Unit 3 Electrochemistry

8/14/2019 Unit 3 Electrochemistry

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UNIT 3: ELECTROCHEMISTRY

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SYNOPSIS

An electrochemical cell and dierence between galvanic and electrolyc cells; Nernst equaon for calculang the emf of cell and dene standard potenal of the cell; Standard Potenal of the Cell, Gibbs energy of cell reacon and its Equilibrium constant;

Dene resisvity (), conducvity () and molar conducvity (m) of ionic soluons; Dierence between ionic (electrolyc) and electronic conducvity; Measurement of conducvity of electrolyc soluons and calculaon of their molar

conducvity;

Variaon of conducvity and molar conducvity of soluons with change in theirconcentraon and dene m (molar conducvity at zero concentraon or innite

diluon); Kohlrausch law and its applicaons; Quantave aspects of electrolysis; Construcon of some primary and secondary baeries and fuel cells;

Corrosion as an electrochemical process.INTRODUCTION:

Electrochemistry is the branch of chemistry which deals with conversion of electrical energy

into chemical energy (in electrolyc cells) and chemical energy into electrical energy

(in galvanic cells) and the study of behaviour of electrolytes.

Electrochemical Cells:

An electrochemical cell is a device in which inter-conversion of chemical energy and electrical

energy takes place. It involves redox reacons.Electrochemical cell is of two types. (i) Electrolyc cell, (ii) Galvanic cell.

ELECTROLYTIC CELL: An electrochemical cell in which electrical energy is converted into

chemical energy. i.e. a device for using electrical energy to carry non-spontaneous chemicalreacons.

GALVANIC CELL: The device in which electrical energy generated by using a spontaneous

redox reacon is called galvanic cellor voltaic cell. The free energy of the reacon is used to

do the electrical work. Eg: Daniel cell, Ni-Cd cell etc.

DANIEL CELL: Daniel cell consists of two beakers, one of which contains 1M ZnSO4soluon &

other 1M CuSO4soluon. A zinc rod is dipped in ZnSO4soluon and a copper rod in CuSO4

soluon. Externally, the zinc rod and copper rod are connected by an insulated wire. The two

soluons are connected by a salt-bridge.

WORKING OF DANIEL CELL: CELL REACTIONS:

When the two electrodes are connected, a spontaneous chemical reacon takes place at

each electrode.(a) Zn has a tendency to lose electrons. Someof the Zn atoms leave electrons on the rod and

come into solution as Zn+2

ions.

: 2; (Oxidaon half)Hence, Zn rod becomes negavely charged. It iscalled anode as oxidaon takes place at thiselectrode.(b) Cu has a tendency to gain electrons. Someof the Cu

+2ions from the solution deposit on the

metal as Cu by gaining electrons.

: 2; (Reducon half)Hence, Cu rod becomes posively charged. It iscalled cathode as reducon takes place at thiselectrode.The net cell reacon is;

: : REPRESENTATION OF CELL:By convenon, the anode is kept on the le and the cathode on

the right while represenng the galvanic cell. A galvanic cell is generally represented by

pung a vercal line between metal and electrolyte soluon and pung a double vercal

line between the two electrolytes connected by a salt bridge.

i.e Zn |Zn2

|| Cu2

| Cu

Fig. 3.1: Daniell cell having electrodes ofzinc and copper dipped in thesoluons of their respecve salts.


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EMF OF A CELL: Under this convenon the emf of the cell is posive and is given by the

potenal of the half-cell on the right hand side minus the potenal of the half-cell on the le

hand side.EMF = ERHE ELHE

OR EMF = Eposive electrode Enegave electrode

= ECathode EAnodeDISTINCTION BETWEEN ELECTROLYTIC CELL AND GALVANIC CELL

ELECTROLYTIC CELL GALVANIC CELL

1. Electrical energy is used to bring a redox

reaction.

1. Electrical energy is produced using a redox

reaction.

2. Oxidation takes place at the +ve electrode.

Hence +ve electrode is anode.

2. Oxidation takes place at the ve electrode.Henceve electrode is anode.

3. Reduction takes place at the ve electrode.Henceve electrode is cathode.

3. Reduction takes place at the + ve electrode.

Hence + ve electrode is cathode.

4. Externally electrons enter from cathode &

come out from anode.

4. Externally electrons move from anode to

cathode.

ELECTRODE POTENTIAL: The potenal developed when a metal is in equilibrium with asoluon of its own ions is called electrode potenal.In fact, it is the measure of the tendency

of an electrode in a half-cell to lose or gain electrons.

STANDARD ELECTRODE POTENTIAL: The potenal developed when a metal is in equilibrium

with a one molar soluon of its ions at 298K is called standard electrode potenal

(or standard reducon potenal).

If the electrode contains a gas, its pressure should be 1 atmosphere).

EMF: The potenal dierence between the two electrodes (cathode and anode) of a galvanic

cell is called the cell potenal and is measured in volts. It is called the electromove force

(emf) of the cell when no current is drawn through the cell.

