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UNIT โ III
ELECTROCHEMISTRY
INTRODUCTION
Electrochemistry is a branch of chemistry that analyse the phenomena
resulting from combined chemical and electrical effects. It covers two
processes. The electrolytic processes where chemical changes occur
on the passage of electric current and galvanic or voltaic processes
where production of electrical energy results due to chemical
reactions. Energy storage as a natural process is as old as the universe
itself. The energy stored in stars such as sun is used directly as solar
energy. To balance the supply and demand of energy, man started
storing energy. The introduction of electricity and chemical fuels like
gasoline and LPG makes the energy storage an important factor in the
economic development of the nation.
Electrolytic and metallic conduction (Distinction)
S. No METALLIC CONDUCTION ELECTROLYTIC
CONDUCTION
1. It involves the flow of electrons
in a conductor.
It involves the movement of ions in
a solution.
2. It does not involve any transfer
of matter.
It involves transfer of electrolyte in
the form of ions.
3. Conduction decreases with the
increase in temperature.
Conduction increases with the
increase in temperature.
4. No change in chemical
properties of the conductor.
Chemical reactions occur at the two
electrodes.
Specific conductance:
Specific conductance can be defined as the conductance of a material of 1
cm in length and 1cm2 in area of cross section, otherwise, generally it is the conductance of 1cm3 of any material. Unit of specific conductance = ohm-1 cm-1 or mho cm-1.
Equivalent conductance:
Equivalent conductance is defined as the conducting power of all the ions
produced by dissolving 1 gm equivalent of an electrolyte in a given volume of
solution. The equivalent conductance is calculated from the relation,
๐ =1000 ๐
๐ถ
ฮป = equivalent conductance at a given concentration
k = specific conductance at that concentration
C = concentration of the solution in Normality.
Unit of equivalent conductance is mho cm2eq-1. Molar conductance:
Molar conductance is the conducting power of all the ions produced by one gram mole of an electrolyte in a given volume of the solution.
๐ = 1000๐
๐ถ
Where k is the specific conductance and C is the concentration of the solution in molarity. Measurement of conductance (Wheatstoneโs Bridge method)
Wheatstone Bridge is as shown in figure. The assembly consists of two resistance arms. In the first arm a resistance box is included where a standard resistance R can be introduced. The solution whose resistance and in turn the conductance is to be measured is taken in a conductivity cell and introduced in the second arm. The Wheatstone Bridge is fed with an alternating source of
current through a key. If a direct current is used, electrolysis will occur at the conductivity cell and this will produce a back emf due to the accumulation of products at the electrode. This also changes the concentration of the solution near the electrodes. The middle of the arm is connected to a head phone which in turn is connected to a sliding contact โJโ which can move along wire AB which is a uniform wire of high resistance. The length of the wire can be read out from the scale fixed below it. When the connections are made, a sound in the head phone is heard. On moving the sliding contact along the wire AB at a particular point a minimum sound is heard which indicates the null point. At this point,
๐ ๐ ๐๐
๐๐ฑ ๐ ๐๐ ๐
๐๐ฑ=
๐๐
๐๐๐๐ฑ = ๐ ร
๐๐
๐๐
The conductance of the electrolyte solution, ๐ถ =๐
๐๐ฑ
GALVANIC CELL
Galvanic cell or voltaic cell is called electrochemical cell. It is a
device which produces electrical energy from a redox chemical reaction. The
decrease in the potential energy of the chemical reaction appears in the form of
electrical energy. Redox reaction is a combination of both oxidation and
reduction half reactions.Example: Daniel cell. Daniel cell consists of two chambers. In the first compartment a zinc plate
is immersed in Zinc sulphate solution (zinc electrode) and copper plate is
immersed in copper sulphate solution(copper electrode) in the second
compartment. The two compartments are connected to each other by means of a
salt bridge and externally by connecting wires. Salt bridge is a bent glass tube
containing a gel of K2SO4 and both the ends are plugged with glass wool. This will
provide the electrical contact between the electrolytes. The zinc rod and copper
rod are connected to an ammeter using connecting wire to check the production
of electrical current. When both electrodes are connected a deflection is noted in
the ammeter which shows the production of the electric current in the circuit.
