Electrolysis and molten binary compounds
Identify the electrolyte and electrodes, then predict which elements form from a molten compound.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Know how positive and negative ions form.
- Explain why solid ionic compounds lack mobile ions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why does a molten ionic compound conduct while its solid does not?
What you will learn
- Define electrolysis and label a simple electrolytic cell.
- Predict electrode products for molten binary ionic compounds and describe lead(II) bromide observations.
A power supply drives decomposition
Electrolysis breaks down an ionic compound when it is molten or in aqueous solution by passing an electric current through it. The molten or aqueous material is the electrolyte. In a simple electrolytic cell attached to a direct-current supply, the anode is positive and the cathode is negative. These signs describe electrolysis; do not transfer them without checking to a different type of cell.
Ions must be able to move. In a solid ionic lattice the ions are fixed, so attaching electrodes to the solid does not give the molten-cell behaviour. Melting releases the ions to move towards electrodes. The diagrams and observations here are classroom models, rather than instructions for carrying out an experiment with hazardous substances.
A molten binary compound has two elements
In a molten binary ionic compound, positive metal ions move towards the cathode and form the metal. Negative non-metal ions move towards the anode and form the non-metal. For molten lead(II) bromide, PbBr₂, with inert platinum or carbon/graphite electrodes, lead forms at the cathode and bromine at the anode. A silvery-grey lead deposit or droplets and reddish-brown bromine fumes are the characteristic observations.
Apply the same pattern to molten magnesium chloride: magnesium at the cathode and chlorine gas at the anode. Metals or hydrogen are cathode products in the cases studied; non-metals other than hydrogen form at the anode. Keep the word molten visible: an aqueous solution introduces water and can give different products, as the next lesson explains.
Sources for this lesson
Pause and explain
In a simple electrolysis cell, which label is correct?
Worked example
Predict the products and their locations when molten lead(II) bromide is electrolysed using inert electrodes. Describe the observations.
Show the worked solution
- The electrolyte contains mobile Pb²⁺ and Br⁻ ions, without the water competition of an aqueous cell.
- The negative cathode attracts lead ions and forms silvery-grey lead, seen as a deposit or droplets.
- The positive anode attracts bromide ions and forms reddish-brown bromine fumes.
Answer Cathode: lead; anode: bromine.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Label a diagram containing a direct-current supply, two electrodes and molten PbBr₂. Add electrode products and observations.
Give me a hint
Use the power-supply signs before locating the ion products.
Compare my reasoning
- Label the positive electrode anode, the negative electrode cathode and molten PbBr₂ electrolyte.
- Lead forms at the cathode, giving a silvery-grey deposit or droplets.
- Bromine forms at the anode, producing reddish-brown fumes.
Positive anode: bromine; negative cathode: lead; molten PbBr₂ is the electrolyte.
Look for these in your work
- All three cell components are labelled.
- Product locations match their observations.
Predict both electrode products from molten sodium chloride with inert electrodes. Explain why sodium, rather than hydrogen, is predicted.
Give me a hint
The molten cell has no water supplying competing hydrogen species.
Compare my reasoning
- Molten NaCl contains mobile sodium and chloride ions.
- Sodium forms at the cathode; chlorine forms at the anode.
- Without water in this model, hydrogen is not the competing cathode product.
Cathode: sodium; anode: chlorine. Molten NaCl contains no water competition.
Look for these in your work
- The molten condition is used.
- The prediction is not confused with aqueous brine.
A student expects a metal deposit from solid MgCl₂ with two attached electrodes. Explain the missing condition and predict products after that condition is changed appropriately.
Give me a hint
For the binary-compound prediction, make the ions mobile by melting the salt.
Compare my reasoning
- Solid MgCl₂ has ions fixed in its lattice, so it lacks mobile ionic charge carriers.
- In the molten state its ions can move through the electrolyte.
- The molten binary compound then gives magnesium at the cathode and chlorine at the anode.
The salt must be molten for this prediction: magnesium at the cathode, chlorine at the anode.
Look for these in your work
- Mobility explains the failed solid-cell prediction.
- The changed condition is linked to the correct products.
Common mistakes
- Calling the anode negative in a simple electrolytic cell.
- Predicting that a solid salt behaves like a molten electrolyte.
- Using the molten prediction for an aqueous salt without considering water.
What can you explain now?
Predict the cathode and anode products for molten magnesium chloride with inert electrodes.
Compare with the explanation
Cathode: magnesium; anode: chlorine.
The molten binary compound supplies magnesium cations and chloride anions; metal forms at the negative cathode and non-metal at the positive anode.
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Make it stick
Draw and label a simple molten electrolysis cell from memory. Explain why the salt must have mobile ions and predict the two products of a new binary ionic compound.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.