Measurement of Electrode Potential:

The potenal of individual half-cell cannot be measured. But the measurement of dierence

between the two half-cell potenals gives the emf of the cell. The potenal of one electrode

(half-cell) can be determined by combining with other electrode (half-cell) of knownelectrode potenal.

Standard Hydrogen Electrode is an electrode with hydrogen gas at a pressure of 1

atmosphere in equilibrium with one molar hydrochloric acid in the presence of platinum.

CONSTRUCTION OF S.H.E:

The standard hydrogen electrode consists of aplanum electrode coated with planum black. Theelectrode is dipped in an acidic soluon and pure

hydrogen gas is bubbled through it. Theconcentraon of both the reduced and oxidised

forms of hydrogen is maintained at unity (Fig. 3.3).That is the pressure of hydrogen gas is one atm.

and the concentraon of hydrogen ion in thesoluon is one molar.

Pure hydrogen gas is passed at 1 atm.pressure through the inlet. The planum foil

adsorbs some hydrogen. Thus there is a core ofplanum with an adsorbed lm of hydrogen in

contact with hydrogen ions in the soluon.

Equilibrium is established between the H2gas and H ions. A potenal develops due to thereacon,

: ; The potenal of S.H.E is taken as zero.

The electrode is represented as,,(1)| :(1)

WORKING OF SHE: SHE is coupled with a given single electrode and an electrochemical cell is

set up. The EMF of the resulng cell is measured using a potenometer. Since the potenal

of SHE is zero, the EMF of the cell gives the potenal of the given electrode.At 298 K the emf of the cell, standard hydrogen electrode || second half-cell constructed

by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell

Cu wireInner tube

Pure H2(1 atm)

Outer tube

Pt foilHg

1MHCl


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as cathode, gives the reducon potenal of the other half-cell. If the concentraons of the

oxidised and the reduced forms of the species in the right hand half-cell are unity, then the

cell potenal is equal to standard electrode potenal, of the given half-cell. =

As for standard hydrogen electrode is zero. = 0 = The measured emf of the cell : Pt(s), H2(g, 1 atm.)H (aq, 1 M)||Cu2 (aq, 1 M)Cu is 0.34 V

and it is also the value for the standard electrode potenal of the half-cell corresponding to

the reacon :Cu2(aq, 1M) 2e

Cu(s)Similarly, the measured emf of the cell: Zn2(aq, 1M)Zn || H(aq, 1 M) H2 (g, 1 atm.), Pt(s)is

0.76 V corresponding to the standard electrode potenal of the half-cell reacon:Zn2(aq, 1 M) 2e

Zn(s)Thus for Daniell cell, the half-cell reacon can be represented as

Le electrode: Zn(s) Zn2(aq, 1 M) 2 e

Right electrode: Cu

2

(aq, 1 M) 2 e

Cu(s)The overall reacon of the cell is the sum of above two reacons and we obtain the

equaon: Zn(s) Cu2(aq) Zn

2(aq) Cu(s)Emf of the cell = =

= 0.34V ( 0.76)V = 1.10 V

Somemes metals like planum or gold are used as inert electrodes. They do not parcipatein the reacon but provide their surface for oxidaon or reducon reacons and for the

conducon of electrons.For example, Pt is used in the following half-cells:

Hydrogen electrode: Pt(s)|H2(g)| H

(aq), with half-cell reacon: H(aq) e

H2(g)Bromine electrode: Pt(s)|Br2(aq)| Br

(aq), with half-cell reacon: Br2(aq) e

Br(aq)

ELECTROCHEMICAL SERIES:

The list of standard reducon potenals arranged in the order of their increasing value is

called electrochemical series. The standard electrode potenals are very important and we

get useful informaon from them.

Table 3.1 The standard electrode potenals at 298 K

ELECTRODE ELECTRODE REACTION STD. RED. POTENTIAL (V)

Li+/ Li : ; 3.05

K+/ K : ; 2.93Ba

+2/ Ba : 2; 2.90

Ca

+2

/ Ca : 2; 2.87Na+/ Na : ; 2.71Mg

+2/ Mg : 2; 2.37

Al+3/ Al : 3; 1.66Zn

+2/ Zn : 2; 0.76

Fe+2/ Fe : 2; 0.44Ni

+2/ Ni : 2; 0.25

H+/ H : ; 0.00Cu

+2/ Cu : 2; + 0.34

Ag+/ Ag : ; + 0.80Au

+3/ Au : 3; + 1.50

Pt, 1/2 F2/ F ; ; + 2.55

SIGNIFICANCE OR IMPORTANCE OF ELECTROCHEMICAL SERIES:

o Metals in the top of the list are good reducing agents. Elements at the bottom of the list aregood oxidising agents.

o A metal in the electrochemical series displaces another below it from its salt solution.o Metals which are above hydrogen in the series displace hydrogen from dilute acids.o The series helps in selective discharge of different elements from a mixture of their ions.


