Electrolysis Principles
Section: 4. Electrochemistry | Syllabus: Cambridge AS Level Physics 9702
What is Electrolysis? Electrolysis is the decomposition of an ionic compound (molten or in aqueous solution) by passing an electric current through it. The electrical energy breaks down the compound into its elements or simpler substances.
Key point: Electrolysis requires electricity to make a chemical reaction happen that wouldn't occur naturally. Why Does Electrolysis Work? Ionic compounds contain positive and negative ions: Cations are positive ions (e.g., Na⁺, Cu²⁺, H⁺) Anions are negative ions (e.g., Cl⁻, O²⁻, OH⁻) When molten or dissolved in water, these ions are free to move.
When we apply an electric current: Positive ions (cations) move to the negative electrode Negative ions (anions) move to the positive electrode At the electrodes, ions gain or lose electrons and turn into atoms or molecules The Electrolysis Cell An electrolysis cell has several key components: Component Description Purpose Electrolyte Molten or aqueous ionic compound Contains free-moving ions Electrodes Two conducting rods (often carbon/graphite or metal) Allow current to pass through the electrolyte Power supply Battery or DC power source Provides the electrical energy The Two Electrodes Cathode: The negative electrode (connected to the negative terminal of the power supply) Anode: The positive electrode (connected to the positive terminal of the power supply) Memory tip: "Cathode" and "negative" both have more letters than "anode" and "positive" Or remember: PANIC - Positive Anode, Negative Is Cathode What Happens at Each Electrode?
Electrode Charge Ions attracted Process Result Cathode Negative (-) Cations (+) Reduction (gain electrons) Metal or hydrogen forms Anode Positive (+) Anions (-) Oxidation (lose electrons) Non-metal or oxygen forms Remember OIL RIG: O xidation I s L oss (of electrons) - happens at anode R eduction I s G ain (of electrons) - happens at cathode Conditions for Electrolysis For electrolysis to occur, the ionic compound must have free-moving ions .
This happens when the compound is: 1. Molten (melted) High temperature breaks the ionic lattice Ions can move freely Example: Molten sodium chloride (NaCl) 2. Dissolved in water (aqueous solution) Water separates the ions Ions can move freely in solution Example: Sodium chloride solution (NaCl(aq)) Solid ionic compounds cannot be electrolysed because their ions are fixed in position in the lattice.
Simple Example: Molten Lead Bromide Electrolyte: Molten lead bromide (PbBr₂) Ions present: Pb²⁺ and Br⁻ At the cathode (negative electrode): Pb²⁺ ions are attracted Each Pb²⁺ gains 2 electrons Pb²⁺ + 2e⁻ → Pb Grey lead metal forms At the anode (positive electrode): Br⁻ ions are attracted Each Br⁻ loses 1 electron 2Br⁻ → Br₂ + 2e⁻ Brown bromine gas forms Overall: PbBr₂ → Pb + Br₂ Why Use Different Electrode Materials?
Material Properties When used Graphite (carbon) Inert, conducts electricity, cheap Most common choice Platinum Inert, very unreactive When graphite would react Copper Conducts well, can participate in reaction Electroplating Inert electrodes (like graphite and platinum) don't react - they just allow current to flow.
Energy Changes in Electrolysis Electrolysis is an endothermic process : Electrical energy is converted into chemical energy Energy is needed to break the ionic bonds The products store more energy than the reactants Important: Electrolysis is the opposite of what happens in a battery or fuel cell (which release energy) Uses of Electrolysis Electrolysis has many important applications: Extracting reactive metals: Aluminium from aluminium oxide Purifying metals: Copper purification Electroplating: Coating objects with thin layers of metal Producing chemicals: Chlorine, hydrogen, sodium hydroxide Anodising: Making protective oxide layers on aluminium Key Terms Summary Term Meaning Electrolysis Decomposition using electricity Electrolyte Molten or aqueous ionic compound being electrolysed Electrode Conducting rod that carries current Cathode Negative electrode Anode Positive electrode Cation Positive ion (goes to cathode) Anion Negative ion (goes to anode) Oxidation Loss of electrons (at anode) Reduction Gain of electrons (at cathode) Common Mistakes to Avoid Mixing up cathode and anode: Remember PANIC - Positive Anode, Negative Is Cathode Forgetting that ions must be free to move: Solid ionic compounds cannot be electrolysed Confusing oxidation and reduction: Remember OIL RIG - Oxidation Is Loss, Reduction Is Gain Wrong electrode products: Cations go to cathode (metals/hydrogen), anions go to anode (non-metals/oxygen) Not balancing electrons: Number of electrons lost must equal number gained Key Points Summary Electrolysis = decomposition of ionic compound using electricity Requires molten or aqueous ionic compound (free-moving ions) Cathode = negative electrode, attracts cations (positive ions) Anode = positive electrode, attracts anions (negative ions) At cathode: reduction occurs (ions gain electrons) At anode: oxidation occurs (ions lose…
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