Pptelectrochemistry-1

ELECTROCHEMISTRY

The branch of chemistry which deals with the relationship between chemical reaction & electrical energy is known as electrochemistry. It has two aspects, the study of chemical reaction which occurs due to the passage of an electric current & the generation of electricity by chemical reaction.

Arrhenius theory of electrolytic dissociation(1887)(i) An electrolyte on dissolving in suitable solvent gives equal no. of +ve charge & -ve charge ions.(ii) Since the no. of cations & anions are equal in given solution , the solution as whole is electrically neutral in nature. (iii) Cations are obtained from metallic atoms or radicals by loss of electrons, while anions are obtained from non-metallic atoms or radicals by gain of electrons.(iv) The process of electrolytic dissociation is a reversible process.(v) The ions are free to move & under the influence of electricity , they are directed towards oppositely charged electrodes.

(vi) The properties of the electrolytes in solution are the properties of the ions produced.

(vii) No. of molecules dissociated into ions

Degree of ionization= --------------------------------------------------

Total of molecules taken

(viii) Strong & weak electrolytes.

(ix) The conduction of an electrolytic solution ,generally increases with increase in temp

Electrolysis

The process in which by conversion of electrical energy into chemical energy or decomposition of an electrolyte takes place is known as electrolysis. This process is carried out in electrolytic cell Electrolytic cell:

A suitable device of glass or steel in convenient shape & size in which electrical energy converted into chemical energy known as electrolytic cell.

 Mechanism of electrolysis:

BA=B+ + A –

At cathode : B+ +e- B(Reduction)

At anode:A –A + e-( Oxidation)

Specific conductivity(K):The resistance of a conductor is directly proportional to its length & inversely proportional to its cross-sectional area.(Ohm’s law) lR= — Where R= resistance in ohms ; = specific A resistance or resistivity; l= length in cm; A= area of cross- section in cm2

Thus, if l=1 cm; A=1 cm2; then R= Sp resistance of a conductor is the resistance between two opposite faces of a cm cube of that substance. Sp cond is the reciprocal of sp resistance of an electrolytic solution.=1/ = l/AR ohm-1 cm-1 If l=1cm; A=1 cm2; then K= 1/R= conductanceSp cond (K) is the cond of 1 cm3 of solution of an electrolyte.

B. Equivalent conductivity(eq):Eq cond of an electrolytic solution is defined as the conductance of all the ions present in 1 eq of the electrolyte in the solution at given dilution. If I eq of electrolyte is contained in v mL, then:

eq = v xSp cond of 1 cm3 of solution= vx K

If normality of electrolytic solution is N, then;

v= (1/N)L=(1000/N)mL(or cm3)

eq =1000K/N ohm-1cm2eq-1

MOLAR CONDUCTIVITY((Λm)

Molar cond of a solution is defined as the conductance of all the ions present in 1 mole of the electrolyte in the solution at given dilution. If M is the molar conc. In mol/L, then

Λm =1000K/M ohm-1cm2 mol-1 

Cond=K

Eq cond =9xK

Variation of conduction with dilution :

Specific conductance: Sp cond of a solution depends on

i) the no. of ions ii) the amt of water present in the

solution. Sp cond decreases with dilution.

Equivalent conductance: Eq cond increases with dilution.

Eq

cond

√C

o.o5 o.1 0.15

140

120

100NaCl

BaCl 2

CH 3 COOH

CH 3 COONa

Conductometric titration:

1. Titration of strong acid with strong base:

2.Titration of weak acid with strong base:

Cond

Vol of alkali added

Cond

Vol of alkali added

End point

Faraday’s first law of electrolysis(1834)

During the electrolysis , the amount of substance liberated or deposited at an electrode is directly proportional to the quantity of electricity passed through an electrolyte.WQ Where W= wt of sub

Q= qty of electricity in coulomb

But Q=it i= current in amp

t= time in sec

W it

W= Zit When i=1 & t=1sec ie Q=1Coulomb

W=Z Z= Electrochemical equivalent( ECE) of sub

ECE is the amount of sub deposited or liberated at an electrode when 1Q of electricity passed through its solution ie current of 1 amp passed for 1 sec through the solution 

Faraday’s second law of electrolysis:

When different electrolytes are arranged in series & same amount of electricity passed through them , the amount of different sub deposited or liberated at the respective electrode are directly proportional to their chemical equivalentW1 E1 Where W1& W2 are the wt of sub

