ICSE Class 10 Chemistry: Complete Conceptual Guide to Electrolysis
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Electrolysis is the process of decomposing a chemical compound in its molten or aqueous state by passing a direct electric current through it. Unlike metallic conduction where electrons travel without altering the metal lattice, electrolytic conduction physically splits ionic bonds and moves matter to the electrodes. Mastering the preferential discharge series and the distinction between active and inert electrodes is essential for scoring top marks in the ICSE board examination.
Fundamentals of Electrolytes and Ionic Dissociation
An electrolyte is a compound that conducts direct electric current in a molten state or in an aqueous solution, undergoing chemical decomposition in the process. In contrast, non-electrolytes such as pure water, glucose, and carbon tetrachloride consist purely of covalent molecules and do not provide mobile ions, rendering them non-conductive.
ICSE distinguishes strictly between two classes of electrolytes based on their degree of dissociation:
- Strong electrolytes (such as dilute sulphuric acid, aqueous sodium hydroxide, and molten lead bromide) dissociate almost completely into ions in solution. They contain almost entirely free ions and conduct current efficiently.
- Weak electrolytes (such as acetic acid, ammonium hydroxide, and carbonic acid) undergo partial dissociation. Their solutions contain both unionized molecules and a relatively small fraction of free ions, resulting in poor electrical conductivity.
Metallic conduction differs fundamentally from electrolytic conduction. In metallic conductors, current flows entirely through the drift of free electrons without any chemical change or transfer of mass, and electrical resistance increases with temperature. In electrolytes, current is transported by the physical migration of cations and anions, accompanied by chemical decomposition at the electrodes, and electrical resistance decreases as temperature increases because ionic mobility rises.
Electrode Mechanics: Redox Reactions and Direct Current
An electrolytic cell consists of two conductive terminals immersed in an electrolyte and powered by a direct current (DC) source. Alternating current (AC) cannot be used for electrolysis because the rapid reversal of polarity causes continuous switching between oxidation and reduction at both electrodes, preventing any net chemical change from taking place.
The two electrodes serve distinct redox functions based on their polarity:
- Cathode (Negative Electrode): Connected to the negative terminal of the DC source, the cathode is electron-rich. Positively charged cations migrate toward the cathode and accept electrons, undergoing reduction (e.g., Pb2+ + 2e- → Pb).
- Anode (Positive Electrode): Connected to the positive terminal, the anode is electron-deficient. Negatively charged anions migrate toward the anode and lose electrons, undergoing oxidation (e.g., 2Br- - 2e- → Br2).
The entire circuit maintains electrical neutrality because the number of electrons released by anions at the anode equals the number of electrons accepted by cations at the cathode. This closed transfer loop features ions carrying charge internally through the solution and electrons carrying charge through the external copper wiring.
Preferential Discharge Theory
When an aqueous solution contains multiple species of cations and anions (due to simultaneous ionization of the solute and water), ions compete for discharge. The preferential discharge theory dictates which ion is discharged based on three governing rules:
- Position in the Electrochemical Series: Ions placed lower in the series have a lower discharge potential and discharge more easily. For cations, the discharge order follows: K+ < Ca2+ < Na+ < Mg2+ < Al3+ < Zn2+ < Fe2+ < Pb2+ < H+ < Cu2+ < Ag+. For anions, ease of discharge increases downward: SO42- < NO3- < Cl- < Br- < I- < OH-. Hence, Cu2+ discharges before H+, and OH- discharges before SO42-.
- Relative Concentration of Ions: If an anion higher in the series is present in a much higher concentration, it can discharge preferentially over a lower ion. For example, in concentrated NaCl (brine), Cl- discharges at the anode instead of OH-. However, this concentration rule applies primarily to anions and does not override the discharge of highly electropositive cations like Na+ in aqueous systems.
- Nature of the Electrodes: Inert electrodes (platinum, graphite) do not take part in chemical reactions. Active electrodes (copper, nickel, silver) participate in the reaction: the anode dissolves by losing electrons instead of discharging the anions present in the electrolyte.
Exemplar ICSE Electrolytic Processes
1. Electrolysis of Molten Lead Bromide (PbBr2): This process is conducted in a silica crucible because silica withstands high temperatures and is chemically non-reactive. Graphite electrodes are used because graphite is inert to corrosive bromine fumes. Molten PbBr2 provides Pb2+ and Br- ions. At the cathode, Pb2+ + 2e- → Pb (silvery-grey metal). At the anode, 2Br- - 2e- → Br2 (reddish-brown fumes).
