped in the solution

with unknown p that is needed to determine and to addsome 5uinonhydrone4s crystalls in this solution.

Ruinonhydrone is a crystalline product which consists of

5uinone9beno5uinone6*ES%2 and hydro5uinone

&E%9$62. It is less solubility in water and decomposesinto 5uinone and hydro5uinone in the solution. In the

saturated solution e5ual molar mixture of 5uinoneand

hydro5uinone is formed.

) i h d l t d


61/68

)uinonhydrone electrode&&the third re"erence electrodethe third re"erence electrode((

34562 345&64(2 7+ 0 !t

4?2 2> 2@ A 4?>2

+

=

(O'!

O''!cell

%%

aEE

2690

2%E

2%Elg20B


62/68

8he scheme o" )uinonhydrone cell8he scheme o" )uinonhydrone cell

9ith one electrolyte9ith one electrolyte

!t 420 )uinhydr 7+ 0/l 0 /l7g2l20 7g

Ecell E )uinhydr- Ecalomel

8he glass electrode "or p7 measurements.The cell consists ofa glass indicator electrode and ) saturated calomel reference


63/68

a glass indicator electrode and ) saturated calomel referenceelectrode* both immersed in the solution whose p+ is to bedetermined. The indicator electrode consists of ) thin* ,-sensitive glass membrane sealed onto one end of ) heavy-

walled glass or plastic tube. small volume of dilutehydrochloric acid saturated with silver chloride is contained inthe tube "in some electrodes this solution is ) buffer containingchloride ion$. % silver wire in this solution forms ) silver/silverchloride reference electrode* which is connected to one of the

terminals of ) potential-measuring device. The calomelelectrode is connected to the other terminal.

0how that the system contains two reference electrodes& "1$ theexternal calomel electrode and "2$ the internal silver/silverchloride electrode. %lthough the internal reference electrode is

part of the glass electrode* it is not the p+-sensing element.nstead* it is the thin glass membrane at the tip of the electrodethat responds to p+.


64/68

The make the cell with glass and calomel electrodes

and measuring its e.m.f can be determined p ofsolution.

glass electrode4s constant which is depended on the

electrode nature. The constant is fined according to the

graph which is plotted between the >celland p

ordinates.

++= )aglass aconstE lg0B


65/68

8he mechanism o" the di""usion

potential

7l 1 ; 7l


66/68

8he scheme o" concentrated cell8he scheme o" concentrated cell

"' 6 "'*366 "'*36 "'

(

C1 B C

2

= +

%g#O 2 %g#O 1

#O-

Ecell E2 %g+>%g? E1 %g+>%g


67/68

The electric circuit without transferis when the

electrodes are dipped in one solution.

>xamples/ ;tL2L&lL!g&l, !g U hydrogen-sil'er-

sil'er chloride electrode

;b L ;b#$% LL 2#$%9a56 L ;b$2 L ;b U the lead storage

battery

The electric circuit with transfer is when the

electrodes are dipped in different solution which

contact with each other.

>xamples/ "nL"n#$%LL&u#$%L&u U aniell cell

;tL2L&lLL)&lLg2&l2,g U hydrogen-calomel cell


68/68

Lacture №10. Electrochemistry.ppt

Documents