Title: ELECTROCHEMISTRY
1ELECTROCHEMISTRY
2- References
- Engg.Chemistry by Jain and Jain
- Engg.Chemistry by Dr. R.V.Gadag and Dr.
A.Nithyananda Shetty - Principles of Physical Chemistry by Puri and
Sharma
3- Electrochemistry is a branch of chemistry which
deals with the properties and behavior of
electrolytes in solution and inter-conversion of
chemical and electrical energies.
4- An electrochemical cell can be defined as a
single arrangement of two electrodes in one or
two electrolytes which converts chemical energy
into electrical energy or electrical energy
into chemical energy. - It can be classified into two types
- Galvanic Cells.
- Electrolytic Cells.
5Galvanic Cells A galvanic cell is an
electrochemical cell that produces electricity as
a result of the spontaneous reaction occurring
inside it. Galvanic cell generally consists of
two electrodes dipped in two electrolyte
solutions which are separated by a porous
diaphragm or connected through a salt bridge.
To illustrate a typical galvanic cell, we can
take the example of Daniel cell.
6Daniel Cell.
7 At the anode Zn ? Zn 2 2e- At the
cathode Cu 2 2e- ? Cu Net reaction
Zn(s)Cu 2 (aq)? Zn 2 (aq) Cu(s)
8An electrolytic cell is an electro chemical cell
in which a non- spontaneous reaction is driven by
an external source of current although the
cathode is still the site of reduction, it is
now the negative electrode whereas the anode, the
site of oxidation is positive.
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10Representation of galvanic cell.
- Anode Representation
- ZnZn2 or Zn Zn2
- Zn ZnSO4 (1M) or Zn ZnSO4 (1M)
- Cathode Representation
- Cu2/Cu or Cu2 Cu
- Cu2 (1M) Cu or CuSO4(1M)/Cu
- Cell Representation
- Zn ZnSO4 (1M) CuSO4(1M)/Cu
11Liquid Junction Potential.
- Difference between the electric potentials
developed in the two solutions across their
interface . - Ej Ø soln, R - Ø soln,L
- Eg Contact between two different
electrolytes (ZnSO4/ CuSO4). - Contact between same electrolyte of
different concentrations(0.1M HCl / 1.0 M HCl).
12Salt Bridge.
- The liquid junction potential can be reduced (to
about 1 to 2 mV) by joining the electrolyte
compartments through a salt bridge. -
13Function Of Salt Bridge.
- It provides electrolytic contact between the two
electrolyte solutions of a cell. - It avoids or at least reduces junction potential
in galvanic cells containing two electrolyte
solutions in contact.
14Emf of a cell.
- The difference of potential, which causes a
current to flow from the electrode of higher
potential to one of lower potential. - Ecell Ecathode- Eanode
- The E Cell depends on
- the nature of the electrodes.
- temperature.
- concentration of the electrolyte
solutions.
15 - Standard emf of a cell(Eo cell) is defined as the
emf of a cell when the reactants products of
the cell reaction are at unit concentration or
unit activity, at 298 K and at 1 atmospheric
pressure. -
16- The emf cannot be measured accurately using a
voltmeter -
- As a part of the cell current is drawn,thereby
causing a change in the emf. -
- As a part of the emf is used to overcome the
internal resistance of the cell.
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18- The emf of the cell Ex is proportional to the
length AD. - Ex a AD
- The emf of the standard cell Es is proportional
to the length AD1. - Es a AD1
-
- Ex - AD
- Es AD1
-
- Ex AD x Es
- AD1
-
19Standard Cell.
- It is one which is capable of giving constant and
reproducible emf. - It has a negligible temperature coefficient of
the emf. - The cell reaction should be reversible.
- It should have no liquid junction potential.
- Eg Weston Cadmium Cell. The emf of the cell is
1.0183 V at 293 K and 1.0181 V at 298 K.
20Sealed wax
Cork
Soturated solution of CdSO4.8/3H2O
CdSO4.8/3H2O crystals
Paste of Hg2SO4
Cd-Hg 12-14 Cd
Mercury, Hg
21- Cell representation
- Cd-Hg/Cd2// Hg2SO4/Hg
- At the anode
- Cd (s) ? Cd2 2e-
- At the cathode
- Hg2SO4(s) 2e- ? 2 Hg (l) SO42-(aq)
- Cell reaction
- Cd Hg22 ? Cd2 2Hg
22Origin of single electrode potential.
