Edexcel GCSE Chemistry: Topic 3 - Chemical Changes: Electrolytic Processes

Edexcel GCSE Chemistry: Topic 3 - Chemical Changes: Electrolytic Processes

3.22 Definition of Electrolytes

  • Electrolytes: Ionic compounds in the molten state or dissolved in water.
  • Free Movement: When an ionic substance is melted or dissolved, the ions can move freely within the liquid or solution.

3.23 Description of Electrolysis

  • Electrolysis: The process where electrical energy from a direct current supply decomposes electrolytes.
  • Mechanism: A current is passed through substances that are molten or in solution, which allows the electrolyte to decompose into its constituent elements.

3.24 Movement of Ions during Electrolysis

  • Cation Movement: Positively charged ions (cations) migrate to the negatively charged electrode known as the cathode.
  • Anion Movement: Negatively charged ions (anions) migrate to the positively charged electrode known as the anode.
  • Discharge of Ions: Ions get discharged at the electrodes, resulting in the production of elements.

3.25 Formation of Products during Electrolysis

  • Electrolytic Solutions: In an ionic solution (not a molten ionic compound), it contains:
    • Ions from the ionic compound.
    • Ions from water: hydroxide ions (OH⁻) and hydrogen ions (H⁺).
  • At the Cathode (-):
    • Hydrogen gas (H₂ from H⁺ in water) is produced unless the cation from the ionic compound is from a metal that is less reactive than hydrogen.
    • If the metal is less reactive, that metal will be produced instead.
  • At the Anode (+):
    • Oxygen gas (O₂ from OH⁻ in water) will be produced unless the ionic compound contains halide ions (Cl⁻, Br⁻, I⁻).
    • If halide ions are present, the halogen will be produced instead (e.g., Cl₂).

3.26 Products of Electrolysis of Binary Ionic Compounds

  • Electrolysis of Molten Ionic Compounds: It is simpler to predict the products since the only ions present are those from the compound itself.
  • Process:
    • Identify the ions in the ionic compound.
    • The positive ions (+) will go to the cathode.
    • The negative ions (-) will go to the anode.

3.27 Half Equations in Electrolysis (HT only)

  • Half Equation Concept: Each half equation represents a reaction occurring at the anode or cathode.
  • Electron Representation: Electrons are represented by the symbol ‘e⁻’.
  • Half Equations for Reactions:
    • Negative Electrode (Cathode): The half equation would be of the form:
      X^+ + e^-
      ightarrow X
    • Where X is the metallic ion being reduced.
    • Positive Electrode (Anode): The half equation would be of the form:
      X^-
      ightarrow e^- + X
    • Where X is the non-metallic ion being oxidized.

3.28 Oxidation and Reduction in Electrolysis (HT only)

  • Definitions:
    • Oxidation: Defined as the loss of electrons.
    • Reduction: Defined as the gain of electrons.
  • Memory Aid: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

3.29 Reactions at Electrodes (HT only)

  • Cathode Reaction: Reduction occurs, where cations gain electrons.
    • Gain of Electrons: Involves cations whose charge must be neutralized by gaining electrons.
  • Anode Reaction: Oxidation occurs, where anions lose electrons.
    • Loss of Electrons: Involves anions whose charge must be neutralized by losing electrons.

3.30 Electrolysis of Copper Sulfate Solution and Purification of Copper

  • Experimental Setup:
    • Anode: Made of impure copper (the copper to be purified).
    • Cathode: Made of pure copper.
    • Electrolyte: Copper sulfate solution (CuSO₄).
  • Process Explanation:
    • Cu²⁺ ions from the anode dissolve and move to the cathode, gaining electrons and discharging as pure copper.
    • The impurities remain at the anode, forming sludge.
    • Mass Changes: The mass of the cathode increases as pure copper is deposited, while the anode loses mass as Cu²⁺ ions are oxidized to copper.

3.31 Core Practical: Investigate Electrolysis of Copper Sulfate Solutions

  • With Inert Electrodes:
    • At the Cathode: Cu (s) is produced because Cu is less reactive than hydrogen.
    • At the Anode: O₂ is produced; SO₄²⁻ ions do not produce a halogen at the anode.
    • The reaction leaves H⁺ and SO₄²⁻ in the solution, which can recombine to form sulfuric acid.
  • With Copper Electrodes: The same process as described in 3.30 occurs, maintaining the concentration of Cu²⁺ in solution constant because Cu²⁺ ions deposited at the cathode are replaced by those lost at the anode.