Electrochemistry JEE Notes | EduRev

JEE : Electrochemistry JEE Notes | EduRev

 Page 1


Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
Key Concepts
Electrochemical Cells
An electrochemical cell consists of two electrodes (metallic conductors) in contact with an electrolyte
(an ionic conductor).
An electrode and its electrolyte comprise an Electrode Compartment.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells which produce electricity as a result of a spontaneous cell reaction .
Note: In a galvanic cell, cathode is positive with respect to anode.
In a electrolytic cell, anode is made positive with respect to cathode.
GALVANIC CELL
This cell converts chemical energy into electrical energy.
Salt
Bridge
Catton
plug
Anode of Zn
(–ve electrode)
(Anodic Cell)
Cathode of Cu
(+ ve electrode)
(Cathodic Cell)
ZnSO
4
CuSO
4
Galvanic cell is made up of two half cells i.e., anodic and cathodic. The cell reaction is of redox kind.
Oxidation takes place at anode and reduction at cathode. It is also known as voltaic cell. It may be
represented as shown in Fig. Zinc rod immersed in  ZnSO
4
 behaves as anode and copper rod immersed
in CuSO
4
 behaves as cathode.
Oxidation takes place at anode.
Zn ? Zn
2+
 + 2e
–
 (loss of electron : oxidation)
Reduction takes place at cathode:
Cu
2+
 + 2e
–
 ? Cu(gain of electron ; reduction)
Overall process : Zn(s) + Cu
2+
 ? Cu(s) + Zn
2+
In galvanic cell like Daniell cell: electrons flow from anode (zinc rod) to the cathode (copper rod)
through external circuit; zinc dissolves as Zn
2+
 ; Cu
2+
 ion in the cathode cell picks up two electron and
become deposited at cathode.
REPRESENTATION OF A CELL (IUPAC CONVENTIONS):
Let us illustrate the convention taking the example of Daniel cell.
(i) Anodic half cell is written on left and cathodic half cell on right hand side.
Zn(s) |ZnSO
4
(sol)||CuSO
4
(sol)|Cu(s)
(ii) Two half cells are separated by double vertical lines: Double vertical lines indicate slat bridge or any
type of porous partition.
(iii) EMF (electromotive force) may be written on the right hand side of the cell.
(iv) Single vertical lines indicate the phase separation between electrode and electrolyte solution.
Zn|Zn
2+
||Cu
2+
|Cu
(v) Invert eletrodes are represented in the bracket
Zn|ZnSO
4
||H
+
|H
2
,Pt
RELATIONSHIP BETWEEN ?G AND ELECTRODE POTENTIAL
Let n, faraday charge is taken out from a cell of e.m.f. (E) then electrical work done by the cell may be
calculated as,
E L E C T R O C H E M I S T R Y
E L E C T R O C H E M I S T R Y
Page 2


Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
Key Concepts
Electrochemical Cells
An electrochemical cell consists of two electrodes (metallic conductors) in contact with an electrolyte
(an ionic conductor).
An electrode and its electrolyte comprise an Electrode Compartment.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells which produce electricity as a result of a spontaneous cell reaction .
Note: In a galvanic cell, cathode is positive with respect to anode.
In a electrolytic cell, anode is made positive with respect to cathode.
GALVANIC CELL
This cell converts chemical energy into electrical energy.
Salt
Bridge
Catton
plug
Anode of Zn
(–ve electrode)
(Anodic Cell)
Cathode of Cu
(+ ve electrode)
(Cathodic Cell)
ZnSO
4
CuSO
4
Galvanic cell is made up of two half cells i.e., anodic and cathodic. The cell reaction is of redox kind.
Oxidation takes place at anode and reduction at cathode. It is also known as voltaic cell. It may be
represented as shown in Fig. Zinc rod immersed in  ZnSO
4
 behaves as anode and copper rod immersed
in CuSO
4
 behaves as cathode.
Oxidation takes place at anode.
Zn ? Zn
2+
 + 2e
–
 (loss of electron : oxidation)
Reduction takes place at cathode:
Cu
2+
 + 2e
–
 ? Cu(gain of electron ; reduction)
Overall process : Zn(s) + Cu
2+
 ? Cu(s) + Zn
2+
In galvanic cell like Daniell cell: electrons flow from anode (zinc rod) to the cathode (copper rod)
through external circuit; zinc dissolves as Zn
2+
 ; Cu
2+
 ion in the cathode cell picks up two electron and
become deposited at cathode.
REPRESENTATION OF A CELL (IUPAC CONVENTIONS):
Let us illustrate the convention taking the example of Daniel cell.
(i) Anodic half cell is written on left and cathodic half cell on right hand side.
Zn(s) |ZnSO
4
(sol)||CuSO
4
(sol)|Cu(s)
(ii) Two half cells are separated by double vertical lines: Double vertical lines indicate slat bridge or any
type of porous partition.
(iii) EMF (electromotive force) may be written on the right hand side of the cell.
(iv) Single vertical lines indicate the phase separation between electrode and electrolyte solution.
Zn|Zn
2+
||Cu
2+
|Cu
(v) Invert eletrodes are represented in the bracket
Zn|ZnSO
4
||H
+
|H
2
,Pt
RELATIONSHIP BETWEEN ?G AND ELECTRODE POTENTIAL
Let n, faraday charge is taken out from a cell of e.m.f. (E) then electrical work done by the cell may be
calculated as,
E L E C T R O C H E M I S T R Y
E L E C T R O C H E M I S T R Y
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
Work done = Charge × Potential = nFE
From thermodynamics we know that decrease in Gibbs free energy of a system is a measure of
reversible or maximum obtainable work by the system if there is no work due to volume expansion
? ? ?G = – nFE
Under standard state ?Gº = – nFEº ............(1)
(i) From thermodynamics we know, ?G = negative for spontaneous process. Thus from eq. (i) it is clear
that the EMF should be + ve for a cell process to be feasible or spontaneous.
(ii) When ?G = positive, E = negative and the cell process will be non spontaneous.
Reactions ?G E
Spontaneous (–) (+)
Non-spontaneous (+) (–)
Equilibrium 0 0
Standard free energy hcange of a cell may be calculated by electrode potential data.
Substituting the value of Eº (i.e., standard reduction potential of cathode-standard reduction potential
of anode) in eq. (i) we may get ?Gº.
CONCENPT OF ELECTROMOTIVE PORCE (EMF) OF A CELL
Electron flows from anode to cathode in external circuit due to a pushing effect called or electromotive
force (e.m.f.). EMF is called as cell potential. Unit of e.m.f. of cell is volt.
EMF of cell may be calculated as:
E
cell
 = reduction potential of cathode – Reduction potential of anode
Similarly, standard e.m.f. of the cell (Eº) may be calculated as
Eº
cell
 = Standard reduction potential of cathode – Standard reduction potential of anode.
SIGN CONVENTION OF EMF
EMF of cell should be positive other wise it will not be feasible in the given direction.
Zn|ZnSO
4
||CuSO
4
|Cu E = + 1.10 volt (Feasible)
Cu|CuSO
4
||ZnSO
4
|Zn E = – 1.10 volt (Not Feasible)
SALT BRIDGE
Two electrolyte solutions in galvanic cells are separated using salt bridge as represented in the Fig.
Salt bridge is a device to minimize or eliminate the liquid junction potential. Saturated solution of salt
like KCl, KNO
3
, NH
4
Cl and NH
4
NO
3
 etc. in agar-agar gel is used in salt bridge. Salt bridge contains high
concentration of ions viz. K
+
 and NO
3
– 
at the junction with electrolyte solution. Thus, salt bridge carries
whole of the current across the boundary; more over the K
+
 and NO
3
–
 ions have same speed. Hence,
salt bridge with uniform and same mobility of cations and anions completes the electrical circuit &
permits the ions to migrate.
NERNST EQUATION
Walter nernst derived a relation between cell potential and concentration or Reaction quotient.
?G = ?Gº + RT ln Q ..........(i)
where ?G and ?Gº are free energy and standard free energy change, 'Q' is reaction quotient.
?  – ?G = nFE and – ?Gº = nFEº
Thus from Eq. (i), we get – nFE = – nFEº + RT lnQ
At 25ºC, above equation may be written as E = Eº – Q log
n
0591 . 0
Where 'n' represents number of moles of electrons involved in process.
E, Eº are e.m.f. and standard e.m.f. of the cell respectively.
In general, for a redox cell reaction involving the transference of n electrons
aA + bB ? cC + dD, the EMF can be calculated as:
E
Cell
 = Eº
Cell 
 – 
b a
d c
] B [ ] A [
] D [ ] C [
log
n
0591 . 0
Page 3


Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
Key Concepts
Electrochemical Cells
An electrochemical cell consists of two electrodes (metallic conductors) in contact with an electrolyte
(an ionic conductor).
An electrode and its electrolyte comprise an Electrode Compartment.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells which produce electricity as a result of a spontaneous cell reaction .
Note: In a galvanic cell, cathode is positive with respect to anode.
In a electrolytic cell, anode is made positive with respect to cathode.
GALVANIC CELL
This cell converts chemical energy into electrical energy.
Salt
Bridge
Catton
plug
Anode of Zn
(–ve electrode)
(Anodic Cell)
Cathode of Cu
(+ ve electrode)
(Cathodic Cell)
ZnSO
4
CuSO
4
Galvanic cell is made up of two half cells i.e., anodic and cathodic. The cell reaction is of redox kind.
Oxidation takes place at anode and reduction at cathode. It is also known as voltaic cell. It may be
represented as shown in Fig. Zinc rod immersed in  ZnSO
4
 behaves as anode and copper rod immersed
in CuSO
4
 behaves as cathode.
Oxidation takes place at anode.
Zn ? Zn
2+
 + 2e
–
 (loss of electron : oxidation)
Reduction takes place at cathode:
Cu
2+
 + 2e
–
 ? Cu(gain of electron ; reduction)
Overall process : Zn(s) + Cu
2+
 ? Cu(s) + Zn
2+
In galvanic cell like Daniell cell: electrons flow from anode (zinc rod) to the cathode (copper rod)
through external circuit; zinc dissolves as Zn
2+
 ; Cu
2+
 ion in the cathode cell picks up two electron and
become deposited at cathode.
REPRESENTATION OF A CELL (IUPAC CONVENTIONS):
Let us illustrate the convention taking the example of Daniel cell.
(i) Anodic half cell is written on left and cathodic half cell on right hand side.
Zn(s) |ZnSO
4
(sol)||CuSO
4
(sol)|Cu(s)
(ii) Two half cells are separated by double vertical lines: Double vertical lines indicate slat bridge or any
type of porous partition.
(iii) EMF (electromotive force) may be written on the right hand side of the cell.
(iv) Single vertical lines indicate the phase separation between electrode and electrolyte solution.
Zn|Zn
2+
||Cu
2+
|Cu
(v) Invert eletrodes are represented in the bracket
Zn|ZnSO
4
||H
+
|H
2
,Pt
RELATIONSHIP BETWEEN ?G AND ELECTRODE POTENTIAL
Let n, faraday charge is taken out from a cell of e.m.f. (E) then electrical work done by the cell may be
calculated as,
E L E C T R O C H E M I S T R Y
E L E C T R O C H E M I S T R Y
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
Work done = Charge × Potential = nFE
From thermodynamics we know that decrease in Gibbs free energy of a system is a measure of
reversible or maximum obtainable work by the system if there is no work due to volume expansion
? ? ?G = – nFE
Under standard state ?Gº = – nFEº ............(1)
(i) From thermodynamics we know, ?G = negative for spontaneous process. Thus from eq. (i) it is clear
that the EMF should be + ve for a cell process to be feasible or spontaneous.
(ii) When ?G = positive, E = negative and the cell process will be non spontaneous.
Reactions ?G E
Spontaneous (–) (+)
Non-spontaneous (+) (–)
Equilibrium 0 0
Standard free energy hcange of a cell may be calculated by electrode potential data.
Substituting the value of Eº (i.e., standard reduction potential of cathode-standard reduction potential
of anode) in eq. (i) we may get ?Gº.
CONCENPT OF ELECTROMOTIVE PORCE (EMF) OF A CELL
Electron flows from anode to cathode in external circuit due to a pushing effect called or electromotive
force (e.m.f.). EMF is called as cell potential. Unit of e.m.f. of cell is volt.
EMF of cell may be calculated as:
E
cell
 = reduction potential of cathode – Reduction potential of anode
Similarly, standard e.m.f. of the cell (Eº) may be calculated as
Eº
cell
 = Standard reduction potential of cathode – Standard reduction potential of anode.
SIGN CONVENTION OF EMF
EMF of cell should be positive other wise it will not be feasible in the given direction.
Zn|ZnSO
4
||CuSO
4
|Cu E = + 1.10 volt (Feasible)
Cu|CuSO
4
||ZnSO
4
|Zn E = – 1.10 volt (Not Feasible)
SALT BRIDGE
Two electrolyte solutions in galvanic cells are separated using salt bridge as represented in the Fig.
Salt bridge is a device to minimize or eliminate the liquid junction potential. Saturated solution of salt
like KCl, KNO
3
, NH
4
Cl and NH
4
NO
3
 etc. in agar-agar gel is used in salt bridge. Salt bridge contains high
concentration of ions viz. K
+
 and NO
3
– 
at the junction with electrolyte solution. Thus, salt bridge carries
whole of the current across the boundary; more over the K
+
 and NO
3
–
 ions have same speed. Hence,
salt bridge with uniform and same mobility of cations and anions completes the electrical circuit &
permits the ions to migrate.
NERNST EQUATION
Walter nernst derived a relation between cell potential and concentration or Reaction quotient.
?G = ?Gº + RT ln Q ..........(i)
where ?G and ?Gº are free energy and standard free energy change, 'Q' is reaction quotient.
?  – ?G = nFE and – ?Gº = nFEº
Thus from Eq. (i), we get – nFE = – nFEº + RT lnQ
At 25ºC, above equation may be written as E = Eº – Q log
n
0591 . 0
Where 'n' represents number of moles of electrons involved in process.
E, Eº are e.m.f. and standard e.m.f. of the cell respectively.
In general, for a redox cell reaction involving the transference of n electrons
aA + bB ? cC + dD, the EMF can be calculated as:
E
Cell
 = Eº
Cell 
 – 
b a
d c
] B [ ] A [
] D [ ] C [
log
n
0591 . 0
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
3
www.learnershabitat.ac.in
THERMODYNAMIC TREATEMENT OF NERNEST EQUATION
(i) Prediction and feasibility of spontaneity of a cell reaction.
Let us see whether the cell (Daniell) is feasible or not; i.e. whether Zinc will displace copper or not.
Zn | (s) | ZnSO
4
(sol) || CuSO
4
(sol) | Cu(s)
volt 34 . 0 E ; volt 76 . 0 E
0
Cu / Cu
0
Zn / Zn
2 2
? ? ? ?
? ?
0
Zn / Zn
0
Cu / Cu
0
cell
2 2
E E E
? ?
? ?
= 0.34 – (– 0.76) = + 1.10 volt
Since E
0
  = + ve, hence the cell will be feasible and zinc will displace copper from its salt solution. In
the other words zinc will reduce copper.
(ii) Determination of equilibrium constant : We know, that
E = E
0
 – Q log
n
0591 . 0
..........(1)
At equilibrium, the cell potential is zero because cell reactions are balanced, i.e. E = 0
? From Eq. (i), we have
0 = E
0
 –
eq
K log
n
0591 . 0
or K
eq
 = anti 
?
?
?
?
?
?
?
?
0591 . 0
nE
log
0
(iii) Heat of Reaction inside the cell : Let n Faraday charge flows out of a cell of e.m.f. E, then
– ?G  = nFE ..........(i)
Gibbs Helmholtz equation (from thermodynamics) may be given as,
 ?G = ?H + T 
P
T
G
?
?
?
?
?
?
?
? ?
..........(ii)
From Eqs. (i) and (ii), we have
– nFE = ?H + T 
? ?
P
T
nFE
?
?
?
?
?
?
?
? ?
 = ?H – nFT
P
T
E
?
?
?
?
?
?
?
?
? ?H = – nFE + nFT
P
T
E
?
?
?
?
?
?
?
?
(iv) Entropy change inside the cell : We know that G = H – TS or ?G = ?H – T ?S  ..........(i) where
?G = Free energy change ; ?H = Enthalpy change and ?S = entropy change.
According to Gibbs Helmoholtz equation,
?G = ?H + T
P
T
G
?
?
?
?
?
?
?
? ?
............(ii)
From Eqs. (i) and (ii), we have
– T ?S = T
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = –
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = nF
P
T
E
?
?
?
?
?
?
?
?
where 
P
T
E
?
?
?
?
?
?
?
?
 is called temperature coefficient of cell e.m.f. .
Page 4


Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
Key Concepts
Electrochemical Cells
An electrochemical cell consists of two electrodes (metallic conductors) in contact with an electrolyte
(an ionic conductor).
An electrode and its electrolyte comprise an Electrode Compartment.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells which produce electricity as a result of a spontaneous cell reaction .
Note: In a galvanic cell, cathode is positive with respect to anode.
In a electrolytic cell, anode is made positive with respect to cathode.
GALVANIC CELL
This cell converts chemical energy into electrical energy.
Salt
Bridge
Catton
plug
Anode of Zn
(–ve electrode)
(Anodic Cell)
Cathode of Cu
(+ ve electrode)
(Cathodic Cell)
ZnSO
4
CuSO
4
Galvanic cell is made up of two half cells i.e., anodic and cathodic. The cell reaction is of redox kind.
Oxidation takes place at anode and reduction at cathode. It is also known as voltaic cell. It may be
represented as shown in Fig. Zinc rod immersed in  ZnSO
4
 behaves as anode and copper rod immersed
in CuSO
4
 behaves as cathode.
Oxidation takes place at anode.
Zn ? Zn
2+
 + 2e
–
 (loss of electron : oxidation)
Reduction takes place at cathode:
Cu
2+
 + 2e
–
 ? Cu(gain of electron ; reduction)
Overall process : Zn(s) + Cu
2+
 ? Cu(s) + Zn
2+
In galvanic cell like Daniell cell: electrons flow from anode (zinc rod) to the cathode (copper rod)
through external circuit; zinc dissolves as Zn
2+
 ; Cu
2+
 ion in the cathode cell picks up two electron and
become deposited at cathode.
REPRESENTATION OF A CELL (IUPAC CONVENTIONS):
Let us illustrate the convention taking the example of Daniel cell.
(i) Anodic half cell is written on left and cathodic half cell on right hand side.
Zn(s) |ZnSO
4
(sol)||CuSO
4
(sol)|Cu(s)
(ii) Two half cells are separated by double vertical lines: Double vertical lines indicate slat bridge or any
type of porous partition.
(iii) EMF (electromotive force) may be written on the right hand side of the cell.
(iv) Single vertical lines indicate the phase separation between electrode and electrolyte solution.
Zn|Zn
2+
||Cu
2+
|Cu
(v) Invert eletrodes are represented in the bracket
Zn|ZnSO
4
||H
+
|H
2
,Pt
RELATIONSHIP BETWEEN ?G AND ELECTRODE POTENTIAL
Let n, faraday charge is taken out from a cell of e.m.f. (E) then electrical work done by the cell may be
calculated as,
E L E C T R O C H E M I S T R Y
E L E C T R O C H E M I S T R Y
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
Work done = Charge × Potential = nFE
From thermodynamics we know that decrease in Gibbs free energy of a system is a measure of
reversible or maximum obtainable work by the system if there is no work due to volume expansion
? ? ?G = – nFE
Under standard state ?Gº = – nFEº ............(1)
(i) From thermodynamics we know, ?G = negative for spontaneous process. Thus from eq. (i) it is clear
that the EMF should be + ve for a cell process to be feasible or spontaneous.
(ii) When ?G = positive, E = negative and the cell process will be non spontaneous.
Reactions ?G E
Spontaneous (–) (+)
Non-spontaneous (+) (–)
Equilibrium 0 0
Standard free energy hcange of a cell may be calculated by electrode potential data.
Substituting the value of Eº (i.e., standard reduction potential of cathode-standard reduction potential
of anode) in eq. (i) we may get ?Gº.
CONCENPT OF ELECTROMOTIVE PORCE (EMF) OF A CELL
Electron flows from anode to cathode in external circuit due to a pushing effect called or electromotive
force (e.m.f.). EMF is called as cell potential. Unit of e.m.f. of cell is volt.
EMF of cell may be calculated as:
E
cell
 = reduction potential of cathode – Reduction potential of anode
Similarly, standard e.m.f. of the cell (Eº) may be calculated as
Eº
cell
 = Standard reduction potential of cathode – Standard reduction potential of anode.
SIGN CONVENTION OF EMF
EMF of cell should be positive other wise it will not be feasible in the given direction.
Zn|ZnSO
4
||CuSO
4
|Cu E = + 1.10 volt (Feasible)
Cu|CuSO
4
||ZnSO
4
|Zn E = – 1.10 volt (Not Feasible)
SALT BRIDGE
Two electrolyte solutions in galvanic cells are separated using salt bridge as represented in the Fig.
Salt bridge is a device to minimize or eliminate the liquid junction potential. Saturated solution of salt
like KCl, KNO
3
, NH
4
Cl and NH
4
NO
3
 etc. in agar-agar gel is used in salt bridge. Salt bridge contains high
concentration of ions viz. K
+
 and NO
3
– 
at the junction with electrolyte solution. Thus, salt bridge carries
whole of the current across the boundary; more over the K
+
 and NO
3
–
 ions have same speed. Hence,
salt bridge with uniform and same mobility of cations and anions completes the electrical circuit &
permits the ions to migrate.
NERNST EQUATION
