ELECTROCHEMISTRY
The branch of chemistry that deals with the study of relationship between electrical energy and chemical energy and their inter-conversion is called electrochemistry.
1) CONDUCTORS
2) REDOX REACTIONS
3) ELECTROCHEMICAL CELL
4) ELECTRODE POTENTIAL
5) STANDARD HYDROGEN ELECTRODE
6) ELECTROCHEMICAL SERIES
7) ELECTROLYSIS
CONDUCTORS
The substances that allow the electric current to pass through them are called conductors.
(i) METALLIC CONDUCTORS
The substances that allow electric current to pass through them without undergoing any chemical change are called conductors. E.g. Cu, Fe, Ag, Al etc.In metallic conductors, current is carried by the electrons. Hence, they are also known as electronic conductors.
(ii)ELECTROLYTES OR ELECTROLYTIC CONDUCTORS
The substances that allow electric current to pass through them in their molten state or in their aqueous solution are called electrolytes.E.g., NaOH,NaCl,H2SO4 etc.In electrolytes current is carried by the ions.
TYPES OF ELECTROLYTES
(I) STRONG ELECTROLYTES
The electrolytes which completely dissociate into ions and hence conduct electricity to a greater extent are called strong electrolytes.E.g NaOH, NaCl, HCl, etc.
NaCl(aq) → Na+(aq)+ Cl-(aq)
For strong electrolytes degree of dissociation is always = 1.
(II) WEAK ELECTROLYTES
The electrolytes which do not completely dissociate into ions and hence conduct electricity to a smaller extent are called weak electrolytes.E.g.CH3COOH, NH4OH etc.
CH3COOH ⇌ CH3COO- + H+
For weak electrolytes, the degree of dissociation is < 1.
Q :What is electrolytic conduction?
Ans: The flow of electric current through an electrolytic solution is known as electrolytic conduction.
REDOX REACTIONS
The reactions that involve transfer of electrons from one substance to another are called redox reactions. A redox reaction involves reduction and oxidation processes. E.g.
Na + Cl → Na+ Cl-
OXIDATION:- The process that involves loss of electrons is called oxidation.
Na → Na+ + e-
REDUCTION: - The process that involves gain of electrons is called reduction.
Cl + e- → Cl-
OXIDIZING AGENT: - It is an electron acceptor.
Na + Cl → Na+ Cl-
Here chlorine accepts an electron hence it is an oxidizing agent.
REDUCING AGENT: - It is an electron donor.
Na + Cl → Na+ Cl-
Here sodium donates an electron hence it is a reducing agent.
COMPLEMENTARY NATURE
This is because both the processes occur simultaneously. In other words, oxidation cannot take place without reduction and reduction cannot take place without oxidation. Hence, both are complementary to each other.
REDOX REACTIONS:TYPES
(I)DIRECT REDOX REACTIONS
The redox reactions in which oxidation and reduction take place in the same vessel are called direct redox reactions.E.g. When Zn rod is dipped in the aqueous solution of CuSO4, Zn is oxidized to Zn2+ ions and Cu2+ ions are reduced to Cu metal in the same vessel.
(II)INDIRECT REDOX REACTIONS
The redox reactions in which oxidation and reduction take place in the separate vessels are called indirect redox reactions. E.g. Electrochemical cell.
ELECTROCHEMICAL CELL
It is a device used to convert chemical energy into electrical energy. CONSTRUCTION:-It consists of two vessels. Left vessel contains zinc electrode dipped in one molar aqueous solution of zinc sulphate . Right vessel contains copper electrode dipped in one molar aqueous solution of copper sulphate. Both the vessels are connected with the help of a copper wire and a salt bridge . Left vessel is called oxidation half-cell and right vessel is known as reduction half-cell.
An electrochemical cell is represented as follow:
WORKING OF CELL
OXIDATION HALF CELL
Here oxidation takes place and each zinc atom loses two electrons to form Zn2+ ions.These electrons move through the wire and reach the copper electrode.
