AQA Chemistry 5.4.3.2 (+ 5.4.3.5 Higher) · Combined and Triple
Electrolysis of molten ionic compounds
A molten binary compound has only two kinds of ion. The metal forms at the cathode and the non-metal at the anode, every time. Split the formula and send each ion home.
Triple Science · Foundation tier
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Opposites attract.
Two electrodes dip into a molten salt: one positive, one negative. A positive metal ion is drifting between them.
If you had to guess, does the positive ion head for the positive electrode or the negative one?
Predict the products
Split the formula, send each ion home.
When a simple ionic compound is melted and electrolysed, the metal is produced at the cathode and the non-metal at the anode. Choose a compound, then say where each ion goes.
Pb²⁺, the metal ion
Where does it go?
Br⁻, the non-metal ion
Where does it go?
Place both ions to see the products. Try at least two compounds.
The classroom demonstration
Molten lead bromide.
Solid lead bromide in a crucible with two graphite electrodes: no current. Heat it until it melts and the circuit completes. A grey bead of lead collects at the cathode; red-brown bromine vapour comes off the anode. Bromine is toxic, so this is a teacher demonstration in a fume cupboard (anhydrous zinc chloride is a safer alternative). It is not a required practical: the electrolysis practical uses aqueous solutions.
PbBr2(l) Pb(l) + Br2(g)
2NaCl(l) 2Na(l) + Cl2(g)
Why does nothing happen until the lead bromide melts?
Pick one.
Why some metals need electrolysis rather than carbon is in Extraction of metals.
Think again · the ion and the element
“Molten sodium chloride gives chloride at the anode.”
Your call
ChooseWhat is produced at the anode when molten sodium chloride is electrolysed?
Sort
Cathode product or anode product?
Place each product.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Predict
- The products at each electrode for a molten binary compound.
- Explain
- Why the compound must be molten; why each ion goes where it does.
Key fact
When a simple ionic compound is electrolysed in the molten state using inert electrodes, the metal is produced at the cathode and the non-metal at the anode.
Key note · AQA 5.4.3.2 (+ 5.4.3.5 HT) · Combined and Triple
Electrolysis of molten ionic compounds
- When a simple ionic compound (such as lead bromide) is electrolysed in the molten state using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.
- Molten: only the compound’s own ions are present, so prediction is simple. NaCl gives sodium and chlorine; PbBr2 gives lead and bromine; Al2O3 gives aluminium and oxygen.
- The solid does not conduct because its ions are fixed; melting frees them. Overall equations: 2NaCl(l) → 2Na(l) + Cl2(g); PbBr2(l) → Pb(l) + Br2(g).
- Lead bromide is a teacher demonstration in a fume cupboard (bromine is toxic; zinc chloride is a safer alternative). It is not a required practical: the electrolysis practical uses aqueous solutions.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Writing chloride where you mean chlorine?
Molten lead bromide is a teacher demonstration, not a required practical; the electrolysis required practical (Combined RP9 = Chemistry RP3) uses aqueous solutions.