Walter nernst derived a relation between cell potential and concentration or Reaction quotient.
?G = ?Gº + RT ln Q ..........(i)
where ?G and ?Gº are free energy and standard free energy change, 'Q' is reaction quotient.
?  – ?G = nFE and – ?Gº = nFEº
Thus from Eq. (i), we get – nFE = – nFEº + RT lnQ
At 25ºC, above equation may be written as E = Eº – Q log
n
0591 . 0
Where 'n' represents number of moles of electrons involved in process.
E, Eº are e.m.f. and standard e.m.f. of the cell respectively.
In general, for a redox cell reaction involving the transference of n electrons
aA + bB ? cC + dD, the EMF can be calculated as:
E
Cell
 = Eº
Cell 
 – 
b a
d c
] B [ ] A [
] D [ ] C [
log
n
0591 . 0
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
3
www.learnershabitat.ac.in
THERMODYNAMIC TREATEMENT OF NERNEST EQUATION
(i) Prediction and feasibility of spontaneity of a cell reaction.
Let us see whether the cell (Daniell) is feasible or not; i.e. whether Zinc will displace copper or not.
Zn | (s) | ZnSO
4
(sol) || CuSO
4
(sol) | Cu(s)
volt 34 . 0 E ; volt 76 . 0 E
0
Cu / Cu
0
Zn / Zn
2 2
? ? ? ?
? ?
0
Zn / Zn
0
Cu / Cu
0
cell
2 2
E E E
? ?
? ?
= 0.34 – (– 0.76) = + 1.10 volt
Since E
0
  = + ve, hence the cell will be feasible and zinc will displace copper from its salt solution. In
the other words zinc will reduce copper.
(ii) Determination of equilibrium constant : We know, that
E = E
0
 – Q log
n
0591 . 0
..........(1)
At equilibrium, the cell potential is zero because cell reactions are balanced, i.e. E = 0
? From Eq. (i), we have
0 = E
0
 –
eq
K log
n
0591 . 0
or K
eq
 = anti 
?
?
?
?
?
?
?
?
0591 . 0
nE
log
0
(iii) Heat of Reaction inside the cell : Let n Faraday charge flows out of a cell of e.m.f. E, then
– ?G  = nFE ..........(i)
Gibbs Helmholtz equation (from thermodynamics) may be given as,
 ?G = ?H + T 
P
T
G
?
?
?
?
?
?
?
? ?
..........(ii)
From Eqs. (i) and (ii), we have
– nFE = ?H + T 
? ?
P
T
nFE
?
?
?
?
?
?
?
? ?
 = ?H – nFT
P
T
E
?
?
?
?
?
?
?
?
? ?H = – nFE + nFT
P
T
E
?
?
?
?
?
?
?
?
(iv) Entropy change inside the cell : We know that G = H – TS or ?G = ?H – T ?S  ..........(i) where
?G = Free energy change ; ?H = Enthalpy change and ?S = entropy change.
According to Gibbs Helmoholtz equation,
?G = ?H + T
P
T
G
?
?
?
?
?
?
?
? ?
............(ii)
From Eqs. (i) and (ii), we have
– T ?S = T
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = –
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = nF
P
T
E
?
?
?
?
?
?
?
?
where 
P
T
E
?
?
?
?
?
?
?
?
 is called temperature coefficient of cell e.m.f. .
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
4
www.learnershabitat.ac.in
DIFFERENT TYPES OF HALF-CELLS AND THEIR REDUCTION POTENTIAL
(1) Gas - Ion Half Cell :
In such a half cell, an inert collector of electrons, platinum or grphite is in contact with gas and a
solution containing a specified ion. One of the most important gas-ion half cell is the hydrogen-gas-
hydrogen ion half cell. In this cell, purified H
2
 gas at a constant pressure is passed over a platinum
electrode which is in contact with an acid solution.
H
+
(aq) + e
–
  1/2 H
2
? ?
? ?
?
? ?
?
?
H
pH
log
1
0591 . 0
E E
2 / 1
2 0
H / H
H / H
2
2
(2) Metal-Metal Ion Half Cell :
This type of cell consist of a metal M is contact with a solution containing M
n+
 ions.
M
n+
 (aq) + ne
– 
  M(s)
n
n
0
M /M
n M /M
0.0591 1
E E log
n
M
?
?
?
? ?
? ?
? ?
(3) Metal-Insoluble Salt-Anion Half Cell :
In this half cell, a metal coated with its insoluble salt  is in contact with a solution containing the anion
of the insoluble salt. eg. Silver-Silver Chloride Half Cell :
This half cell is represented as Cl
–
/AgCl/Ag. The equilibrium reaction that occurs at the electrode is
AgCl(s) + e
–
  Ag(s) + Cl
–
(aq)
?
?
?
?
?
?
? ?
?
? ? Cl log
1
0591 . 0
E E
0
Ag / AgCl / Cl Ag / AgCl / Cl
, 
AgCl
] Cl [
) K (
log
1
059 . 0
E
sp 0
Ag / Ag ?
? ?
(4) Oxidation-reduction Half Cell :
This type of half cell is made by using an inert metal collector, usually platinum, immersed in a solution
which contains two ions of the same element in different states of oxidation. eg. Fe
2+
 - Fe
3+
 half cell.
Fe
3+
(aq) + e
–
  Fe
2+
(aq)
3 2
3 2
2
0
Fe /Fe 3
Fe /Fe
0.0591 [Fe ]
E E log
1
[Fe ]
? ?
? ?
?
?
? ?
CONCENTRATION CELL
The cells in which electrical current is produced due to transport of a substance from higher to lower
concentration. Concentration gradient may arise either in electrode material or in electrolyte. Thus
there are two types of concentration cell.
(i) Electrode Gas concentrtion cell :
Pt, H
2
(P
1
)|H
+
(C)|H
2
(P
2
), Pt
Here, hydrogen gas is ubbled at two different partial pressures at electrode dipped in the solution of
same electrolyte.
Cell Process : 1/2H
2
(p
1
) ? H
+
(c) + e
–
 (Anode process)
H (c) + e    ½ H (p )
+ –
 
