Redox Reactions and Electrochemistry

Redox Reactions and Electrochemistry PDF Notes, Important Questions and Synopsis

SYNOPSIS

Electrochemical cell	Galvanic cell
This cell converts electrical energy to chemical energy.	This cell converts chemical energy to electrical energy.
It requires a source of external energy.	It is a source of energy.
It has a cathode as the negative electrode.	It has a cathode as the positive electrode.
It has an anode as the positive electrode.	It has an anode as the negative electrode.

Electrode potential:

For any electrode → oxidation potential (OP) = −Reduction potential (RP)
E_cell = RP of the cathode – RP of the anode
E_cell = RP of the cathode + OP of the anode
E°_cell is always a +ve quality and the anode will be an electrode of low RP.

Greater the SRP value, greater will be the oxidising power.

Concentration cell: A cell in which both electrodes are made of the same material.
For all concentration cells, E°_cell = 0.
Electrolyte concentration cell:
Example: Zn(s) / Zn²⁺ (c_¹) || Zn²⁺(c₂) / Zn(s)

Different types of electrodes:

Metal–metal ion electrode
Example: M(s)/Mⁿ⁺Mⁿ⁺+ ne→M(s)
Gas–ion electrode
Pt/H₂(Patm)/Hⁿ⁺(XM) as a reduction electrode.
Oxidation –reduction electrode
Example: Pt/Fe²⁺ , Fe³⁺As a reduction electrode: AgCl(s)+e → Ag((s) +Cl
Metal-metal insoluble salt electrode
Example: Ag/AgCl,Cl^-
As a reduction Electode: AgCl(s) + e → Ag(s) +Cl^-

Electrolysis:

$begin mathsize 12px style rightwards arrow from text Increasing order of deposition end text to text K end text to the power of text + end text end exponent text , Ca end text to the power of text +2 end text end exponent text , Na end text to the power of text + end text end exponent text , Mg end text to the power of text +2 end text end exponent text , Al end text to the power of text +3 end text end exponent text , Zn end text to the power of text +2 end text end exponent text , Fe end text to the power of text +2 end text end exponent text , H end text to the power of text + end text end exponent text , Cu end text to the power of text +2 end text end exponent text , Ag end text to the power of text + end text end exponent text , Au end text to the power of text +3 end text end exponent text. end text of end style$

Similarly, the anion which is a stronger reducing agent (low value of SRP) is liberated first at the anode.

$begin mathsize 12px style rightwards arrow from text Increasing order of deposition end text to text SO end text subscript text 4 end text end subscript superscript text -2 end text end superscript text ,NO end text subscript text 3 end text end subscript superscript text-end text end superscript text ,OH end text to the power of text-end text end exponent text ,Cl end text to the power of text-end text end exponent text , Br end text to the power of text-end text end exponent text ,I end text to the power of text-end text end exponent of end style$

Faraday’s law of electrolysis:

First law:
The mass of an atom or ion oxidised or reduced at either electrode (during electrolysis) is directly proportional to the quantity of electricity passed through the electrolyte.

Second law:
When the same quantity of electricity is passed through different electrolytes, the mass of substances deposited is proportional to their respective chemical equivalent or equivalent weight.

Conductance:

Conductance is the property of a conductor which facilitates the flow of electricity through it.
Specific conductors or conductivity: Conductance of a solution of definite dilution enclosed in a cell having two electrodes of unit area separated by 1 cm.
Equivalent conductance: Conductance of all the ions produced by 1 gram equivalent of an electrolyte in a given solution.
Molar conductance: Conductance of all the ions produced by ionisation of 1 g mole of an electrolyte when present in V mL of solution.

Kohlrausch’s law: Equivalent conductivity of an electrolyte at infinite dilution is equal to the sum of conductances of the anions and cations.

Ionic mobility: Distance travelled by the ion per second under the potential gradient of 1 volt per cm. Its unit is cm²S^-1V^-1.

Battery: A battery consists of two or more voltaic cells connected in series.

Primary batteries:
- In primary batteries, the reaction occurs only once and cannot be reused again.
  Examples:
  Leclanché cell: A zinc container acts as the anode, and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon.
  Anode: $begin mathsize 12px style Zn subscript left parenthesis straight s right parenthesis end subscript rightwards arrow Zn to the power of plus 2 end exponent plus 2 straight e to the power of minus end style$
  Cathode: $begin mathsize 12px style MnO subscript 2 plus NH subscript 4 to the power of plus plus straight e to the power of minus rightwards arrow MnO left parenthesis OH right parenthesis plus NH subscript 3 end style$
  Mercury cell: It consists of zinc and mercury amalgam as the anode and a paste of HgO and carbon as the cathode.

Secondary batteries:

Secondary batteries are portable voltaic cells which are rechargeable.
The most important secondary cell is the lead storage battery commonly used in automobiles and invertors.
Anode:
$begin mathsize 12px style Pb left parenthesis straight s right parenthesis plus SO subscript 4 to the power of negative 2 end exponent left parenthesis aq right parenthesis rightwards arrow PbSO subscript 4 left parenthesis straight s right parenthesis plus 2 straight e to the power of minus double-struck k end style$
Cathode:
$begin mathsize 12px style PbO subscript 2 left parenthesis straight s right parenthesis plus SO subscript 4 to the power of negative 2 end exponent left parenthesis aq right parenthesis plus 4 straight H to the power of plus left parenthesis aq right parenthesis text end text plus text end text 2 straight e to the power of minus rightwards arrow PbSO subscript 4 left parenthesis straight s right parenthesis plus 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis end style$
Overall reaction:
$begin mathsize 12px style 2 straight H subscript 2 left parenthesis straight g right parenthesis plus straight O subscript 2 left parenthesis straight g right parenthesis rightwards arrow 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis end style$

Corrosion:

In corrosion, a metal is oxidised by loss of electrons to oxygen and the formation of oxides.
It is an electrochemical phenomenon.
Corrosion of iron:
Oxidation:
$begin mathsize 12px style Fe left parenthesis straight s right parenthesis rightwards arrow Fe to the power of plus 2 end exponent left parenthesis aq right parenthesis plus 2 straight e to the power of minus end style$
Reduction:
$begin mathsize 12px style straight O subscript 2 left parenthesis straight g right parenthesis plus 4 straight H to the power of plus left parenthesis aq right parenthesis plus 4 straight e to the power of minus rightwards arrow 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis end style$
Atmospheric oxidation:
$begin mathsize 12px style 2 Fe left parenthesis straight s right parenthesis plus 2 straight H subscript 2 straight O text end text left parenthesis straight l right parenthesis plus 1 half text end text straight O subscript 2 left parenthesis straight g right parenthesis rightwards arrow Fe subscript 2 straight O subscript 3 left parenthesis straight s right parenthesis plus 4 straight H to the power of plus left parenthesis aq right parenthesis end style$