From my observations in my weekly Sec 4 O-Level Pure Chemistry Tuition Classes, i noticed that many of the secondary 4 Chemistry students are very weak when it comes to application questions involving Selective Discharge of Ions in Electrolytic Cells. This is covered in the topic of Electrolysis in their school.
In an aqueous solution, more than one type of cation and anion are present in the electrolyte. However, only one cation and one anion are preferentially or selectively being discharged during the process of electrolysis.
If the electrodes used during electrolysis are inert, the ions being preferentially discharged will depend on three factors:
1) Selective Discharge of Cations
2) Selective Discharge of Anions
3) Effect of Concentration on the Selective Discharge of Anions
3 Factors Affecting Selective Discharge of Ions (using Inert Electrodes)
1. Selective Discharge of Cations
This is linked to the topic of Metals & Reactivity Series.
The ease of discharge of cations will depend on the position of the metals in the reactivity series.
You learnt that the more reactive a metal, the greater its tendency to react and form ions.
This simply means that in electrolysis, the more reactive the metal, the least tendency for the metal ions to be preferentially discharged at the cathode.
Ions of reactive metals like potassium and sodium will remain in the electrolyte solution and will not be discharged.
Hydrogen ion and ions of less reactive metals like copper and silver will be preferentially discharged.
In aqueous solution, hydrogen ions will be preferentially discharged over the ions of the metals above hydrogen in the reactivity series. Hydrogen gas will be liberated.
Likewise, if the cations come from a metal which is below hydrogen in the reactivity series, then the metal ions will be preferentially discharged. A metal will be produced.
2. Selective Discharge of Anions
Ease of discharge of anions increases down the list:
- Sulfate ion, SO42-
- Nitrate ion, NO3–
- Chloride ion, Cl–
- Bromide ion, Br–
- Iodide ion, I–
- Hydroxide ion, OH–
Note that sulfate and nitrate ions will remain in the electrolyte solution. They are not electrolysed during electrolysis.
Hydroxide ions has the highest ease of discharge because it gives out electrons most readily (easy to be oxidised) during electrolysis. Oxygen gas will be produced.
Half equation for the discharge of hydroxide ions: 4OH–(aq) => 2H2O(l) + O2(g) + 4e–
3. Effect of Concentration on the Selective Discharge of Anions
It is important to note that an anion in higher concentration is always being preferentially discharged, regardless of the ease of discharge of anions (factor 2).
Let me give you a simple example to discuss the effect of concentration on the selective discharge of anions.
In the electrolysis of concentrated sodium chloride solution, the ions available in the electrolyte are:
- Sodium ion, Na+
- Chloride ion, Cl–
- Hydrogen ion, H+ (due to partial dissociation of water)
- Hydroxide ion, OH– (due to partial dissociation of water)
Both chloride and hydroxide ions are attracted to the positive anode.
Based on the ease of discharge of anions (factor 2), hydroxide ions should be preferentially discharged.
However, in concentrated solution, there are a lot more chloride ions than hydroxide ions.
Therefore, chloride ions are preferentially being discharged over the hydroxide ions at the anode.
YouTube Video Tutorial on Selective Discharge of Ions in Electrolytic Cells – Part 1: Position of Reactivity Series and Concentration Effect
In the YouTube Video below, you will learn in a step-by-step manner on how Position of Reactivity Series and Concentration Effect will determine the selective discharge of ions in Electrolysis.
Length of Video: 14.16 minutes
Direct Video Link:https://youtu.be/gQVimP_UtJg
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 and learn the key O-Level Pure Chemistry concepts together.
In the next blog post, i will be sharing a video on Selective Discharge of Ions in Electrolytic Cells based on the Nature of Electrodes. Do stay tuned to it!
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