Edexcel GCSE Chemistry (1CH0) — Chemical Changes
You can easily dissolve table salt in water, but forcing its elements apart requires a continuous, directed flow of electricity. Electrolysis is a chemical process that uses electrical energy from a direct current (d.c.) supply to decompose an electrolyte into its constituent elements. Through this process, electrical energy is actively converted into chemical energy.
An electrolyte must be an ionic compound in a molten liquid state or dissolved in water (aqueous). In a solid state, ionic compounds do not conduct electricity because their ions are locked into a fixed, giant ionic lattice by strong electrostatic forces. When melted or dissolved, this lattice breaks apart. The ions are free to move and can carry electrical charge to the electrodes.
A reliable direct current supply, such as a battery or power pack, is mandatory for electrolysis. Direct current provides a continuous, one-way flow of electrons through the external circuit. This maintains a fixed polarity at the electrodes: the anode remains permanently positive, and the cathode remains permanently negative.
If an alternating current (a.c.) were used, the polarity of the electrodes would rapidly swap back and forth many times a second. Because of this, the ions in the electrolyte would constantly change direction. They would never successfully reach an electrode to be discharged and separated, meaning decomposition would not occur.
To fully describe how electrolysis breaks down a substance, you must follow the correct sequential steps:
A student uses a 6 V direct current battery and inert graphite electrodes to perform electrolysis on a sample of molten lead(II) bromide (). Describe the step-by-step decomposition of this electrolyte and provide the half-equations for each electrode.
Step 1: Identify the mobile ions present in the electrolyte.
Step 2: Describe the migration of the ions.
Step 3: Describe the discharge at the cathode (reduction).
Step 4: Describe the discharge at the anode (oxidation).
Step 5: State the overall decomposition equation.
Students often incorrectly state that electrons flow through the electrolyte. Remember, charge is carried by mobile ions in the liquid, and by electrons only in the external wires.
When asked to describe electrolysis, you must explicitly use the terms 'direct current' and 'decompose' to secure full marks according to Edexcel mark schemes.
When answering 'Why is direct current used?', state that it maintains a constant electrode polarity, which ensures products form and collect at specific electrodes.
To secure marks for conductivity questions, always use the exact phrase 'ions are free to move' when explaining why molten or aqueous ionic substances conduct electricity.
Use the mnemonics PANIC (Positive Anode, Negative Is Cathode) and OIL RIG (Oxidation Is Loss, Reduction Is Gain) to confidently navigate electrode questions.
Electrolysis
A chemical process in which electrical energy from a direct current supply decomposes an electrolyte into its constituent elements.
Direct current (d.c.)
An electric current that flows in only one direction, essential for maintaining constant polarity at the electrodes during electrolysis.
Electrolyte
An ionic compound in a molten state or dissolved in water that contains mobile ions and can be decomposed by an electric current.
Decompose
The chemical breakdown of a compound into its constituent elements or simpler substances.
Molten
The liquid state of a substance produced by heating a solid until it melts.
Aqueous
A solution in which the solvent is water.
Ion
An electrically charged atom or group of atoms formed by the loss or gain of electrons.
Giant ionic lattice
A regular, repeating three-dimensional structure of oppositely charged ions held together by strong electrostatic forces.
Anode
The positive electrode in an electrolysis cell, where oxidation (loss of electrons) occurs.
Cathode
The negative electrode in an electrolysis cell, where reduction (gain of electrons) occurs.
Cation
A positively charged ion that migrates toward the cathode during electrolysis.
Anion
A negatively charged ion that migrates toward the anode during electrolysis.
Oxidation
A chemical reaction involving the loss of electrons.
Reduction
A chemical reaction involving the gain of electrons.
Inert electrodes
Electrodes that allow the transfer of electrons but do not react with the electrolyte or products (e.g. graphite or platinum).
Half-equation
A balanced chemical equation showing the loss or gain of electrons by an ion at an electrode during electrolysis.
Put your knowledge into practice — try past paper questions for Chemistry
Electrolysis
A chemical process in which electrical energy from a direct current supply decomposes an electrolyte into its constituent elements.
Direct current (d.c.)
An electric current that flows in only one direction, essential for maintaining constant polarity at the electrodes during electrolysis.
Electrolyte
An ionic compound in a molten state or dissolved in water that contains mobile ions and can be decomposed by an electric current.
Decompose
The chemical breakdown of a compound into its constituent elements or simpler substances.
Molten
The liquid state of a substance produced by heating a solid until it melts.
Aqueous
A solution in which the solvent is water.
Ion
An electrically charged atom or group of atoms formed by the loss or gain of electrons.
Giant ionic lattice
A regular, repeating three-dimensional structure of oppositely charged ions held together by strong electrostatic forces.
Anode
The positive electrode in an electrolysis cell, where oxidation (loss of electrons) occurs.
Cathode
The negative electrode in an electrolysis cell, where reduction (gain of electrons) occurs.
Cation
A positively charged ion that migrates toward the cathode during electrolysis.
Anion
A negatively charged ion that migrates toward the anode during electrolysis.
Oxidation
A chemical reaction involving the loss of electrons.
Reduction
A chemical reaction involving the gain of electrons.
Inert electrodes
Electrodes that allow the transfer of electrons but do not react with the electrolyte or products (e.g. graphite or platinum).
Half-equation
A balanced chemical equation showing the loss or gain of electrons by an ion at an electrode during electrolysis.