Mechanism: Reactions at anode:Oxidation half reactionZn(๐ ) โ Zn2+ + 2eโ
Reaction at cathode:Reduction half reactionCu2+ + 2eโ โ Cu(๐ ) The overall cell reaction is
Zn(๐ ) + Cu2+(๐๐) โ Zn2+
(๐๐) + Cu(๐ )
Redox reaction
The electrode at which oxidation occurs is called anode while electrode at
which reduction occurs is called cathode. In Daniel Cell zinc electrode is the negative terminal as it gives out electrons and copper electrode is positive terminal as it accepts electrons. Cell representation or cell notation: By convention we always represent anode at the left side and cathode at the right side. The above galvanic cell represented by
Zn/Zn2+(M)//Cu2+(๐)/Cu Reversible and irreversible cells A cell which follows the following conditions is a reversible cell. (i) If an external emf exactly equal to the emf of the cell is applied, the cell
reaction is stopped. (ii) If an external emf slightly greater than the emf of the cell is applied, the cell
reaction gets reversed. (iii) If an external emf slightly lesser than the emf of the cell is applied, the
current flows from the cell. If any cell does not follow these conditions then it is called as an irreversible cell.
Example: Reversible cell: Daniel cell Zn/Zn2+//Cu2+/Cu. Irreversible cell: Zn / H2SO4 / Ag
Zn + H2SO4 โ ZnSO4 + H2 โ
In this case, Zinc dissolves with the liberation of H2 gas.Hydrogen has already escaped; the cell reaction cannot be reversed Emf
The difference in potential which causes the flow of current from an electrode of higher potential to another electrode of lower potential in a galvanic cell is called the electromotive force (emf) of the cell. Unit of emf is volt.
By convention we compare the reduction potential of all electrodes with
respect to each other and the reduction potential of the right hand electrode should be always higher than the reduction potential of the left hand electrode The emf of the cell
๐ธ๐๐๐๐ = ๐ธ๐ โ ๐ธ๐ฟ Standard electrode potential
The tendency of the electrode to lose or gain electrons when it is in contact with 1M concentration of its own salt solution at 250C is called as the standard electrode potential. Redox Potential
The potential difference that arises due to the presence of ions of a substance in two oxidation states is called redox potential. For example, a platinum wire
immersed in a solution containing ๐น๐2+ ๐๐๐ ๐น๐3+ions. The reduction equation is,
๐น๐3+ + ๐โ โ ๐น๐2+ The redox potential of the electrode:
E๐๐๐๐๐ฅ = E๐๐๐๐๐ฅ0 โ
RT
nF๐๐
๐น๐2+
๐น๐3+
Nernst Equation for Electrode Potential Consider the following redox reaction,
Mn+ + neโ โ M
For such a redox reversible reaction, the free energy change (ฮG)and its equilibrium constant (K) are interrelated as,
โG = โRT๐๐๐ + RT๐๐[๐๐๐๐๐ข๐๐ก]
[๐๐๐๐๐ก๐๐๐ก]
โG = โG0 + RT๐๐[๐๐๐๐๐ข๐๐ก]
[๐๐๐๐๐ก๐๐๐ก]
(1) Since, โG0 = โRT๐๐๐
Where, ฮG0 = standard free energy change.
The above equation (1) is known as Vanโt Hoff isotherm. The decrease in free energy (-ฮG) in the above reaction will produce
electrical energy. In the cell, if the reaction involves the transfer of โnโ number of
electrons, then โnโFaraday of electricity will flow. If E is the emf of the cell, then
the total electrical energy nFE is produced in the cell. Where โFโ is the Faradayโs
constant which is equal to 96500 coulombs.
