Electrochemistry class 12 notes 2021

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Electrochemistry Introduction

 

(1) The branch of chemistry under which the study of the conversion of electrical energy into chemical energy and the conversion of chemical energy into electrical energy is called electrochemistry.

 

(2) The device through which chemical energy is converted into electrical energy and electrical energy into chemical energy is called cell.

Cells are of the following two types

(1) – electrochemical cell
(2) – electrolytic cell

 

(1) – Electrochemical cell – The cells which convert chemical energy into electrical energy are called electrochemical cells.

E.g. Daniel cell, dry cell, lead accumulator cell, mercury cell

 

(2) – Electrolytic cells :- Those cells which convert electrical energy into chemical energy are called electrolytic cells.

 

For example, a cell formed by the electrolysis of a salt (electrolyte)

 

(3) – Whether it is an electrochemical cell or an electrolytic cell, in both types of cells, both oxidation and reduction reactions, ie redox reaction, continue simultaneously.

 

Oxidation – losing electrons
reduction – taking up electrons

 

(4) The place in the cell where oxidation and reduction reactions take place is called electrode.

(5) – The electrode at which the oxidation reaction takes place is called anode.

(6) The electrode at which the reduction reaction takes place is called the cathode.

(7) – In some elements, the tendency to be oxidised by losing electrons is found to be more (eg – Zn) and in some elements, the tendency to be reduced by accepting electrons is found more like- Cu

image

 

(8) – Hydrogen is the only element in which both the tendency to be oxidized and the tendency to be reduced are equal, so it is used for making standard electrode potential.

 

Function and structure of Daniel cell

image Cu2+ Zn 2+

 

(1) – Zn has more tendency to get oxidised, Zn 2+ ions come into solution from the rod of zn and the electrons obtained from oxidation remain on the rod, due to which the rod becomes negatively charged.

(2) – The electrons from the negatively charged zn rod pass through the external circuit to the cu rod.

(3) – Cu 2+ ions come to the rod from the solution due to the tendency of Cu to be reduced more and remain the same after accepting electrons.

(4) – Similarly, there is a rod oxidation of Zn and reduction of Cu rod due to which electrons flow from Zn to Cu rod and current flows from Cu to Zn.

The reaction taking place in Daniel’s cell

at anode-
Zn Zn 2+ +2e- (oxidation)

at cathode –
Cu 2+ +2e- Cu (reduction)

complete redox reaction
Zn + Cu +2 Zn +2 + Cu

Note – Lavan Setu works to balance both.

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Electrochemistry Class 12 Notes

 

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Function and structure of Daniel cell

image Cu2+ Zn 2+

 

(1) – Zn has more tendency to get oxidised, Zn 2+ ions come into solution from the rod of zn and the electrons obtained from oxidation remain on the rod, due to which the rod becomes negatively charged.

(2) – The electrons from the negatively charged zn rod pass through the external circuit to the cu rod.

(3) – Cu 2+ ions come to the rod from the solution due to the tendency of Cu to be reduced more and remain the same after accepting electrons.

(4) – Similarly, there is a rod oxidation of Zn and reduction of Cu rod due to which electrons flow from Zn to Cu rod and current flows from Cu to Zn.

 

The reaction taking place in Daniel’s cell

 

at anode-
Zn Zn 2+ +2e- (oxidation)

at cathode –
Cu 2+ +2e- Cu (reduction)

complete redox reaction
Zn + Cu +2 Zn +2 + Cu

Note – Lavan Setu works to balance both.

 

The convention for the performance of an electrochemical cell:-
(1)- The anode of the cell is written on the left side and the cathode on the right side

(2) – While performing the anode a vertical line is drawn between the metal and the metal cation

Image

 

(3) – While displaying the cathode, give a vertical line between the metal ion and the metal

Image

 

(4) – To show the complete cell, write the anode and cathode and draw 2 parallel lines between the two, which shows the salt bridge

 

whole cell display

Image

(5)- Whole cell reaction of Daniel cell

Image

 

* Electrode potential

 

(1)- When a metal rod is immersed in an aqueous solution of a metal salt, the metal rod becomes positively charged or negatively charged relative to the solution, similarly the potential difference between the rod and the solution is called electro-potential.

