Free Chemistry NEET Notes for Electrochemistry


Free Chemistry Notes for Electrochemistry (NEET)


Power up your NEET Exam prep with Chapter-Wise Chemistry Notes for Electrochemistry at Onlineneetcoaching.in. Crafted by Brilliant Tutorial experts with 25 years of NEET insights.

- Electrochemistry -

Important Chemistry Notes for IITJEE/NEET Preparation- Electrochemistry


Class 12 Chemistry is a broad subject that requires a thorough understanding of the concepts and topics covered. As a result, we have provided Chemistry Notes PDF for IIT JEE/NEET to students and NEET aspirants. Electrochemistry Class 12 Notes PDF for NEET can be found below. With the help of detailed syllabus,  Class 12 students learn what they need to study, how many points are assigned to each unit, and how much time is allotted for each unit. As a result, they can easily plan their study schedule.

Check out the Electrochemistry Class 12 notes PDF for your IIT JEE/NEET Preparation based on the IIT JEE/NEET Chemistry Syllabus. The Electrochemistry notes PDF is designed in such a way that it is very useful for IIT JEE/NEET aspirants.


ELECTROCHEMISTRY

ELECTROLYTES AND ELECTROLYSIS

A substance which decomposes as a result of the passage of electric current is called an electrolyte and phenomenon of decomposition by electricity is called electrolysis.

ELECTROLYTIC CELL

The apparatus used to carry out electrolysis is known as electrolytic cell. The main features of electrolytic cell are :
Feature
Cathode
Anode
Sign
Negative since attached to negative end of external battery
Positive since attached to positive end of external battery
Direction of movement of electrons
Into the cell
Out of the cell
Direction of movement of ions
Cations
Anions
Half-Reaction
Reduction
Oxidation

Oxidation : Loss of electrons is called oxidation.
neet chemistry notes for Electrochemistry class 12

Reduction : Gain of electrons is called reduction.
neet chemistry notes for Electrochemistry class 12

FARADAY'S LAWS OF ELECTROLYSIS

FIRST LAW

The amount of the substance deposited or liberated at an electrode is directly proportional to the quantity of electricity passed through an electrolyte
neet chemistry notes for Electrochemistry class 12
I = Current strength in amp., t = time in sec.,
Q = Quantity of charge (coulombs)
Z is a constant known as Electrochemical equivalent
When I = 1 amp., t = 1 sec. then Q = 1 coulomb, then W = Z.

Thus, Electrochemical equivalent is the amount of the substance deposited or liberated by 1 ampere current passing for 1 second (i.e. 1 coulomb, I × t = Q)

SECOND LAW

When the same quantity of electricity is passed through different electrolytes, the amounts of the products obtained at the electrodes are directly proportional to their chemical equivalents or equivalent weights. Thus
neet chemistry notes for Electrochemistry class 12
Hence, Electrochemical equivalent ∝ Equivalent wt.

ONE FARADAY
neet chemistry notes for Electrochemistry class 12  F = Known as Faraday's constant
One Faraday is the quantity of charge carried by one mole of electrons
neet chemistry notes for Electrochemistry class 12

AMOUNT OF THE SUBSTANCE 'm' LIBERATED OR DEPOSITED AT AN ELECTRODE
neet chemistry notes for Electrochemistry class 12

COMPARISON OF ELECTROLYTIC AND METALLIC CONDUCTORS

Property
Metallic Conductor
Electrolytic Conductor
Conduction
Due to movement of electrons
Due to movement of ions
Transfer of matter
No transfer of matter
There is transfer of matter
Chemical property
No change in chemical property
There is chemical change
Resistance
Resistance is offered by atomic Kernels
Resistance is offered by interionic attractions, viscosity of solvent
Temperature
Decreases with increase of temperature
Increases with increase of temperature
Magnitude
Generally high
Generally low
Solvation
No solvation
Ions are solvated

FACTORS AFFECTING ELECTROLYTIC CONDUCTION

Please refer to the properties discussed above viz. viscosity, temperature, solvation of ions etc.

