Q24
E⁰_cell for the given redox reaction is 2.71 V
Mg(s) + Cu²⁺(0.01 M) –––> Mg²⁺ (0.001 M) + Cu(s)
Calculate E_cell for the reaction. Write the direction of flow of current when an external opposite potential applied is
(i) less than 2.71 V and
(ii) greater than 2.71 V
(a) A steady current of 2 amperes was passed through two electrolytic cells X and Y connected in series containing electrolytes FeSO₄ and ZnSO₄ until 2.8 g of Fe deposited at the cathode of cell X. How long did the current flow? Calculate the mass of Zn deposited at the cathode of cell Y.
(Molar mass : Fe = 56 g mol⁻¹, Zn = 65.3 g mol⁻¹, 1F
Solution
The cell can be represented as:
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<p><img alt="" src="" style=height:198px; width:270px
(i) When the external opposite potential is less than 2.71 V then electron flows from Mg rod to Cu rod hence current flows from Cu to Mg i.e., the direction of flow of current is from cathode to anode.
(ii) When the external opposite potential is greater than 2.71 V then electron flows from Cu rod to Mg rod and current flows from Mg to Cu i.e., the direction of flow of current is from anode to cathode.
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(b) (i) The electrolyte A is a strong electrolyte, and the electrolyte B is a weak electrolyte.
(ii) On extrapolation, for electrolyte, A limiting value of conductance is obtained that is conductance at zero concentration.
The curve obtained for a strong electrolyte shows that there is a small decrease in molar conductivity with increase in concentration. In other words, the molar conductivity is increased only slightly on dilution (for observing dilution effects, go towards zero on X-axis). A strong electrolyte is completely dissociated in solution and thus, furnishes all ions for conductance. However, at higher concentrations, the dissociated ions are close to each other and thus, the inter-ionic attractions are greater. These forces retard the motion of the ions and thus, conductivity is low. With a decrease in concentration (dilution), the ions move away from each other thereby feeling less attractive forces from the counterions. This results in an increase in molar conductivity with dilution. The molar conductivity approaches a maximum limiting value at infinite dilution designated as ∧ₘ⁰.
For electrolyte B:
The curve obtained for B shows that there is a large increase in the value of molar conductivity with dilution, especially near-infinite dilution. This is because as the solution of a weak electrolyte is diluted, its ionization is increased. This results in more number of ions in solution and thus, there is an increase in molar conductivity. However, the conductance of a weak electrolyte never approaches a limiting value. Or in other words, it is not possible to find conductance at zero concentration.
