In the previous blogpost, we have discussed on the 2nd criterion for predicting products of electrolysis for aqueous solutions under the Series on “Secrets of Redox and Electrolysis“.

Today we will end the series by introducing the 3rd criterion:

Consider: Nature of Electrodes Used in Electrolysis

Different products are discharged at the electrode when different type of electrodes are used.
When we say different types of electrodes used, it simply means:

Inert Electrode (such as Carbon and Platinum) vs Reactive Electrode (such as Cu or Silver)

We shall once again look at it in a step-by-step simplified approach.

Lets consider 2 electrolysis reactions using Copper(II) Sulphate as the electrolyte.

Set-Up A: Inert Electrodes such as Platinum

At the +ve Anode:

• 4OH-(aq) –> 2H2O(l) + O2(g) + 4e-
• SO42- ions remain in solution

At the –ve Cathode:

• Cu2+(aq) + 2e- –> Cu(l)
• H+ ions remain in solution

Summary:

• Overall reaction is: 2CuSO4(aq) + 2H2O(l) –> 2Cu(s) + O2(g) + 2H2SO4(aq)
• Acidic solution of H2SO4(aq) formed at end of reaction

Set-Up B: Reactive Electrodes such as Copper

At the +ve Anode:

• Cu(s) –> Cu2+(aq) + 2e-
• Anode dissolves to form Cu2+ ions in aqueous solution

At the –ve Cathode:

• Cu2+(aq) + 2e- –> Cu(l)
• H+ ions remain in solution
• Cathode becomes coated with a layer of reddish-brown Cu

Summary:

• Cu is transferred from the anode to the cathode
• Cu cathode slowly increases in mass
• Cu anode decreases in mass
• Colour & Concentration of CuSO4 remain unchanged
• No effective loss of Cu2+ ions from the CuSO4 solution

Let’s check out 1 question to see how much you understand.

Question:
Aqueous copper(II) sulphate is electrolyzed using copper electrodes. Which observations will be made?
Anode (+ve) Cathode (-ve) the electrolyte
A. anode dissolves           pink solid forms          blue colour fades
B. anode dissolves           pink solid forms           no change
C. colourless gas forms   colourless gas forms    no change
D. colourless gas forms   pink solid forms blue   colour fades

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