Chemistry Notes & Tips

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^–
• SO₄^2- 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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PPS: To learn everything on Redox & Electrolysis, please visit the section right below called “Table of contents for Secrets of Redox & Electrolysis“