Q. 32

Aqueous copper sulphate solution and aqueous silver nitrate solution are electrolysed by 1 ampere current for 10 minutes in separate electrolytic cells. Will, the mass of copper and silver, deposited on the cathode be the same or different? Explain your answer.

Answer :

Faraday’s second law states that the mass of substances deposited on the electrode is proportional to their equivalent weight if the same amount of electricity is passed through different electrolytic solutions.

Faraday’s law : M E


M=nE


Where,


M is the mass of substance deposited


n is the number of moles


E is the equivalent weight of the substance


Q=It


where,


Q is the charge


I is the current and


t is time(in seconds)


Putting the values in the above formula , we get


1A x (10 x 60) sec = 600C


The electrochemical reactions for silver and copper are as given below:


Cu2+ + 2e- Cu


Ag+ + e- Ag


1 mole of electrons have a charge equal to 1 Faraday= 96500 C


For copper, we need 2 moles of electrons or (2F of charge) and for silver only one mole of the electron (1F of charge)


So, mass deposited


For silver,


If 1 F (96500 C) or 1 mole of electrons can deposit 1 mole i.e., 108 g of Ag, then


600 C can deposit,


g = 0.671 g of silver


For copper,


If 2 Faraday(96500 C) or 2 moles of electrons can deposit 1 mole i.e., 63g of Cu, then


600 C can deposit,


g = 0.195 g of copper


The highlighted fractions represent the equivalent weight.


Equivalent mass of Cu2+ is different from the equivalent mass of Ag+ so obviously the mass of copper deposited will not be the same as the mass of silver deposited.


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