Elechtrochemistry

• Electrolysis occurs when an electric current causes the decomposition of an ionic compound (molten or aqueous).
• Electrolytes are molten or aqueous substances undergoing electrolysis, examples of these are molten salts, salt solutions, acids and alkalis.
• Electrodes are metallic or graphite rods that conduct current in/out of a solution.
• Anode is a positive electrode.
• Cathode is a negative electrode.

Anode

• Electrons flow from the anode to the battery.
• Oxidation occurs at anode.
• Anions (negative ions, excluding non-metals except hydrogen) are drawn to the anode.

Cathode

• Electrons flow from the battery to the cathode.
• Reduction occurs at cathode.
• Cations (metal and hydrogen ions) in the electrolyte are drawn to the cathode.

Types of Electrodes

• Inert electrodes (graphite or platinum): Graphite and platinum don't undergo chemical reactions and conduct electricity well.
• Platinum is an inert metal electrode.
• Graphite is an inert nonmetal electrode.
• Active Electrodes: Electrodes that participate in the reaction, examples of these are copper and zinc.

Metallic Conduction

• Metallic conduction involves electron flow.
• Metallic conduction occurs in solids and liquids without chemical change.

Electrolytic Conduction

• Electrolytic conduction involves ion movement
• Electrolytic conduction occurs only in molten or aqueous solutions.
• Electrolytic conduction involves chemical change.

Electrolysis of Molten Compounds Using Inert Electrodes

• A molten substance is a melted substance.
• Metals form at the cathode and nonmetals at the anode.
• Electrons move in the external circuit (anode to cathode).
• Loss or gain of electrons always occurs at the electrodes.
• Ions always move within the electrolyte.
• Sodium Chloride Electrolysis: NaCl (molten) → Na+ + Cl-
• At Cathode (-): Na+ + e- → Na
• At Anode (+): 2Cl- → Cl2 + 2e- or 2Cl- - 2e- → Cl2
• Calcium chloride Electrolysis: CaCl2 consists of Ca2+ and Cl- ions.
• At Cathode (-): Ca2+ + 2e- → Ca
• At Anode (+): 2Cl- → Cl2 + 2e- OR 2Cl- - 2e- → Cl2
• Lead(II) Bromide Electrolysis: Molten lead bromide contains Pb2+ and Br- ions.
• At Cathode (-): Pb2+ ions are attracted towards the cathode where they will accept two electrons, Pb2+ + 2e- → Pb.
• At Anode (+): Bromide ions are attracted towards the positive electrode, Br ions give away extra electron to anode and form bromine atoms.
• Bromine atoms joins up in pairs and form diatomic bromine molecules: 2Br- → Br2 + 2e- OR 2Br- - 2e- → Br2.

Electrolysis of Molten Alumina (Al2O3)- Extraction of Aluminum

• Aluminum is extracted from its purified oxide ore through electrolysis, where the ore Bauxite is purified by treating it with sodium hydroxide, removing iron oxide and sand impurities.
• Purified bauxite is called alumina.
• Alumina is dissolved in molten cryolite (Na3AlF6).
• Cryolite Advantages: Cryolite reduces melting point, increases conductivity, and acts as a solvent for the ore alumina.
• Electrolysis in Hall Heroult cell uses inert electrodes (graphite).
• Reaction at Cathode (-): Al3+ + 3e- → Al (reduction)
• Reaction at Anode (+): 2 02- → O2+ 4e (oxidation)
• Released oxygen reacts with graphite to form CO2, causing anode rods replacement.
• C + O2 → CO2
• Aluminum Use: Aircraft manufacturing (low density) and food containers/foil (corrosion resistant).

Electrolysis of Aqueous Solutions

• H+ and OH- ions from water compete with acid or salt ions to discharge.

At Cathode

• If the metal is more reactive than hydrogen, the metal exists as ions while H+ ions take electrons and become hydrogen gas.
• If the metal is less reactive than hydrogen, the metal ions take electrons and become metal while H+ ions remain in solution.

