1.4Unit 2 · Bonding & Electrolysis
Electrolysis — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 4 of 4 1 quiz
Lesson summary
Explain why some substances conduct and others do not.At the cathode (aqueous)the metal forms if it is less reactive than hydrogen; otherwise hydrogen forms.At the anode (aqueous)a halogen forms if halide ions are present (concentrated); otherwise oxygen forms from hydroxide ions…
- Cathode half-equations
- Anode half-equations
- Electric current is the flow of charged particles
- Explaining conductivity
- Conductivity of covalent compounds
- Electrolysis of molten compounds
Section 1 of 4Electrolysis
Electric current is the flow of charged particles
in metals and graphite it is delocalised electrons; in electrolytes it is moving ions. A substance conducts only if it has charged particles that are free to move.Explaining conductivity
solid ionic compounds do not conduct because ions are held in fixed positions. When melted or dissolved in water, the ions are free to move towards the electrodes, so they conduct and are decomposed.Conductivity of covalent compounds
simple molecular substances (e.g. sulfur, ethanol, sugar solution) do not conduct because their molecules are uncharged and there are no free electrons.- Some covalent compounds such as HCl react with water to produce ions, so their solutions do conduct.
Electrolysis of molten compounds
the only products are the metal at the cathode and the non-metal at the anode, e.g. molten lead(II) bromide gives silvery lead and brown bromine vapour; molten sodium chloride gives sodium and chlorine.Electrolysis of aqueous solutions
water supplies H+ and OH− ions as well.- At the cathode, the less reactive of the metal and hydrogen is discharged.
- At the anode, halide ions are discharged if present in reasonable concentration; otherwise OH− is discharged to give oxygen.
Half-equations
write the ion, add electrons to the side that balances charge.- Cations gain electrons at the cathode (e.g. Na+ + e− → Na); anions lose electrons at the anode (2Cl− → Cl2 + 2e−).
- The total charge on each side must be equal.
Practical
electrolyse solutions such as copper(II) sulfate, sodium chloride and dilute sulfuric acid with graphite electrodes; collect gases in test tubes and identify them.- With copper(II) sulfate, pink copper coats the cathode and the blue colour fades as Cu2+ ions are removed.
Covalent insulators
with no mobile charged particles, covalent substances are used as insulators: plastic coatings on wires, oils in electrical equipment, and the dense gas sulfur hexafluoride, SF6, in transformers.Movement in the circuit
in the wires, electrons flow from the anode, through the power supply, to the cathode. In the electrolyte, cations move to the cathode and anions to the anode, because opposite charges attract.Binary molten compounds
a binary ionic compound contains just two elements.- When molten, the metal always forms at the cathode and the non-metal at the anode, e.g. molten zinc chloride gives zinc (Zn2+ + 2e− → Zn) and chlorine (2Cl− → Cl2 + 2e−).
Practical: molten lead(II) bromide
heat solid lead(II) bromide in a crucible until molten, dip in two graphite electrodes connected to a d.c. supply, and work in a fume cupboard.- Brown bromine gas forms at the anode; molten grey lead collects below the cathode.
Oxygen from water
the anode equation for oxygen can also be written using water: 2H2O → O2 + 4H+ + 4e−. This form suits acidic solutions such as dilute sulfuric acid and copper(II) sulfate.Writing electrons
an oxidation half-equation can show electrons added on the right (2Br− → Br2 + 2e−) or subtracted on the left (2Br− − 2e− → Br2). Both are accepted; check that the charges balance.Predicting aqueous products
first list every ion present, including H+ and OH− from water. Then compare reactivity at the cathode and check for halide ions at the anode. Ions that are not discharged stay in solution.
Section 2 of 4Process & sequence
- Pour the aqueous electrolyte into a beaker (or electrolysis cell) and insert two inert graphite electrodes.
- Connect the electrodes to a low-voltage d.c. supply; the negative terminal goes to the cathode.
- Invert small test tubes of electrolyte over each electrode to collect any gas.
- Switch on and observe bubbles, deposits or colour changes at each electrode.
- Test gases: squeaky pop (H2), relights glowing splint (O2), bleaches damp litmus (Cl2).
- Record observations and write half-equations for each electrode.
Electrolysis of an aqueous solution — write these steps in this order; each step is a marking point.
- Ions free to move
- Cations to cathode
- Anions to anode
- Reduction at cathode
- Oxidation at anode
- Collect and test products
Section 3 of 4Compare
| Electrolyte | Cathode (−) | Anode (+) |
|---|---|---|
| Molten lead(II) bromide | Lead | Bromine |
| Concentrated sodium chloride solution | Hydrogen | Chlorine |
| Dilute sulfuric acid | Hydrogen | Oxygen |
| Copper(II) sulfate solution | Copper | Oxygen |
| Dilute sodium chloride solution | Hydrogen | Oxygen (mainly) |
| Substance | Conducts? | Why |
|---|---|---|
| Metal / graphite (solid) | Yes | Delocalised electrons move |
| Ionic solid | No | Ions fixed in lattice |
| Ionic compound molten or in solution | Yes | Ions free to move |
| Covalent molecular substance | No | No ions or free electrons |
Section 4 of 4Worked examples
Predict the products and write half-equations for electrolysis of concentrated sodium chloride solution.
- Cathode: hydrogen (sodium is more reactive than hydrogen): 2H+ + 2e− → H2.
- Anode: chlorine : 2Cl− → Cl2 + 2e−.
- (chloride is present)
- Sodium hydroxide solution remains.
- Anode: chlorine : 2Cl− → Cl2 + 2e−.
- Cathode: hydrogen (sodium is more reactive than hydrogen): 2H+ + 2e− → H2.
Explain why solid lead(II) bromide does not conduct but molten lead(II) bromide does.
- In the solid, Pb2+ and Br− ions are fixed in the lattice and cannot move.
- When molten, the ions are free to move to the electrodes and carry the charge.
- In the solid, Pb2+ and Br− ions are fixed in the lattice and cannot move.
Marked practiceQuizzes for this lesson
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