OHM'S LAW

The potential difference across the conductor is directly proportional to the current flowing through it.
Potential difference ∝ Current
neet chemistry notes for Electrochemistry class 12
where R is a constant and known as resistance of the conductor
neet chemistry notes for Electrochemistry class 12  
Law is applicable to metallic as well as electrolytic conductors. R is expressed in Ohms also designated as neet chemistry notes for Electrochemistry class 12.

RESISTANCE (R)

It offers obstruction to the passage of electric current. It is directly proportional to the length (l) and inversely proportional to the area of cross section (a) of the conductor.
neet chemistry notes for Electrochemistry class 12
where r is a constant, called resistivity or specific resistance.
when l = 1 cm and a = 1 cm2 then neet chemistry notes for Electrochemistry class 12 = R

SPECIFIC RESISTANCE (ρ)

The resistance offered by 1 cm3 of the conductor is known as specific resistance.
Units of neet chemistry notes for Electrochemistry class 12

CONDUCTANCE (C)

It is ease of flow of electric current through the conductor and reciprocal of resistance R.
C = neet chemistry notes for Electrochemistry class 12 units ohm–1, mho or neet chemistry notes for Electrochemistry class 12–1

SPECIFIC CONDUCTIVITY 𝛋 (KAPPA)

It is the reciprocal of specific resistance.
neet chemistry notes for Electrochemistry class 12
Hence specific conductivity 𝛋 (Kappa) = Conductance × cell constant
Units  = Ohm–1 cm–1 = neet chemistry notes for Electrochemistry class 12–1 cm–1 = S cm–1 (neet chemistry notes for Electrochemistry class 12–1 = S, Siemens)

EFFECT OF DILUTION ON CONDUCTANCE

The number of current carrying particles or ions per ml decrease on dilution and specific conductivity, being the conductance of one centimetre cube of solution, decreases with dilution.

CELL CONSTANT AND ITS DETERMINATION

The quantity neet chemistry notes for Electrochemistry class 12 is known as cell constant. Its direct measurement is very difficult. It is measured by using standard solution of KCl whose conductivity is known at different concentrations and temperatures.
Hence cell constant = neet chemistry notes for Electrochemistry class 12

EQUIVALENT CONDUCTIVITY neet chemistry notes for Electrochemistry class 12

The conductivity of all the ions produced when 1 gram equivalent of an electrolyte is dissolved in V ml of solution is known as equivalent conductivity.
or
The conductance of a solution containing 1 gm. equivalent of electrolyte placed between two large electrodes one centimetre apart.

RELATION BETWEEN EQUIVALENT CONDUCTIVITYneet chemistry notes for Electrochemistry class 12 AND SPECIFIC CONDUCTIVITY (neet chemistry notes for Electrochemistry class 12)

neet chemistry notes for Electrochemistry class 12
where V is the volume in cm3 or ml containing 1 gev. of the electrolyte.

UNITS OF EQUIVALENT CONDUCTIVITY

Ohm–1 cm2 eq–1 or neet chemistry notes for Electrochemistry class 12–1 cm2 eq–1 or S cm2 eq–1

EFFECT OF DILUTION ON EQUIVALENT CONDUCTIVITY

Since the degree of dissociation of the electrolyte increases with dilution, the equivalent conductivity also increases. The increase is more in case of weak electrolytes than strong electrolytes.
The equivalent conductivity increases and specific conductivity decreases with dilution.

IMPORTANCE OF EQUIVALENT CONDUCTIVITY

It helps to compare the conductivity of different electrolytes, since solutions of different electrolytes having 1 gram equivalent each in the same volume will have the same total charge of electricity. One mole of NaCl yields ions carrying 2 faradays of electricity and one mole of Na2SO4 yields ions carrying 4 faradays of electricity. But 1 gram equivalent of each will produce ions carrying 2 faradays of electricity.
Equivalent wt. of Na2SO4 = neet chemistry notes for Electrochemistry class 12
Hence conductivity of different electrolytes can only be compared if their solutions have equivalent concentrations.

