42. Electrolysis: Aqueous Solutions

Electrolysis: Aqueous Solutions

Based on the video, here are the notes on the electrolysis of substances dissolved in water and the specific rules for predicting which products will form at each electrode.

1. The Challenge of Aqueous Solutions

In an aqueous solution, the water molecules (H₂O) also split into ions. This means that in addition to the ions from the ionic compound, there are always Hydrogen ions (H⁺) and Hydroxide ions (OH⁻) present in the electrolyte.

2. Rules for the Negative Cathode (-)

Both the positive metal ion and the H⁺ ion are attracted to the cathode, but only one is discharged.

  • The Rule: The least reactive element is discharged.

  • If the metal is more reactive than hydrogen (e.g., Sodium, Magnesium): Hydrogen gas (H₂) is produced.

  • If the metal is less reactive than hydrogen (e.g., Copper, Silver): The pure metal is produced.

3. Rules for the Positive Anode (+)

Both the negative ion from the compound and the OH⁻ ion are attracted to the anode.

  • The Rule: If a halide ion is present (Cl⁻, Br⁻, I⁻), the halogen gas (Cl₂, Br₂, I₂) is produced.

  • If no halide is present (e.g., Sulfate, Nitrate): Oxygen gas (O₂) and water are produced.

    • Equation: 4OH⁻ → 2H₂O + O₂ + 4e⁻.

4. Worked Examples

Electrolyte

Ions Present

Product at Cathode (-)

Product at Anode (+)

Copper Sulfate (CuSO₄)

Cu²⁺, SO₄²⁻, H⁺, OH⁻

Copper metal (Cu is less reactive than H)

Oxygen gas (No halide present)

Sodium Chloride (NaCl)

Na⁺, Cl⁻, H⁺, OH⁻

Hydrogen gas (H is less reactive than Na)

Chlorine gas (Chloride is a halide)

5. Summary Checklist

  • Check reactivity: Use the reactivity series to compare the metal ion against hydrogen to determine the cathode product.

  • Identify halides: Look for Group 7 elements to determine if a halogen or oxygen is produced at the anode.