Zn (s)→ Zn2+ (aq) + 2e- (oxidation)
REDUCTION HALF CELL
Here reduction takes place and cupric ions from copper sulphate solution accept electrons and are converted into copper metal
Cu2+(aq)+ 2e- → Cu (s) (reduction)
The overall reaction would be:
Zn (s)→ Zn2+ (aq) + 2e- (oxidation)
Cu2+(aq)+ 2e- → Cu (s) (reduction)
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Zn(s) + Cu2+(aq) → Zn2+(aq)+ Cu(s) (redox reaction)
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SALT BRIDGE :FUNCTIONS
It is a U-shaped inverted tube contains solution of inert electrolyte in agar- agar. It is plugged with cotton on both the ends.
(i)It helps to complete electrical circuit of the cell.
(ii)It helps to maintain electrical neutrality of the cell.
ELECTRODE POTENTIAL
It is the tendency of an electrode to lose or gain electrons. It is of two types.
(1)REDUCTION POTENTIAL: - It is the tendency of an electrode to gain electrons. It is expressed in volts. E.g. in electrochemical cell copper electrode has reduction potential.
(2)OXIDATION POTENTIAL: - It is the tendency of an electrode to lose electrons .It is expressed in volts. E.g. in electrochemical cell zinc electrode has oxidation potential.
STANDARD REDUCTION POTENTIAL
If an electrode undergoes reduction with respect to standard hydrogen electrode then the electrode potential is called standard reduction potential .It is expressed in volts with positive sign . On the other hand, if an electrode undergoes oxidation with respect to the SHE then standard reduction potential is expressed in volts with negative sign. E.g.the standard reduction potential of Cu electrode is+0.34 volts and that of Zinc electrode is -0.76 volts.
ELECTROMOTIVE FORCE (EMF)
It is also known as Standard Cell Potential.It is the maximum potential difference between the two electrodes of a cell when no current flows through the circuit. It is expressed in volts. It is calculated as follow :
E°Cell or EMF = E°Cathode -E°Anode
STANDARD HYDROGEN ELECTRODE
It is used to find out electrode potential of the electrodes of various elements. The standard reduction potential of NHE is taken as zero. It is a reversible electrode. It is a reversible electrode. It consists of a platinum wire sealed in a glass tube and has a platinum foil attached to it. The foil is coated with finely divided platinum. It is dipped in one molar HCl solution. Hydrogen gas is constantly bubbled through the solution at one atmospheric pressure and 298 K.
ELECTROCHEMICAL SERIES
The arrangement of elements in the decreasing order of their standard reduction potential values is called electrochemical series.
APPLICATIONS
(1) DETERMINATION OF EMF
Electrochemical series is very helpful in the calculation of emf of an electrochemical cell. It is calculated as follow :
E°Cell or EMF = E°Cathode -E°Anode
(2) COMPARISON OF STRENGTH
As we move down the series the strength oxidizing agents decreases and that of reducing agents increases. Hence the strongest oxidizing agent is present at the top of the series (F-) and strongest reducing agent is present at the bottom of the series (Li+).
(3) PREDICTION OF LIBERATION OF HYDROGEN
Metals with negative value of standard reduction potential liberate hydrogen from protic acids. E.g.
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
Fe (s) + H2SO4 (aq) → FeSO4 (aq) + H2 (g)
(4) COMPARISON OF REACTIVITY OF METALS
Metals with negative value of standard reduction potential like Na, K, Zn, Fe etc are called active metals whereas metals with positive value of standard reduction potential like Cu, Ag, Hg etc are called inactive metals.
(5) PREDICTING THE FEASIBILITY OF RXN
A redox reaction takes place if an element with lower value of standard reduction potential loses electrons and the element with higher value of standard reduction potential gains electrons. E.g. following rxn is not feasible.
Cu (s) + Zn2+ (aq) → Cu2+ (aq) + Zn (s)
Whereas reverse reaction is feasible, that can take place.
Zn (s) + Cu2+ (aq) → Zn 2+ (aq) + Cu (s)
(6) DISPLACEMENT OF A METAL FROM ITS SALT SOLUTION BY OTHER METAL
A metal can replace other metal from its salt solution if that metal has low reduction potential value than the metal present in the salt solution. E.g. Zn can displace Cu from copper sulphate solution but Cu cannot displace Zinc from zinc sulphate solution.
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Cu (s) + ZnSO4 (aq) → No reaction
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