Title SP-2_Ch-9-Electrochemistry.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Chapter Contents Chapter 9 Electrochemistry Metallic / Electrolytic Conductors Equivalent Conductivity Molar Conductivity Variation of Molar Conductivity with Concentration Kohlrausch Law of Independent Migration of Ions Electrolysis Quantitative Aspects of Electrolysis and Faraday's Laws Electrode Potential and EMF of a Cell Nernst Equation Commercial Cells Corrosion METALLIC / ELECTROLYTIC CONDUCTORS Conductors Substances which allow electric current to flow through them are called conductors while those which do not permit the flow of electric current are called insulators. There are two types of conductors namely (i) metallic conductors and (ii) electrolytic conductors. Differences between Metallic and Electrolytic Conduction Metallic conduction 1. Metallic conduction is carried by the movement of electrons. 2. It involves no change in the chemical properties of the conductor. 3. It does not involve the transfer of any matter. 4. Metallic conduction decreases with increase in temperature. Electrolytic conduction Electrolytic conduction is carried by the movement of ions. It involves the decomposition of the electrolyte as a result of the chemical reaction. It involves the transfer of matter as ions. Electrolytic conduction increases with increase in temperature. Factors affecting electrolytic conduction a. Nature of the electrolyte : Conductivity  extent of ionization b. Size of the ions produced and their solvation : Greater the size of the ions or greater the solvation of the ions, lesser is the conductance. c. Nature of the solvent and its viscosity : Greater the polarity of the solvent, greater is the ionization and hence greater is the conductance. Similarly, greater is the viscosity of a solvent, lesser is the conductance. d. Concentration of the solution : Higher the conc. of the solution lesser is the conduction. e. Temperature Conductance  Temperature.
68 Electrochemistry NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Conductance (G) : The reciprocal of resistance is called as conductance i.e. 1 G R  It measures the ease with which current flows through a conductor. Units. 1 G ohm ohm    1 or mho or siemen (S) (1S = 1–1) (S.I. unit : siemen (S)) Resistivity (Specific Resistance) The resistance of a conductor is (i) Directly proportional to its length R  l (ii) Inversely proportional to its area of cross section. 1 R a  Combining equations (i) and (ii) R a  l or R a   l where  = Constant of proportionality known as resistivity or specific resistance. If l = 1 cm; a = 1 cm2 then R =  Thus resistivity may be defined as, "The resistance offered by a conductor of unit length with unit area of cross-section." In other words, specific resistance is the resistance offered by all the ions present in 1 cm3 of an electrolytic solution. Units : R a   l or l = Ra or 2 Ra ohm cm cm   l = ohm - cm Specific Conductance (Conductivity) The reciprocal of the specific resistance is called as specific conductance. It is denoted by  (Kappa) 1    We know, Ra   l 1 1 Ra a R     l l Now 1 G R   G a   l If l = 1 cm, a = 1 cm2 , then  = G Hence conductivity or specific conductance of a solution may be defined as the conductance of a conductor of unit length with unity area of cross section. It may be defined as conducting power of all the ions present in 1 cm3 of an electrolytic solution. Units : 1 1 ohm cm ohm cm        1 1 S.I. units : Conductivity : Sm–1.
NEET Electrochemistry 69 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Cell Constant For a particular cell, l/a is constant and this constant is called Cell constant, l = distance between two electrodes a = area of cross section of the electrodes  Specific conductivity () = Conductance × Cell constant EQUIVALENT CONDUCTIVITY Equivalent conductivity (eq) of an electrolyte in solution is defined as the conductance of a solution containing one gram equivalent of an electrolyte. If V ml is the volume of a solution containing 1 gm equivalent of an electrolyte and  is its conductivity in S cm–1, then equivalent conductance (eq) of the electrolyte is given by eq =  × V or 2 1 eq 1000 Scm equiv. Normality      The SI unit of equivalent conductivity is S m2 equiv–1. MOLAR CONDUCTIVITY It is defined as, "the conducting power of all the ions produced by dissolving one gram mole of an electrolyte in solution". It is expressed as m and is defined as M  1000 m  where M is the concentration in moles per litre. Units : Molar conductance has unit ohm–1 cm2 mol–1 or S cm2 mol–1. Affecting parameters of molar conductivity 1. Nature of electrolyte 2. Concentration of the solution 3. Temperature Relation of m and eq m = eq × n-factor Example 1 : The conductivity of 0.25 M solution of KCl at 300 K is 0.0275 S cm–1. Calculate molar conductivity. Solution : Conductivity 1000 Molar conductivity Molarity   2 1 m 0.0275 1000 110 S cm mol 0.25     Example 2 : The resistance of conductivity cell containing 0.001 M KCl solution at 298 K is 1500 ohm. What is the cell constant if the conductivity of 0.001 M KCl solution at 298 K is 0.146 × 10–3 S cm–1? Solution : Cell constant = Conductivity × Resistance = 0.146 × 10–3 × 1500 = 0.219 cm–1
70 Electrochemistry NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 VARIATION OF MOLAR CONDUCTIVITY WITH CONCENTRATION The values of both conductivity and molar conductivity of an electrolyte in solution change with the concentration of the electrolyte. Conductivity of all electrolytes always decreases with decrease in concentration because it is the conductance of all the ions present in unit volume and the number of ions per unit volume decreases with the decrease in concentration. Molar conductivity of an electrolyte not only depends on the nature of electrolyte but also on its concentration. Depending upon the values of molar conductivity, the electrolytes can be divided into two groups, namely (i) strong electrolytes and (ii) weak electrolytes. For the same concentration, the molar conductivity of a strong electrolyte is higher than that of weak electrolyte because a strong electrolyte is completely ionised at all concentrations and a weak electrolyte is partially ionised in the high concentration region. As concentration of an electrolyte decreases the molar conductivity of a strong electrolyte marginally increases whereas it increases appreciably in case of a weak electrolyte. The former is due to increase in interionic distance on dilution which reduces the influence of cation on anion and vice versa whereas the latter is due to increase in the extent of ionisation of weak electrolytes on dilution. The plot of a graph between molar conductivity (m) vs conc. for a strong electrolyte and a weak electrolyte is shown in the figure. Strong electrolyte m conc. Weak electrolyte m conc. Molar conductivity of a strong electrolyte varies linearly with concentration . It can be extrapolated when concentration approaches zero. The molar conductivity at infinite dilution is known as limiting molar conductivity and is represented by o m m or    . It can be experimentally determined for strong electrolytes only because m slowly increases linearly with dilution and can be represented as o    m m A C The intercept of the plot along Y-axis is o m and slope equal to (–A). The magnitude of slope for a given solvent and temperature depends on the charges on cations and anions produced on dissociation of the electrolyte in the solution. Thus, all electrolytes having same type of charges have same value for A. Explanation for the Variation of Molar Conductivity with Concentration Conductance Behaviour of Weak Electrolytes The variation of  with dilution can be explained on the basis of number of ions in solution. The number of ions furnished by the electrolyte in solution depends upon the degree of dissociation with dilution. With the increase in dilution, the degree of dissociation increases and as a result molar conductance increases. The limiting value of molar conductance (m  ) corresponds to degree of dissociation equal to 1 i.e., the whole of the electrolyte dissociates.