The shape of the lesson
By the end of the lesson, learners can
- Anode, cathode, electrolyte: Select molten lead(II) bromide in the simulator and switch on the current.
- Molten binary compounds: A binary compound contains only two elements.
- Where the charge goes: Extended.
- Cations (positive) go to the cathode (negative).
- Anions (negative) go to the anode (positive).
- Molten binary compound: metal at the cathode, non-metal at the anode.
- Extended: reduction at the cathode, oxidation at the anode.
One picture every idea returns to
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. The electrolyte is the liquid that conducts; the anode is the positive electrode and the cathode the negative electrode. In a molten binary compound, positive metal ions are attracted to the cathode and form the metal; negative non-metal ions are attracted to the anode and form the non-metal. Molten lead(II) bromide gives lead at the cathode and bromine at the anode.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Splitting compounds with electricity.. Pass a current through a molten ionic compound and it breaks down into its elements: the metal at one electrode, the non-metal at the other. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Anode, cathode, electrolyte | Select molten lead(II) bromide in the simulator and switch on the current. Project tab 5 of the lab (Electrochemistry lab, electrolysis) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Molten binary compounds | A binary compound contains only two elements. | Pairs |
| 21 to 30 min | Teach and check: Where the charge goes | Extended. | Jot Thoughts |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Where the charge goes, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Metals always form at the cathode and non-metals at the anode in a molten compound. | Remember that opposite charges attract: positive metal ions go to the negative electrode. Also, a solid electrolyte does not conduct; if the question says the compound is solid, nothing happens until it melts. |
Electrolysis of molten compounds: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define electrolysis.
Molten lead(II) bromide is electrolysed using graphite electrodes. Name the product at each electrode and describe what is seen at the anode.
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
Predict the products at the cathode and at the anode when molten potassium chloride is electrolysed.
(Extended) Write the half-equations for the reactions at each electrode during the electrolysis of molten sodium chloride, and state which electrode reaction is oxidation.
(Extended) Describe how electric charge is transferred during the electrolysis of molten lead(II) bromide.
Answer key and marking notes
- The decomposition (breakdown) of an ionic compound, when molten or in aqueous solution,
- by the passage of an electric current.
- Cathode: lead.
- Anode: bromine.
- A red-brown gas (vapour) is seen at the anode.
- In the solid, the Na+ and Cl− ions are held in fixed positions in the lattice and cannot move.
- In the molten state the ions are free to move and carry the charge.
- Cathode: potassium.
- Anode: chlorine.
- Cathode: Na+ + e− → Na.
- Anode: 2Cl− → Cl2 + 2e−.
- The anode reaction is oxidation, because the chloride ions lose electrons.
- Electrons flow through the external circuit (the wires) from the anode, through the power supply, to the cathode.
- In the molten electrolyte the charge is carried by ions: Pb2+ moves to the cathode and Br− moves to the anode.
- At the cathode Pb2+ ions gain electrons; at the anode Br− ions lose electrons.
9 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Molten sodium chloride is electrolysed. Which particles move towards the cathode through the molten electrolyte?
- Asodium ions, Na+
- Bchloride ions, Cl−
- Celectrons
- Dsodium atoms, Na
Molten calcium chloride, CaCl2, is electrolysed using graphite electrodes. What are the products?
- Acalcium at the cathode and chlorine at the anode
- Bchlorine at the cathode and calcium at the anode
- Chydrogen at the cathode and chlorine at the anode
- Dcalcium at the cathode and oxygen at the anode
The diagram shows the electrolysis of molten sodium chloride. At which electrode does sodium form, and why?
- AY, because it is connected to the negative terminal and attracts Na+ ions
- BX, because it is connected to the positive terminal and attracts Na+ ions
- CY, because sodium ions are negative and are attracted to it
- DX, because sodium is a metal and metals always form at the anode
Which substance is an electrolyte?
- Amolten sodium chloride
- Ba copper wire
- Csolid sodium chloride
- Dliquid ethanol
Molten lead(II) bromide is electrolysed in a school laboratory. Why must this be done in a fume cupboard?
- ABromine vapour, which is toxic, is released at the anode
- BHydrogen, which is explosive, is released at the cathode
- CChlorine gas is released at the anode
- DLead turns into a gas at the cathode
A molten binary compound is electrolysed. A silvery liquid collects at the negative electrode and a pale yellow-green gas bubbles off at the positive electrode. Which compound was electrolysed?
