4 Electrochemistry | IGCSE Chemistry 0620

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🎯What you need to be able to do

Define electrolysis, and identify the anode (+), cathode (−) and electrolyte.
Identify the products and observations for molten lead(II) bromide, concentrated aqueous sodium chloride and dilute sulfuric acid, with inert electrodes.
Predict the products from a molten binary compound; state that metals or hydrogen form at the cathode and non-metals at the anode.
Describe electroplating and why it is used.
Describe the transfer of charge; electrolysis of aqueous copper(II) sulfate with carbon and with copper electrodes; halides in dilute or concentrated solution EXTENDED.
Write ionic half-equations at the anode (oxidation) and cathode (reduction) EXTENDED.
State what a hydrogen–oxygen fuel cell does; compare fuel cells with petrol engines EXTENDED.

📚The chemistry

What electrolysis is

Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by passing an electric current through it. The electrolyte is the molten or aqueous substance; the anode is the positive electrode and the cathode the negative one. Solid ionic compounds cannot be electrolysed because their ions cannot move. Inert electrodes (graphite or platinum) do not react.

Positive ions are attracted to the cathode and negative ions to the anode. Metals or hydrogen form at the cathode; non-metals (other than hydrogen) form at the anode. For a molten binary compound (two elements) the products are simply those two elements: molten zinc chloride gives zinc and chlorine.

An electrolysis cell containing molten lead(II) bromide with two graphite electrodes connected to a d.c. power supply. Electrons flow through the wires from the anode (positive) to the cathode (negative). In the melt, Pb2+ ions move to the cathode where molten lead forms, and Br- ions move to the anode where red-brown bromine vapour is given off. — Electrons flow in the wires; ions carry the charge in the electrolyte. Lead forms at the cathode, bromine at the anode.

EXTENDED Charge transfer: electrons travel through the external circuit to the cathode, where positive ions gain electrons (reduction); negative ions lose electrons at the anode (oxidation), and those electrons return round the circuit. Half-equations show this:

\[ \text{cathode: } \mathrm{Pb^{2+} + 2e^{-} \rightarrow Pb} \] \[ \text{anode: } \mathrm{2Br^{-} \rightarrow Br_2 + 2e^{-}} \]

Aqueous solutions

Water provides H⁺ and OH⁻ ions as well, so there is competition at each electrode:

Cathode: hydrogen forms unless the metal is less reactive than hydrogen (copper, silver, gold), in which case the metal forms.
Anode: from a concentrated halide solution the halogen forms; EXTENDED from a dilute halide, or from sulfates and nitrates, oxygen forms (from OH⁻).

molten PbBr₂: lead (silvery liquid) at the cathode; bromine (red-brown vapour) at the anode

concentrated NaCl(aq): hydrogen (bubbles) at the cathode; chlorine (pale yellow-green gas, bleaches damp litmus) at the anode; sodium hydroxide remains

dilute H₂SO₄: hydrogen at the cathode; oxygen at the anode, in a 2 : 1 volume ratio

Electrolysis of dilute sulfuric acid in an apparatus with two inverted collecting tubes over inert electrodes. The tube over the cathode holds twice the volume of gas (hydrogen) as the tube over the anode (oxygen). — Dilute sulfuric acid: twice as much hydrogen as oxygen, because water is decomposed, \( \mathrm{2H_2O \rightarrow 2H_2 + O_2} \).

EXTENDED Copper(II) sulfate: with graphite electrodes, copper is deposited on the cathode (pink-brown), oxygen bubbles off the anode, and the blue colour fades as Cu²⁺ ions are removed. With copper electrodes, the anode dissolves (\( \mathrm{Cu \rightarrow Cu^{2+} + 2e^{-}} \)) as fast as copper deposits on the cathode, so the blue colour stays the same and the anode loses exactly the mass the cathode gains — the basis of copper purification.

Electroplating

Metal objects are electroplated to improve their appearance and resistance to corrosion. Make the object the cathode, the plating metal the anode, and use an electrolyte containing ions of the plating metal.

