Overview
This lecture explains the electrolysis of aqueous solutions, detailing how to predict which ions are discharged at the electrodes using basic rules and the reactivity series.
Electrolysis Basics
- Electrolysis splits compounds into elements by passing electric current through an electrolyte.
- Ions in the electrolyte must be free to move; metal oxides must be molten, while soluble compounds can be dissolved in water.
- Standard setup includes a beaker of electrolyte, positive anode, negative cathode, connecting wire, and power supply.
Ions in Aqueous Solutions
- Aqueous solutions contain ions from both the dissolved compound and water (H⁺ and OH⁻).
- Identifying which ions are present is the first step in predicting electrolysis outcomes.
Rules for Ion Discharge
- At the cathode (negative): positive ions (metal ions and H⁺) are attracted; the least reactive ion is discharged.
- At the anode (positive): negative ions (anions and OH⁻) are attracted; a halide ion (e.g., Cl⁻, Br⁻) is discharged if present, otherwise OH⁻ is discharged.
Example 1: Electrolysis of Copper Sulfate Solution
- Ions present: Cu²⁺, SO₄²⁻, H⁺, OH⁻.
- At the cathode: Cu²⁺ ions are discharged (less reactive than H⁺), forming solid copper.
- At the anode: OH⁻ ions are discharged (no halide present), producing oxygen gas and water.
Example 2: Electrolysis of Sodium Chloride Solution
- Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
- At the cathode: H⁺ ions are discharged (less reactive than Na⁺), forming hydrogen gas.
- At the anode: Cl⁻ ions are discharged (halide present), forming chlorine gas.
Key Terms & Definitions
- Electrolysis — The process of using electricity to split compounds into elements.
- Electrolyte — A liquid containing free-moving ions that conducts electricity.
- Cathode — The negative electrode where reduction occurs (gains electrons).
- Anode — The positive electrode where oxidation occurs (loses electrons).
- Reactivity Series — List ranking elements by how easily they react, used to predict discharge at the cathode.
- Halide — A negative ion formed from halogen elements (e.g., Cl⁻, Br⁻).
Action Items / Next Steps
- Review the reactivity series and memorize discharge rules for both cathode and anode.
- Practice applying these rules to predict products of electrolysis in different aqueous solutions.