GCSE · Chemistry · AQA · Spec 8462

Electrolysis of molten ionic compounds

Pass a current through solid lead bromide and nothing happens. Melt it, and lead appears at one electrode while brown bromine comes off the other. Why that way round?

Watch the ions choose their electrode

Electrolyte: Molten lead bromide, PbBr₂ (molten). Cathode (−), made of Graphite (inert), half-equation Higher only: Pb²⁺ + 2e⁻ → Pb, product Lead, observation Silvery molten lead collects at the bottom. Anode (+), made of Graphite (inert), half-equation Higher only: 2Br⁻ → Br₂ + 2e⁻, product Bromine, observation Brown bromine vapour is given off. Pb²⁺ (positive) migrates to the cathode. Br⁻ (negative) migrates to the anode. e⁻ in the wires in the external circuit: anode to cathode. static

opposites attract: + to −, − to +

Cathode (−)

Higher only: Pb²⁺ + 2e⁻ → Pb

Product: Lead

Observation: Silvery molten lead collects at the bottom

Anode (+)

Higher only: 2Br⁻ → Br₂ + 2e⁻

Product: Bromine

Observation: Brown bromine vapour is given off

Lead bromide is a compound of a metal and a non-metal, so it is made of positive lead ions (Pb²⁺) and negative bromide ions (Br⁻). Once it is melted, the ions can move. The lead ions drift to the negative electrode, the bromide ions drift to the positive one, and at each electrode the ions turn into the element. The two lines in bold are the half equations for each electrode.

Exam line: Metal ions are positive, so the metal forms at the cathode (−). Non-metal ions are negative, so the non-metal forms at the anode (+).

Watch out: Melting doesn't split the compound. It only frees the ions to move. The splitting happens at the electrodes, and only while the current is flowing.

Your turn · Predict the products

Where does each element form?

Each compound is melted and electrolysed with inert electrodes. Pick an element, then put it where it forms.

Still to sort

Forms at the cathode (−) (0)

Where the positive ions end up.

Where the line is: A metal always forms positive ions, so every metal lands here, whatever it is combined with.

Forms at the anode (+) (0)

Where the negative ions end up.

Where the line is: A non-metal in these compounds forms negative ions, so every non-metal lands here.

6 of 6 still to sort.

Three new molten compounds, each made of one metal and one non-metal. You haven't been told any of these answers. You don't need to be.

Watch out: Name the element, not the ion. Iodide ions (I⁻) arrive at the anode, but what forms there is iodine.

Check your thinking

Which idea would you back?

A student is about to electrolyse a molten compound of a metal and a non-metal, using two inert electrodes. Four friends each say what they think will happen.

Only one of these ideas is right. Which is closest to what you think?

The metal forms at the positive electrode. Metals are positive, so that's their side.It doesn't need melting. The solid has exactly the same ions, so it would work just as well.Electrons flow through the molten compound from one electrode to the other.The non-metal forms at the positive electrode, because its ions are negative.

How sure are you?

I'm sureI think soI'm guessing

The metal forms at the positive electrode. Metals are positive, so that's their side.

This is a common misconception

You've got the charge right and the attraction backwards. Metal ions are positive, and opposite charges attract, so they are pulled to the NEGATIVE electrode, the cathode. The positive anode actually pushes them away, because like charges repel.

It doesn't need melting. The solid has exactly the same ions, so it would work just as well.

This is a common misconception

Same ions, yes, but in the solid they are locked in place and can't move to the electrodes, so nothing happens. Melting sets them free to move. That's the only job melting does: it doesn't split the compound. The splitting happens at the electrodes.

Electrons flow through the molten compound from one electrode to the other.

This is a common misconception

Electrons travel through the wires, not through the melt. Inside the liquid, the charge is carried by ions moving. Electrons only change hands at the surface of each electrode, where the ions gain or lose them.

The non-metal forms at the positive electrode, because its ions are negative.

This matches the evidence

Yes. Non-metal ions are negative, so they are attracted to the positive anode. There they lose their extra electrons and become the element. The metal ions do the mirror image at the negative cathode, gaining electrons to become metal atoms.

If you were sure about a wrong one, that's the one to fix now. Picture the cell: + goes to −, − goes to +.