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Nernst Equation:

Nernst showed that for the electrode reacon: (): ; ()The electrode potenal at any concentraon measured with respect to standard hydrogen

electrode can be represented by:

(+

/ ) = (+

/ )

[]

[+]

But the concentraon of solid M is taken as unity and we have

(+/ ) = (+/ )

[+] (3.8)

Where (+/ ) is standard electrode potenal, R is gas constant (8.314 JK1 mol1), F isFaraday constant (96487 C mol1), T is temperature in kelvin and *Mn+ is the concentraon

of the species, Mn.In Daniell cell, the electrode potenal for any given concentraon of Cu2and Zn2ions, we

write

For Cathode: (+/ ) = (+/ )

[+] (3.9)

For Anode: (+/ ) = (+/ )

[+] (3.10)

The cell potenal, = (+/ ) (+/ ) = (+/ )

[+] (+/ )

[+]= (+/ ) (+/ )

[+]

[+]

= [+][+] (3.11)

Thus E(cell) depends on the concentraon of both Cu2 and Zn2 ions. It increases with

increase in the concentraon of Cu2ions and decrease in the concentraon of Zn2ions.

By converng the natural logarithm in Eq. (3.11) to the base 10 and substung the values of

R, F and T = 298 K, it reduces to

= . [+][+]

We should use the same number of electrons (n) for both the electrodes. For eg.Ni(s)| Ni

2(aq)|| Ag

(aq) | Ag

The cell reacon is Ni(s) 2Ag

(aq)Ni2

(aq) 2Ag(s)

The Nernst equaon can be wrien as

= [+][+]

Thus for a general electrochemical reacon of the type:

aA bB ne cC dDNernst equaon can be wrien as:

= =

[][][][] (3.13)

Example 3.1:Represent the cell in which the following reacon takes place

Mg(s) 2Ag

(0.0001M)Mg2(0.130M) 2Ag(s). Calculate its E(cell)if = 3.17 V.Soluton: The cell can be wrien as Mg|Mg2(0.130M)||Ag

(0.0001M)|Ag.

= [+][+]

= 3.17 0.05912 0.130

(0.0001) = 3.17 0.21 = 2.96 Equilibrium Constant from Nernst Equation:

In Daniell cell, when the circuit is closed, the redox reacon

Zn(s) Cu2

(aq) Zn2

(aq) Cu(s) (3.1)

takes place and as me passes, the concentraon of Zn2 keeps on increasing while the

concentraon of Cu2 decreases. At the same me voltage of the cell as read on thevoltmeter keeps on decreasing. Aer some me, there is no change in the concentraon of

Cu2and Zn2ions and voltmeter gives zero reading. This indicates that equilibrium has been

aained and for which the Nernst equaon may be wrien as:

= 0 = . [+][+]

OR = .

[+]

[+

]


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But at equilibrium,[+][+] = for the reacon 3.1.

and at T = 298K the above equaon can be wrien as

= . = 1.10 ( = 1.10 )

= . .

= 37.288 = (37.288) = 2 10at 298 K

In general, = . Example 3.2:Calculate the equilibrium constant of the reacon:

Cu(s) 2Ag

(aq) Cu2

(aq) 2Ag(s) (Given = 0.46 V)Soluton: Given: = 0.46 V

WKT, = . OR = .

= . . =15.6 = (15.6) = 3.92 10

Electrochemical Cell and Gibbs Energy of the Reaction:

Electrical work done in one second is equal to electrical potenal mulplied by total chargepassed. To obtain maximum work from a galvanic cell then charge has to be passed

reversibly. The reversible work done by a galvanic cell is equal to decrease in its Gibbs

energy and therefore, if the emf of the cell is E and nF is the amount of charge passed and

rG is the Gibbs energy of the reacon, then

rG = nFE(cell) (3.15)

It may be remembered that E(cell) is an intensive parameter but rG is an extensive

thermodynamic property and the value depends on n.Thus, for the reaconZn(s) Cu

2(aq)Zn

2(aq) Cu(s) rG = -2FE(cell)

And for the reacon 2Zn(s) 2Cu2

(aq)2Zn2

(aq) 2Cu(s) rG = -4FE(cell)

If the concentraon of all the reacng species is unity, then E (cell)= Eo

(cell)and we haverG

o= nFEo(cell) (3.16).Also rG

o= RT ln K = -2.303RT logK.

Example 3.3:The standard electrode potenal for Daniell cell is 1.1V. Calculate the standard

Gibbs energy for the reacon: Zn(s) Cu2

(aq) Zn2(aq) Cu(s).Soluton: rG

o= nFEo(cell)

For the given reacon n = 2, F = 96487 C mol1 and Eo(cell)= 1.10 V

Therefore, rGo= 2 x 1.1V x 96487 C mol1

= 21227 J mol1

= 21.227 kJ mol1

Conductance of Electrolytic Solutions:

DEFINITIONS:

Resistance: The electrical resistance (R) is the opposing force to the ow of current oered

by the substance and it is measured in ohm () (SI unit is (kg m 2)/(s3A2). It is measured with

the help of a Wheatstone bridge.The electrical resistance of any object is directly proporonal to its length, l, and inversely

proporonal to its area of cross secon, A. That is,

OR = (3.17)

