Electrochemistry: Ohm՚s Law, Conductance and Conductivity, Molar Conductivity (For CBSE, ICSE, IAS, NET, NRA 2022)

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Ohm՚s Law

According to this law, current of amperes flows through a solution which offers a resistance of ohms and a potential difference of volts is applied, then

If the solution is taken in a conductivity cell which has two parallel electrodes apart and each having an area of cross section , the resistance of the electrolyte is found to be directly proportional to and inversely proportional to i.e..

“rho” is a constant of proportionality and is called specific resistance or restively.

Conductance and Conductivity

  • The conductance is reciprocal of resistance.
  • The conductivity is reciprocal of specific resistance or resistivity.
  • Conductance is denoted by and is measured in the unit of and now known as named as siemens, .
  • The conductivity is denoted by k “kappa” .

The unit of conductivity (K) is expressed in .

The Value of Conductivity of Some Selected Substance at

The Value of Conductivity

Molar Conductivity

  • Molar conductivity is the conducting power of all the ions furnished by one mole of an electrolyte in a solution of specified concentration.
  • It is denoted by and is related to by the relation.

  • is the molarity of the solution.
  • Its units are .

Factor Affecting Conductivity

Nature of Electrolyte

  • Conductivity of an electrolyte depends upon the Nature of electrolyte.
  • A weak electrolyte furnishes fewer ions therefore it has lower conductivity than a strong electrolyte of same concentration.
  • The ions in electrolyte with higher valency carry more charge and therefore they conduct more charge than the ion of lower valency.
  • The ion which can move faster will carry the charge also faster and therefore has more conducting power.

Temperature

  • Conductivity of an electrolyte generally increases by 2 – 3 percent for each degree rise in temperature.
  • With increase in temperature viscosity of the solvent decreases and thus ion can move faster.

Concentration

  • Variation of conductivity with concentration
  • When the solution diluted its conductivity also decreases. It is because is the conducting power of all the ions present per of the solution.
Concentration Conductivity

Variation of Molar and Equivalent Conductivity with Concentration

As the solution is diluted its molar conductivity increases.

It is given by

But strong and weak electrolyte as show different type of Behaviour on dilution

Variation of Molar
  • Fig. show that molar conductivity for a strong electrolyte like is very gradual on dilution and also the value is high at all concentrations.
  • In case of weak electrolyte like , there is gradual increase in on dilution which is followed by a sharper increase on further dilution.
  • In strong electrolyte, attraction between opposite ions is large and their conducting ability is less. On dilution inter-ionic forces decrease and ions can move faster leading to the increase in molar conductivity.
  • In weak electrolyte, molar conductivity is low in concentrated solution. This is due to partial dissociation (ionization) of weak electrolytes . On diluting the solution, the degree of ionization increases, which increases the number of ions and leads to a sharp increase in molar conductivity in weak electrolytes.

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