- Asodium chloride
- Blead(II) bromide
- Czinc iodide
- Daqueous sodium chloride
Which statement describes what happens at the anode during the electrolysis of a molten ionic compound?
- ANegative ions lose electrons, so they are oxidised
- BNegative ions gain electrons, so they are reduced
- CPositive ions lose electrons, so they are oxidised
- DPositive ions gain electrons, so they are reduced
During the electrolysis of molten lead(II) bromide, 4 moles of electrons pass through the circuit. How many moles of lead and of bromine, Br2, are produced?
- A2 mol of lead and 2 mol of bromine
- B4 mol of lead and 2 mol of bromine
- C2 mol of lead and 4 mol of bromine
- D4 mol of lead and 4 mol of bromine
Molten aluminium oxide contains Al3+ and O2− ions. Which half-equation shows the reaction at the anode during its electrolysis?
- A2O2− → O2 + 4e−
- BO2− → O2 + 2e−
- C2O2− + 4e− → O2
- DAl3+ + 3e− → Al
Quiz key
The cathode is negative, so it attracts positive ions: Na+ ions move towards it and are turned into sodium there. Chloride ions are negative and move the other way, to the positive anode. Electrons carry the current in the wires, not through the molten electrolyte, where the charge is carried by moving ions. Sodium atoms only exist after the ions have reached the cathode; they are the product, not the particles that move through the melt.
Molten CaCl2 contains only Ca2+ and Cl− ions. The positive calcium ions go to the negative cathode and form calcium metal; the negative chloride ions go to the positive anode and form chlorine gas. Swapping the electrodes reverses the attraction of opposite charges. Hydrogen and oxygen can only form when water is present, and there is no water in a molten compound.
Electrode Y is wired to the short, thick plate of the cell, the negative terminal, so Y is the cathode. Sodium ions are positive, so they are attracted to the negative cathode, gain electrons and form sodium. X is joined to the positive terminal, so it is the anode; a positive electrode repels Na+ ions and chlorine forms there instead. Sodium ions are not negative, and in a molten compound the metal always forms at the cathode, not the anode.
An electrolyte is a molten or aqueous ionic compound that conducts and is decomposed by the current. Molten sodium chloride has mobile ions, so it qualifies. A copper wire conducts by moving electrons and is not decomposed, so it is a conductor, not an electrolyte. Solid sodium chloride has ions, but they are fixed in the lattice and cannot move. Ethanol is a covalent liquid with no ions, so it does not conduct at all.
The anode product is bromine, which is released as a red-brown, toxic vapour, so the experiment is carried out in a fume cupboard. There is no water in molten lead(II) bromide, so no hydrogen forms. There are no chloride ions, so no chlorine can form. Lead forms at the cathode as a molten liquid that collects at the bottom; it does not boil away.
A pale yellow-green gas is chlorine, so the compound must be a chloride; the silvery liquid is the metal, here sodium. Lead(II) bromide would give a red-brown bromine vapour at the anode and zinc iodide would give purple-brown iodine. Aqueous sodium chloride is not molten, and it gives bubbles of hydrogen at the cathode, not a silvery liquid metal.
The anode is positive, so it attracts negative ions (anions). An anion has extra electrons; to become a neutral atom or molecule it must lose them, for example 2Br− → Br2 + 2e−. Loss of electrons is oxidation. Gain of electrons by positive ions (reduction) is what happens at the cathode. A negative ion cannot become neutral by gaining more electrons, and positive ions are not discharged at the anode.
Cathode: Pb2+ + 2e− → Pb, so 2 mol of electrons make 1 mol of lead and 4 mol of electrons make 2 mol of lead. Anode: 2Br− → Br2 + 2e−, so 1 mol of Br2 releases 2 mol of electrons and 4 mol of electrons correspond to 2 mol of Br2. Answers with 4 mol treat each electron as producing one atom or molecule, ignoring the 2+ charge on lead or the fact that each Br2 needs two bromide ions.
Oxide ions are attracted to the positive anode and lose electrons (oxidation). Two O2− ions are needed to make one O2 molecule, and together they carry four extra electrons, so 2O2− → O2 + 4e− (charge −4 on both sides). The second equation does not balance the oxygen atoms or the charge. The third puts electrons on the wrong side, which would be reduction. The aluminium equation is correct chemistry but it happens at the cathode.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.