The proporonality constant, (rho), is called resisvity (specic resistance). Its SI units are

ohm metre ( m) or its submulple, ohm cenmetre ( cm) is also used.The resisvity of a substance is its resistance when it is one metre long and its area of cross

secon is one m2. It can be seen that:1 m = 100 cm or 1 cm = 0.01 m

The inverse of resistance, R, is called conductance, C, and we have the relaon:

= = =

(3.18)

The SI unit of conductance is Siemens (S) OR ohm1(also known as mho) or 1. The inverse

of resisvity is conducvity (specic conductance), (kappa). The SI units of conducvity areS m1but oen S cm1. Conducvity of a material is its conductance when it is 1 m long and

its area of cross secon is 1 m2. It may be noted that 1 S cm1= 100 S m1.


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Table 3.2: The values of Conducvity of some Selected Materials at 298.15 K

Certain materials called superconductors by denion have zero resisvity or innite

conducvity. Earlier, only metals and their alloys at very low temperatures (0 to 15 K) wereknown to behave as superconductors, but nowadays a number of ceramic materials and

mixed oxides are also known to show superconducvity at temperatures as high as 150 K.

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

to the movement of electrons. The electronic conductance depends on

(i) The nature and structure of the metal(ii) The number of valence electrons per atom(iii) Temperature (it decreases with increase of temperature).

The conductance of electricity by ions present in the soluons is called electrolyc or ionic

conductance. The conducvity of electrolyc (ionic) soluons depends on:

(i) The nature of the electrolyte added(ii) Size of the ions produced and their solvaon

(iii) The nature of the solvent and its viscosity(iv) Concentraon of the electrolyte(v) Temperature (it increases with the increase of temperature).

Passage of direct current through ionic soluon over a prolonged period can lead to change

in its composion due to electrochemical reacons.

Measurement of the Conductivity of Ionic Solutions:

Accurate measurement of an unknown resistance can be performed by Wheatstone bridge.

But, this cannot be used for measuring the resistance of an ionic soluon. Since, passing

direct current (DC) changes the composion of the soluon and a soluon cannot be

connected to the bridge like a metallic wire or other solid conductor.The rst diculty is resolved by using an alternang current (AC) source of power. The

second problem is solved by using a specially designed vessel called conducvity cell.

It consists of two planum electrodes coated with planum black. Their cross seconal area

is A and are kept at distance l. Therefore, soluon in between these electrodes is a column

of length l and area of cross secon A. The resistance of such a column of soluon is then

given by the equaon:

= =


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The quantyis called cell constant denoted by the symbol, C. It depends on the distance

between the electrodes and their area of cross-secon and has the dimension of length1

and calculated by knowing l and A. Measurement of l and A is not only inconvenient but also

unreliable. The cell constant is usually determined by measuring the resistance of the cell

containing a soluon whose conducvity is already known. Generally, we use KCl soluons

whose conducvity is known accurately at various concentraons (Table 3.3) and at dierent

temperatures. The cell constant, C, is then given by the equaon:

= = =

Table 3.3: Conducvity and Molar conducvity of KCl soluons at 298.15K

Once the cell constant is determined, we can use it for measuring the resistance or

conducvity of any soluon. The set up for the measurement of the resistance is shown in

Fig. 3.5.

=Ce Cnstant

=C

(3.20)MOLAR CONDUCTANCE(): Molar conductance of an electrolyte is the conductance of thesolution due to all the ions produced by one mole of the electrolyte.

= (3.21)Unit of Molar conductance: = =

. = Smmol;.

1 Smmol; = 10S cmmol; OR 1 S cmmol; = 10;S mmol;.Example 3.4: Resistance of a conducvity cell lled with 0.1 mol L1KCl soluon is 100 . If

the resistance of the same cell when lled with 0.02 mol L1KCl soluon is 520 , calculate

the conducvity and molar conducvity of 0.02 mol L1KCl soluon. The conducvity of 0.1

mol L1KCl soluon is 1.29 S/m.Soluton: The cell constant is given by the equaon:

Cell constant = C = conducvity resistance= 1.29 S/m 100 = 129 m1= 1.29 cm1

Conducvity of 0.02 mol L1KCl soluon = cell constant / resistance

= =

= 0.248 S mConcentraon = 0.02 mol L1

= 1000 0.02 mol m3= 20 mol m3

Molar conducvity = = =

= 124 10; ;

It consists of two resistances R3 and R4, a variable

resistance R1 and the conducvity cell having the

unknown resistance R2. The Wheatstone bridge is fed

by an oscillator O (a source of A.C. power in the audiofrequency range 550 to 5000 cycles per second). P is a

suitable detector (a headphone or other electronic

device) and the bridge is balanced when no currentpasses through the detector. Under these condions:

Unknown resistance, R = (3.19)Conducvity meters are available which can directly

read the conductance or resistance of the soluon in

the conducvity cell. Once the cell constant and the

resistance of the soluon in the cell are determined,

the conducvity of the soluon is given by the

equaon:

Fig.3.5: Arrangement formeasurement of resistanceof a soluon of anelectrolyte.


