In the previous blog post on Selective Discharge of Ions and Reactivity Series – Part 1, we have looked at the 3 factors on how ions in the electrolyte are preferentially being discharged based on (1)Selective Discharge of Cations & Anions and (2)Concentration Effect of Anions. Today, we will look at the last factor known as Nature of Electrodes when it comes to selective discharge of ions in electrolysis.

We will compare the electrolysis of copper (II) sulfate solution using two different types of electrodes: Inert Electrodes and Reactive Electrodes.

Nature of Electrodes: (A) INERT versus (B) REACTIVE

(A) Using Inert Electrodes e.g. Platinum or Graphite electrodes

At the anode:

• Hydroxide and sulfate ions are attracted to the positive anode
• Hydroxide ions are preferentially discharged to liberate oxygen gas
• Sulfate ions are not discharged and remains in the electrolyte solution

At the cathode:

• Hydrogen and copper (II) ions are attracted to the negative cathode
• Copper is lower than hydrogen in the reactivity series
• Thus, copper (II) ions are preferentially discharged
• Reddish-brown copper metal is deposited at the cathode

In the electrolyte solution:

• Blue colour of copper (II) sulfate solution gradually becomes lighter
• Remaining hydrogen and sulfate ions combine to form sulfuric acid
• Resulting electrolyte becomes increasingly more acidic

(B) Using Reactive Electrodes e.g. Copper electrodes

At the anode:

• Hydroxide and sulfate ions are attracted to the positive anode
• We would expect hydroxide ions to be preferentially discharged over sulfate ions based on selective discharge of anions
• However, copper is a reactive electrode and itself will be preferentially discharged (oxidised) to form copper (II) ions
• Cu(s) => Cu²⁺(aq) + 2e^–
• Anode will decrease in size and mass

At the cathode:

• Hydrogen and copper (II) ions are attracted to the negative cathode
• Copper is lower than hydrogen in the reactivity series
• Thus, copper (II) ions are preferentially discharged
• Reddish-brown copper metal is deposited at the cathode

In the electrolyte solution:

• There is a net transfer of copper (II) ions from the anode to the cathode
• There is no net loss of copper (II) ions from the copper (II) sulfate solution
• Blue colour and concentration of copper (II) sulfate solution remain unchanged

The table below further summarises the difference in products obtained from the electrolysis of copper (II) sulfate solution using inert (platinum or graphite) electrodes and reactive (copper) electrodes.

This is an important concept in Electrolysis. We always ensure that O-Level and IP Chemistry students in our recommended weekly Pure Chemistry Tuition Classes know it well. If you are still unsure about it, you can take a look at the video below.YouTube Video Tutorial on Selective Discharge of Ions in Electrolytic Cells – Part 2: Nature of Electrodes

In the YouTube Video below, you will learn in a step-by-step manner on how Nature of Electrodes will determine the selective discharge of ions in Electrolysis.

Length of Video: 11.59 minutes

Direct Video Link:https://youtu.be/njX27H_tZc8

 

I hope you find the content easy for your understanding and if you have anything valuable to add, leave me a comment below.Feel free to share this blog post with your friends.In the next blog post, i will be sharing a video on Why   Different Sodium Chloride Electrolytes will give you Different Products when Inert Electrodes are used in Electrolytic Cells. Do stay tuned to it!

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Electrode	Cathode	Anode	Reaction at Cathode	Reaction at Anode
Copper (II) sulfate solution	Platinum	Platinum	Reddish-brown copper metal is deposited. Cathode increase in size and mass.	Oxygen gas is liberated. Effervescence observed.
Copper (II) sulfate solution	Copper	Copper	Reddish-brown copper metal is deposited. Cathode increase in size and mass.	Copper anode dissolves. Anode increase in size and mass.