(2)- It is denoted by E and its unit is Volt.

(3)- If the rod is oxidised when it is put in a solution, then the potential produced between it is called oxidation potential, it is denoted by Eo.p.

(4)- If the rod is oxidised when it is put in a solution, then the electro-potential produced between it is called oxidation electro-potential, it is denoted by Er.p.

(5) – Oxidation potential and reduction potential for a metal both have equal values ​​but their signs are opposite.
like –
Eo.p = Er.p

 

Cell potential or electromotive force of electrochemical cell (Em.f)

 

Anode oxidation of electrochemical cell and reduction takes place at cathode, so the potential produced at anode is called oxidation potential and produced at cathode. The reducing potential is called

The seed mathematical sum of the oxidation potential of an electrochemical cell that reduction potential is called the cell potential or electromotive force of the cell.

Image

 

Note:- If the type of electro-potential is not mentioned, then it is considered as reduction potential.

Standard electrode potential

 

298k When a metal rod is immersed in a 1M/(molar) aqueous solution of its salt, then the potential difference between the rod and the solution is called standard electrode potential.
It is denoted by E°, its unit is also volt.

 

Calculation of single electro potential

 

(1)- By electrochemical cell, the difference of the two electrode potentials is easily detected by the volt meter.

E{cel} = Er.p(anode) – Er.p(cathode)

 

(2)- If the potential of one electrode of an electrochemical cell is to be determined, then the other electrode potential is assumed to be zero (0) of its own accord, thus the cell potential that comes is called a single electrode potential.

E{cel} = Er.p(cathode) – Er.p(anode)

If Er.p(anode) = 0

E{cel} = Er.p(cathode) = Er.p(anode)

 

(3)- When the electrode potential of the cell becomes zero when the single electrode potential is used, it is called the standard electrode or the reference electrode, as this electrode is used for hydrogen.

 

imp*

Structure of Standard Hydrogen Electrode

 

(1)- Hydrogen gas is passed on 1Atm pressure on pt black coated pt plate, due to which the electrons of hydrogen gas adsorbed on the plate give up and come to the solution in the form of H+ ions. The electrons obtained from oxidation remain on the rod.

Image

 

(2)- Oxidation half-reaction

 

H2(Gas) —-> 2H+ +2e-

 

(3)- The electrons present on the negatively charged pt plate reduce the H+ ions present in the solution with the same rate with which the H2 molecule gets oxidised.

reduction half reaction
2H+(aq) +2e- —-> H2(ges)

 

(4)- Hence H2 hence the rate of oxidation and reduction of H2 is equal, so the electrode potential of H2 is zero

 

(5)- Demonstration of electrodes of standard hydrogen :-
as a cathode
Pt | H+(aq) | H2 (Gas)

as anode
H2 (Gas) | H+(aq) | Pt

 

Electrochemical Series

image

(1)- If different elements are arranged on the basis of various reduction potentials, then a series is formed, it is called electrochemical series.

 

(2)- Elements above hydrogen in electrochemical series tend to get oxidised by giving up electrons i.e. these elements are of reducing nature.

 

(3)- The more negative the value of the standard reduction potential of the element, the greater will be its tendency to lose electrons.

 

(4)- Elements falling below hydrogen in electrochemical series tend to be reduced by accepting electrons, that is, these elements are oxidising in nature.

 

(5)- The more positive the value of the standard reduction potential of the element, the greater will be the tendency to accept electrons.

 

(6) Elements above hydrogen in the electrochemical series act as anodes.

 

(7) Elements higher in the electrochemical series replace the metal coming down with its salt.

Image

 

(8) In the electrochemical series, a metal coming up from hydrogen reacts with an acid to liberate hydrogen gas, whereas a metal coming down does not react with an acid to liberate hydrogen gas.