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Alternavely, = . = 0.248 10; ;and = x 1000

= . . = 124 ;Example 3.5:The electrical resistance of a column of 0.05 mol L1NaOH soluon of diameter

1 cm and length 50 cm is 5.55 103 ohm. Calculate its resisvity, conducvity and molar

conducvity.Soluton: A = r2= 3.14 0.52cm2= 0.785 cm2= 0.785 104m2

l = 50 cm = 0.5 m

= OR =

=. .

= 87.135 cm = = =

. = 0.01148 ;

, =

= . . = 229.6 ;

Variation of Conductivity and Molar Conductivity with

Concentration:

Conducvity and molar conducvity change with the concentraon of the electrolyte. TheConducvity of both weak and strong electrolytes decreases with decrease in concentraon.

This is because the number of ions per unit volume that carry the current in a soluon

decreases on diluon. The conducvity of a soluon at any given concentraon is the

conductance of one unit volume of soluon kept between two planum electrodes with unit

area of cross secon and at a distance of unit length. This is clear from the equaon:

= = When A = m2and l=m.

Molar conducvity of a soluon at a given concentraon is the conductance of the volume Vof soluon containing one mole of electrolyte kept between two electrodes with area of

cross secon A and distance of unit length. Therefore,

= = Since l=1 and A = V (volume containing 1 mole of electrolyte) = (3.22)

Molar conducvity increases with decrease in concentraon. This is because the total

volume, V, of soluon containing one mole of electrolyte also increases.When concentraon approaches zero, the molar conducvity is known as liming molar

conducvity and is represented by the symbol . The variaon in mwith concentraon isdierent (Fig. 3.6) for strong and weak electrolytes.

Strong Electrolytes: For strong electrolytes, increases slowly with diluonand can be represented by the equaon:

= / (3.23)It can be seen that if we plot (Fig.

3.6) against /, we obtain a

straight line with intercept equal to

and slope equal to . The valueof the constant for a given solvent

and temperature depends on the

type of electrolyte i.e., the charges

on the caon and anion produced on

the dissociaon of the electrolyte inthe soluon. Thus, NaCl, CaCl2,

MgSO4 are known as 1-1, 2-1 and

2-2 electrolytes respecvely. All

electrolytes of a parcular type have

the same value for A.

Fig. 3.6: Molar conducvity versus c for acecacid (weak electrolyte) and potassiumchloride (strong electrolyte) in aqueoussoluons.


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Kohlrausch law of independent migration of ions:

The law states that liming molar conducvity of an electrolyte can be represented as the

sum of the individual contribuons of the anion and caon of the electrolyte. Thus, if + and are liming molar conducvity of the sodium and chloride ionsrespecvely, then the liming molar conducvity for sodium chloride is given by the

equaon:(NaC) = + (3.24)

In general, if an electrolyte on dissociaon gives v caons and v anions then its liming

molar conducvity is given by: (NaC) = :: ;; (3.25)Table 3.4: Liming molar conducvity for some ions in water at 298 K

Weak electrolytes:

Weak electrolytes like acec acid have lower degree of dissociaon at higher concentraons

and hence for such electrolytes, the change in m with diluon is due to increase in the

degree of dissociaon and consequently the number of ions in total volume of soluon thatcontains 1 mol of electrolyte. In such cases m increases steeply (Fig. 3.6) on diluon,

especially near lower concentraons. Therefore cannot be obtained by extrapolaon ofm to zero concentraon. At innite diluon (i.e., concentraon c zero) electrolyte

dissociates completely ( =1), but at such low concentraon the conducvity of the soluonis so low that it cannot be measured accurately. Therefore, for weak electrolytes isobtained by using Kohlrausch law of independent migraon of ions (Example 3.8). At any

concentraon c, if is the degree of dissociaon then it can be approximated to the rao of

molar conducvity m at the concentraon c to liming molar conducvity, . Thus wehave:

Using Kohlrausch law of independent migraon of ions, it is possible to calculate for anyelectrolyte from the oof individual ions.

Example 3.7: for NaCl, HCl and NaAc are 126.4, 425.9 and 91.0 S cm2mol1 respecvely.Calculate ofor HAc.

Soluton:Given:

()

= 126.4

;

() = 425.9 ;() = 91.0 ;

From Kohlrausch Law,

WKT, (HA) = + = + + + = () () () = (425.9 91.0 126.4 ) S cm2mol1= 390.5 S cm2mol1.

= (3.26)Also

=

(;) (3.27)

Electrolytic Cells and Electrolysis:

Electrolyc cell is one in which an external source of voltage is used to bring about a

chemical reacon. For eg: Consider an electrolyc cell consisng of two copper strips dipping

in an aqueous soluon of copper sulphate. If a DC voltage is applied to the two electrodes,

then Cu2ions discharge at the cathode (negavely charged) and the following reacon takesplace:

Cu2(aq) 2e-- Cu (s) (3.28)

Copper metal is deposited on the cathode.