- Consider Zn(s)/ ZnSO4
- Anodic process Zn(s) ? Zn2(aq)
- Cathodic process Zn2(aq) ? Zn(s)
- At equilibrium Zn(s) ? Zn2(aq)
-
- Metal has net negative charge and solution has
equal positive charge leading to the formation of
an Helmholtz electrical layer.
23Single electrode potential.
- Electric layer on the metal has a potential Ø
(M). - Electric layer on the solution has a potential
Ø (aq) - Electric potential difference between the
electric double layer existing across the
electrode /electrolyte interface of a single
electrode or half cell.
24De-electronation
Electronation
Helmholtz double layer
25MEASUREMENT OF ELECTRODE POTENTIAL.
- It is not possible to determine experimentally
the potential of a single electrode. - It is only the difference of potentials between
two electrodes that we can measure by combining
them to give a complete cell. - By arbitrarily fixing the potential of reversible
hydrogen electrode as zero it is possible to
assign numerical values to potentials of the
various other electrodes.
26Sign Of Electrode Potential.
- The electrode potential of an electrode
- Is positive If the electrode reaction is
reduction when coupled with the standard
hydrogen electrode - Is negative If the electrode reaction is
oxidation when coupled with standard hydrogen
electrode. According to latest accepted
conventions, all single electrode potential
values represent reduction tendency of
electrodes.
27- when copper electrode is combined with SHE,
copper electrode acts as cathode and undergoes
reduction hydrogen electrode acts as anode. - H2(g) ? 2H 2e- (oxidation)
- Cu2 2e- ? Cu (reduction)
- Hence electrode potential of copper is assigned
a positive sign. Its standard electrode
potential is 0.34 V.
28- When zinc is coupled with S.H.E. zinc electrode
acts as anode and hydrogen electrode acts as
cathode. - Zn ? Zn2 2e-
- 2H 2e-? H2.
- Hence, electrode potential of zinc is negative.
The standard electrode potential of zinc
electrode is -0.74 V.
29Nernst Equation.
- It is a quantitative relationship between
electrode potential and concentration of the
electrolyte species. - Consider a general redox reaction
- Mn(aq) ne- ? M(s) ----(1)
- We know that, ?G -nFE ----- (2)
- ?Go-nFEo-----(3)
- ?G ?Go RT ln K
-
30- ?G ?Go RT ln K
- ?G ?Go RT lnM/Mn-----(4)
- -nFE -nFEo RT ln M/Mn----(5)
- E Eo RT/nF ln 1/Mn------(6)
- EEo- 2.303 RT/nF log 1/Mn---(7)
- At 298K,
- E Eo-0.0592/n log 1/Mn-------(8)
31problems
- 1. A galvanic cell consists of copper plate
immersed in 10 M solution of CuSO4 and iron plate
immersed in 1M FeSO4 at 298K. If E0cell0.78 V,
write the cell reaction and calculate E.M.F. of
the cell.
32- Solution
- Cell reaction
- Fe Cu2 ? Fe2 Cu
- ECell E0Cell-0.0592/2 log Fe2 /Cu2
- ECell 0.78 0.0296 log 10/1
- 0.8096V
33- Calculate E.M.F. of the zinc silver cell at
25C when Zn2 1.0 M and Ag 10 M
(E0cell1.56V at 25C). Write the cell
representation and cell reaction
34- Solution
- Cell representation
- Zn/ Zn2((1M)//Ag(10M) /Ag
- Cell reaction
- Zn 2Ag ? Zn2 2Ag
- ECell E0Cell-0.0592/2 log Zn2 /Ag2
- ECell 1.56 0.0592 log 10/1.0
- 1.6192 V
35- The emf of the cell
- Mg Mg 2 (0.01M) Cu 2 /Cu is measured to
be 2.78 V at 298K. The standard eletrode
potential of magnesium electrode is -2.37 V.
