GCSE Chemistry revision

Representation of reactions at electrodes as half equations

Electrolysis

AQA 4.4.3.5 Higher Tier only

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Movement in Circuit and Electrolyte: Electrons flow through the metal wiring of the external circuit from the anode to the cathode, while mobile ions move through the liquid electrolyte—cations migrate toward the negative cathode and anions toward the positive anode.
Electron Loss and Gain at Electrodes: At the positive anode, negative ions lose electrons to become neutral species, and these released electrons enter the external wire; at the negative cathode, positive ions gain electrons supplied by the circuit.
Example: During the electrolysis of molten lead(II) bromide (PbBr₂), bromide ions (Br⁻) move through the electrolyte to the anode to lose electrons, while lead ions (Pb²⁺) move to the cathode to collect electrons.

1. Charge Transfer in Electrolytic Cells AQA

Movement in Circuit and Electrolyte: Electrons flow through the metal wiring of the external circuit from the anode to the cathode, while mobile ions move through the liquid electrolyte—cations migrate toward the negative cathode and anions toward the positive anode.
Electron Loss and Gain at Electrodes: At the positive anode, negative ions lose electrons to become neutral species, and these released electrons enter the external wire; at the negative cathode, positive ions gain electrons supplied by the circuit.
Example: During the electrolysis of molten lead(II) bromide (PbBr₂), bromide ions (Br⁻) move through the electrolyte to the anode to lose electrons, while lead ions (Pb²⁺) move to the cathode to collect electrons.

Oxidation at the Anode: Oxidation always occurs at the positive anode, represented by a half-equation showing negative ions losing electrons to form neutral elements.
Example: 2Cl⁻ → Cl₂ + 2e⁻ (Chloride ions lose electrons to form chlorine gas).
Reduction at the Cathode: Reduction always occurs at the negative cathode, represented by a half-equation showing positive ions gaining electrons to form neutral elements.
Example: Cu²⁺ + 2e⁻ → Cu (Copper ions gain electrons to deposit as solid copper).

2. Constructing Anode and Cathode Half-Equations AQA

Oxidation at the Anode: Oxidation always occurs at the positive anode, represented by a half-equation showing negative ions losing electrons to form neutral elements.
Example: 2Cl⁻ → Cl₂ + 2e⁻ (Chloride ions lose electrons to form chlorine gas).
Reduction at the Cathode: Reduction always occurs at the negative cathode, represented by a half-equation showing positive ions gaining electrons to form neutral elements.
Example: Cu²⁺ + 2e⁻ → Cu (Copper ions gain electrons to deposit as solid copper).

Gain of Electrons: Reduction is defined as any process in which an atom, ion, or molecule gains one or more electrons during a reaction.
Example: In the conversion of iron(III) to iron(II), the Fe³⁺ ion is reduced because it gains an electron: Fe³⁺ + e⁻ → Fe²⁺
Decrease in Oxidation Number: Reduction is also defined as a decrease in the oxidation number (state) of an element as it changes from reactant to product.
Example: In the reaction Fe³⁺ + e⁻ → Fe²⁺, the oxidation state of iron decreases from +3 to +2.

3. Defining Reduction via Electrons and Oxidation Numbers AQA

Gain of Electrons: Reduction is defined as any process in which an atom, ion, or molecule gains one or more electrons during a reaction.
Example: In the conversion of iron(III) to iron(II), the Fe³⁺ ion is reduced because it gains an electron: Fe³⁺ + e⁻ → Fe²⁺
Decrease in Oxidation Number: Reduction is also defined as a decrease in the oxidation number (state) of an element as it changes from reactant to product.
Example: In the reaction Fe³⁺ + e⁻ → Fe²⁺, the oxidation state of iron decreases from +3 to +2.