Electrolysis & Ion Discharge

• Electrolysis is the decomposition of a chemical compound in an aqueous solution or molten state, using direct current and causing a chemical change.
• An electrolyte is an ionic compound that allows electric current to pass through it, decomposing in the fused state or aqueous solution.

Strong Electrolytes

• Strong electrolytes are compounds that are almost completely ionized in their aqueous solution or fused state
• Examples of strong electrolytes include mineral acids, alkalis, and salts.

Weak Electrolytes

• Weak electrolytes are compounds that are feebly ionized in their fused state or aqueous solution and conduct electricity poorly.
• Examples of weak electrolytes include acetic acid and oxalic acid.

Non-electrolytes

• Non-electrolytes are compounds that do not allow an electric current to pass through them in solution or in the molten state.
• Examples of non-electrolytes include kerosene and carbon disulfide.

Electrolytic Cell

• An electrolytic cell converts electrical energy into chemical energy.

Electrochemical Cell

• An electrochemical cell converts chemical energy into electrical energy.

Electrodes

• Electrodes are graphite, metal plates, or gas carbon rods immersed in the electrolyte of an electrolytic cell, through which current enters and leaves.

Cathode

• Cathode is the electrode connected to the negative terminal of the battery.

Anode

• Anode is the electrode connected to the positive terminal of the battery.

Ions

• Ions are atoms or groups of atoms carrying a positive or negative charge.

Cations

• Cations are atoms carrying a positive charge.

Anions

• Anions are atoms carrying a negative charge.

Oxidation

• Oxidation is a chemical process which involves the addition of oxygen or the removal of hydrogen.

Oxidizing Agents

• An oxidizing agent oxidizes other substances by accepting electrons, providing oxygen, or providing an electronegative ion.

Reduction

• Reduction is a chemical process which involves the removal of oxygen or the addition of hydrogen.

Reducing Agent

• A reducing agent reduces other substances by providing electrons, hydrogen, or an electropositive ion.

Dissociation

• Dissociation, or electrolytic dissociation, is the process by which an ionic compound separates into ions in the fused state or aqueous solution.
• Example: NaCl dissociates into Na+ + Cl-

Ionisation

• Ionization is the process by which polar covalent compounds are converted into ions in a water solution.
• Example: HCl ionizes into H+ + Cl-

Electrochemical Series

• The electrochemical series arranges metals based on how easily their atoms lose electrons to form positively charged ions.

Selective Discharge of Ions

• Selective discharge refers to the preferential discharge of ions present in an electrolyte at their respective electrodes.
• Selective discharge is affected by:
  • Relative position of ions in the electrochemical series
  • Concentration of the ions
  • Nature of the electrode

Electrolysis of Fused Lead Bromide

• Electrolyte: Molten lead bromide (PbBr2)
• Anode: Graphite
• Cathode: Graphite
• Overall reaction: PbBr2 → Pb2+ + 2Br-
• Reaction at the cathode: Pb2+ + 2e- → Pb
• Reaction at the anode: Br- - e- → Br, then Br + Br → Br2

Electrolysis of Acidified Water

• Electrolyte: Acidified water
• Anode: Platinum
• Cathode: Platinum
• Ionization of acidified water: H2O ⇌ H+ + OH-
• H2SO4 ⇌ 2H+ + SO42-
• Ions present: H+, SO42-, OH-
• Reaction at the cathode: H+ + e- → H, then H + H → H2 (Hydrogen molecule)
• Reaction at the anode: OH- - e- → OH × 4
• OH- ion discharge in preference to SO42- which leads to 4OH → 2H2O + O2 (Oxygen molecule)

Electrolysis of Aqueous Copper Sulphate

• Electrolyte: Aqueous copper sulphate solution
• Anode: Copper
• Cathode: Copper
• Dissociation of aqueous copper sulphate: CuSO4 ⇌ Cu2+ + SO42-
• H2O ⇌ H+ + OH-
• Reaction at the cathode: Cu2+ + 2e- → Cu, as Cu is lower in the electrochemical series.
• Reaction at the anode: Cu - 2e- → Cu2+
• SO42- and OH- are not discharged.

Applications of Electrolysis

• Electroplating with metals
• Electrorefining of metals
• Extraction of metals

Electroplating

• Electroplating is a process where a thin film of a metal (gold, silver, or nickel) is deposited on another metallic article using electricity.

Reasons for Electroplating

• Decoration purposes
• Protection from rusting and corrosion

Electroplating with Nickel

• Electrolyte: Aqueous solution of nickel sulphate
• Cathode: Article to be electroplated
• Anode: Block of pure nickel
• Dissociation in the solution: NiSO4 ⇌ Ni2+ + SO42-
• H2O ⇌ H+ + OH-
• Reaction at the cathode: Ni2+ + 2e- → Ni (deposited)
• Reaction at the anode: Ni - 2e- → Ni2+
• Article to be electroplated is always kept at the cathode.

Electrolytic Refining of Metals

• A process by which metals containing impurities are purified electrolytically to give a pure metal.

Electrolytic Refining of Copper

• Electrolyte: Copper sulphate solution and dilute sulphuric acid
• Cathode: Thin strip of pure copper
• Anode: Impure copper
• Reaction at the cathode: Cu2+ + 2e- → Cu
• Reaction at the anode: Cu - 2e- → Cu2+

Electrometallurgy

• Electrometallurgy involves extracting metals via electrolysis.

Activity series

• Metals like K, Na, Ca, Mg, Al, extracted by electrolysis of their fused salts
• Metals like Zn, Fe, Pb, [H], Cu, Hg, Ag extracted by reducing agents like carbon, carbon monoxide.
• Metals like Pt, and Au extracted by thermal decomposition

Acids, Bases and Salts as Electrolytes

• Acids, bases, and salts can be classified as either strong or weak electrolytes based on their degree of dissociation.
• The degree of dissociation formula is: (Number of molecules dissociated / Total number of molecules) × 100

Test your knowledge of electrolysis. Questions cover factors affecting ion discharge, the electrochemical series, and the dissociation of hydrochloric acid. Also includes matching ions with their discharge tendencies.