โG = โnFE (or) โโG0 = nF๐ธ0 (2) Where, E = Electrode potential
E0 = Standard electrode potential
โโG = decrease in free energy change
โโG0 = decrease in standard free energy change Comparing equations (1) and (2), It becomes
โ๐๐น๐ธ = โ๐๐น๐ธ0 + ๐ ๐๐๐[๐]
[๐๐+] (3)
Dividing the above equation (3) by - nF,
๐ธ = ๐ธ0 โ ๐ ๐
๐๐น๐๐
1
[๐๐+] (4)
Since the activity of solid metal [M] = 1.
In general, ๐ธ = ๐ธ0 +๐ ๐
๐๐น๐๐[๐๐+]
๐ธ = ๐ธ0 +2.303๐ ๐
๐๐น๐๐๐[๐๐+]
When R =8.314 J/K/Mol, F = 96500 coulombs, T = 298K (250C), the above
equation becomes,
๐ธ = ๐ธ0 +0.0591
๐๐๐๐[๐๐+]
This is known as the Nernst Equation. Calculate the reduction potential ofCu/Cu2+(0.5M)at 250E0 Cu2+/Cu = 0.337V. The reduction potential of copper electrode can be calculated as follows Given data: E0 Cu2+/Cu =0.337 V
Cu2+ + 2eโ โ Cu
ECu2+
Cu= E0
Cu2+
Cuโ
RT
nF๐๐
1
[Cu2+]
Substitute the values
ECu2+
Cu= 0.337 โ
8.314 ร 298 ร 2.303
2x96500๐๐๐
1
0.5
= 0.337 โ0.591
2๐๐๐2
= 0.337 โ 0.02955 ร 0.3010
๐๐๐ฎ๐+
๐๐ฎ= ๐. ๐๐๐๐ ๐
Electrochemical series
The reduction potential of most of the electrodes are found with respect to the standard hydrogen electrode. A series of various electrodes arranged in the increasing order of the standard reduction potentials is called electrochemical series or emf series. Significance:
(i) Relative Tendency of oxidation or reduction of electrodes can be predicted.
(ii) Predicting the feasibility of an electrochemical cell. Determination of emf (Poggendroffโs compensation principle)
The emf of a cell can be measured using poggendroffโs compensation principle. Here the emf of the cell is just opposed or balanced by an external emf (emf of a standard cell), so that no current flows in the circuit. A potentiometer is used to measure the emf of a cell.
The potentiometer consists of a uniform wire AB. A storage battery is connected
to the ends A and B of the wire through a rheostat (R). The cell of unknown
emf(x) is connected in the circuit by connecting its positive pole to A and the
negative pole is connected to a sliding contact D through a galvanometer G. The
sliding contact is freely moved along the wire ABtill no current flows through the
galvanometer. Then the distance AD is measured. The emf of unknown cell is
directly proportional to the distance AD.
E๐ฅ ฮฑ AD
Then the unknown cell(x) is replaced by a standard cell (c) in the circuit.
The sliding contact is again moved till there is null deflection in the galvanometer. Then the distance AD|is measured. The emf of standard cell Ec is directly proportional to the distance AD|
E๐ฅ ฮฑ AD|
Then the emf of the unknown cell can be calculated from the following equation. ๐๐๐ ๐๐ ๐ข๐๐๐๐๐ค๐ ๐๐๐๐ ๐ธ๐ฅ
๐๐๐ ๐๐ ๐ ๐ก๐๐๐๐๐๐ ๐๐๐๐ ๐ธ๐
=๐๐๐๐๐กโ ๐ด๐ท
length AD|
๐ธ๐ฅ
๐ธ๐
=๐ด๐ท
AD|
Therefore, emf of the unknown cell,
๐ธ๐ฅ =๐ด๐ท
AD|ร ๐ธ๐
Application of EMF
1. Solubility of sparingly soluble salts:
The solubility of a sparingly soluble salt like AgCl can be determined by measuring the emf of the cell. For this purpose a concentration cell containing two Ag electrodes is constructed.
