MOLAR CONDUCTIVITY

 

MOLAR CONDUCTIVITY 

• The conductivity  is defined as the conductivity of an electrolyte solution divided by molar concentration
• Molar conductivity = λ_m = κ/c
•  if k is expressed in Sm^–1 and the concentration, c in mol m^–3 then the unit of λ_m will be Sm²mol^–1.
•  If we use Scm^–1 as the units for k and mol cm^–3, the units of
•  concentration, then the units for  λ_m  are Scm²mol^–1. It can be calculated by using the equation:
•  λ_m (Scm²mol^–1) = κ(S cm^–1) × 1000 (cm³/L)/molarity (mol/L).

ž OR, 1 Sm²mol^–1 = 10⁴ Scm²mol^–1

Variation of molar conductivity of an ionic solution with concentration

 

Variation of molar conductivity of an ionic solution with concentration

•  conductivity increases with increase in concentration since it depends on number of ions per volume.
• Molar conductivity increases with decrease in concentration since it depends on mobility of ions.
•  When concentration approaches zero, the molar conductivity is known as limiting molar conductivity and is represented by  E°_m

Kohlrausch law of independent migration of ions and its applications

It states that limiting molar conductivity of an electrolyte  is the sum of the individual contributions of the anion and cation of the electrolyte.

ü  Thus, if λ°_Na+ and λ°_{Cl -}  are limiting molar conductivity of the sodium and  chloride ions , then the limiting molar conductivity for sodium  chloride is given by the equation:   

                     Ë°_m(NaCl) = λ°_Na+ + λ°_{Cl -} 

Applications of Kohlrausch law 

This law may be used to determine the limiting molar conductivity, ‘λ°_m’ degree of dissociation ‘α’ and dissociation constant ‘K_a’ of a weak electrolyte

Faraday’s Laws of Electrolysis

 

 (i) First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

 (ii)Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic Mass of Metal ÷ Number of electrons required to reduce the cation).

Electrolytic cell

☝This is a device  which brings about a chemical change by means of electric current supplied. 

☝In this cell two electrodes are  dipped in same container.

☝ electricity from outside source supplies electrons for the non-spontaneous reaction.R

☝Reduction takes place cathode where metal from electrolytic solution gets deposited.

ü  oxidation takes place anode  from where metal goes into the electrolytic solution.

Corrosion And Prevention of corrosion

•   Corrosion is the oxidative deterioration of metal.
•   25% of steel produced goes to replace steel structures and products destroyed by corrosion.
•  Rusting of iron requires the presence of both oxygen and water.
•   Rusting results from tiny galvanic cells formed by water droplets.

Prevention of corrosion

•  Galvanizing: is the coating of iron with zinc. Zinc is more easily oxidized than iron, which protects and reverses oxidation of the iron.
•   Cathodic Protection: is the protection of a metal from corrosion by connecting it to a metal (a sacrificial anode e.g. Mg or Zn) that is more easily oxidized.
•   Electroplating.
•   By applying paint, grease, rubber to prevent contact of metal surface from air.

Electrolytic cell and electrolysis

•  Electrolysis: It is the process in which electrical energy is used to drive a non-spontaneous chemical reaction.
•   An electrolytic cell is an apparatus for carrying out electrolysis.
•   Processes in an electrolytic cell are the reverse of those in a galvanic cell.

Electrolysis process is used in  Manufacture of Cl₂ and NaOH, Electro-refining and Electroplating, Electrolysis of water