MOLAR CONDUCTIVITY (neet chemistry notes for Electrochemistry class 12)

The conductivity of all the ions produced when 1 mole of an electrolyte is dissolved in V ml of solution is known as molar conductivity.
or
The conductance of a solution containing one gram-mole of electrolyte placed between two large electrodes one centimeter apart.

RELATION BETWEEN MOLAR CONDUCTIVITY (neet chemistry notes for Electrochemistry class 12) & SPECIFIC CONDUCTIVITY (neet chemistry notes for Electrochemistry class 12)

neet chemistry notes for Electrochemistry class 12
where V is the volume in cm3 or ml containing 1 mole of the electrolyte.

UNITS OF MOLAR CONDUCTIVITY (neet chemistry notes for Electrochemistry class 12)

Ohm–1 cm2 mol–1  or neet chemistry notes for Electrochemistry class 12–1 cm2 mol–1 or S cm2 mol–1

EFFECT OF DILUTION ON MOLAR CONDUCTIVITY

Since the degree of dissociation of electrolyte increases with dilution,  neet chemistry notes for Electrochemistry class 12 also increases but less in case of strong electrolytes and more in case of weak electrolytes.

FACTORS AFFECTING THE MOLAR CONDUCTIVITY

  • Nature of electrolyte : The strong electrolytes like KNO3, KCl, NH4NO3, HCl, H2SO4, NaOH, KOH etc are completely ionised in aqueous solution and have high values of  neet chemistry notes for Electrochemistry class 12. The weak electrolytes are ionised to lesser extent in aqueous solution and have lower values of neet chemistry notes for Electrochemistry class 12.
  • Concentration of the solution : The concentrated solutions of strong electrolytes have significant inter-ionic attractions, which reduce the speed of ions and lower the value of Lm. The dilution decreases such attractions and increases the value of neet chemistry notes for Electrochemistry class 12. The limiting value neet chemistry notes for Electrochemistry class 12 (neet chemistry notes for Electrochemistry class 12 the molar conductivity at zero concentration or at infinite dilution) can be obtained by extrapolating the graph.
neet chemistry notes for Electrochemistry class 12
In case of weak electrolytes, the degree of ionisation increases which increases the value of neet chemistry notes for Electrochemistry class 12. The limiting value  cannot be obtained by extrapolating the graph. The limiting value,neet chemistry notes for Electrochemistry class 12 , for weak electrolytes is obtained by Kohlrausch law.
  • Temperature : The increase of temperature decreases interionic attractions, solvation of ions, viscosity and increases kinetic energy of ions and their speed. Thusneet chemistry notes for Electrochemistry class 12increases with temperature.
  • Viscosity of solvent : The higher the value of viscosity the lower is the value ofneet chemistry notes for Electrochemistry class 12.
  • Dielectric constant of solvent : The higher the value of dielectric constant of solvent, the more is the value ofneet chemistry notes for Electrochemistry class 12. The former decreases interionic attractions.

DEBYE-HUCKEL ONSAGER EQUATION

Relation between molar conductivity neet chemistry notes for Electrochemistry class 12 at a particular concentration and molar conductivity neet chemistry notes for Electrochemistry class 12 at infinite dilution is given by
neet chemistry notes for Electrochemistry class 12where b is constant.
It depends upon nature of solvent and temperature.

DEGREE OF DISSOCIATION (neet chemistry notes for Electrochemistry class 12)

For weak electrolytes
neet chemistry notes for Electrochemistry class 12

KOHLRAUSCH'S LAW

At infinite dilution the molar conductivity of an electrolyte is the sum of the ionic conductivities of the cations and anions. e.g. for AxBy.
neet chemistry notes for Electrochemistry class 12
It is important to note that the source of cations (A+) and anions (B) may be electrolyte itself or any other electrolyte.
neet chemistry notes for Electrochemistry class 12  (CH3COOH) =neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

IONIC MOBILITY

The distance travelled by an ion per second under a potential gradient of 1 volt per cm is known as ionic mobility.
Ionic Mobility U = neet chemistry notes for Electrochemistry class 12

POTENTIAL GRADIENT

Potential difference applied at the electrodes divided by the distance between the electrodes is known as Potential gradient.