Calculate the electrode potential of copper
electrode
36- Cell reaction
- Mg Cu2 ? Mg2 Cu
- E Eo-0.0592/n log 1/Mn
- EMg EoMg-0.0592/2 log 1/Mg2
- -2.4291V
- EcellECu-EMg
- 2.78 ECu--2.429
- ECu 2.78-2.429
- 0.3509 V
37- The emf of the cell
- Cu Cu 2 (0.02M) Ag /Ag is measured to be
0.46 V at 298K. The standard eletrode potential
of copper electrode is 0.34 V. Calculate the
electrode potential of silver. electrode
38Energetics of Cell Reactions.
- Net electrical work performed by the cell
reaction of a galvanic cell - W QE ------(1)
- Charge on 1mol electrons is
F(96,500)Coulombs. - When n electrons are involved in the cell
reaction, - the charge on n mole of electrons nF
39- Q nF
- Substituting for Q in eqn (1)
- W nFE ----------(2)
- The cell does net work at the expense of
- ?G accompanying. ?G -nFE
- ?G nFE
-
40- From Gibbs Helmholtz equation.
- ?G ?H T d(?G)/ dTP ------- (2)
- -nFE ?H nFT (d E/ dT)P
- ?H nFT (d E/ d T)P nFE
- ?H nFT(d E/ dT)P E
- We know that, d (?G)/ dTP - ?S
- ?S nF (dE/ dT)P
-
41- Problem Emf of Weston Cadmium cell is 1.0183 V
at 293 K and 1.0l81 V at 298 K. - Calculate ?G, ?H and ?S of the cell reaction
at 298 K. - Solution- ?G ?G - n FE
- n 2 for the cell reaction F 96,500 C
E 1.0181 V at 298 K - ?G -2 x 96,500 x 1.0181 J -196.5 KJ
42- ?H ?H nF T (dE /dT)P E
- (dE/dT)p 1.0181 1.0183 / 298-293 -0.0002 /
5 - -0.00004VK-1
- T 298 K
- ?H 2 x 96,500 298 x (-0.00004) 1.0181)
- -198. 8 KJ
- ?S ?S nF (dE / dT) P
- 2 x 96,500 x (0-00004) -7.72JK-1
43Classification of Electrodes.
- Gas electrode ( Hydrogen electrode).
- Metal-metal insoluble salt (Calomel electrode).
- Ion selective electrode.(Glass electrode).
44Gas electrode.
- It consists of gas bubbling over an inert metal
wire or foil immersed in a solution containing
ions of the gas. - Standard hydrogen electrode is the primary
reference electrode, whose electrode potential at
all temperature is taken as zero arbitrarily.
45Construction.
46- Representation Pt,H2(g)/ H
- Electrode reaction H e- ?1/2 H2(g)
- The electrode reaction is reversible as it can
undergo either oxidation or reduction depending
on the other half cell. - If the concentration of the H ions is 1M,
pressure of H2 is 1atm at 298K it is called as
standard hydrogen electrode (SHE).
47Applications.
- To determine electrode potential of other unknown
electrodes. - To determine the pH of a solution.
- EEo- 2.303 RT/nF log H21/2/H
- 0 -0.0591 log 1/H
- -0.0591pH.
- Cell Scheme Pt,H2,H(x)// SHE
48- The emf of the cell is determined.
- E (cell) E (c) E(A)
-
- 0 (- 0.0592 pH)
- E (cell) 0.0592 pH
- pH E(cell)/ 0.0592
-
49Limitations.
- Constuction and working is difficult.
- Pt is susceptible for poisoning.
- Cannot be used in the presence of oxidising
agents.
50Metal metal salt ion electrode.
- These electrodes consist of a metal and a
sparingly soluble salt of the same metal dipping
in a solution of a soluble salt having the same
anion. - Eg Calomel electrode.
- Ag/AgCl electrode.
51Construction.