2. Electrolysis of Acidified Water: Pure water ionizes poorly. Adding dilute H2SO4 provides mobile H+, SO42-, and OH- ions. Using platinum electrodes: at the cathode, 2H+ + 2e- → H2; at the anode, OH- discharges preferentially over SO42-: 4OH- - 4e- → 2H2O + O2. Because the formation of one O2 molecule requires 4 electrons while one H2 molecule requires 2 electrons, the gases evolve in a 2:1 volume ratio of Hydrogen to Oxygen.
3. Electrolysis of Aqueous Copper(II) Sulphate:
- With Inert Platinum Electrodes: Cu2+ discharges at the cathode (pink deposit), and OH- discharges at the anode evolving O2. As Cu2+ ions are removed without replenishment, the blue colour of the solution gradually fades, leaving behind dilute sulphuric acid.
- With Active Copper Electrodes: Cu2+ deposits at the cathode (Cu2+ + 2e- → Cu). At the active anode, copper atoms oxidize and dissolve into solution (Cu - 2e- → Cu2+). The rate of copper loss at the anode equals the rate of copper gain at the cathode; therefore, the blue colour intensity of the solution remains entirely unchanged.
Industrial Applications: Electroplating and Electrorefining
Electroplating is the electrolytic deposition of a thin, superior metal layer (such as silver, gold, or nickel) onto a cheaper base metal object (like iron or brass) for corrosion resistance or aesthetic finish. Four mandatory operational rules apply:
- The article to be electroplated is always made the cathode so that metal cations migrate to it and deposit as atoms.
- The pure plating metal is always made the anode to continually replenish the electrolyte.
- The electrolyte must contain ions of the plating metal. For silver plating, sodium argentocyanide [NaAg(CN)2] is used instead of silver nitrate because it ionizes slowly to provide a low, uniform concentration of Ag+ ions, resulting in a smooth, compact, and adherent coating rather than a loose, crystalline deposit.
- A low direct current must be passed for a prolonged duration to ensure uniform crystal deposition.
In the electrorefining of blister copper, an impure copper block serves as the anode, and a pure copper strip serves as the cathode in an acidified CuSO4 bath. Pure copper transfers from the anode to the cathode. Insoluble impurities (such as gold and silver) drop beneath the anode as anode mud, while active impurities (like zinc and iron) dissolve into the electrolyte as ions.
Key takeaways
- Direct current (DC) is mandatory; alternating current (AC) reverses polarity continuously, causing zero net electrolysis.
- Reduction always occurs at the cathode (electron gain), while oxidation always occurs at the anode (electron loss).
- Ions lower in the electrochemical series require lower activation energy and discharge preferentially over higher ions.
- Active electrodes dissolve during electrolysis to replenish the electrolyte, keeping its concentration and colour constant.
- Acidified water yields hydrogen and oxygen in a 2:1 volume ratio due to the stoichiometric electron transfer (4e⁻ per O₂ vs 2e⁻ per H₂).
Test yourself
Why can sodium metal not be extracted by electrolyzing an aqueous solution of sodium chloride?
In an aqueous solution, H⁺ ions from water are lower in the electrochemical series than Na⁺ ions, so H⁺ ions discharge preferentially at the cathode, releasing hydrogen gas instead of sodium.
Why is sodium argentocyanide preferred over silver nitrate as the electrolyte for silver plating?
Sodium argentocyanide dissociates slowly to supply a low and steady concentration of Ag⁺ ions, which produces a smooth, uniform, and adherent silver coating rather than a rapid, uneven, and flaky deposit.
What happens to the blue colour of aqueous copper(II) sulphate when electrolyzed with platinum electrodes versus copper electrodes?
With platinum electrodes, the blue colour fades as Cu²⁺ ions are consumed; with copper electrodes, the blue colour remains unchanged because the copper anode dissolves at the same rate that Cu²⁺ deposits at the cathode.
Why must lead bromide be heated in a silica crucible with graphite electrodes during electrolysis?
Silica withstands high temperatures without cracking or reacting with molten lead bromide, and graphite electrodes are chemically unreactive toward the corrosive bromine gas evolved at the anode.
State the volume ratio of gases collected at the cathode and anode during the electrolysis of acidified water and explain why.
The ratio is 2:1 (Hydrogen : Oxygen) because liberating 1 molecule of O₂ at the anode releases 4 electrons, which simultaneously discharge 2 molecules of H₂ (2 × 2e⁻) at the cathode.