At the anode, copper is converted into Cu2ions by the reacon:

Cu(s) Cu2

(s) 2e (3.29)


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Thus copper is dissolved (oxidised) at anode and deposited (reduced) at cathode. This is the

basis for puricaon of the metals. The impure copper is made an anode that dissolves on

passing current and pure copper is deposited at the cathode. Many metals like Na, Mg, Al,

etc. are produced on large scale by electrochemical reducon of their respecve caons

where no suitable chemical reducing agents are available for this purpose.

Sodium and magnesium metals are produced by the electrolysis of their fused chlorides andaluminium is produced by electrolysis of aluminium oxide in presence of cryolite.

ELECTROLYSIS: The decomposition of an electrolyte on passing electric current through its

molten state or its solution is called electrolysis.

FARADAYS LAWS OF ELECTROLYSIS:

FIRST LAW: The mass of a substance liberated or deposited at an electrode during

electrolysis is proportional to the quantity of electricity passed.

i.e, W Q Where,W = Mass of substance liberated or deposited at an electrode.Q = Quanty of electricity in Coulombs.

Since, Charge = Current x Time,

Q = I x tHence, W I x t

OR W = z x I x t Where, Z is called Electrochemical equivalent (e.c.e) of the

substance.

SECOND LAW: When the same quantity of electricity is passed through the solutions of

different electrolytes connected in series, the masses of the substances deposited or

liberated at respective electrodes are directly proportional to their equivalent masses.

i.e, =

The amount of electricity (or charge) required for oxidaon or reducon depends on the

stoichiometry of the electrode reacon. For example, in the reacon:

Ag

(aq) e

Ag(s) (3.30)One mole of the electron is required for the reducon of one mole of silver ions. We knowthat charge on one electron is equal to 1.6021 x 1019C.

Therefore, the charge on one mole of electrons is equal to: NAx 1.6021 x 1019C

C = 6.022 x 1023mol1x 1.6021 x 1019 C = 96487 C mol1.This quanty of electricity is called Faraday and is represented by the symbol F.

1F 96500 C mol1.Example 3.10: A soluon of CuSO4 is electrolysed for 10 minutes with a current of 1.5

amperes. What is the mass of copper deposited at the cathode?

Soluton: Given: t = 10 mins = 600 s I = 1.5 A

Charge (Q) = Current (I) me (t) = 1.5 A 600 s = 900 C

According to the reacon:Cu2(aq) 2e

= Cu(s)We require 2F or 2 96487 C to deposit 1 mol or 63 g of Cu.For 900 C, the mass of Cu deposited = (63 g mol1 900 C)/(2 96487 C mol1) = 0.2938 g.

Products of Electrolysis:

Products of electrolysis depend on the nature of material being electrolysed and the type of

electrodes being used. If the electrode is inert (e.g., planum or gold), it does not parcipate

in the chemical reacon and acts only as source or sink for electrons. While, if the electrodeis reacve, it parcipates in the electrode reacon. Thus, the products of electrolysis may be

dierent for reacve and inert electrodes. The products of electrolysis depend on the

dierent oxidising and reducing species present in the electrolyc cell and their standardelectrode potenals. Moreover, some of the electrochemical processes although feasible, are

so slow kinecally that at lower voltages these dont seem to take place and extra potenal

(called overpotenal) has to be applied, which makes such process more dicult to occur.For example, When molten NaCl is electrolysed, the products are sodium metal and Cl 2gas.

At cathode: : ; At anode: ; ;.

But, during the electrolysis of aqueous sodium chloride soluon, the products are NaOH, Cl 2

and H2. In this case besides Na and Cl ions we also have H and OH ions along with the

solvent molecules, H2O.


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Sodium hydroxide is manufactured by the electrolysis of brine. During electrolysis, Hand Cl

ions with lower discharge potenal are evolved at the cathode and anode respecvely. Na

and OHions which are not discharged combine to form sodium hydroxide.Reacons taking place during electrolysis

Ionisaon: NaCl Na Cl ; H2O H OH

At the cathode: 2H

2e

H2At the anode : 2Cl Cl2 2e

Na OH NaOH

Batteries:

Any baery (actually it may have one or more than one cell connected in series) or cell that

we use as a source of electrical energy is basically a galvanic cell where the chemical energy

of the redox reacon is converted into electrical energy. However, for a baery to be ofpraccal use it should be reasonably light, compact and its voltage should not vary

appreciably during its use. There are mainly two types of baeries.