W2 E2 deposited or liberated & E1& E2 are the

chemical equivalent of sub W1 E1

----- = -----

W2 E2

 CuSO4

AgNO3

B

RELATION BETWEEN ECE & CE:ACCORDING TO FARADAY’S FIRST LAWWAA= ZAA i t WBB= ZBB i t WA A ZA A

---------- = -------- ---------- = --------

WB B ZBB

ACCORDING TO FARADAY’S SECOND LAW

WA A EA A

---------- = -------- ---------- = --------

WB B EBB

EA A Z Z A A

-------= -------- -------= --------

EEB B ZZBB

EE Z ; E/Z= CONST; E/Z= 96500C( 1F=96500C); E/Z= CONST; E/Z= 96500C( 1F=96500C)

E/Z of Cu=31.75/3.29x10-4=96504 coulombs[1F=96500 coulombs] E/Z of Ag=107.9/1.110x10-3 = 96469 coulombs E/Z of I= 127/1.31x10-3 =96500 coulomb E/Z of Zn=32.7/3.39x10-4=96500 coulomb

Kohlrausch’s law of independent migration of ions:

At infinite dilution, when dissociation is complete & all interionic effects vanish , each ion contributes to the total molar cond. of an electrolyte at a definite share. Thus molar cond at infinite dilution[m

] of any

electrolyte is equal to the sum of the molar cond of its cation & anion.

m =+ +

+ - -

Where + &- the no. of cations & anion per formula of

electrolyte & + & -

are molar cond at infinite dilution

of cation & anion respectively.

e.g. (MgCl2), += 1& -=2;

m (MgCl2)= ( Mg 2+) + 2 (Cl-)

Applications of Kohlrausch’s law:

A.Determination of molar cond of a weak electrolyte (Acetic acid):

= m (HCl)+ m

(CH3COONa)- m (NaCl)

=[ ( H+)+ (Cl-)]+[ (CH3COO-) + ( Na+)]-

[ ( Na+)+ (Cl-)]

=[ ( H+)+(CH3COO-)= m (CH3COOH)

 B.Determination of degree of dissociation:

No of molecules dissociated into ions

c = -------------------------------------------------

Total of molecules taken

mc, the molar cond at a particular dilution No of molecules

dissociated into ions at this dilution ……(i)

m Total of molecules taken ….(ii)

Dividing (i) by (ii) we get;

mc No of molecules dissociated into ions at this dilution

------= -------------------------------------------------------------------

m Total of molecules taken

The value ofmc can be obtained by direct measurement; while

m can be obtained with the help of Kohlrausch’s law

Determine of solubility of sparingly soluble salt:

AgCl, BaSO4,CaCO3,PbS, etc

For very sparingly soluble salts: eq c = eq

= Kv

v= eq /K= + +

+ - - /K

But v cm3 of saturated solution contains= I eq

1000 cm3 or 1 L sat. sol contains=(1000/v)eq

Hence , solubility of salt, S= 1000/v eq/L

=1000E/v g/L

=1000xE(eq wt) xK/ [+ + + - -

] g/L

~

Ostwald’s dilution law:

Binary electrolyte BA, dissociates as:

BA B+ +A-

Suppose 1 mole BA is dissolved in V L of water & at equilibrium a fraction of it is dissociated into ions BA B+ + A-

Amt at equilibriumm (1-) mol mol mol

Equi concentrtion (1-/V) mol /Lit /V mol /Lit /V mol /Lit

On applying law of mass action, we get:

[B+] [A-] /V./V 2 2 2 C

--------- = ----------- = ------- = K, K=-------- = --------

[BA] 1-/V (1-)V (1-)V (1-) 

Where C is the conc in mol/Lit

 For a wk electrolyte

K= 2 /V (Where is very small)

pH & pOH pH value of a solution is defined as negative of logarithm of the hydrogen-ion conc, expressed in mole/litre pH= -log10[H

+] = log101/[H+]At 25 C , the ionic product of water, Kw= [H+][OH-] = 10-14. Also in pure water: [H+]=[OH-] In neutral solution, [H+]=[OH-] = 1x 10-14= 1x10-7 or pH of neutral solution = -log10 1x10-7 = 7pOH= = -log10[OH-] pH + pOH=14pH=7 neutral solutionpH< 7 acidic solutionpH> 7 basic solution

Buffer solutions :

A buffer solutions is one which maintains a fairly constant pH, even when small amt of acid or alkali is added to it (i) Acidic Buffer : Acidic buffer can be made from acetic acid & sodium acetateCH3 COOH H++ CH3COO- (feebly ionized)CH3 COONa Na++ CH3COO- (highly ionized)

When a few drops of acid are added to it HCl H++ Cl-

CH3COONa + H Cl CH3 COOH + NaCl(from buffer) (from added acid)When a few drops of base are added to it,NaOH Na++ OH-

CH3 COOH + NaOH H2O + CH3COONa(from buffer) (from added base)

(ii) Basic buffer: A basic buffer can be made from mixture of ammonium hydroxide & ammonium chlorideNH4 OH NH4