2 2
½ H (p )      ½ H (p )
2 1 2 2
? E =  
2 / 1
1
2
p
p
log
F
RT 303 . 2
?
?
?
?
?
?
?
or E = –
2
1
p 2.303RT
log
2F p
? ?
? ?
? ?
? ?
? ?
? ?
, At 25ºC, E = – ?
?
?
?
?
?
1
2
p
p
log
F 2
059 . 0
For spontanity of such cell reaction p
1
 > p
2
(2) Electrolyte concentration cells:
Zn(s) |ZnSO
4
(C
1
)||ZnSO
4
(C
2
) | Zn(s)
In such cells, concentration gradient arise in electrolyte solutions. Cell process may be given as,
Page 5


Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
Key Concepts
Electrochemical Cells
An electrochemical cell consists of two electrodes (metallic conductors) in contact with an electrolyte
(an ionic conductor).
An electrode and its electrolyte comprise an Electrode Compartment.
Electrochemical Cells can be classified as:
(i) Electrolytic Cells in which a non-spontaneous reaction is driven by an external source of current.
(ii) Galvanic Cells which produce electricity as a result of a spontaneous cell reaction .
Note: In a galvanic cell, cathode is positive with respect to anode.
In a electrolytic cell, anode is made positive with respect to cathode.
GALVANIC CELL
This cell converts chemical energy into electrical energy.
Salt
Bridge
Catton
plug
Anode of Zn
(–ve electrode)
(Anodic Cell)
Cathode of Cu
(+ ve electrode)
(Cathodic Cell)
ZnSO
4
CuSO
4
Galvanic cell is made up of two half cells i.e., anodic and cathodic. The cell reaction is of redox kind.
Oxidation takes place at anode and reduction at cathode. It is also known as voltaic cell. It may be
represented as shown in Fig. Zinc rod immersed in  ZnSO
4
 behaves as anode and copper rod immersed
in CuSO
4
 behaves as cathode.
Oxidation takes place at anode.
Zn ? Zn
2+
 + 2e
–
 (loss of electron : oxidation)
Reduction takes place at cathode:
Cu
2+
 + 2e
–
 ? Cu(gain of electron ; reduction)
Overall process : Zn(s) + Cu
2+
 ? Cu(s) + Zn
2+
In galvanic cell like Daniell cell: electrons flow from anode (zinc rod) to the cathode (copper rod)
through external circuit; zinc dissolves as Zn
2+
 ; Cu
2+
 ion in the cathode cell picks up two electron and
become deposited at cathode.
REPRESENTATION OF A CELL (IUPAC CONVENTIONS):
Let us illustrate the convention taking the example of Daniel cell.
(i) Anodic half cell is written on left and cathodic half cell on right hand side.
Zn(s) |ZnSO
4
(sol)||CuSO
4
(sol)|Cu(s)
(ii) Two half cells are separated by double vertical lines: Double vertical lines indicate slat bridge or any
type of porous partition.
(iii) EMF (electromotive force) may be written on the right hand side of the cell.
(iv) Single vertical lines indicate the phase separation between electrode and electrolyte solution.
Zn|Zn
2+
||Cu
2+
|Cu
(v) Invert eletrodes are represented in the bracket
Zn|ZnSO
4
||H
+
|H
2
,Pt
RELATIONSHIP BETWEEN ?G AND ELECTRODE POTENTIAL
Let n, faraday charge is taken out from a cell of e.m.f. (E) then electrical work done by the cell may be
calculated as,
E L E C T R O C H E M I S T R Y
E L E C T R O C H E M I S T R Y
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
Work done = Charge × Potential = nFE
From thermodynamics we know that decrease in Gibbs free energy of a system is a measure of
reversible or maximum obtainable work by the system if there is no work due to volume expansion
? ? ?G = – nFE
Under standard state ?Gº = – nFEº ............(1)
(i) From thermodynamics we know, ?G = negative for spontaneous process. Thus from eq. (i) it is clear
that the EMF should be + ve for a cell process to be feasible or spontaneous.
(ii) When ?G = positive, E = negative and the cell process will be non spontaneous.
Reactions ?G E
Spontaneous (–) (+)
Non-spontaneous (+) (–)
Equilibrium 0 0
Standard free energy hcange of a cell may be calculated by electrode potential data.
Substituting the value of Eº (i.e., standard reduction potential of cathode-standard reduction potential
of anode) in eq. (i) we may get ?Gº.
CONCENPT OF ELECTROMOTIVE PORCE (EMF) OF A CELL
Electron flows from anode to cathode in external circuit due to a pushing effect called or electromotive
force (e.m.f.). EMF is called as cell potential. Unit of e.m.f. of cell is volt.
EMF of cell may be calculated as:
E
cell
 = reduction potential of cathode – Reduction potential of anode
Similarly, standard e.m.f. of the cell (Eº) may be calculated as
Eº
cell
 = Standard reduction potential of cathode – Standard reduction potential of anode.
SIGN CONVENTION OF EMF
EMF of cell should be positive other wise it will not be feasible in the given direction.
Zn|ZnSO
4
||CuSO
4
|Cu E = + 1.10 volt (Feasible)
Cu|CuSO
4
||ZnSO
4
|Zn E = – 1.10 volt (Not Feasible)
SALT BRIDGE
Two electrolyte solutions in galvanic cells are separated using salt bridge as represented in the Fig.
Salt bridge is a device to minimize or eliminate the liquid junction potential. Saturated solution of salt
like KCl, KNO
3
, NH
4
Cl and NH
4
NO
3
 etc. in agar-agar gel is used in salt bridge. Salt bridge contains high
concentration of ions viz. K
+
 and NO
3
– 
at the junction with electrolyte solution. Thus, salt bridge carries