52- Representation Hg Hg2Cl2 / KCl
- It can act as anode or cathode depending on the
nature of the other electrode. - As anode 2Hg 2Cl- ? Hg2Cl2 2e-
- As Cathode Hg2Cl2 2e- ? 2Hg 2 Cl-
-
53- E Eo 2.303 RT/2F log Cl-)2
-
- Eo -0.0591 log Cl- at 298 K
- Its electrode potential depends on the
concentration of KCl. - Conc. of Cl- Electrode potential
- 0.1M
0.3335 V - 1.0 M 0.2810 V
- Saturated 0.2422 V
-
54Applications.
- Since the electrode potential is a constant it
can be used as a secondary reference electrode. - To determine electrode potential of other unknown
electrodes. - To determine the pH of a solution.
- Pt,H2/H(X) // KCl,Hg2Cl2,Hg
- pH E(cell) 0.2422/ 0.0592
55Ion Selective Electrode.
- It is sensitive to a specific ion present in an
electrolyte. - The potential of this depends upon the activity
of this ion in the electrolyte. - Magnitude of potential of this electrode is an
indicator of the activity of the specific ion in
the electrolyte. - This type of electrode is called indicator
electrode. -
56 57- Scheme of typical pH glass electrode
- a sensing part of electrode,
- a bulb made from a specific glass
- sometimes electrode contain small amount
- of AgCl precipitate inside the glass
- electrode
- 3 internal solution, usually 0.1M HCl for pH
- electrodes
- 4.internal electrode, usually silver chloride
- electrode or calomel electrode
- 5.body of electrode, made from non-
- conductive glass or plastics.
- 6.reference electrode, usually the same type
- as 4
- 7.junction with studied solution, usually made
from ceramics or capillary with asbestos or
quartz fiber.
58- The hydration of a pH sensitive glass membrane
involves an ion-exchange reaction between singly
charged cations in the interstices of the glass
lattice and protons from the solution. - H Na Na H
- Soln. glass soln.
glass - Eg Eog 0.0592 pH
59Electrode Potential of glass electrode.
- The overall potential of the glass electrode
has three components - The boundary potential Eb,
- Internal reference electrode potential Eref.
- Asymetric potential Easy.- due to the difference
in response of the inner and outer surface of the
glass bulb to changes in H. - Eg Eb Eref. Easy.
60- Eb E1 E2
- RT/nF ln C1 RT/nF ln C2
- L RT/nF ln C1
- Eb depends upon H
- Eg Eb EAg/AgCl Easy.
- L RT/nF ln C1 EAg/AgCl Easy.
- Eog RT/nF ln C1
- Eog 0.0592 log H
- Eg Eog 0.0592 pH.
61- Advantages
- It can be used without interference in solutions
containing strong oxidants, strong reductants,
proteins, viscos fluids and gases as the glass is
chemically robust. - It can be used for solutions having pH values 2
to 10. With some special glass (by incorporation
of Al2O3 or B2O3) measurements can be extended to
pH values up to 12. - It is immune to poisoning and is simple to
operate - The equilibrium is reached quickly the response
is rapid
62- 5. It can be used for very small quantities of
the solutions. Small electrodes can be used for
pH measurement in one drop of solution in a tooth
cavity or in the sweat of the skin (micro
determinations using microelectrodes) - 6. If recently calibrated, the glass electrode
gives an accurate response. - 7. The glass electrode is much more convenient to
handle than the inconvenient hydrogen gas
electrode.
63- Disadvantages
- The bulb of this electrode is very fragile and
has to be used with great care. - The alkaline error arises when a glass electrode
is employed to measure the pH of solutions having
pH values in the 10-12 range or greater. In the
presence of alkali ions, the glass surface
becomes responsive to both hydrogen and alkali
ions. Low pH values arise as a consequence and
thus the glass pH electrode gives erroneous
results in highly alkaline solutions.
64- The acid error results in highly acidic solutions
(pH less than zero)Measured pH values are high. - Dehydration of the working surface may cause
erratic electrode performance. It is crucial
that the pH electrode be sufficiently hydrated
before being used. When not in use, the
electrode should be stored in an aqueous
solution because once it is dehydrated, several
hours are required to rehydrate it fully.