At Anode

• For concentrated aqueous halides (Cl, Br, I), halide ions lose electrons an become oxidized.
• With dilute halide solutions, OH ions lose electrons and oxygen gas will be formed
• 4OH →2 H2O + O2 + 4e

Electrolysis of Concentrated Sodium Chloride Solution (Brine)

• H⁺ and OH⁻ from water, and Na⁺ and Cl⁻ from sodium chloride are present in the solution.
• At cathode (-):2H+ + 2e- → H2, Sodium is more reactive than hydrogen, so H⁺ ions are discharged.
• Observation: Bubbles of colourless gas.
• At anode (+):2Cl → Cl2+2e-, Chlorine gas is produced, which appears as pale green bubbles.
• The gas produced burns with a pop sound if a lighted splint is introduced.
• This reaction leaves behind Na⁺ and OH⁻ ions (positive and negative, respectively) in the electrolyte, leading to the formation of sodium hydroxide in the cathodic compartment.
• Overall this produces: Na+ + OH → NaOH (Thymolphthalein turns blue)
• Products: Chlorine gas (disinfectant, HCl synthesis, plastics), hydrogen gas, and sodium hydroxide (HCl and paper manufacture, pollution free fuel).

Electrolysis of Dilute Sodium Chloride Solution

• Electrolysis of dilute sodium chloride solution includes H⁺ and OH⁻ from water, and Na⁺ and Cl⁻ from sodium chloride.
• Cathode (-): 2H+ + 2e- → H2 produces hydrogen gas
• Anode (+): 4OH- → 2H2O + O2 + 4e produces oxygen gas
• Observation: Bulb glows and generation of gases is present.

Electrolysis of Concentrated Hydrochloric Acid (HCl)

• Cathode (-): H+ attracts to cathode and forms hydrogen gas which is colourless, and burns with a pop sound.
• 2H+ + 2e- → H2
• Anode (+): Chloride ions discharge to form pale green gas that bleaches damp litmus paper.
• 2Cl → Cl2 + 2e

Electrolysis of Dilute Sulfuric Acid (H2SO4)

• Electrolysis of dilute sulfuric acid (H2SO4) creates the following reactions.
• Cathode (-): 2H+ + 2e- → H2, Hydrogen gas is produced.
• Anode (+): 4OH →2 H2O + O2 + 4e, The reaction produces oxygen gas.
• As the electrolysis carries on the sulfuric acid concentration increases as the water is used up.

General Observations in a Circuit

• Bubbles of gases are formed (note the nature or color of gas)
• Deposition of metal (note the color of metal)
• Bulb glows
• The electrolyte changes
• As the electrolyte concentration increases, the bulb glows brighter and bubble formation becomes vigorous.

Electrolysis of Copper(II) Sulfate Solution Using Inert Electrode

• Ions in the electrolyte include H⁺ and OH⁻ from water, and Cu+2 and SO42- ions from copper(II) sulfate.
• Cathode: Cu+2 + 2e- → Cu, Copper ions are discharged, forming a red/brown metal layer.
• Anode: 4OH → 2H2O + O2 + 4e, Oxygen is formed from OH- ions, creating bubbles of colorless gas, and if positive, will relight.
• Products: The blue copper sulfate changes to colorless sulfuric acid (H⁺ and SO42-).

Electrolysis Using Active Electrodes (Copper)

• Electrolysis refines metals using a unique cell.
• Cathode: Pure copper.
• Anode: Impure copper sample.
• Electrolyte: Copper salt solution (copper sulfate/nitrate)
• At the anode: the copper anode loses electrons to give Cu2+ ions in the solution
• Cu → Cu+2 + 2e-
• At the Cathode: copper ions are discharged at the cathode as neutral copper atom.
• Cu+2 + 2e- → Cu
• Anode copper dissolves and concentrates the Cu2+ ions which remains unchanged.
• Anode impurities settle at the cell's bottom through refining/purifying.
• The anode decreases in size as copper becomes positive ions.
• The cathode increases in size as copper metal is formed.
• Electrolyte color (blue) remains unaltered from the electrolyte's/Cu2+ ion's concentration.

Electroplating

• It is a process where one metal coats another improving appearance & corrosion resistance.
• Plate the object by connecting to the cathode.
• The plating metal connects as the anode.
• The electrolyte has plating metal ions.
• Example: Silver coating on a steel spoon.
• Anode: Ag-e → Ag+ (gets thinner).
• Cathode: Ag+ + e → Ag (covered with silver, grows larger).
• Electrolyte: no change
• The object must be clean (rub with sandpaper) and grease-free for metal adhesion.
• Keep rotating or moving the object for equal layer forming.