whole of the current across the boundary; more over the K
+
 and NO
3
–
 ions have same speed. Hence,
salt bridge with uniform and same mobility of cations and anions completes the electrical circuit &
permits the ions to migrate.
NERNST EQUATION
Walter nernst derived a relation between cell potential and concentration or Reaction quotient.
?G = ?Gº + RT ln Q ..........(i)
where ?G and ?Gº are free energy and standard free energy change, 'Q' is reaction quotient.
?  – ?G = nFE and – ?Gº = nFEº
Thus from Eq. (i), we get – nFE = – nFEº + RT lnQ
At 25ºC, above equation may be written as E = Eº – Q log
n
0591 . 0
Where 'n' represents number of moles of electrons involved in process.
E, Eº are e.m.f. and standard e.m.f. of the cell respectively.
In general, for a redox cell reaction involving the transference of n electrons
aA + bB ? cC + dD, the EMF can be calculated as:
E
Cell
 = Eº
Cell 
 – 
b a
d c
] B [ ] A [
] D [ ] C [
log
n
0591 . 0
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
3
www.learnershabitat.ac.in
THERMODYNAMIC TREATEMENT OF NERNEST EQUATION
(i) Prediction and feasibility of spontaneity of a cell reaction.
Let us see whether the cell (Daniell) is feasible or not; i.e. whether Zinc will displace copper or not.
Zn | (s) | ZnSO
4
(sol) || CuSO
4
(sol) | Cu(s)
volt 34 . 0 E ; volt 76 . 0 E
0
Cu / Cu
0
Zn / Zn
2 2
? ? ? ?
? ?
0
Zn / Zn
0
Cu / Cu
0
cell
2 2
E E E
? ?
? ?
= 0.34 – (– 0.76) = + 1.10 volt
Since E
0
  = + ve, hence the cell will be feasible and zinc will displace copper from its salt solution. In
the other words zinc will reduce copper.
(ii) Determination of equilibrium constant : We know, that
E = E
0
 – Q log
n
0591 . 0
..........(1)
At equilibrium, the cell potential is zero because cell reactions are balanced, i.e. E = 0
? From Eq. (i), we have
0 = E
0
 –
eq
K log
n
0591 . 0
or K
eq
 = anti 
?
?
?
?
?
?
?
?
0591 . 0
nE
log
0
(iii) Heat of Reaction inside the cell : Let n Faraday charge flows out of a cell of e.m.f. E, then
– ?G  = nFE ..........(i)
Gibbs Helmholtz equation (from thermodynamics) may be given as,
 ?G = ?H + T 
P
T
G
?
?
?
?
?
?
?
? ?
..........(ii)
From Eqs. (i) and (ii), we have
– nFE = ?H + T 
? ?
P
T
nFE
?
?
?
?
?
?
?
? ?
 = ?H – nFT
P
T
E
?
?
?
?
?
?
?
?
? ?H = – nFE + nFT
P
T
E
?
?
?
?
?
?
?
?
(iv) Entropy change inside the cell : We know that G = H – TS or ?G = ?H – T ?S  ..........(i) where
?G = Free energy change ; ?H = Enthalpy change and ?S = entropy change.
According to Gibbs Helmoholtz equation,
?G = ?H + T
P
T
G
?
?
?
?
?
?
?
? ?
............(ii)
From Eqs. (i) and (ii), we have
– T ?S = T
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = –
P
T
G
?
?
?
?
?
?
?
? ?
or ?S = nF
P
T
E
?
?
?
?
?
?
?
?
where 
P
T
E
?
?
?
?
?
?
?
?
 is called temperature coefficient of cell e.m.f. .
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
4
www.learnershabitat.ac.in
DIFFERENT TYPES OF HALF-CELLS AND THEIR REDUCTION POTENTIAL
(1) Gas - Ion Half Cell :
In such a half cell, an inert collector of electrons, platinum or grphite is in contact with gas and a
solution containing a specified ion. One of the most important gas-ion half cell is the hydrogen-gas-
hydrogen ion half cell. In this cell, purified H
2
 gas at a constant pressure is passed over a platinum
electrode which is in contact with an acid solution.
H
+
(aq) + e
–
  1/2 H
2
? ?
? ?
?
? ?
?
?
H
pH
log
1
0591 . 0
E E
2 / 1
2 0
H / H
H / H
2
2
(2) Metal-Metal Ion Half Cell :
This type of cell consist of a metal M is contact with a solution containing M
n+
 ions.
M
n+
 (aq) + ne
– 
  M(s)
n
n
0
M /M
n M /M
0.0591 1
E E log
n
M
?
?
?
? ?
? ?
? ?
(3) Metal-Insoluble Salt-Anion Half Cell :
In this half cell, a metal coated with its insoluble salt  is in contact with a solution containing the anion
of the insoluble salt. eg. Silver-Silver Chloride Half Cell :
This half cell is represented as Cl
–
/AgCl/Ag. The equilibrium reaction that occurs at the electrode is
AgCl(s) + e
–
  Ag(s) + Cl
–
(aq)
?
?
?
?
?
?
? ?
?
? ? Cl log
1
0591 . 0
E E
0
Ag / AgCl / Cl Ag / AgCl / Cl
, 
AgCl
] Cl [
) K (
log
1
059 . 0
E
sp 0
Ag / Ag ?
? ?
(4) Oxidation-reduction Half Cell :
This type of half cell is made by using an inert metal collector, usually platinum, immersed in a solution
which contains two ions of the same element in different states of oxidation. eg. Fe
2+
 - Fe
3+
 half cell.
Fe
3+
(aq) + e
–
  Fe
2+
(aq)
3 2
3 2
2
0
Fe /Fe 3
Fe /Fe
0.0591 [Fe ]
E E log
1
[Fe ]
? ?
? ?
?
?
? ?
CONCENTRATION CELL
The cells in which electrical current is produced due to transport of a substance from higher to lower
concentration. Concentration gradient may arise either in electrode material or in electrolyte. Thus
there are two types of concentration cell.
(i) Electrode Gas concentrtion cell :
Pt, H
2
(P
1
)|H
+
(C)|H
2
(P
2
), Pt
Here, hydrogen gas is ubbled at two different partial pressures at electrode dipped in the solution of
same electrolyte.
Cell Process : 1/2H
2
(p
1
) ? H
+
(c) + e
–
 (Anode process)
H (c) + e    ½ H (p )
+ –
 