Image

 

Calculation of work done by electrochemical cell

Chemical energy is converted into electrical energy by electrochemical cell
Electrical energy is used to do useful work, so the maximum work done by the cell is equal to the reduction in the cell.

W (max) = -∆G

G –> Gibbs free energy

Maximum work done by the cell = charge × potential difference
W(max) = Q×E

If n electrons flow through the cell, then the charge on them is nF.

W (max) = nF×E
q = nF

W (max) = nFE
G = nFE
G° = -(nFE)°

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Nernst Equation :- If electrochemical cell is not present under standard conditions i.e. the concentration of the metal salt is not 1M and there is no temperature of 298k, in this case Nernst equation is used to find the potential of the cell. Origin of this Nernst equation is done as follows

 

Note :- If cell is in normal state then E(cel) = 0

According to thermodynamics – G = G° + RT loge(kc)

G = G° + RT loge(kc)={Helm Holtz equation)

-nFE = nFE° + RT loge(concentration of products/concentration of reactants)

Image

Image

 

Electrolytic cell

 

When an electric current is passed through an aqueous solution of an electrolyte or in a molten state, then an electric current is conducted in it, the cell made from it is called an electrolytic cell.

 

Important definitions related to electrolytic cell

 

(1)- Resistance – According to Ohm’s law, the potential of the electrode of an electrolytic cell is proportional to the current flowing in it.
The Ba produced in the flow of electric current is called resistance
E I
E RI
R = E/I
( R – is the constant called resistance)

Unit – Ohm (Ω)

 

(2)- Conductivity {G} – The reciprocal of resistance is called conductivity

G = 1/R
(G – Gradient G)

Unit – ohm- [(Ω-)]

 

(3)- Specific resistance (P)– The specific resistance of an electrolytic cell is proportional to the distance (l) between its electrodes and inversely proportional to the area of ​​cross-section {R} of the electrodes

R L/A
R = P.(L/A)
P = (R×A)/L

Unit – (ohm×m2)/m
=ohm×m
Unit in CGS system – om×cm

 

(4)-Specific Conductivity : – Just as resistance has inverse conductivity, in the same way, specific resistance has inverse specific conductivity, it is denoted by K.

K = 1/p
K = L/R×A
K = (1/R)×(L/A)= G.(L/A)

{G = 1/R}

Unit – om- m-.
Unit in CGS system – om- cm-

 

(5)- Cell constant :- The ratio of the distance between the two electrodes of the electrolytic cell and the area of ​​cross section of the electrode is called cell constant.
By
Cell constant = L/A
V1Efficient conductivity (K) = (1/R)×(L/A)
= (1/R)×cell constant
= cell constant × conductivity

(conductivity – 1/R)

 

(6)- Molar conductivity (λm) : – The ratio of the conductivity of the electrolytic cell and the concentration in moles per liter is called molar conductivity, it is represented by m.

m = K/(mol/L)

1L = 1000ml = 1000cm3

m = K/(mol/1000cm3)

m = (K × 1000cm3)/mol

Unit – om- cm- ×cm3/mol
Om- cm2 mol-

Om- = S (Siemens)

Unit – S×cm2 mol-

 

Types of substances on the basis of conductivity

 

Substances are divided into three parts on the basis of conductivity

(1)- Conducting material

-> Those substances which conduct electricity easily are called good conductors like solution of all metals and electrolytes.
-> The value of conductivity for conductors ranges from 10^4 to 10^7 om- m-

(2)- semiconductor material

-> Those materials which conduct little electric current are called semiconductors Ex – Si,Ge
-> For semiconductors the value of the way wa conductivity is 10^-6 to 10^4 om- cm-

 

(3)- Insulators

-> Those substances which do not conduct electric current are called non-conducting materials.
Ex- wood, rubber, cotton cloth

 

-> The value of conductivity for inductors is 10^-20 to 10^-10 om-cm

 

Types of Drivers

 

(1)-Electronic conductor/Metallic conductor :- Those conductors day’s conductivity is due to free electrons are called electronic.
like all metals

The conductivity of metals depends on the following factors
The value of a metallic conductor increases as the number of valence electrons per atom increases.