GALVANIC CELL, ELECTROCHEMICAL CELL OR VOLTAIC CELL

A device which converts chemical energy into electrical energy is called Galvanic, Electrochemical or voltaic cell.

General representation of an electrochemical cell
neet chemistry notes for Electrochemistry class 12


Cathode
Anode
Sign
Positive due to consumption of electrons
Negative due to release of electrons
Reaction
Reduction
Oxidation
Movement of electrons
Into the cell
Out of cell

Other features of the electrochemical cell are
  • There is no evolution of heat
  • The solutions remain neutral on both side
  • The reaction and flow of electrons stops after some time.

DANIELL CELL

An electrochemical cell of Zinc and copper metals is known as Daniell Cell. It is represented as
neet chemistry notes for Electrochemistry class 12
Zn (s) | Zn++ (aq.) | | Cu2+ (aqs.) | Cu (s)
LHS oxidation : neet chemistry notes for Electrochemistry class 12
RHS reduction : neet chemistry notes for Electrochemistry class 12
Overall reaction : neet chemistry notes for Electrochemistry class 12
By convention cathode is represented on the RHS and anode on the LHS. Two vertical lines represent the salt bridge.

FUNCTION OF SALT BRIDGE

  • Completes the circuit and allows the flow of current
  • It maintain the electrical neutrality on both sides.

Salt-bridge generally contains solution of strong electrolyte such as KNO3, KCl etc. KCl is preferred because the transport numbers of K+ and Cl are almost same.

ELECTRODE POTENTIAL

When an electrode is in contact with the solution of its own ions in a half cell, it has a tendency to lose or gain electrons which is known as electrode potential. It is expressed in volts.

OXIDATION POTENTIAL

The tendency to lose electrons in above case is known as Oxidation potential.
Oxidation potential of a half-cell is inversely proportional to the concentration of ions in the solution.

REDUCTION POTENTIAL

The tendency to gain electrons in above case is known as reduction potential.
It is not possible to determine the absolute value of electrode potential. For this a reference electrode is required. The electrode potential is only the difference of potentials between two electrodes that we can measure by combining them to give a complete cell.

STANDARD REDUCTION POTENTIAL (neet chemistry notes for Electrochemistry class 12 )

According to latest convention the electrode potential is always represented as reduction potential. If its value is negative, it means electrode has oxidation potential. The standard conditions are 1 molal solution, 298K temperature and 1atm. pressure. According to IUPAC convention, the reduction potential alone be called as the electrode potential unless it is specifically mentioned.

REFERENCE ELECTRODE

The following electrodes are used as reference electrodes for determining the standard reduction potentials.

STANDARD HYDROGEN ELECTRODE (SHE)

Standard hydrogen electrode (SHE) also known as Normal Hydrogen Electrode (NHE), consists of platinum wire, carrying platinum foil coated with finely divided platinum black. The wire is sealed into a glass tube, placed in a beaker containing 1M HCl. The hydrogen gas at 1 atm pressure is bubbled through the solution at 298K. Half cell is Pt, H2(1 atm)/H+ (1M)
In SHE, at the surface of platinum, either of the following reactions can take place
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12 Reduction
neet chemistry notes for Electrochemistry class 12 Oxidation
The electrode potential of SHE has been fixed as zero.
All other single electrode potentials are referred to as potentials on hydrogen scale.

Drawbacks of SHE
  • It is difficult to maintain H2 at 1 atm. pressure.
  • It is difficult to maintain H+ ion concentration at 1M
  • The platinum electrode is easily poisoned by traces of impurities.
Hence calomel and silver chloride electrodes are conveniently used as reference electrodes.