2 2
½ H (p )      ½ H (p )
2 1 2 2
? E =  
2 / 1
1
2
p
p
log
F
RT 303 . 2
?
?
?
?
?
?
?
or E = –
2
1
p 2.303RT
log
2F p
? ?
? ?
? ?
? ?
? ?
? ?
, At 25ºC, E = – ?
?
?
?
?
?
1
2
p
p
log
F 2
059 . 0
For spontanity of such cell reaction p
1
 > p
2
(2) Electrolyte concentration cells:
Zn(s) |ZnSO
4
(C
1
)||ZnSO
4
(C
2
) | Zn(s)
In such cells, concentration gradient arise in electrolyte solutions. Cell process may be given as,
Electrochemistry – Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
5
www.learnershabitat.ac.in
Zn(s) ? Zn
2+
(C
1
) + 2e
–
(Anodic process)
Zn (C ) + 2e     Zn(s)
2+
2
—
Zn (C )  
2+ 2+
2     1
Zn (C )
(Over all process)
? From Nernst equation, we have
?
?
?
?
?
?
? ?
2
1
C
C
log
F 2
RT 303 . 2
0 E
or
2
1
C 2.303RT
E log
2F C
? ?
?
? ?
? ?
For spontanity of such cell reaction, C
2
 > C
1
 .
COMMERCIAL VOLTAIC CELLS
Batteries can be blassified as primary and secondary. Primary batteries can not be returned to their
original state by recharging, so when the reactants are consumed, the battery is "dead" and must be
discarded. Secondary batteries are often called storage batteries or rechargeable batteries. The
reactions in these batteries can be reversed; thus, the batteries can be recharged.
PRIMARY BATTERIES:
DRY CELLS AND ALKALINE BATTERIES
Zinc serves as the anode, and the cathode is a
graphite rod placed down the center of the device.
These cells are often called "dry cells" because
there is no visible liquid phase. However, water is
present, so the cell contains a moist paste of
NH
4
Cl, ZnCl
2
 and MnO
2
. The moisture is necessary
because the ions present must be in a medium in
which they can migrate from one electrode to
the other. The cell generates a potential of 1.5 V
using the following half-reactins.
Paper or card board
salt bridge
Moist electrolyte paste
Anode
(usually zinc metal)
(–)
(+)
Cathode
Dry Cell battery
Cathode, reductions : 2NH
4
+
(aq) + 2e
–
 ? 2NH
3
(g) + H
2
(g)
Anode, oxidation : Zn(s) ? Zn
2+
(aq) + 2e
–
The two gases formed at the cathode will build up
pressure and could cause the cell to rupture. This
problem is avoided, however, by two other reactions
that take place in the cell. Ammonia molecules bind
to Zn
2+
 ions, and hydrogen gas is oxidized by MnO
2
to water.
Anode 
[Zinc inner case]
Cathode
[graphite rod]
Paste of MnO
NH Cl and 
carbon
2
3
Zn
2+
(aq) + 2NH
3
(g) + 2Cl
–
(aq) ? Zn(NH
3
)
2
Cl
2
(s)
2MnO
2
(s) + H
2
(g) ? ?Mn
2
O
3
(s) + H
2
O(l)
LeClanche cells were widely used because of their
low cost, but they have several disadvantages. If
current is drawn from the battery rapidly, the gaseous
products cannot be consumed rapidly enough, so the
cell resistance rises, and the voltage drops. In addition,
the zinc electrode and ammonium ions are in contact
in the cell, and these chemicals react slowly.
Recall that zinc reacts with acide to form hydrogen. The ammonium ion, NH
4
+
(aq), is a weak Bronsted
acid and reacts slowly with zinc. Because of this reaction, these voltaic cells connot be stored
indifinitely. When the zinc outer shell deteriorates, the battery can leak acid and perhaps damage the
appliance in which it is contained.
At the present time alkaline batteries are used the chemistry of alkaline cells is quite similar to that in
a LeClanche cell, except that the material inside the cell is basic (alkaline). Alkaline cells use the
oxidation of zinc and the reduction of MnO
2
 to generate a current, but NaOH or KOH is used in the cell
instead of the acidic salt NH
4
Cl.
Read More
Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!

Complete Syllabus of JEE

JEE

Content Category

Related Searches

MCQs

,

Electrochemistry JEE Notes | EduRev

,

shortcuts and tricks

,

Free

,

Sample Paper

,

Objective type Questions

,

Extra Questions

,

Electrochemistry JEE Notes | EduRev

,

pdf

,

mock tests for examination

,

ppt

,

study material

,

past year papers

,

Electrochemistry JEE Notes | EduRev

,

video lectures

,

Exam

,

Viva Questions

,

Semester Notes

,

Previous Year Questions with Solutions

,

practice quizzes

,

Summary

,

Important questions

;