Tiêu đề ELECTROCHEMISTRY.pdf ✅

 Digital www.allendigital.in [ 131 ] Introduction Electrochemistry is the branch of physical chemistry which deals with the study of interconversion of chemical energy and electrical energy, Chemical energy → Electrical energy occurs Electrical energy → Chemical energy occurs through a Spontaneous redox reaction through a non-spontaneous redox reaction Conductors Substances which allow electric current to flow through them are called conductors. Examples - Metals, Aqueous solution of acids, bases and salts, fused salts, impure water etc. Conductors are of two types – (1) Metallic conductors or electronic conductors (2) Electrolytic conductors or ionic conductors Metallic conductors Electrolytic conductors 04 Electrochemistry Electrolytic cell Electrochemical cell Galvanic cell or voltaic cell
NEET : Chemistry [ 132 ] www.allendigital.in  Digital (1) Metallic conductors: The conductors which conduct electric current by the movement of electrons without undergoing any chemical change are known as metallic conductors. Metals - Cu, Ag, Fe, Al etc, non metals - graphite (2) Electrolytic conductors: Those substances which conduct the electric current through ions in aqueous solution or in molten state are called as electrolytic conductors. Strong electrolytes: Electrolytes which get ionized completely in an aqueous solution are called as strong electrolytes. Ex. : Salts, strong acids and strong bases in water. Weak electrolytes:- Electrolytes which do not get ionized completely in an aqueous solution are called as weak electrolytes. Ex. : CH3COOH, HCN, NH3, amine, etc. Difference between metallic and electrolytic conductors Metallic conductor Electrolytic conductor (i) Charge carriers are free electrons. Charge carriers are free ions. (ii) Flow of electricity takes place without any chemical change. Flow of electricity takes place by chemical changes at electrodes. (iii) No transfer of matter takes place. Transfer of matter takes place in the form of ions. (iv) Resistance is due to vibration of Kernels. Resistance is due to inter ionic attraction and viscosity of medium. (v) The resistance increases with the increase in temperature The resistance decreases with the increase in temperature. (vi) Faraday’s laws of electrolysis are not followed. Faraday’s laws of electrolysis are followed. Electrolytic Conductance (a) Resistance (R):-Metallic and electrolytic conductors obey ohm's law according to which the resistance of a conductor is the ratio of the applied potential difference (V) to the follow of current (I). R is expressed in ohms. (b) Conductance (G):- It is the property by virtue of which it favours flow of current. The conductance of a conductor is equal to reciprocal of resistance unit of G is mho or ohm–1 or Siemens (S). (f) Equivalent conductivity or Equivalent Conductance ( eq or  eq ) (e) Molar conductivity or Molar conductance (m or m) (d) Specific conductance/ Conductivity () (c) Specific resistance/Resistivity () (b) Conductance (G) (a) Resistance (R) R=V I G=1 R Pt electrode Molten NaCl Electron
Electrochemistry  Digital www.allendigital.in [ 133 ] (c) Specific resistance/Resistivity ():- The resistance (R) of a conductor is directly proportional to its length (l) and inversely proportional to its area of cross section (A). R  A ; R =  A R =  A ;  = R. A If l = 1 cm, A = 1 cm2, therefore  = R ∵ V = A × l = 1 cm3 Therefore, the resistance offered by 1 cm3 electrolytic solution is known as resistivity. Unit of  → ohm cm (d) Specific conductance/ Conductivity () :- It is defined as the reciprocal of specific resistance,  =  1 R =  A  =  1 . R A Specific conductance = Conductance × Cell constant Hence specific conductivity of a solution is defined as the conductance offered by 1 cm3 of electrolytic solution. Cell constant: G* = A ; Its unit is cm–1 (e) Molar conductivity or Molar conductance (m or m) :- It is defined as the conductance of all the ions produced by one mole of electrolyte present in the given volume of solution. m =  × V V = Volume of solution containing 1 mol of electrolyte. If concentration of solution is M - mol per litre then m = 1000 M = 1cm Solution Electrode plate 1 cm 1 cm ~ κ = G × G ∗ Unit of  is ohm–1 cm–1 Unit → ohm–1 cm2 mol–1 V cm3 1 Mole Electrolyte
NEET : Chemistry [ 134 ] www.allendigital.in  Digital (f) Equivalent conductivity or Equivalent conductance (eq or eq) :- It is defined as the conductance of all the ions produced by one gram equivalent of the electrolyte in the given volume of solution.  eq =  × V V = Volume of solution containing 1 g-eq of electrolyte. If concentration of solution is N - gram equivalent per litre then eq = 1000 N Relation between eq. and m   = m 1000 M and   = eq. 1000 N We know that Normality = Valency Factor × Molarity. So Illustration 1: The resistance of a 1N solution of salt is 50 , Calculate the equivalent conductance of the solution, if the two platinum electrodes in solution are 2.1 cm apart and each having an area of 4.2 cm2. Solution: 1 1 R A    = =      = 1 2.1 1 50 4.2 100  = and   = eq. 1000 N =  1 1000 100 1 = 10 ohm–1 cm2 eq–1 Illustration 2: Which of the following has maximum molar conductivity? (i) 0.08 M solution and its specific conductivity is 2 × 10–2 –1 cm–1. (ii) 0.1 M solution and its resistivity is 50  cm. Solution: (i)   −  = =  2 m 1000 1000 2 10 M 0.08 = 250 –1 cm2 mol–1 (ii)   = m 1000 M , ∵ =  1 ,  m = 1 50 × 1000 0.1 = 200 –1 cm2 mol–1 So, the molar conductivity of 0.08 M solution is greater than 0.1 M solution Illustration 3: The conductance of a salt solution (AB) measured by two parallel electrodes of area 100 cm2 separated by 10 cm was found to be 0.0001 –1. If the volume enclosed between the two electrodes contain 0.5 mol of salt. What is the molar conductivity (Scm2 mol–1) of the salt at same concentration? (1) 0.01 (2) 0.02 (3) 2 × 10–5 (4) None of these Solution: G* = = 10 A 100 = 0.1; G = 0.0001 S; V = 100 × 10 = 1000 cm3 = 1 litre  = G G* = 0.1 × 0.0001 = 10–5 m = −   = 5 1000 10 1000 M 0.5 = 0.02 S cm2 mol–1 Unit → ohm–1 cm2 eq–1 Λeq = Λm V. F. V cm3 1 Gram Equivalent Electrolyte