Ag/AgCl ๐2, 0.01N KCl //0.01N ๐1 AgNO3/Ag
The cell can be constructed by placing one of the silver electrode in
contact with 0.01 N solution of silver nitrate and the other in contact with
0.01N solution of potassium chloride. The two solutions are connected by a
salt bridge. A drop of silver nitrate solution is added to the KCl solution.
The small amount of AgCl formed is sufficient to give its saturated solution.
The cell so constructed is a concentration cell with respect to silver ions.
The emf of the above cell is given by,
E = E0 +0.0591
๐๐๐๐
C1
C2
E = emf of the cell
E0 = emf of the cell
n = 1
C2 = 0.01 ๐ (๐๐ ๐ ๐ข๐๐)
C1 = ๐๐ ๐๐ ๐๐๐ข๐๐ ๐๐ข๐ก
From E value the value of c1 is calculated.
Ksp, the solubility product of AgCl = [ Ag+] [Clโ] = C1 x 0.01
Solubility of AgCl = [ K๐ ๐]1
2โ = (C1 x 0.01)1
2โ
2. Determination of thermodynamic functions
It is used to calculate H and S of a redox reaction taking place in a cell. From the relation
โ๐บ = โ๐๐น๐ธ
According to Gibbโs Helmholtz equation,
โ๐บ = โ๐ป + ๐ (๐(โ๐บ)
๐๐)
๐
We get,
โ๐๐น๐ธ = โ๐ป + ๐ (๐(โ๐๐น๐ธ)
๐๐)
๐
รท ๐กโ๐ ๐๐๐ข๐๐ก๐๐๐ ๐๐ฆ โ ๐๐น
๐ธ =โ๐ป
โ๐๐น+
๐๐๐ธ
๐๐
๐ธ โ๐๐๐ธ
๐๐=
โ๐ป
โ๐๐น
โ๐๐น (๐ธ โ๐๐๐ธ
๐๐) = โ๐ป
โ๐บ = โ๐ป โ ๐โ๐
Comparing equation (7) and (3),
โ๐โ๐ = ๐ [๐(โ๐๐น๐ธ)
๐๐]
๐
โ๐โ๐ = โ๐๐น๐ธ (๐๐ธ
๐๐)
๐
โ๐ = ๐๐น (๐๐ธ
๐๐)
๐
Calculate the emf of the following cell at 25โ Zn/Zn2+, 0.1M // ๐ถ๐ข2+0.5M /Cu, given that the standard emf of the cell is 1.1 V.
Cell reaction is ๐๐ + ๐ถ๐ข2+ โ ๐ถ๐ข + ๐๐2+ The Nernst equation for emf of the cell is
๐ธ = ๐ธ0 โ๐ ๐
๐๐น๐๐
[๐๐2+]
[๐ถ๐ข2+]
Substitute the values
๐ธ0 = 1.1๐ ๐ = 8.314
๐ = 25 = 273 + 25 = 298 ๐พ
๐ = 2
๐น = 96500 ๐๐๐ข๐๐๐๐๐
๐ธ = 1.1 โ8.314 ร 298 ร 2.303
2 ร 96500๐๐๐
0.1
0.5
๐ธ = 1.1 โ0.0591
2 ร 96500๐๐๐
0.1
0.5
๐ธ = 1.1207 ๐
TYPES OF ELECTRODES
i) Metal-Metal ion electrode.
eg: Zn-Zn 2+
Cu-Cu 2+
ii) Metal- Metal sparingly soluble salt
electrode
eg: Calomel electrode.
iii) Gas electrode.
eg: Hydrogen electrode.
