When aqueous solutions of metal halides are subjected to electrolysis, the situation becomes more complicated due to the presence of water. Water is a very weak electrolyte and upon ionization it gives very small amount of hydrogen and hydroxide ions.
This means that the electrolyte will consist of more than one type of ions which will approach both cathode and anode. Hence there can be a choice over which ions will get discharged.
Electrochemical principle
For example, if an aqueous solution of sodium chloride is electrolyzed, both sodium and hydrogen cations are attracted towards the cathode and both chloride and hydroxide ions are attracted towards the anode. Which ion is preferentially discharged depends upon the tendency of the elements to lose or gain electrons. The elements present at the top of electrochemical series (Table 16.2) possess more tendency to lose electrons as compared to elements at the bottom.
If a metal is below hydrogen in the electrochemical series, then the metal ion is discharged. Metals like this include copper and silver. If dilute copper chloride is electrolyzed, copper metal is obtained at the cathode.
If a metal is placed higher in the series then hydrogen ion is discharged. Metals like lithium, sodium, magnesium. If dilute NaCl is electrolyzed, hydrogen gas is obtained at the cathode.
Intermediate case: lead & nickel
Metals like lead and nickel present a more complicated picture. If aqueous solutions of halides of these metals are electrolyzed, then the products obtained at the cathode depends on the concentrations. If the solution is concentrated then metal is deposited at the cathode. If the solution is very dilute, hydrogen is evolved. At in-between concentrations both metal and hydrogen are obtained.
At anode, the electrolysis of dilute solutions of halides will deposit oxygen because hydroxide ions are preferentially discharged due to their higher concentrations.
Cathode reactions
2Mⁿ⁺ + 2e⁻ → 2M(s)
2H⁺ + 2e⁻ → H₂ or 2H₂O + 2e⁻ → H₂ + 2OH⁻
Anode reactions
2X⁻ → X₂ + 2e⁻ (halogen)
4OH⁻ → O₂ + 2H₂O + 4e⁻
Electrolysis played a crucial role in the discovery and industrial production of elements like fluorine and aluminum.
📊 Table 16.2 – Electrochemical Series
| Electrode | Half Reaction | Standard Potential (V) |
|---|---|---|
| Lithium | Li⁺ + e⁻ → Li | -3.04 |
| Potassium | K⁺ + e⁻ → K | -2.93 |
| Calcium | Ca²⁺ + 2e⁻ → Ca | -2.86 |
| Sodium | Na⁺ + e⁻ → Na | -2.71 |
| Magnesium | Mg²⁺ + 2e⁻ → Mg | -2.37 |
| Aluminum | Al³⁺ + 3e⁻ → Al | -1.66 |
| Zinc | Zn²⁺ + 2e⁻ → Zn | -0.82 |
| Iron (II) | Fe²⁺ + 2e⁻ → Fe | -0.44 |
| Cadmium | Cd²⁺ + 2e⁻ → Cd | -0.40 |
| Nickel | Ni²⁺ + 2e⁻ → Ni | -0.25 |
| Lead | Pb²⁺ + 2e⁻ → Pb | -0.12 |
| Hydrogen | 2H⁺ + 2e⁻ → H₂ | 0.00 |
| Copper | Cu²⁺ + 2e⁻ → Cu | +0.34 |
| Iodine | I₂ + 2e⁻ → 2I⁻ | +0.53 |
| Bromine | Br₂ + 2e⁻ → 2Br⁻ | +1.06 |
| Chlorine | Cl₂ + 2e⁻ → 2Cl⁻ | +1.36 |
| Fluorine | F₂ + 2e⁻ → 2F⁻ | +2.87 |
Chapter-16 Chemistry – X
KEYNOTE: Dilute halides: at anode O₂ (OH⁻ discharged) ; at cathode: metal if below H, H₂ if above H.
TRICK: “Higher on series, hydrogen cheers; lower on series, metal appears.”
MEMORIZE: For Pb, Ni: concentrated → metal; dilute → hydrogen; in-between → both.