++ OH- (feebly ionized)

NH4 Cl NH4++ Cl- (highly ionized)

 When a few drops of base are added to it,

NaOH Na++ OH-

NH4 Cl + NaOH NH4OH + NaCl

(from buffer) (from added base)

When a few drops of acid are added to it ,

HCl H++ Cl-

NH4 OH + H Cl NH4 Cl + H2O

(from buffer) (from added acid)

pH of acidic buffer: The dissociation of a wk acid HA is expressed as:HA H+ + A-

  [H+] [A-] [HA]

Ka= ------------ or [H+]= Ka----------- ---------(i) [HA] [A-]

Where Ka is dissociation const of acidFor preparing a buffer , a highly ionized salt of acid, BA is added to itBA B+ + A-

It can be assumed that all the ions have come from the salt itself ie,[A-] =[ BA] -------- (ii) From (i) & (ii) , we get:

[HA] [ Acid]

[H+]= Ka-------- = Ka-------- ; [BA] [salt]

Taking log & reversing the sign, we get: [Acid] [Salt]

-log [H+]= -log Ka – log--------;pH = p Ka +log -------- [Salt] [Acid]

Henderson- Hasselbatch equation [base] [Salt]

-log [OH-]= -log Kb–log--------;pOH =p Kb + log --------- [Salt] [Base]

Henderson- Hasselbatch equation 

Solubility product: In a saturated solution of any sparingly soluble electrolyte, the product of the conc of the ions, raised to a power equal to the noof times the ion appears in the equation representing the ionization of the electrolyte , is a const at a given temp.A saturated solution is a solution in which the dissolved & undissolved solute are in equilibrium.The solubility of a substance in a solvent is the conc in the saturated solution.Exa AgCl Ag+ + Cl- Applying the law of mass action:

K=[Ag+] [Cl-] /[ AgCl] Ksp= [Ag+] [Cl-] where Ksp solubility product, [Ag+] & [Cl-] are expressed in mole/lit

Ksp= [ Ag+] [Cl-] solubility product S mol / L S mol /L

=S2 mol2/L2=S2

The product [Ag+] [Cl-] in the Ksp expression above is called Ionic product or ion product.

Electrochemical cell:An electrochemical cell is a device in which a redox reaction is utilized to get electrical energy.The electrode where oxidation occurs , is called anode; while the electrode where reduction occurs , is called cathode.

At anode (-ve electrode): Zn Zn+++ 2e- ( Oxidation)At cathode(+ ve electrode): Cu++ +2e- Cu ( Reduction)____________________________________________Cell reaction: Zn + Cu ++ Zn+++ Cu

Representation of galvanic cell:

Zn; Zn++(1M) II Cu++ (1M);Cu

ZnSO4

Zn anode -Electron flow

Voltmeter

Cu cathode+

CuSO4

Salt-bridge

EMF of electrochemical cell:Zn- I Zn(aq) II Cu(aq);Cu+

Ecell =E right- E left

Ecell= e.m.f. of cellE right= reduction pot of right hand side electrodeE left= reduction pot of left hand side electrode Nernst’s equation for a cell reaction:Cell reaction: Zn (s) + Cu ++

(aq) Zn++ (aq) + Cu (s)

2.303RT V [Zn++ ] E cell= E0

cell - --------------- log --------- nF [Cu ++ ] At 298 K, the Nernst’s equation can be written as: 0.0592 V [Zn++ ] Ecell= (E0

cathode- E0

anode) - ---------------log --------- n [Cu ++ ]

Concentration cell:A conc cell is made up of two half-cells having identical electrodes, identical electrolytes, but conc of the reactive ions at the two electrodes are different. The two half-cells may be joined by a salt bridge.

-AgAgNO3(C1M) Dil AgNO3(C2M) concAg+

  2.303RT C2 0.0592 V C2

Ecell= ------------log-------- = --------- log ------ nF C1 1 C1

At left electrode(anode) :Ag Ag+(C1) + 1e- ( Oxidation)At right electrode(cathode):Ag+(C2)+1e-Ag( Reduction)

AgNO3

Ag- Ag+

AgNO3

Electron flowVoltmeter

Salt bridge

Std electrode potentials(reduction)at 25 0C

Metal ion Potential in volts

Li++e- Li

K++e- K

Zn+++2e- Zn

Fe+++2e- Fe

Sn +++2e- Sn

H ++e- H

Cu +++2e- Cu

Ag ++e- Ag

Pt 4++4e- Pt

Au ++e- Au

½ F2 +e- F-

-3.05 (anode)

-2.93

-0.76

-0.44

-0.14

0.00 ( Reference)

+0.34

+0.80

+0.86

+1.69

+2.87 (Cathode)