AQA Chemistry 5.4.3.1 (+ 5.4.3.5 Higher) · Combined and Triple

The process of electrolysis

Melt or dissolve an ionic compound and its ions can move. Pass a direct current and the ions travel to the oppositely charged electrodes and are discharged as elements.

Combined Science · Higher tier

Higher layer · electrons at the electrodes

Start here

Salt and a bulb.

A simple circuit with a gap in it, a bulb, and two things to bridge the gap: a pile of dry table salt, or a beaker of salt dissolved in water.

If you had to guess, which one lets the bulb light: the dry salt, or the salt water?

The cell

Switch it on.

Two electrodes dip into the compound and connect to a direct current supply. The negative electrode is the cathode; the positive electrode is the anode. Step through from solid to products.

Solid lead bromide. The Pb²⁺ and Br⁻ ions are there, but held in fixed positions in the lattice. Switch on: nothing happens, no current flows. Predict: why does the solid not conduct?

The words

Electrolyte, cathode, anode.

When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes. In the liquid the charge is carried by the moving ions; in the wires it is carried by electrons. The electrodes are usually inert (graphite or platinum) so they do not react. Tap each.

Molten lead bromide contains Pb²⁺ and Br⁻ ions. Where do the Br⁻ ions go?

Pick one.

Electrolysis is used to extract metals too reactive to be extracted with carbon: Extraction of metals. Which product forms at each electrode for a given compound is the next lesson, Electrolysis of molten ionic compounds.

Higher · what happens at the electrode

Gain at the cathode, loss at the anode.

At the cathode, positively charged ions gain electrons, so the reactions are reductions. At the anode, negatively charged ions lose electrons, so the reactions are oxidations. The electrons that the anions give up flow round the external circuit to the cathode, where the cations collect them.

cathode (−) · Pb²⁺ + 2e⁻ Pb gains electrons: reduction

anode (+) · 2Br⁻ Br₂ + 2e⁻ loses electrons: oxidation

Where does oxidation take place during electrolysis?

Pick one.

Oxidation and reduction as electron transfer: Oxidation and reduction.

Think again · which way?

“Positive ions go to the positive electrode.”

Your call

Choose

Which electrode do the positive metal ions move towards?

Sort

Cathode, anode or electrolyte?

Place each statement.

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Command words in this lesson

Explain: Why the compound must be molten or dissolved; why ions move where they do.
Describe: The process of electrolysis and the parts of the cell.
Name: The electrodes and the electrolyte.

Key fact

Positively charged ions move to the negative electrode (the cathode); negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements.

Key note · AQA 5.4.3.1 (+ 5.4.3.5 HT) · Combined and Triple

The process of electrolysis

When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode); negatively charged ions move to the positive electrode (the anode).
Ions are discharged at the electrodes producing elements. This process is called electrolysis. Ions carry the charge in the liquid; electrons carry it in the wires. Electrodes are usually inert (graphite or platinum).
A solid ionic compound cannot be electrolysed: its ions are fixed in the lattice.
Higher: at the cathode positive ions gain electrons (reduction); at the anode negative ions lose electrons (oxidation). Pb²⁺ + 2e⁻ → Pb; 2Br⁻ → Br₂ + 2e⁻.

Photograph this card. Tomorrow, cover it and say each line first.

Previous and next

Connects to

Ask MrBadmus

Sending positive ions to the positive electrode?

The electrolysis required practical (Combined RP9 = Chemistry RP3) uses aqueous solutions and belongs to its own lesson.