65- As the glass membrane has a very high electrical
resistance (50 to 500 mO), the ordinary
potentiometer cannot be used for measurement of
the potential of the glass electrode. Thus
special electronic potentiometers are used which
require practically no current for their
operation.
66- Standardization has to be carried out frequently
because asymmetry potential changes gradually
with time. Because of an asymmetry potential, not
all glass electrodes in a particular assembly
have the same value of EoG . For this reason, it
is best to determine EoG for each electrode
before use. - The commercial verson is moderately expensive
67Limitations.
- The bulb is very fragile and has to be used with
great care. - In the presence of alkali ions, the glass surface
becomes responsive to both hydrogen and alkali
ions. Measured pH values are low. - In highly acidic solutions (pH less than zero)
measured pH values are high. - When not in use, the electrode should be stored
in an aqueous solution.
68Applications.
- Determination of pH
- Cell SCE Test solution / GE
- E cell Eg Ecal.
- E cell Eog 0.0592 pH 0.2422
-
- pH Eog -Ecell Ecal. / 0.0592
69Problems
- The cell SCE ?? (0.1M) HCl ? AgCl(s) /Ag
- gave emf of 0.24 V and 0.26 V with buffer
having pH value 2.8 and unknown pH value
respectively. Calculate the pH value of unknown
buffer solution. Given ESCE 0.2422 V
70- Eog 0.0592pH Ecell Ecal.
- 0.0592x2.8 0.24 0.2422
- 0.648 V
- pH Eog -Ecell Ecal. / 0.0592
- 0.648 -0.26-0.2422/0.0592
- 2.46
71CONCENTRATION CELLS.
- Two electrodes of the same metal are in
contact with solutions of different
concentrations. - Emf arises due to the difference in
concentrations. - Cell Representation
- M/ MnC1 Mn/MC2
72Construction.
73- At anode Zn ?Zn2(C1) 2e-
- At cathode Zn2(C2) 2e-? Zn
- Ecell EC-EA
- E0 (2.303RT/ nF)logC2-
E0(2.303RT/nF)logC1 - Ecell (0.0592/n) log C2/C1
- Ecell is positive only if C2 gt C1
74- Anode - electrode with lower electrolyte
concentration. - Cathode electrode with higher electrolyte
concentration. - Higher the ratio C2/C1 higher is the emf.
- Emf becomes zero when C1 C2.
75Problems
- Zn/ZnSO4(0.001M)ZnSO4(x)/Zn is 0.09V at 25C.
Find the concentration of the unknown solution.
76- Ecell 0.0592/n log C2/C1
- 0.09 (0.0592/2) log ( x / 0.001)
- x 1.097M
77- 2. Calculate the valency of mercurous ions
- with the help of the following cell.
- Hg/ Mercurous Mercurous /Hg
- nitrate (0.001N) nitrate (0.01N)
when the emf observed at 18 C is 0.029 V - Ecell(2.303 RT/nF) log C2/C1
78- Ecell(2.303 RT/nF) log C2/C1
- 0.029 2.303RT/n) log (0.01/0.001)
- 0.029 0.057 x 1/ n
- n 0.057/0.029 2
- Valency of mercurous ions is 2, Hg2 2
79- Assignment
- Answer the following questions
- 1.Distinguish between electrolytic and galvanic
cells. - 2.Explain the origin of electrode potential.
What are the sign conventions for electrode
potential? - 3.Give reasons for the following.
- i) The glass electrode changes its emf over a
period - of time.
- ii) KCl is preferred instead of NaCl as an
electrolyte - in the preparation of salt bridge
- 4. What is meant by a standard cell? Give an
example
80- 5. Quote any four limitations of glass electrode
- 6.Define liquid junction potential. How it can be
eliminated or minimized? - 7.Derive Nernst equation for the single electrode
potential. - 8.Describe potentiometric determination of emf
of a cell. - 9.Writ e construction and working of Calomel
Electrode - 10.What are concentration cells? Show that emf of
concentration cell becomes zero at a certain
point of its working. -