Understanding Electrolysis

• Electrolysis decomposes ionic substances into simpler substances via an electric current.

Electrolytic Cell

• An electrolytic cell is the apparatus for electrolysis, consisting of electrodes (cathode and anode) and an electrolyte.

Electrodes

• Electrodes are conductive materials (metals or graphite) facilitating electric current flow into and out of the electrolyte.
• The cathode is the negatively charged electrode where reduction (gain of electrons) occurs.
• The anode is the positively charged electrode where oxidation (loss of electrons) occurs.

Electrolyte

• The electrolyte contains free ions, conducts electricity, and can be a molten ionic compound or an ionic solution.

The Process of Electrolysis

• Ions move towards oppositely charged electrodes.
• Positive ions (cations) gain electrons (reduction) and are discharged (neutralized) at the cathode.
• Negative ions (anions) lose electrons (oxidation) and are discharged at the anode.
• Ion and electron flow constitutes an electric current, driving electrolyte decomposition.

Factors Affecting Electrolysis

• Electrolyte Nature: Different ions in electrolytes yield varying products at the electrodes.
• Ion Concentration: Higher ion concentration influences preferential discharge.
• Electrode Material: Inert electrodes (platinum, graphite) do not participate, while active electrodes (copper) can.
• Applied Voltage: Electrolysis requires a minimum voltage (decomposition potential).
• Temperature: Affects electrolyte conductivity and reaction rate.

Electrolysis of Molten Ionic Compounds

• In molten ionic compounds, metal ions are reduced at the cathode, and non-metal ions are oxidized at the anode.
• Molten lead(II) bromide (PbBr2) electrolysis:
  • Cathode: Pb2+ + 2e- → Pb (lead metal forms).
  • Anode: 2Br- → Br2 + 2e- (bromine gas forms).

Electrolysis of Aqueous Solutions

• Aqueous solutions electrolysis is complex as water molecules can also be electrolyzed.
• Ions from the salt and water molecules compete to be discharged at the electrodes.
• Products depend on the relative ease of oxidation or reduction of ions and water molecules.

Electrolysis of Brine (Concentrated NaCl Solution)

• Brine electrolysis yields chlorine gas, hydrogen gas, and sodium hydroxide solution.
  • Cathode: 2H+ + 2e- → H2 (hydrogen gas forms).
  • Anode: 2Cl- → Cl2 + 2e- (chlorine gas forms).
• Remaining Na+ and OH- ions form sodium hydroxide (NaOH).

Preferential Discharge of Ions

• Preferential discharge depends on the electrochemical series.
• Cations lower in the series are more readily reduced at the cathode.
• Anions higher in the series are more readily oxidized at the anode.
• High concentration can lead to the discharge of ions, even if not favored by the electrochemical series.

Applications of Electrolysis

• Extraction of Metals: Reactive metals like sodium, potassium, and aluminum are extracted from ores.
• Purification of Metals: Metals like copper are purified, with impure copper as the anode and pure copper deposited at the cathode.
• Electroplating: A metal object is coated with a thin layer of another metal for protection or decoration.
• Production of Chemicals: Chlorine, hydrogen, and sodium hydroxide are produced via brine electrolysis.
• Anodizing: A protective oxide layer is formed on metals like aluminum to enhance corrosion resistance.

Quantitative Electrolysis

• Faraday's Laws govern the quantitative aspects of electrolysis.
• Faraday's First Law: The mass of a substance produced or consumed at an electrode is directly proportional to the electricity quantity passed.
• Faraday's Second Law: The mass of different substances produced or consumed by the same electricity quantity is proportional to their equivalent weights.

Faraday Constant

• The Faraday constant (F) is the electric charge of one mole of electrons, ≈ 96,485 coulombs per mole (C/mol).
• Substance amount calculation: mass = (Q x M) / (n x F), where Q is charge, M is molar mass, n is the number of electrons transferred, and F is the Faraday constant.

Industrial Applications: Aluminum Extraction

• Aluminum is extracted from bauxite via electrolysis.
• Bauxite is purified to obtain aluminum oxide (alumina, Al2O3).
• Alumina is dissolved in molten cryolite (Na3AlF6) to lower its melting point and improve conductivity.
  • Cathode: Al3+ + 3e- → Al (molten aluminum forms).
  • Anode: 2O2- → O2 + 4e- (oxygen gas forms), reacting with the carbon anode to form carbon dioxide.
• The Hall–Héroult process is the primary method for aluminum extraction.

Industrial Applications: Chlorine Production

• Chlorine is produced by brine electrolysis.
• Electrolytic cells used include the diaphragm cell, membrane cell, or mercury cell.
  • Anode: 2Cl- → Cl2 + 2e- (chlorine gas is produced).
  • Cathode: 2H+ + 2e- → H2 (hydrogen gas is produced).
• Sodium hydroxide (NaOH) is a valuable byproduct.

Electroplating Process

• Electroplating coats a base metal with a thin layer of another via electrolysis.
• The object to be plated is the cathode, and the plating metal is the anode.
• The electrolyte contains ions of the plating metal.
• Silver plating example: the object is the cathode, a silver bar is the anode, and silver nitrate solution is the electrolyte.
• Silver ions are reduced at the cathode, depositing a silver layer.
• Silver from the anode replenishes the silver ions in solution.

Refining of Copper

• Electrolytic refining purifies impure copper.
• Impure copper is the anode, and a thin sheet of pure copper is the cathode.
• The electrolyte is a copper(II) sulfate solution.
  • Anode: Cu → Cu2+ + 2e- (copper dissolves).
  • Cathode: Cu2+ + 2e- → Cu (pure copper deposits).
• Impurities (gold, silver, platinum) settle as "anode mud."

Limitations and Considerations

• Energy Consumption: Electrolysis is energy-intensive.
• Environmental Concerns: Toxic gas production (chlorine) and waste disposal require careful management.
• Electrode Material Stability: Electrode materials degrade, requiring replacement.
• Overpotential: Actual voltage can be higher than theoretical due to electrode kinetics.