Electrolysis II: Factors Affecting Preferential Discharge of Ions – SS2 Chemistry Lesson Note (First Term)

Preferential discharge is the process by which one ion is discharged at an electrode in preference to other ions present in the electrolyte. Three factors decide which ion is discharged: the position of the ion in the electrochemical series, the concentration of the ion, and the nature of the electrodes.

Behavioural objectives

By the end of the lesson, students should be able to:

• State the three factors that affect the preferential discharge of ions.
• Explain how the position of an ion in the electrochemical series decides whether it is discharged.
• Name the ions that are not discharged from aqueous solution.
• Explain the effect of concentration on the discharge of ions.
• Describe how the nature of the electrode affects discharge, with examples.

What factors affect the preferential discharge of ions?

The discharge of ions at the electrodes is governed by three conditions:

1. The position of ions in the electrochemical series.
2. The concentration of ions.
3. The nature of the electrodes.

How does the position of ions in the electrochemical series affect discharge?

Cations

When all other factors are constant, a cation (an ion that moves to the cathode) which is lowest in the series (less electropositive) will be discharged in preference to one higher up (more electropositive). The less electropositive ion gains electron(s) more readily from the cathode, so it is reduced and discharged as a neutral atom. The more electropositive ion remains in the solution as a positive ion.

NOTE: K⁺, Na⁺ and Ca²⁺ are never discharged from aqueous solution, because of the large gap between them and H⁺. However, K⁺, Na⁺ and Ca²⁺ are discharged during the electrolysis of their molten salts.

Anions

An anion which is higher up in the series (less electronegative) is discharged in preference to one lower down the series (more electronegative). This is because the less electronegative ion loses electron(s) more readily than the more electronegative ion.

NOTE: F^–, SO₄^2- and NO₃^– are never discharged from aqueous solution, because of the large gap between them and OH^–.

How does the concentration of ions affect discharge?

When the concentration of an ion in the electrolyte is increased, the ion is more likely to be discharged. The influence of concentration is effective only when the two competing ions are closely positioned in the electrochemical series. The effect of concentration becomes less important as the positions of the competing ions become further apart in the series.

How does the nature of the electrodes affect discharge?

Inert electrodes

Inert electrodes (for example platinum and graphite) do not take part in the electrolytic reactions. However, platinum is attacked by liberated chlorine and graphite is attacked by liberated oxygen.

Electrodes that influence discharge

Some electrodes have a strong affinity for certain ions and influence the discharge of those ions. For example, in the electrolysis of aqueous NaCl using a mercury cathode, Na⁺ is discharged at the cathode to form sodium amalgam, Na/Hg:

Na⁺(aq) + Hg(l) + e^– → Na/Hg(l)

Also, in the electrolysis of CuSO₄ solution using a copper anode, neither SO₄^2- nor OH^– is discharged. Instead, the Cu atoms lose electrons more readily and go into solution as Cu²⁺. For this reason, the copper anode is known as a reactive electrode.

Evaluation

1. State the three factors that affect the preferential discharge of ions.
2. Why are K⁺, Na⁺ and Ca²⁺ not discharged from aqueous solution?
3. When does the concentration of an ion influence which ion is discharged?
4. What is a reactive electrode? Give one example.

Assignment

1. Explain what happens at the cathode when aqueous NaCl is electrolysed using a mercury cathode. Write the equation.
2. Explain what happens at the anode when CuSO₄ solution is electrolysed using a copper anode.