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

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

What key terms must I know?

These seven terms are used throughout the topic.

• Electrolyte: a compound that conducts electricity when molten or dissolved in water, and is decomposed by it.
• Cathode: the negative electrode, where reduction (gain of electrons) takes place.
• Anode: the positive electrode, where oxidation (loss of electrons) takes place.
• Cation: a positive ion that moves to the cathode, such as Cu²⁺ and H⁺.
• Anion: a negative ion that moves to the anode, such as Cl^– and OH^–.
• Discharge: the change of an ion into a neutral atom or molecule by gaining or losing electrons at an electrode.
• Electrochemical series: a list of ions arranged in order of how easily they gain or lose electrons.

Why does preferential discharge happen?

It happens because an aqueous electrolyte contains more than one ion of each charge.

• Several ions present: an aqueous solution contains ions from the compound and also H⁺ and OH^– from water.
• Competition: at each electrode, the ions of the same charge compete to be discharged.
• Result: usually only one ion is discharged at each electrode, and the others stay in solution.

What factors affect the preferential discharge of ions?

Three factors affect the preferential discharge of ions.

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

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

When other factors are constant, the ion that gains or loses electrons more easily is discharged first.

Cations

• Rule: when other factors are constant, the cation lower in the series (less electropositive) is discharged in preference to one higher up (more electropositive).
• Reason: the less electropositive ion gains electrons more readily from the cathode.
• At the cathode: the ion is reduced and discharged as a neutral atom.
• The other ion: the more electropositive ion stays in the solution as a positive ion.
• Example: in aqueous copper(II) sulphate, Cu²⁺ is discharged before H⁺, and copper is deposited.

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

Anions

• Rule: when other factors are constant, the less electronegative anion is discharged in preference to the more electronegative one.
• Reason: the less electronegative ion loses electrons more readily.
• At the anode: the ion is oxidised and discharged as a neutral atom or molecule.
• Example: OH^– from water is discharged before SO₄^2-, so oxygen is given off.

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

Ions that are not discharged from aqueous solution

Six common ions are not discharged from aqueous solution: three cations and three anions.

How does the concentration of ions affect discharge?

An ion is more likely to be discharged when its concentration in the electrolyte is high.

• Main rule: when the concentration of an ion in the electrolyte is increased, the ion is more likely to be discharged.
• When it matters: the effect works only when the two competing ions are close together in the electrochemical series.
• When it matters less: the effect becomes less important as the competing ions are placed further apart in the series.
• Example, concentrated NaCl: Cl^– is plentiful, so chlorine is given off at the anode.
• Example, dilute NaCl: Cl^– is few, so OH^– from water is discharged and oxygen is given off.
• Cathode in both cases: Na⁺ is not discharged from aqueous solution, so H⁺ is discharged and hydrogen is given off.

How does the nature of the electrodes affect discharge?

The electrode material decides whether only the ions in solution are discharged or the electrode itself takes part.

Inert electrodes

• Meaning: inert electrodes do not take part in the electrolytic reactions.
• Examples: platinum and graphite (carbon).
• Limits: platinum is attacked by liberated chlorine, and graphite is attacked by liberated oxygen.
• Effect on discharge: only the ions of the electrolyte and water are discharged.

Electrodes that influence discharge

Some electrodes have a strong affinity for certain ions and influence the discharge of those ions.

• Mercury cathode with aqueous NaCl: Na⁺ is discharged at the cathode to form sodium amalgam, Na/Hg.
• Equation at the mercury cathode: Na⁺(aq) + Hg(l) + e^– → Na/Hg(l)
• Copper anode with CuSO₄ solution: neither SO₄^2- nor OH^– is discharged.
• What happens instead: copper atoms lose electrons more readily and go into solution as Cu²⁺.
• Reactive electrode: an electrode that takes part in the reaction, such as the copper anode here.

Comparing the two kinds of electrode

An inert electrode only carries current, while a reactive electrode takes part in the reaction.

What are some worked examples of preferential discharge?

The table applies the three factors to common electrolytes.

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.
5. Name the products at each graphite electrode when dilute NaCl solution is electrolysed.

Assignment

Explain how the nature of the electrode changes the products of electrolysis in these two cases:

1. Aqueous NaCl electrolysed with a mercury cathode: state what happens at the cathode and write the equation.
2. CuSO₄ solution electrolysed with a copper anode: state what happens at the anode.