Overview
This lecture covers half equations, electrolysis in molten and aqueous solutions (brine), balancing redox reactions, and how to answer extended response questions using clear, concise bullet points or tables.
Half Equations
- A half equation shows the gain or loss of electrons during redox reactions.
- Example: Na⁺ + e⁻ → Na (sodium ion gains 1 electron to form sodium metal).
- Example: 2Cl⁻ → Cl₂ + 2e⁻ (chloride ions lose electrons to form chlorine gas).
- Both ways of writing half equations for diatomic nonmetals (e.g., Br⁻, Cl⁻, O²⁻) are accepted by exam boards, provided charges and atoms balance.
- Always balance both atoms and charges in the half equation.
Balancing Complex Half Equations
- For O²⁻ → O₂, balance oxygen first, then balance charges by adding electrons: 2O²⁻ → O₂ + 4e⁻.
Electrolysis in Aqueous Solutions (Brine)
- NaCl dissolved in water ("brine") dissociates into Na⁺, Cl⁻, H⁺, and OH⁻ ions.
- Water acts as an amphoteric substance (acts as both acid and base).
- In aqueous electrolysis, H⁺ is reduced at the cathode (not Na⁺), forming H₂ gas.
- Cl⁻ ions are oxidized at the anode, forming Cl₂ gas.
- Na⁺ and OH⁻ remain in solution after electrolysis.
Electrolysis: Molten vs Aqueous
- In molten NaCl, Na⁺ is reduced to Na metal; Cl⁻ is oxidized to Cl₂ gas.
- In aqueous NaCl, H₂ gas forms at the cathode; Cl₂ at the anode; Na⁺ and OH⁻ remain in solution.
- Energy usage is higher for molten electrolysis due to the need for heating.
Answering Six Mark Questions
- Use bullet points or tables rather than paragraphs for clarity in extended answers.
- Compare properties like bonding, ion dissociation, melting point, state, and products between molten and aqueous sodium chloride.
Key Terms & Definitions
- Half Equation — an equation showing electron transfer in a redox reaction.
- Amphoteric — a substance that can act as both an acid and a base.
- Brine — an aqueous solution of sodium chloride (salty water).
- Cathode — electrode where reduction (gain of electrons) occurs.
- Anode — electrode where oxidation (loss of electrons) occurs.
Action Items / Next Steps
- Complete practice half equations for various ions (e.g., K⁺, Mg²⁺, Al³⁺, Br⁻, O²⁻, H⁺).
- Write dissociation and half equations for NaCl in water.
- Fill in the comparison table for molten vs aqueous NaCl (brine) electrolysis (states, ions, products, energy).
- Prepare bullet points or tables for six mark questions as instructed.