Primary Batteries:

In the primary baeries, the reacon occurs only once andaer use over a period of me baery becomes dead and

cannot be reused again. The most familiar example of this

type is the dry cell (known as Leclanche cell) which is used

commonly in our transistors and clocks. The cell consists of a

zinc container that also acts as anode and the cathode is acarbon (graphite) rod surrounded by powdered manganese

dioxide and carbon (Fig.3.8). The space between the

electrodes is lled by a moist paste of ammonium chloride

(NH4Cl) and zinc chloride (ZnCl2). The electrode reacons are

complex, but they can be wrien approximately as follows :

Anode: Zn(s) Zn2 2eCathode: MnO2 NH4

e MnO(OH) NH3In the reacon at cathode, manganese is reduced from the 4 oxidaon state to the 3

state. Ammonia produced in the reacon forms a complex with Zn2to give *Zn (NH3)4+2. The

cell has a potenal of nearly 1.5 V.Mercury cell, (Fig. 3.9) suitable for low current devices like

hearing aids, watches, etc. consists of zinc mercury

amalgam as anode and a paste of HgO and carbon as the

cathode. The electrolyte is a paste of KOH and ZnO. The

electrode reacons for the cell are given below:

Anode: Zn(Hg) 2OH

ZnO(s) H2O 2e

Cathode: HgO H2O 2e

Hg(l) 2OHThe overall reacon is represented byZn(Hg) HgO(s) ZnO(s) Hg(l)The cell potenal is approximately 1.35 V and remains

constant during its life as the overall reacon does not

involve any ion in soluon whose concentraon can

change during its life me.

Secondary Batteries:

A secondary cell aer use can be

recharged by passing current throughit in the opposite direcon so that it

can be used again. A good secondary

cell can undergo a large number of

discharging and charging cycles. The

most important secondary cell is the

lead storage baery (Fig. 3.10)

commonly used in automobiles and

invertors. It consists of a lead anode

and a grid of lead packed with leaddioxide (PbO2) as cathode.


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A 38% soluon of sulphuric acid is used as an electrolyte.The cell reacons when the baery is in use are given below:

Anode: Pb(s) SO42

(aq) PbSO4 (s) 2eCathode: PbO2 (s) SO4

2(aq) 4H

(aq) 2e

PbSO4 (s) 2H2O(l)i.e., overall cell reacon consisng of cathode and anode reacons is:

Pb(s)PbO2 (s) 2H2SO4 (aq) 2PbSO4 (s) 2H2O(l)

On charging the baery the reacon is reversed and

PbSO4 (s)on anode and cathode is converted into Pb

and PbO2, respecvely.

Another important secondary cell is the nickel-

cadmium cell (Fig. 3.11) which has longer life than

the lead storage cell but more expensive tomanufacture.

The overall reacon during discharge is:Cd(s) 2Ni(OH)3 (s) CdO(s) 2Ni(OH)2 (s) H2O(l)

Fuel Cells:

Producon of electricity by thermal plants is not a very ecient method and is a majorsource of polluon. In such plants, the chemical energy (heat of combuson) of fossil fuels

(coal, gas or oil) is rst used for converng water into high pressure steam. This is then used

to run a turbine to produce electricity. We know that a galvanic cell directly converts

chemical energy into electricity and is highly ecient. It is now possible to make such cells in

which reactants are fed connuously to the electrodes and products are removed

connuously from the electrolyte compartment. Galvanic cells that are designed to convertthe energy of combuson of fuels like hydrogen, methane, methanol, etc. directly into

electrical energy are called fuel cells. One of the most successful fuel cells uses the reacon

of hydrogen with oxygen to form water (Fig. 3.12).

The cell was used for providing electrical power in the Apollo space programme. The water

vapours produced during the reacon were condensed and added to the drinking watersupply for the astronauts. In the cell, hydrogen and oxygen are bubbled through porous

carbon electrodes into concentrated aqueous sodium hydroxide soluon. Catalysts like nely

divided planum or palladium metal are incorporated into the electrodes for increasing therate of electrode reacons. The electrode reacons are given below:

Cathode: O2 (g) 2H2O(l) 4e- 4OH(aq)Anode: 2H2(g) 4OH

(aq) 4H2O(l) 4e

Overall reacon being: 2H2 (g) O2 (g) 2H2O(l).The cell runs connuously as long as the reactants are supplied. Fuel cells produce electricity

with an eciency of about 70 % compared to thermal plants whose eciency is about 40%.

There has been tremendous progress in the development of new electrode materials, beercatalysts and electrolytes for increasing the eciency of fuel cells. These have been used in

automobiles on an experimental basis. Fuel cells are polluon free and in view of their future

importance, a variety of fuel cells have been fabricated and tried.


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CORROSION:The chemical attack on a metal by the constituents in the environment of

the metal is called corrosion.

Eg: 1. Rusng of iron on exposure to moist air.

2. Formaon of a green coang of basic copper carbonate (CaCO3.Ca(OH)2) on copper.

3. Tarnishing of silver.

RUSTING: When iron is exposed to moist air for a long time, a brown coating of hydratedferric oxide Fe2O3.3H2O is formed on the surface. This is called rusting.

MECHANISM OF RUSTING: The impure iron surface behaves like a small electrochemical cell

in the presence of water containing dissolved O2 or CO2. In these cells, pure iron acts as

anode and impure surface acts as cathode. Moisture containing dissolved O2or CO2acts as

the electrolyte.

At anode iron gets oxidised to Fe2+

ions.

: 2;At cathode moisture and oxygen of air are reduced to OH

ions.