CALOMEL ELECTRODE (Hg, Hg2Cl2, KCl)

Half cell is Hg, Hg2Cl2(s) KCl(solution)
Oxidation :
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Reduction :
neet chemistry notes for Electrochemistry class 12
Overall reaction :
neet chemistry notes for Electrochemistry class 12
EoRed = 0.2422 V

SILVER-SILVER SALT ELECTRODE

Half cell is Ag, AgCl(s), Cl (KCl or HCl)
Oxidation :
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Reduction :
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

ELECTROMOTIVE FORCE (EMF) OF A CELL

It is the difference between the electrode potentials of two half-cells and cause of flow of current from electrode at higher potential to electrode at lower potential. It is also the measure of free energy change. Standard EMF of a cell
neet chemistry notes for Electrochemistry class 12

ELECTROCHEMICAL SERIES

It is the arrangement of electrodes in the increasing order of their Standard Reduction potential . neet chemistry notes for Electrochemistry class 12

APPLICATIONS OF ELECTROCHEMICAL SERIES

  • The lower the value of E°, the greater the tendency to form cation
neet chemistry notes for Electrochemistry class 12
  • Replacement (or evolution) of H2 from hydro acids by metals.
    Metals placed below hydrogen in E.C.S. can not replace hydrogen from dil. acids but metals placed above hydrogen can replace hydrogen from dil. acids.
Ca + H2SO4 CaSO4 + H2  
possible
neet chemistry notes for Electrochemistry class 12
Cu + H2SO4 CuSO4 + H2  
not possible
neet chemistry notes for Electrochemistry class 12
  • Metals placed above hydrogen evolve H2 with H2O or steam, but metals placed below hydrogen cannot.
  • Oxides of metals placed above hydrogen are not reduced by H2 but oxides of iron and metals placed below iron are reduced by H2
SnO, PbO, Fe3O4, CuO are reduced with H2
CaO, MgO, K2O are not reduced with H2
  • Oxides of Ag and metals below Ag are decomposed
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
  • Reducing character decreases down the series.
  • Reactivity decreases down the series.
  • Feasibility of redox reaction : A redox reaction can occur if the species losing the electrons lie above that which gains the electrons.
  • Determination of emf : Emf is the difference of reduction potentials of two half cells.
Eemf = ERHS – ELHS
If the value of e.m.f. is positive the reaction can take place spontaneously, otherwise not.

FUNCTIONING OF ELECTROCHEMICAL CELL

With the passage of time the electrode potential of the cathode decreases and that of anode increases the difference becomes zero, the driving force of emf becomes zero and reaction stops.

TYPES OF ELECTRODES AND HALF-CELLS

GAS ELECTRODES OR GAS ION HALF CELLS

Half-cell Reaction
Oxidation
neet chemistry notes for Electrochemistry class 12 : neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12  2H+ + 2e
neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12  H2(g)
Reduction
neet chemistry notes for Electrochemistry class 12 : neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12 neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12

METAL-METAL ION ELECTRODE OR HALF-CELL

A metal rod dipped in the solution of its own ions
Zn++(aq) / Zn  : neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Ag+(aq) / Ag  :neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Cu++(aq) / Cu  :neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

METAL-METAL INSOLUBLE SALT-SALT ANION

Cl(aq) / AgCl / Ag: neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Br–(aq) / AgBr / Ag : neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Cl(aq) / Hg2Cl2(s)  / Hg (Pt) : neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
OH(aq) / Cu(OH)2(s) / Cu : neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12:neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

REDOX ELECTRODES HALF CELLS

An inert metal such as platinum wire dipped in a solution of ions of the same metal in different oxidation states
Fe3+(aq), Fe2+(aq) / Pt :neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Sn4+(aq), Sn2+(aq) / Pt: neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Cr2O72–(aq), Cr3+(aq), H+(aq) / Pt :neet chemistry notes for Electrochemistry class 12
MnO4 (aq); Mn2+(aq), H+(aq) / Pt :neet chemistry notes for Electrochemistry class 12

NERNST EQUATION FOR ELECTRODE POTENTIAL

The relationship between the concentration of ions and electrode potential is given by Nernst equation.
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
If we write the electrode reaction, in general, as neet chemistry notes for Electrochemistry class 12
The potential of electrode is given by
neet chemistry notes for Electrochemistry class 12
or neet chemistry notes for Electrochemistry class 12

NERNST EQUATION FOR CELL POTENTIAL

Consider a general cell reaction involving n electrons.
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Concentration of solids and liquids is taken as unity, concentration of ions Mol L–1 and concentration of gases as partial pressures in Atmosphere.