iv) Redox electrode
Pt/Fe 2+, Fe 3+
STANDARD HYDROGEN ELECTRODE
Hydrogen electrode consists of a platinum foil that is connected to a platinum
wire and sealed in a glass tube. Hydrogen gas is passed through the side arm of
the glass tube. This electrode, when dipped in a 1N HCl and hydrogen gas at 1
atmospheric pressure is passed forms the standard hydrogen electrode. The
electrode potential of SHE is taken as zero It is represented as,
๐๐ก, ๐ป2 (1 ๐๐ก๐) /๐ป + (1๐); ๐ธ0 = 0๐
In a cell, when this electrode acts as anode, the electrode reaction can be written as
H2(g)โ 2 H+ + 2e-
When this electrode acts as cathode, the electrode reaction can be written as
2 H+ + 2e-โ H2(g)
Limitations (i) It
requires hydrogen gas and is difficult to set up and transport.
(ii) It requires considerable volume of test solution.
(iii) The solution may poison the surface of the platinum electrode.
(iv) The potential of the electrode is altered by changes in barometric pressure.
CALOMEL ELECTRODE
A calomel electrode is commonly used as a secondary reference electrode
for electrode potential measurements. A calomel electrode consists of a glass tube
with side tubes on both sides of it. The tube consists of a layer of pure mercury at
the bottom, over which mercurous chloride is placed. The remaining portion of
the tube is filled with saturated solution of KCl. The bottom of the tube is sealed.
A platinum wire is inserted for electrical contact.
This electrode is represented as
๐ป๐, ๐ป๐2๐ถ๐2(๐ ) //๐พ๐ถ๐ ๐ ๐๐๐ข๐ก๐๐๐
๐ป๐2๐ถ๐2 + 2๐โ โ 2๐ป๐(๐) + 2๐ถ๐โ( ๐๐) (๐ ๐๐๐ข๐๐ก๐๐๐)
2๐ถ๐โ (๐๐) + 2 ๐ป๐(๐) โ ๐ป๐2๐ถ๐2(๐) + 2๐โ (๐๐ฅ๐๐๐๐ก๐๐๐)
The standard electrode potential of saturated calomel electrode is 0.2422 vlots. The electrode is reversible with respect to chloride ions. The potential of the
electrode depends upon the concentration of KCl solution taken. Determination of pH using calomel electrode. The cell of following type is constructed.
๐๐ก / ๐ป2 (๐) (1 ๐๐ก๐), ๐ป+(๐ = ๐ข๐๐๐๐๐ค๐)// ๐พ๐ถ๐, ๐ป๐2๐ถ๐2 / ๐ป๐
๐ธ๐๐๐๐ = ๐ธ๐ โ ๐ธ๐ฟ
= 0.2422 โ ๐ธ๐ฟ
๐ธ๐ฟ = ๐ธ๐ฟ0 โ 0.0591๐๐๐ 1
[๐ป+]โ
๐ธ๐ฟ = 0.0591log[๐ป+]
๐ธ๐ฟ = โ0.0591๐๐ป
๐ธ๐๐๐๐ = 0.2422 + 0.0591 ๐๐ป
๐๐ป =๐ธ๐๐๐๐ โ 0.2422
0.0591
QUINHYDRONE ELECTRODE
Quinhydrone is a 1:1 molecular complex of quinone (represented by Q) and
hydroquinone (represented by H2Q). The electrode may be represented as Pt/Q, H2Q, H+. It is reversible with respect to H+ ions. The reduction reaction at this electrode may be represented as:
๐ + 2๐ป+ + 2๐โ โ ๐ป2๐ The emf of the electrode is 0.6994V Determination of pH using quinhydrone electrode The Quinhydrone electrode is coupled with a calomel electrode. The cell may be represented as
๐ป๐, ๐ป๐2๐ถ๐2/๐พ๐ถ๐// ๐ป + ๐, ๐๐ป2, ๐๐ก The emf of the cell is given as
๐ธ๐๐๐๐ = ๐ธ๐ โ ๐ธ๐ฟ = ๐ธ๐ โ ๐ธ๐๐๐๐
= ๐ธ๐ โ 0.2422
๐ธ๐ = ๐ธ0๐ โ
0.0591
2๐๐๐
[๐๐ป2]
[๐ป+]2[๐]
๐ธ๐ = ๐ธ0๐ โ
0.0591
2๐๐๐
1
[๐ป+]2
๐ธ๐ = ๐ธ0๐ +
0.0591
2๐๐๐[๐ป+]2
๐ธ๐ = ๐ธ0
๐ + 0.0591 ๐๐๐[๐ป+]
๐ธ๐ = ๐ธ0๐ โ 0.0591 ๐๐ป
๐ธ๐ = 0.6694 โ 0.0591 ๐๐ป
๐ธ๐๐๐๐ = 0.6694 โ 0.0591 ๐๐ป โ 0.2422
๐๐ป =0.6694 โ 0.2422โ๐ธ๐๐๐๐
0.0591
๐๐ป =0.4572โ๐ธ๐๐๐๐
0.0591
ENERGY STORAGE DEVICES
Battery storage technology provides the
most wide spread satisfactory method as storage device in the current
scenario. Electrochemical batteries are of several types. Depending on
the type of battery their usage also varies. There is a growing trend
and need for the rechargeable batteries.