Hydrogen–oxygen fuel cells

A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product. EXTENDED Compared with a petrol engine: no carbon dioxide or other pollutants from the vehicle, and more efficient; but hydrogen is hard to store (a flammable gas needing high-pressure tanks), there are few refuelling stations, and hydrogen is often made from fossil fuels or with electricity that may itself produce CO₂.

✏️Worked example

(a) Molten lead(II) bromide is electrolysed with graphite electrodes. Name the product at each electrode and describe what is seen. [4] (b) EXTENDED Write the half-equations at each electrode and state which process is oxidation. [3] (c) EXTENDED Concentrated aqueous sodium chloride is replaced by a very dilute solution. State how the anode product changes. [1]

(a) Cathode: lead — a silvery liquid collects under the cathode. Anode: bromine — red-brown vapour (gas) is seen.

(b) \( \mathrm{Pb^{2+} + 2e^{-} \rightarrow Pb} \) at the cathode; \( \mathrm{2Br^{-} \rightarrow Br_2 + 2e^{-}} \) at the anode. The anode reaction is oxidation (loss of electrons).

(c) Oxygen is formed instead of chlorine.

Check it. Electrons balance: 2 gained at the cathode for every 2 lost at the anode. Oxidation happens at the Anode, Reduction at the Cathode — “an ox, red cat”.

Sodium at the cathode. From aqueous sodium chloride, sodium is never produced — it is more reactive than hydrogen, so hydrogen forms instead. Only the molten salt gives sodium.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Concentrated aqueous potassium bromide is electrolysed with inert electrodes. What forms at the cathode? A: potassium. B: hydrogen. C: bromine. D: oxygen.

B. Potassium is more reactive than hydrogen, so hydrogen forms. (Bromine forms at the anode.)

2. (Theory.) Name the products at the cathode and anode when molten zinc chloride is electrolysed. [2]

Cathode: zinc. Anode: chlorine.

3. (Theory.) Explain why solid sodium chloride cannot be electrolysed but molten sodium chloride can. [2]

In the solid the ions are fixed in the lattice and cannot move; when molten the ions are free to move to the electrodes and carry the current.

4. (Theory.) Describe how a steel spoon could be electroplated with silver. [3]

Make the spoon the cathode (negative) and a piece of silver the anode (positive); dip both into a solution of a silver salt (e.g. aqueous silver nitrate) and pass a direct current.

5. (Theory.) EXTENDED Write the half-equation for the reaction at the anode when concentrated aqueous sodium chloride is electrolysed, and state whether it is oxidation or reduction. [2]

\( \mathrm{2Cl^{-} \rightarrow Cl_2 + 2e^{-}} \); oxidation (electrons lost).

6. (Theory.) EXTENDED Aqueous copper(II) sulfate is electrolysed with copper electrodes. Describe what happens to the mass of each electrode and to the colour of the solution. [3]

The anode loses mass (Cu → Cu²⁺ + 2e⁻); the cathode gains the same mass (Cu²⁺ + 2e⁻ → Cu). The blue colour stays the same, because Cu²⁺ ions are replaced as fast as they are removed.

7. (Theory.) EXTENDED Give one advantage and two disadvantages of hydrogen–oxygen fuel cells compared with petrol engines in vehicles. [3]

Advantage: only water is produced (no CO₂, CO or oxides of nitrogen). Disadvantages: hydrogen is difficult to store safely (flammable, needs high pressure); few refuelling stations; hydrogen is often produced using fossil fuels.

8. (Multiple choice.) In the electrolysis of dilute sulfuric acid, 40 cm³ of gas collects at the cathode. What volume collects at the anode? A: 10 cm³. B: 20 cm³. C: 40 cm³. D: 80 cm³.

B. Hydrogen : oxygen \( = 2 : 1 \), so 20 cm³ of oxygen.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Royal Society of Chemistry — electrolysis of copper(II) sulfate class practical
Chemistry LibreTexts — background on fuel cells