NERNST EQUATION AND EQUILIBRIUM CONSTANT (Kc)

When Ecell drops to zero the concentration of ions will be equilibrium concentrations. We have for general cell reaction
neet chemistry notes for Electrochemistry class 12 neet chemistry notes for Electrochemistry class 12  cC + dD
neet chemistry notes for Electrochemistry class 12
 neet chemistry notes for Electrochemistry class 12   [Since at equilibrium Ecell = 0]
neet chemistry notes for Electrochemistry class 12 at 298K.

RELATION BETWEEN ELECTRICAL ENERGY AND GIBBS FREE ENERGY (neet chemistry notes for Electrochemistry class 12G)

If n is the number of electrons liberated (or taken up) in a particular cell reaction, then n faradays (nF) of electricity will be generated in the complete reaction. If E is the EMF of the cell, then
Electrical energy supplied by the cell = nFE

According to Gibbs and Helmholtz, the decrease of free energy (–neet chemistry notes for Electrochemistry class 12G) of the reaction occuring in the cell is equal to electrical energy
Hence, –neet chemistry notes for Electrochemistry class 12G = nFE
The standard free energy andneet chemistry notes for Electrochemistry class 12 are related as
neet chemistry notes for Electrochemistry class 12

RELATIONSHIP BETWEEN FREE ENERGY CHANGE AND EQUILIBRIUM CONSTANT :

ΔGo = -2.303RTlog KC

CONCENTRATION CELLS

ELECTRODE CONCENTRATION CELLS

Two electrodes of different concentrations are dipped in the same solution of electrolyte e.g.
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

ELECTROLYTE CONCENTRATION CELLS

Electrodes are the same but electrolyte solutions have different concentrations e.g.
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12

APPLICATIONS OF THE CONCENTRATION CELLS

  • Determination of valency
  • Determination of solubility of sparingly soluble salts.
  • Determination of transition point.

OVERVOLTAGE

The difference between the voltage at which a gas is actually evolved and theoretical value at which it ought to have been evolved during electrolysis is known as overvoltage.

REVERSIBLE CELL

When the cell reaction can be stopped or reversed by applying an emf exactly equal to or infinitesimally greater than that of the cell, it is called reversible cell.

EXAMPLES OF REVERSIBLE ELECTRODES

METAL - METAL ION ELECTRODE
Metal rod dipped into a solution of its own ions.
neet chemistry notes for Electrochemistry class 12
The negative electrode (electrode reaction involving oxidation) increases M+ ions in solution
The positive electrode (electrode reaction involving reduction) decreases M+ ions in solution.
Thus, electrode is reversible with respect to M+ ions

GAS ELECTRODES
neet chemistry notes for Electrochemistry class 12
  • Hydrogen electrode : Pt, H2(g), H+
Reaction
neet chemistry notes for Electrochemistry class 12
The electrode is reversible with respect to H+ ions
  • Chlorine electrode : Pt, Cl2(g), Cl
Reaction  
neet chemistry notes for Electrochemistry class 12
The electrode is reversible with respect to Cl ion.

METAL - METAL SALT ION ELECTRODE
  • Calomel Electrode : Hg, Hg2Cl2(s) ; KCl(solution)
Oxidation :
neet chemistry notes for Electrochemistry class 12
Oxidation decreases concentration of chloride ions.
Reduction :
neet chemistry notes for Electrochemistry class 12
Reduction increases the concentration of chloride ions
The electrode is reversible with respect to Cl ions.
  • Silver-Silver Chloride Electrode : Ag, AgCl(s), Cl(KCl or HCl)
Oxidation :
neet chemistry notes for Electrochemistry class 12
Oxidation decreases the concentration of Cl ions.
Reduction :
neet chemistry notes for Electrochemistry class 12
Reduction increases the concentration of Cl

OXIDATION - REDUCTION ELECTRODES
Such electrodes are set up by inserting unattackable metal (eg: Platinum) into a solution of ions in different oxidation states. The metal acquires a potential due to tendency of ions in one oxidation state to change into another stable oxidation state.
Electrode reaction :
neet chemistry notes for Electrochemistry class 12

IRREVERSIBLE CELL

When the cell reaction cannot be stopped or reversed it is called irreversible cell e.g. cell of Zn and Ag electrodes immersed in solution of H2SO4.

