Electrolysis Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
Electrolysis uses electricity to pull a compound apart. It only works on ionic compounds that are molten or dissolved, because then the ions can move. Positive ions drift to the negative electrode (the cathode) and negative ions to the positive one (the anode), and each turns back into an element there.
Three things to know
- Molten lead(II) bromide gives lead at the cathode and bromine at the anode. A molten compound of two elements always splits into those two elements.
- In solution the water joins in. Concentrated sodium chloride solution gives hydrogen at the cathode and chlorine at the anode. Dilute sulfuric acid gives hydrogen and oxygen.
- At the cathode ions gain electrons (reduction). At the anode ions lose electrons (oxidation). In a solution, a metal only forms at the cathode if it is less reactive than hydrogen, as copper is.
Worked example
Predict the products when molten zinc chloride is electrolysed.
- The ions are Zn²⁺ and Cl⁻.
- Positive zinc ions go to the negative electrode (cathode) and become zinc metal.
- Negative chloride ions go to the positive electrode (anode) and become chlorine gas.
Tips and tricks
- PANIC: Positive Anode, Negative Is Cathode. And OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
- Solid ionic compounds don't conduct "because the ions cannot move". Say ions, not electrons.
It lands in your notebook with its questions as flashcards.
Electrolysis: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
To which electrode do positive ions move?
The cathode is negative, and opposite charges attract.
What forms at the cathode when molten lead(II) bromide is electrolysed?
Lead ions are positive, so they go to the negative electrode.
What forms at the anode when concentrated sodium chloride solution is electrolysed?
Chloride ions lose electrons at the positive electrode.
Why must an ionic compound be molten or dissolved before it can be electrolysed?
In a solid the ions are locked in place.
A spoon is to be coated with silver by electrolysis. Which electrode should the spoon be?
Positive silver ions are attracted to the negative electrode and coat it.
Copper(II) sulfate solution is electrolysed with carbon electrodes. What forms at the cathode?
Copper is less reactive than hydrogen, so copper ions are discharged.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Electrolysis
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Name the product formed at each electrode when molten lead(II) bromide is electrolysed.[2]
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Lead at the cathode (negative electrode); bromine at the anode (positive electrode).
Explain why solid lead(II) bromide does not conduct electricity but molten lead(II) bromide does.[2]
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In the solid the ions are held in fixed positions. When it is molten the ions are free to move and carry the charge.
Concentrated aqueous sodium chloride is electrolysed. Name the product at the cathode, the product at the anode, and the substance left in solution.[3]
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Hydrogen at the cathode, chlorine at the anode, and sodium hydroxide left in solution.
Write the half-equation for the reaction at the cathode when molten lead(II) bromide is electrolysed, and state whether it is oxidation or reduction.[2]
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Pb²⁺ + 2e⁻ → Pb. Reduction, because the ions gain electrons.
Answers: Electrolysis
- 1. Lead at the cathode (negative electrode); bromine at the anode (positive electrode).
- 2. In the solid the ions are held in fixed positions. When it is molten the ions are free to move and carry the charge.
- 3. Hydrogen at the cathode, chlorine at the anode, and sodium hydroxide left in solution.
- 4. Pb²⁺ + 2e⁻ → Pb. Reduction, because the ions gain electrons.