2; 2 ;The Fe

2+

ions formed are further oxidised to Fe3+

ions by atmospheric oxygen. It formshydrated iron oxide Fe2O3.xH2O which is called rust.

PREVENTION OF CORROSION:

o PAINTING: Coating the metal surface with paints and enamels.o Coating the metal surface with a thin film of oil or grease.o GALVANISING: Coating the metal with other metals like zinc.o CHEMICAL PROTECTION: Coating the metal surface with the chemical like Ferric

Phosphate.


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Exercises3.1 Arrange the following metals in the order in which they displace each other from the

soluon of their salts. Al, Cu, Fe, Mg and Zn.3.2 Given the standard electrode potenals, K/K = 2.93V, Ag/Ag = 0.80V,

Hg

2

/Hg = 0.79V Mg

2

/Mg = 2.37 V, Cr

3

/Cr = 0.74VArrange these metals in their increasing order of reducing power.3.3 Depict the galvanic cell in which the reacon Zn(s) 2Ag

(aq) Zn

2(aq) 2Ag(s) takes

place. Further show:

(i) Which of the electrode is negavely charged?(ii) The carriers of the current in the cell. (iii) Individual reacon at each electrode.

3.4 Calculate the standard cell potenals of galvanic cell in which the following reacons

take place:(i) 2Cr(s) 3Cd

2(aq) 2Cr

3(aq) 3Cd (ii) Fe

2(aq) Ag

(aq) Fe

3(aq) Ag(s)

Calculate the rG and equilibrium constant of the reacons.3.5 Write the Nernst equaon and emf of the following cells at 298 K:

(i) Mg(s)|Mg2(0.001M)||Cu2(0.0001 M)|Cu(s) (ii) Fe(s)|Fe2(0.001M)||H(1M)|H2 (g) (1bar)| Pt(s)(iii) Sn(s)|Sn

2(0.050 M)||H

(0.020 M)|H2 (g)(1 bar)|Pt(s)

(iv) Pt(s)|Br2 (l)|Br

(0.010 M)||H

(0.030 M)| H2 (g) (1 bar)|Pt(s).

3.6 In the buon cells widely used in watches and other devices the following reacon

takes place: Zn(s) Ag2O(s) H2O(l)Zn2(aq) 2Ag(s) 2OH(aq)Determine rGoand Eofor the reacon.

3.7 Dene conducvity and molar conducvity for the soluon of an electrolyte.Discuss their variaon with concentraon.

3.8 The conducvity of 0.20 M soluon of KCl at 298 K is 0.0248 S cm1.Calculate its molar

conducvity.

3.9 The resistance of a conducvity cell containing 0.001M KCl soluon at 298 K is 1500 .What is the cell constant if conducvity of 0.001M KCl soluon at 298 K is 0.146 x103S cm1.

3.10 The conducvity of sodium chloride at 298 K has been determined at dierent

concentraons and the results are given below:

Concentration/M 0.001 0.010 0.020 0.050 0.100

102

x /S m1 1.237 11.85 23.15 55.53 106.74Calculate mfor all concentraons and draw a plot between mand

. Find the value of om.

3.11 Conducvity of 0.00241 M acec acid is 7.896 x 105 S cm1. Calculate its molar

conducvity and if om for acec acid is 390.5 S cm2 mol1, what is its dissociaonconstant?3.12 How much charge is required for the following reducons:(i) 1 mol of Al3to Al. (ii) 1 mol of Cu2to Cu. (iii) 1 mol of MnO4

to Mn2.3.13 How much electricity in terms of Faraday is required to produce

(i) 20.0 g of Ca from molten CaCl2. (ii) 40.0 g of Al from molten Al2O3.3.14 How much electricity is required in coulomb for the oxidaon of

(i) 1 mol of H2O to O2. (ii) 1 mol of FeO to Fe2O3.

3.15 A soluon of Ni(NO3)2is electrolysed between planum electrodes using a current of 5

amperes for 20 minutes. What mass of Ni is deposited at the cathode?3.16 Three electrolyc cells A, B, C containing soluons of ZnSO4, AgNO3 and CuSO4,

respecvely are connected in series. A steady current of 1.5 amperes was passed

through them unl 1.45 g of silver deposited at the cathode of cell B. How long did thecurrent ow? What mass of copper and zinc were deposited?

3.17 Using the standard electrode potenals given in Table 3.1, predict if the reacon

between the following is feasible:(i) Fe3(aq)and I

(aq) (ii) Ag

(aq)and Cu(s) (iii) Fe

3(aq)and Br

(aq) (iv) Ag(s)and Fe

3(aq)

(v) Br2(aq)and Fe2(aq).

3.18 Predict the products of electrolysis in each of the following:

(i) An aqueous soluon of AgNO3with silver electrodes.(ii) An aqueous soluon of AgNO3with planum electrodes.(iii) A dilute soluon of H2SO4with planum electrodes.

(iv) An aqueous soluon of CuCl2with planum electrodes.

Unit 3 Electrochemistry

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Transcript of Unit 3 Electrochemistry