Metallic conductivity decreases with increasing temperature because with increasing temperature, the movement of metal ions and free electrons increases, due to which the metal ions obstruct free electronic movement.

 

(2)- Electrolytic conductors :- Those conductors whose conductivity is due to free ions are called electrolytic conductors.
Free ions are present in aqueous solution and molten state of electrolysis, so the solution conducts electricity in these states.

 

Factors affecting the conductivity of electrolytic conductors

 

(1):- Nature of electrolyte If the electrolyte is completely ionized (100%) after dissolving it in water, then it is called Abel’s hydrolytic solution, due to the large number of ions in such electrolyte solutions, its conductivity is also high. it occurs

Image

 

Note:- On electrolysis of NaCl in molten state, Na metal is deposited at the cathode and Cl2 gas is liberated at the anode.

*:- image

 

Reaction of the remaining medium

2Na+2oH ———–> 2NaoH

 

On electrolysis of aqueous NaCl solution, H2 gas is liberated from the cathode and Cl2 gas at the anode and the remaining Na+ and OH- ions in the medium react to form NaoH.
The medium in the above process is alkaline in nature and can also be used in the manufacture of NaOH.

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Laws of electrolysis : – By establishing the relation between the current flowing during electrolysis and the amount of matter received at different electrodes, Faraday gave laws, which are called Faraday’s laws.

 

(1) Faraday’s first law :- This law is applicable for only one electrolyte at a time.

According to this law, the amount of matter obtained at different electrons is proportional to the total current flowing through it in the solution.

If the amount of substance obtained when q coulomb of charge is passed through it is w gm

Image

From Equation 3, the amount of substance obtained at the electrochemical equivalent electrode is equal to its electrochemical equivalent when a current of 1 amp in the solution is applied for 1 sec.
Image

 

(2) Faraday’s second law :- This law is applicable for more than one electrolyte.

According to this law, if the same value of current is passed for the same amount of time, then the amount of substance obtained at different electrodes is proportional to its chemical equivalent.
Mass of metal deflected at the electrode of a cell W 1= E1

Image

 

Choleraus’s law : – Cholera that the molar conductivity at infinite dilution of an electrolyte is equal to the molar conductivity at infinite dilution of the cation obtained from its ionization and the molar conductivity at infinite dilution of the anion as follows
To find the molar conductivity at infinite dilution of weak electrolyte by Coleraus’s law

 

Image

Cholera Application of his law

(1) To find the molar conductivity of weak electrolyte at infinite dilution:-

Acetic acid is a weak acid, it does not undergo complete ionization, so the infinite dilute molar conductivity of acetic acid cannot be determined by the following formula

Image

 

(2):- Calculation of the amount of weak electrolyte dissociation and the dissociation constant

 

With the help of above formula the organizing constant k c is calculated

cell /battery

cells are of the following two types

 

(A) Primary Cell :- Cells which cannot be charged again after being discharged are called primary cells.
Eg:- dry cell or mercury cell,

Structure and working mechanism of dry cell

Image
The dry cell consists of a ZN (zinc) vessel with a graphite rod in the center and a brass cap on it.

The powder of Mno2 and carbon is filled around the graphite rod and the mixture (paste) of Zncl2 is mixed in the remaining vessel.
The dry cell acts as the anode and the graphite rod acts as the cathode.

When the cell is connected to the circuit, the following reaction on the kephod and aphod takes place further

 

Image

Dry cell gives an electromotive force of 1.5
dry cell is used in remonde in wall stick

 

(B) Mercury cell :- Mercury cell consists of a vessel of zn in which mercury oxide Hgo is filled and a solution of NaOH and kOH is filled in it as electrolyte.

-:- image

 

=> An electromotive force of 1.35 volt is obtained from the mercury cell

=> Mercury cell is used in hand watch and hearing machines

 

(2)- Secondary cell :- Cells which can be charged again after being discharged are called secondary cells.
Eg:- lead accumulator cell

 

Construction Function and System of Lead Accumulator Cell

 

(1) Lead Accumulator Cell consists of a hard plastic container which is filled with 38% H2so4 whose density is 1.3gm/m.l!