CRITERIA OF THE FORMATION OF PRODUCTS IN ELECTROLYSIS (PREFERENTIAL DISCHARGE THEORY)

Ions present in large excess conduct electricity and ions having lower discharge potentials are discharged at respective electrodes.

Electrolysis of aqueous NaOH
neet chemistry notes for Electrochemistry class 12 Completely dissociated
neet chemistry notes for Electrochemistry class 12 Slightly dissociated
At cathode : neet chemistry notes for Electrochemistry class 12
At anode  : neet chemistry notes for Electrochemistry class 12
Current is carried by Na+ and OH ions.

Electrolysis of aqueous H2SO4
neet chemistry notes for Electrochemistry class 12 Completely dissociated
neet chemistry notes for Electrochemistry class 12 Slightly dissociated
At cathode : neet chemistry notes for Electrochemistry class 12
At anode : neet chemistry notes for Electrochemistry class 12

Electrolysis of aqueous NaCl
neet chemistry notes for Electrochemistry class 12Completely dissociated
neet chemistry notes for Electrochemistry class 12 Slightly dissociated
At cathode : neet chemistry notes for Electrochemistry class 12
At anode : neet chemistry notes for Electrochemistry class 12
Although the oxidation potential of OH is more than Cl yet Cl2  is formed at anode due to overvoltage.

Electrolysis of aqueous CuSO4 using Pt electrodes
neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
At cathode   neet chemistry notes for Electrochemistry class 12
At anode      neet chemistry notes for Electrochemistry class 12

Electrolysis of CuSO4 solution using copper electrodes
neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12     
At Cathode neet chemistry notes for Electrochemistry class 12
At Anode neet chemistry notes for Electrochemistry class 12
Oxidation potential neet chemistry notes for Electrochemistry class 12

FACTORS GOVERNING THE DISCHARGE POTENTIAL

  • Position in the electrochemical series
  • Concentration of ions in the solution
  • Nature of electrodes

Discharge potential of some ions
neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12

TRANSPORT NUMBER

The fraction of the total current carried by each ion is called its transport number.
The amount of electricity carried by a particular ion  speed of particular ion.
Transport number of the cation nc =  neet chemistry notes for Electrochemistry class 12= neet chemistry notes for Electrochemistry class 12
Transport number of anion na = neet chemistry notes for Electrochemistry class 12
Uc = speed of cation and Ua = speed of anion.
Further na + nc = 1

SOME COMMERCIAL CELLS AND THEIR TYPES

They are broadly classified into two groups.

PRIMARY CELLS

They cannot be recharged and used again.
DRY CELL OR LECLANCHE CELL
Anode : Zinc Container
Cathode : graphite rod surrounded by MnO2 powder
Electrolyte : paste of NH4Cl + ZnCl2
Cathode Reaction : MnO2 + neet chemistry notes for Electrochemistry class 12 + neet chemistry notes for Electrochemistry class 12e  MnO(OH) + NH3
Anode Reaction : neet chemistry notes for Electrochemistry class 12
Net Reaction : Zn2+ + 2NH3neet chemistry notes for Electrochemistry class 12 [Zn(NH3)2]2+
Cell Potential :  1.25V to 1.5V

MERCURY CELL
Anode : Zn-Hg amalgam
Cathode : paste of (HgO + C)
Electrolyte : moist paste of KOH-ZnO
Cathode Reaction :neet chemistry notes for Electrochemistry class 12
Anode Reaction : neet chemistry notes for Electrochemistry class 12
Net Reaction  : neet chemistry notes for Electrochemistry class 12
Cell Potential : 1.30V