Exam line: Molten, so the ions can move. Opposite charges attract, so each ion heads for the electrode of the other sign. There it gains or loses electrons and becomes an element.

Higher

Higher only · Half equations

Finish the half equations

Molten sodium chloride, NaCl, is electrolysed. Write the half equation at each electrode.

The ions in the melt are Na⁺ and Cl⁻.Sodium is a metal, so its ion is positive. Chlorine is a non-metal, so its ion is negative.
missing step

Which line is step 2?

Na⁺ ions go to the cathode (−). Cl⁻ ions go to the anode (+).Na⁺ ions go to the anode (+). Cl⁻ ions go to the cathode (−).

Exam line: Positive ions GAIN electrons at the cathode. Negative ions LOSE electrons at the anode. Check that the total charge is the same on both sides.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Melt an ionic compound, pass a current through it, and it splits into its elements. Which element turns up at which electrode is no mystery: the ions tell you.

What you need to know

Electrolysis uses electricity to split an ionic compound into its elements.
The compound must be molten so its ions are free to move. In a solid they are locked in place.
Inert electrodes, such as graphite, carry the current but don't react themselves.
The cathode is the negative electrode. The anode is the positive electrode.
Metal ions are positive, so they go to the cathode and form the metal.
Non-metal ions are negative, so they go to the anode and form the non-metal.
Molten lead bromide with inert electrodes gives lead at the cathode and bromine at the anode.
Higher only: write a half equation for each electrode, balancing both atoms and charge with electrons.

The big picture

An ionic compound of a metal and a non-metal can be split into its elements by electrolysis once it is molten, because then its ions are free to move. Positive metal ions are attracted to the negative electrode (the cathode) and form the metal there. Negative non-metal ions are attracted to the positive electrode (the anode) and form the non-metal. So molten lead bromide gives lead at the cathode and bromine at the anode, and the same reasoning predicts the products for any molten binary ionic compound. At Higher level you also write a balanced half equation for each electrode.

Key points

1Melting frees the ions to move. It doesn't split the compound; the electrodes do.

2Opposite charges attract: positive metal ions to the negative cathode, negative non-metal ions to the positive anode.

3Predict the products of any molten binary ionic compound: the metal at the cathode, the non-metal at the anode.

4Name the element that forms (bromine), not the ion that arrived (bromide).

5Higher only: at the cathode positive ions gain electrons (Pb²⁺ + 2e⁻ → Pb); at the anode negative ions lose them (2Br⁻ → Br₂ + 2e⁻).

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Worked example

Problem

Molten magnesium chloride, MgCl₂, is electrolysed using inert electrodes. Predict the product at each electrode and explain your answer.

⚠ Watch out

Swapping the electrodes: writing that the metal forms at the anode. The anode is positive, and positive metal ions are repelled by it. They go to the negative cathode.

🧠

Memory hook

PANiC: Positive is the Anode, Negative is the Cathode. Then let opposites attract: metals (positive ions) to the Cathode, non-metals (negative ions) to the Anode.

✓

Check yourself

Close the page. Sketch a cell for any molten metal and non-metal compound. Mark + and −, then draw where each ion goes and what forms there. Why would the solid not work?

Flashcards

(7)

Why must an ionic compound be molten to be electrolysed?

When molten, its ions are free to move to the electrodes. In the solid they are locked in place.

Which electrode is the cathode?

The negative electrode.

Which electrode is the anode?

The positive electrode.

Molten binary ionic compound: where do the metal and the non-metal form?

Metal at the cathode, because its ions are positive. Non-metal at the anode, because its ions are negative. Opposite charges attract.

What carries the charge through the molten compound?

Moving ions. Electrons flow only in the wires and change hands at the electrode surfaces.

Higher only: what happens to positive ions at the cathode, and to negative ions at the anode?

Positive ions gain electrons to become atoms. Negative ions lose electrons, and non-metal atoms pair up into molecules such as Br₂.

Higher only: two things that must balance in a half equation?

The atoms of each element, and the total charge on each side (balanced using electrons, e⁻).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Watch the ions choose their electrode

Where does each element form?

Which idea would you back?

Finish the half equations

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