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Chemistry Notes for class 12 Chapter 3
Electrochemistry
Electrochemistry is that branch of chemistry which deals with the study of production of
electricity from energy released during spontaneous chemical reactions and the use of electrical
energy to bring about non-spontaneous chemical transformations
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2
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4
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Ca and Al
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Purification of metals
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Conductors
Substances that allow electric current to pass through them are known as conductors
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g
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Electrolytic Conductors or Electrolytes
Substances which allow the passage of electricity through their fused state or aqueous solution
and undergo chemical decomposition are called electrolytic conductors, e
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, aqueous solution
of acids
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Electrolytes are of two types:
1
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e
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, HCl, NaOH, K2SO4
2
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g
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Electrochemical Cell and Electrolytic

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A cell of almost constant emf is called standard cell
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Galvanic cell is also called voltaic cell
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There is no evolution of heat
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The solution remains neutral on both sides
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The reaction and now of electrons stops after sometime
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It is represented as

By convention cathode is represented on the RHS and anode on the LHS
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It completes the circuit and allows the flow of current
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It maintains the electrical neutrality on both sides
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KCI is preferred because the
transport numbers of K+ and Cl-are almost same
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The fraction of the current carried by an ion is called its
transport number or transference number
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Transport number of cation
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na = (current carried by anion/total current)
Evidently nc + na = 1
Electrode Potential

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When an electrode is in contact with the solution of its ions in a half-cell, it has a tendency to
lose or gain electrons which is known as electrode potential
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It is an
intensive property, i
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, independent of the amount of species in the reaction
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Oxidation potential of a half-cell is inversely proportional to the concentration of ions
in the solution
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According to IUPAC convention, the reduction potential alone be called as the
electrode potential unless it is specifically mentioned
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For this a reference
electrode [NHE or SHE] is required
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Standard electrode potential The potential difference developed between metal electrode and
solution of ions of unit molarity (1M) at 1 atm pressure and 25°C (298 K) is called standard
electrode potential
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Reference Electrode
The electrode of known potential is called reference electrode
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Standard hydrogen electrode (SHE) Standard hydrogen electrode (SHE)
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The wire is sealed into a glass tube
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The hydrogen gas at 1 atm pressure is bubbled through the solution at 298K
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at the surface of plantinum, either of (he following reaction can take place
2H+(ag) + 2e- → H2G Reduction
H2(g) → 2H+(ag) + 2e- Oxidation
The electrode potential of SHE has been fixed as zero at all temperatures
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It is difficult to maintain 1 atm pressure of H2 gas
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It is difficult to maintain H+ ion concentration 1 M
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The platinum electrode is easily poisoned by traces of impurities
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Electromotive Force (emf) of a Cell
It is the difference between the electrode potentials of two half-cells and cause flow of current
from electrode at higher potential to electrode at lower potential
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Standard emf of a cell,

Electrochemical Series
It is the arrangement of electrodes in the increasing order of their standard reduction potentials
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The lower the value of E°, the greater the tendency to form cation
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Reducing character increases down the series
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Reactivity increases down the series
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Determination of emf; emf is the difference of reduction potentials of two half-cells
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then reaction take place spontaneously, otherwise not
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Greater the reduction potential of a substance, oxidising power
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g
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A negative value of standard reduction potential shows that it is the site of oxidation
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Oxides of metals having E°red ≥ 0
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HgO (s) → Hg(l)(1/2)O2(g)
(E°Hg2+/Hg = 0
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For a electrochemical cell,