SECONDARY CELLS

It can be recharged and can be used again and again.
LEAD STORAGE BATTERY
Anode : Spongy lead
Cathode : grid of lead packed with PbO2
Electrolyte : 38% H2SO4 by mass
Anode Reaction : neet chemistry notes for Electrochemistry class 12
Cathode Reaction :neet chemistry notes for Electrochemistry class 12
Net Reaction :neet chemistry notes for Electrochemistry class 12
When recharged the cell reactions are reversed.

NICKEL-CADMIUM STORAGE CELL
Anode : Cadmium
Cathode : metal grid containing NiO2
Electrolyte : KOH solution
Anode Reaction :   neet chemistry notes for Electrochemistry class 12
Cathode Reaction : neet chemistry notes for Electrochemistry class 12
Net Reaction :  neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12
Cell Potential : 1.4 V

FUEL CELLS

They convert chemical energy into electrical energy and the reactants are continuously fed and products are removed.

HYDROGEN-OXYGEN FUEL CELL

Electrodes are made of porous graphite impregnated with catalyst (Pt, Ag or a metal oxide).
neet chemistry notes for Electrochemistry class 12
Electrolyte-aqueous solution of KOH or NaOH
Oxygen and hydrogen are continuously fed into the cell.
Oxidation (half-cell  reaction) : neet chemistry notes for Electrochemistry class 12
Reduction (half-cell reaction) : neet chemistry notes for Electrochemistry class 12
Net Reaction :neet chemistry notes for Electrochemistry class 12
EMF of the cell 1 volt.

HYDROCARBON-OXYGEN FUEL CELL

Based upon the combustion of hydrocarbons such as methane, ethane propane etc.
Oxidation (half cell reaction) : neet chemistry notes for Electrochemistry class 12
Reduction (half cell reaction) :neet chemistry notes for Electrochemistry class 12
Net Reaction : C3H8 + 5O2 neet chemistry notes for Electrochemistry class 123CO2 + 4H2O

ADVANTAGES OF FUEL CELLS

  • Pollution free
  • High efficiency

THERMODYNAMIC EFFICIENCY OF FUEL CELLS

neet chemistry notes for Electrochemistry class 12
For H2–O2 fuel cells it is 95%.

CORROSION

Slow formation of undesirable compounds such as oxides, sulphides or carbonates at the surface of metals by reaction with moisture and other atmospheric gases is known as corrosion.

FACTORS AFFECTING THE CORROSION

  • Reactivity of metals
  • Presence of moisture and atmospheric gases like CO2, SO2 etc.
  • Presence of impurities
  • Strains in the metal
  • Presence of electrolyte

RUSTING OF IRON - ELECTROCHEMICAL THEORY

An electrochemical cell known as corrosion cell is developed at the surface of iron.
Anode : Pure iron
Cathode : Impure Surface
Electrolyte : neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12    neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12neet chemistry notes for Electrochemistry class 12
Anode Reaction :neet chemistry notes for Electrochemistry class 12
Cathode Reaction :neet chemistry notes for Electrochemistry class 12
neet chemistry notes for Electrochemistry class 12
Net Reaction : neet chemistry notes for Electrochemistry class 12
At surface : neet chemistry notes for Electrochemistry class 12
       neet chemistry notes for Electrochemistry class 12
Net reaction at surface : neet chemistry notes for Electrochemistry class 12

PREVENTION OF CORROSION

  • Barrier protection : By painting, coating, electroplating
  • Sacrificial protection : By galvanization, sherardising
  • Electrical protection
  • Use of anti rust compounds.

CALCULATION OF POTENTIAL OF INTERMEDIATE REACTION

When two half-reactions having potential E1 and E2 which are combined to yield a third half reaction having potential E3. Then E3 is given by
neet chemistry notes for Electrochemistry class 12

Remember : The cell potentials are not thermodynamic functions and should not be added.
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