IGCSE Chemistry 0620 · Common mistakes
Common mistakes in IGCSE Chemistry electrolysis
Updated · by the Fahmazing team
Electrolysis questions reward careful wording. Most lost marks come from mixing up the electrodes, saying the wrong particles move, or naming an ion instead of an element.
Swapping the anode and cathode
Remember PANIC: Positive Anode, Negative Is Cathode. Positive metal ions are attracted to the negative cathode, because opposite charges attract.
Molten lead(II) bromide is electrolysed. Where does lead form?
✗ Wrong: At the anode
✓ Right: At the cathode (negative electrode). Bromine forms at the anode.
Free lesson: Electrolysis
Saying electrons move through the electrolyte
Electrons flow in the wires. In the electrolyte, the charge is carried by moving ions.
Why does molten sodium chloride conduct electricity?
✗ Wrong: Because electrons are free to move.
✓ Right: Because the ions are free to move and carry the charge.
Free lesson: Electrolysis
Why the solid does not conduct
An ionic solid has ions, but they are held in a lattice and can only vibrate. They must be able to move.
Explain why solid lead(II) bromide does not conduct.
✗ Wrong: It has no charged particles.
✓ Right: Its ions are held in fixed positions in the lattice, so they cannot move to carry the charge.
Free lesson: Electrolysis
Electrons on the wrong side of a half-equation
At the cathode, ions gain electrons: electrons go on the left. At the anode, ions lose electrons: electrons go on the right.
Half-equation at the anode for bromide ions
✗ Wrong: 2Br⁻ + 2e⁻ → Br₂
✓ Right: 2Br⁻ → Br₂ + 2e⁻
Free lesson: Electrolysis
Naming the ion instead of the product
The product at an electrode is the element, not the ion.
Name the product at the anode in molten lead(II) bromide.
✗ Wrong: Br⁻ (bromide)
✓ Right: Bromine
Free lesson: Electrolysis
Writing the metal at the cathode for aqueous solutions
In a solution, water provides H⁺ ions. If the metal is more reactive than hydrogen, hydrogen forms at the cathode instead of the metal.
Concentrated aqueous sodium chloride is electrolysed. Name the cathode product.
✗ Wrong: Sodium
✓ Right: Hydrogen. Sodium is more reactive than hydrogen. Chlorine forms at the anode.
Free lesson: Electrolysis of aqueous solutions and electroplating
Forgetting that concentration matters at the anode
A concentrated halide solution gives the halogen at the anode. With no halide, as with a sulfate, oxygen forms.
Aqueous copper(II) sulfate with graphite electrodes
✗ Wrong: Copper at the cathode, sulfur dioxide at the anode
✓ Right: Copper at the cathode, oxygen at the anode. The blue colour fades as Cu²⁺ ions are used up.
Free lesson: Electrolysis of aqueous solutions and electroplating
Setting up electroplating the wrong way round
The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.
Silver-plating a spoon
✗ Wrong: Spoon as the anode
✓ Right: Spoon as the cathode, silver as the anode, a solution of a silver salt as the electrolyte
Free lesson: Electrolysis of aqueous solutions and electroplating
Questions students ask
How do I remember which electrode is which?
Use PANIC: Positive Anode, Negative Is Cathode. Metals and hydrogen form at the cathode; non-metals like chlorine, bromine and oxygen form at the anode.
Why does hydrogen form instead of sodium?
In aqueous solution, H⁺ ions from water are discharged instead of ions of metals more reactive than hydrogen, such as sodium.
What should I write for an observation in electrolysis?
What you see: bubbles of gas, a pink-brown solid coating the cathode, or a blue solution fading. Not just "copper is made".