Concentration of pure solids and liquids is taken as unity
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Relationship between free energy change and equilibrium constant
ΔG° = – 2
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g
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g
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It is reciprocal of resistance (R)
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Unit of cell constant is cm-1 or m-1
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This is because concentration of ions per cc
decreases upon dilution
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It is related to specific conductance as
Λm = (k x 1000/M)
where
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It units are Ω-1 cm2 mol-1 or S cm2 mol-1
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It is related to specific
conductance as
Λm = (k x 1000/N)
where
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Its units are ohm-1 cm2 (equiv-1) or mho cm2 (equiv-1) or S cm2 (g-equiv-1)
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Λm = Λ0m – √C
where, b is a constant
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Factors Affecting Conductivity
(i) Nature of electrolyte The strong electrolytes like KNO3 KCl
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etc
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The weak electrolytes are ionised to a lesser extent in aqueous solution and have lower values
of conductivity (molar as well as equivalent)
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which reduce the speed of ions and lower the value of Λ m
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The dilution decreases such attractions and increase the value of Λm and Λeq
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(the molar conductivity at zero concentration (or at infinite
dilution) can be obtained extrapolating the graph
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The liminting value Λ0m cannot be obtained by extrapolating the graph
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(iii) Temperature The increase of temperature decreases inter-ionic attractions and increases
kinetic energy of ions and their speed
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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
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, for AxBy
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g
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The dissociation constant (K) of the weak electrolyte at concentration C of the solution can be
calculated by using the formula
kc = (Cα2/1 – α)
where, α is the degree of dissociation of the electrolyte
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, PbSO4‘ AgCl, AgBr and AgI which do not dissolve to a large extent in
water are called sparingly soluble salts
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In electrolytic cell both oxidation and reduction takes place in the same cell
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Anode is positively charged and cathode is negatively charged, In electrolytic cell
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During electrolysis of molten electrolyte, cations are liberated at cathode
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4
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the ion with higher reduction
potential gets liberated at the cathode while the ion with lower reduction potential at the
anode
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However for liberation of gases, some extra voltage is required than the
theoretical value of the standard electrode potential
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How to Predict the Products of Electrolysis?
When an aqueous solution of an electrolyte is electrolysed, if the cation has higher reduction
potential than water (-0
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g
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g
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) Similarly if anion has higher oxidation potential than water
(- 1
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g
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g
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2 V)
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Faraday’s Laws of Electrolysis
1
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W∝Ixt=IxtxZ=QxZ
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

I current in amp, t = time in sec,
Q = quantity of charge (coulomb)
Z is a constant known as electrochemical equivalent
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Thus, electrochemical equivalent I” the amount of the substance deposited or liberated by
passing 1A current for 1 sec (i
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1 coulomb, I x t = Q)
2
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the amounts of
the substance deposited or liberated at the electrodes arc directly proportional to their
equivalent weights, Thus,

Hence, electrochemical equivalent ∝ equivalent weight
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For a good quality battery it should be reasonably light
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Primary Batteries
In the primary batteries
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(i) Dry cell or Leclanehe cell
Anode-Zinc container
Cathode-Graphite rod surrounded by MnO2 powder
Electrolyte-Paste of NH4Cl + ZnCl2
Cathode reaction,
2MnO2(s) + 2 NH+4(aq) + 2e- → Mn2O3(s) + 2NH3(g) + H2O(l)
Anode reaction,
Zn(s) → Zn2+(aq) + 2eCell potential 1
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5 V
(ii) Mercury cell
Anode-Zn-Hg amalgam
Cathode-Paste of (HgO + C)
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Electrolyte-Moist paste of KOH-ZnO

Secondary Batteries
These cells can be recharged and can be used again and again, e
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,
(i) Lead Storage battery
Anode-Spongy lead
Cathode-Grid of lead packed with PbO2
Electrolyte-38% H2SO4 by mass

When recharged the cell reactions are reversed
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, as the
source to produce electrical energy are called fuel cells
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Hydrogen-Oxygen Fuel Cell
Electrodes-Made of porous graphite impregnated with catalyst (Pt, Ag or a metal oxide)
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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
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2
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4
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Presence of impurities
Strains in the metal

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5
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Anode- Pure iron
Cathode-Impure surface

Rusting of iron can be prevented by the following methods :
1
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3
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Barrier protection through coating of paints or electroplating
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By the use of antirust solutions (bis phenol)
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