State of Electrolyte

Solid ionic compounds cannot conduct electricity because there are no free-moving ions. In molten or aqueous states, ions are free to move.

Example: Molten PbBr₂

Pb²⁺ ions move to the cathode and receive electrons to form lead metal.

Br⁻ ions move to the anode and donate electrons to form bromine gas (Br₂).

Factor 1: Position of Ions in the Electrochemical Series

Ions located lower in the electrochemical series have a higher tendency to be discharged.

Cations (Ease of discharge increases downwards):

\[\text{K}^+ \rightarrow \text{Na}^+ \rightarrow \text{Ca}^{2+} \rightarrow \text{Mg}^{2+} \rightarrow \text{Al}^{3+} \rightarrow \text{Zn}^{2+} \rightarrow \text{Fe}^{2+} \rightarrow \text{Sn}^{2+} \rightarrow \text{Pb}^{2+} \rightarrow \text{H}^+ \rightarrow \text{Cu}^{2+} \rightarrow \text{Ag}^+\]

Anions (Ease of discharge increases downwards):

\[\text{F}^- \rightarrow \text{SO}_4^{2-} \rightarrow \text{NO}_3^- \rightarrow \text{Cl}^- \rightarrow \text{Br}^- \rightarrow \text{I}^- \rightarrow \text{OH}^-\]

Example 1 — Sodium sulphate solution (Na₂SO₄):

Ions present: Na⁺, H⁺, SO₄²⁻, OH⁻.

Cathode: H⁺ is discharged because it is less electropositive than Na⁺; hydrogen gas (H₂) is released (gives 'pop' sound with burning splinter).

Anode: OH⁻ is discharged because it is less electronegative than SO₄²⁻; oxygen gas (O₂) is released (relights glowing splinter).

Example 2 — Copper(II) sulphate solution (CuSO₄):

Ions present: Cu²⁺, H⁺, SO₄²⁻, OH⁻.

Cathode: Cu²⁺ is discharged because it is less electropositive than H⁺; copper metal is deposited.

Anode: OH⁻ is discharged; oxygen gas is released.

Factor 2: Concentration of Electrolyte

Influences the selection of negative ions discharged at the anode:

Negative ions of higher concentration are selected to be discharged at the anode even if they are higher in the electrochemical series.

Cathode selection is not affected by concentration — positive ions are still discharged based on their position in the electrochemical series.

Example — Hydrochloric acid (HCl) or Sodium chloride (NaCl):

Concentrated (1.0 mol dm⁻³): Concentration of Cl⁻ is higher than OH⁻. Chloride ions (Cl⁻) are discharged at the anode, forming chlorine gas (Cl₂), which bleaches moist blue litmus paper.

Dilute (0.0001 mol dm⁻³): Hydroxide ions (OH⁻) are discharged at the anode because they are lower in the electrochemical series; oxygen gas (O₂) is formed.

Factor 3: Types of Electrode

If the metal anode is the same as the metal ion in the electrolyte:

Anode: Metal atoms ionise and dissolve into the electrolyte forming positive ions (metal → metal ion + e⁻).

Cathode: Metal ions in the electrolyte discharge and deposit on the cathode.

Concentration of metal ions in the electrolyte remains unchanged.

If the anode is inert / not the same (e.g., carbon):

Anode does not dissolve; negative ions in the electrolyte are discharged.

Cathode: Less electropositive cation is discharged.

Example — Silver nitrate solution (AgNO₃):

With silver electrodes: Silver anode dissolves (Ag → Ag⁺ + e⁻); silver is deposited at cathode; Ag⁺ concentration remains constant.

With carbon electrodes: OH⁻ discharged at anode to produce oxygen gas; silver is deposited at cathode; Ag⁺ concentration decreases.