Lead storage battery Lead storage battery is also known as lead โ acid battery.
A battery or storage cell is a combination of two or more cells arranged in series
or parallel in which electrical energy is stored as chemical energy. When
required, this chemical energy can be reconverted into electrical energy. Ex:
Lead โ storage cell.
Lead storage cell consists of a lead anode and lead dioxide cathode (lead dioxide
is packed on a metal plate) immersed in 20% sulphuric acid solution. Actually a
number of lead and lead oxide plates are arranged alternatively, with insulating
material in between them. The lead storage battery consists of six identical cells
joined together in series.
Discharging
When the storage cell acts as a voltaic cell it is said to be discharging. In this
process sulphuric acid is consumed and water is generated. The following
reaction takes place during discharging.
Anode: Pb(s)+SO4(aq)2-โ PbSO4(s)+2e-
Cathode: ๐๐๐2(๐ ) + 4๐ป(๐๐)+ + ๐๐4(๐๐)
2โ + 2๐โ โ ๐๐๐๐4(๐)+ 2๐ป2๐(๐)
Cell reaction:
๐๐(๐ ) + ๐๐๐2(๐)+ 4๐ป(๐๐)
+ + 2๐๐4(๐๐)2โ โ 2๐๐๐๐4(๐)
+ 2๐ป2๐(๐) + ๐๐๐๐๐๐ฆ
Recharging:
The lead storage battery is rechargeable. This is done by applying a voltage
slightly higher than the voltage of the battery, across the electrodes. In this
process the sulphuric acid consumed during discharging is reformed. The
recharging involves exactly the reverse process of the normal cell reaction. The
recharging reactions are
Cathode: ๐๐๐๐4(๐ ) + 2๐โ โ ๐๐(๐ ) + ๐๐4(๐๐)2โ
Anode:๐๐๐๐4(๐ ) + 2๐ป2๐(๐) โ ๐๐๐2๐+ 4๐ป(๐๐)
+ + ๐๐4(๐๐)2โ + 2๐โ
Cell reaction:
2๐๐๐๐4(๐) + 2๐ป2๐(๐) + ๐๐๐๐๐๐ฆ โ ๐๐(๐ ) + ๐๐๐2(๐ ) + 4๐ป(๐๐)
+ + 2๐๐4(๐๐)2โ
The emf of each cell is 2 volts. In Automobiles six such cells are connected in series to form a battery with an emf of 12 volts. Nickel Cadmium cell or Nicad Battery Like a lead storage cell this is a rechargeable battery. Anode : Cd (metal) Cathode : NiO2 Electrolyte : KOH Nicad battery consists of a cadmium anode and a metal grid containing a paste of NiO2 acting as cathode. KOH solution is the electrolyte. It gives a constant voltage of 1.4 V. The cell can be represented as Cd/Cd(OH)2 // KOH(aq)/NiO2/Ni Working: Discharging (or current production) At Anode : Cadmium is oxidized to Cd2+ and this combines with OH- ions to form Cd(OH)2. In this reaction two electrons are released at the anode. Cd(s) + 2 OH- โ Cd(OH)2 + 2e-
At Cathode: NiO2 receives the two electrons from the circuit and undergoes reduction
(Ni4+ โ Ni2+) . The Ni2+ ions combines with OH- ions to form Ni(OH)2 NiO2(s) + 2 H2O + 2e- โ Ni(OH)2 + 2 OH- Overall cell reaction is Cd(s) + NiO2(s) + 2 H2O โ Cd(OH)2 + Ni(OH)2 As no gaseous products are produced, the cell reaction is completely reversible. Recharging: Like the lead โacid battery, Nicad battery can be recharged by sending the current in the opposite direction. The electrode reaction gets reversed and as a result Cd metal gets deposited on the anode and NiO2 at the cathode. Charging Reaction: Cd(OH)2 + Ni(OH)2 + energy โ Cd(s) + NiO2(s) + 2 H2O Advantages:
1. It is lighter and smaller.
2. It has longer life than lead storage battery.
3. Like a dry cell, it can be sealed inside a container.
4. Gives a constant voltage of 1.4 V Disadvantage:
1. It is more expensive than lead-acid battery.
Uses:
It is used in calculators, Electronic camera flashes, rechargeable flash lights s
and cordless electronic appliances.
Lithium Battery Lithium battery is a rechargeable battery. It is considered to be the cell of the future. It is a solid state battery. Lithium is the anode and TiS2 (Titanium
disulfide) is the cathode. The electrolyte is solid made of polymer. The polymer allows the passage of ions but not the electrons.
Anode Solid Electrolyte Cathode
Li(s) โ Li+ + e- TiS2(s) + e- โ TiS2
-
Cell Reactions: Anode : Li(s) โ Li+ + e- Cathode: TiS2(s) + e- โ TiS2
-
Overall reaction: Li(s) + TiS2(s) โ Li+ + TiS2
-
The cell is rechargeable and produces a cell voltage of 3V. Advantages:
1. Lithium is a light weight metal (7g). One mole of material is enough to produce one mole of electrons. It is rechargeable.
2. Cell voltage is high (3V). 3. The constituents are solids and there is no risk of leakage. 4. Battery can be made into various shapes and sizes.
Hydrogen โ Oxygen fuel cell
Fuel cells are galvanic cells in which chemical energy of fuels is directly converted into electrical energy. It is an energy conversion device or electricity generator. Unlike a storage cell it cannot be reversed. It is similar to an electric generator set which converts chemical energy of fuels into electricity.
Example: Hydrogen โ Oxygen fuel cell. In this cell combustion of H2 in O2 takesplace to from water
2๐ป2 + ๐2 โ 2๐ป2๐
Description:
The cell consists of two electrodes made of porous graphite,
impregnated with platinum catalyst. These electrodes are placed in aqueous
KOH or NaOH solution. Oxygen and hydrogen gases are continuously fed in it
at high pressure of 50 atmospheres. The reaction taking place is,
Anode 2๐ป2 + 4๐๐ปโ โ 4๐ป2๐ + 4๐โ Cathode ๐2 + 4๐โ + 2๐ป2๐ โ 4๐๐ปโ Cell reaction 2๐ป2 + ๐2 โ 2๐ป2๐
The emf of the cell is found to be 1V . Advantages of fuel cells
1. Very efficient and converts 75% of chemical energy to electrical energy. 2. The cell is compact and easy to maintain. 3. The fuels hydrogen and oxygen are easily available and cheap 4. The product is only H2O vapours and hence does not cause pollution. 5. Because of its light weight it is used in space vehicles.
Disadvantages
1. The catalyst is easily losing their activity. 2. The catalyst is very expensive.
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