 

(2) In the lead accumulator cell, there are plates of Pb which act as anode and some plates are layered of Pbo2 which acts as the anode!

 

Image

 

When the cell is completely discharged, it becomes lead sulfate and water, in which case current cannot be received from the cell.
In order to get full current from the cell, it needs to be charged. While charging, the opposite reaction already takes place.

Image

 

Shortcoming / Dos :- While charging the lead accumulator cell, the entire PbSo4 is not decomposed, some lead sulphate gets deposited on the electrodes, due to which the cell stops its work gradually, thus excessive lead sulphate on the electrodes in 3-4 years. When it freezes, it closes its function

 

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Corrosion

 

(1)- In the presence of atmospheric moisture and O2, unwanted oxides are formed on the metal surface, which causes corrosion of the metal surface, this process is called corrosion.

(2)- Rusting of iron, silver turning black, copper turning green are examples of corrosion.

(3)- Corrosion process takes place in the presence of moisture

(4)- Corrosion process is more during rainy days because free H+ ionization is present in the atmosphere.

 

Method of rusting iron

 

In the process of rusting, an electrochemical cell (galvanic cell) is formed on the surface of iron, this process is done in the following steps

 

(1) Since the standard reduction potential of iron is negative, the Fe atoms from the iron surface get oxidised to Fe+2 ions, thus the iron surface acts as the anode.

As the electrons obtained from oxidation get collected on the surface, the surface becomes negatively charged.

 

(2) The surface of negatively charged Fe attracts H+ ions present in the atmosphere

(3) Coming on the surface of iron, H+ ions combine with atmospheric oxygen to form chemical water, thus the reduction reaction takes place on the surface.

Image

 

(4) Fe+2 ions from the iron surface come in contact with atmospheric O2 and get further oxidised to form Fe2O3 which gets hydrated to form a reddish brown powder called Fe2O3 .x H2o which is called rust

 

Measures to prevent transmission

 

(1)- The metal should be protected from direct exposure to atmospheric moisture to prevent transmission (pants should be worn)

 

(2)- Varnish oil should be coated on the metal surface.

 

(3)- The metal should not be allowed to come in contact with cotton cloth and newspaper.

 

(4)- To protect the metal coming down in the electric series from transmitting, the anode of the upper metal is put through which the anode is transmitted, this is called sacrificial protection.

– As Zn anode is put to protect the iron from transmitting

Imp*
Manas Western Cell

Manas Western Cell has the following characteristics

 

–> this is a stationary cell
–> it is more irreversible
–>heat coefficient of this cell is very low

 

Composition of Manas Western Cell

 

It is a H shaped vessel made of glass, with a platinum fuse wire in its bottom end, in the bottom of its right tube, we have filled gum Cd – Hg in which Cd is 12%.
CdSo4.(8/3)H2o salt is filled over it and a solution of CdSo4 is filled over it.

Hg is filled on the left and bottom, and above it the paste of Hg and Hg2So4 (Hg2So4 + Hg calomal) and above it the salt of solid CdSo4.(8/3)H2o. And in the remaining part, the solution of CdSo4 is filled.

Image

 

Fuel Cell : – The cells in which the energy of fuel is converted into electrical energy is called ‘Fuel Cell’. H2, O2, Ch4, C2H6 are used as fuel in a fuel cell.

 

Structure of fuel cell : – In a vessel, two helical (with holes) electrodes of graphite are arranged in such a way that the vessel is divided into three parts, the middle part of the vessel is filled with NaOH or KOH. and oxygen gas is passed from one side to the hydrogen gas in the vessel from the other side, due to which the combustion of hydrogen takes place in the presence of oxygen, as a result of which the following reactions take place.

Image

use of fuel
-> fuel cells are lighter
-> Fuel cells are used to generate electricity in spacecraft
-> Fuel cells are pollution free
-> This is used in the Apollo spacecraft launched by India

 

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