The shape of the lesson
By the end of the lesson, learners can
- Concentrated brine: Switch the simulator to sodium chloride solution.
- Dilute sulfuric acid and copper(II) sulfate: Sulfate ions are never discharged at the anode, so oxygen forms there instead.
- Half-equations for solutions: Extended.
- Cathode: metal only if less reactive than hydrogen; otherwise hydrogen.
- Anode: halogen from a concentrated halide; otherwise oxygen.
- Tests: hydrogen pops, oxygen relights a glowing splint, chlorine bleaches damp litmus.
- Extended: electrons on the left at the cathode, on the right at the anode.
One picture every idea returns to
An aqueous solution also contains H+ and OH− ions from water. At the cathode, the metal forms only if it is less reactive than hydrogen (such as copper or silver); otherwise hydrogen gas forms. At the anode, a halogen forms from a concentrated halide solution; otherwise oxygen gas forms. So concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode, and dilute sulfuric acid gives hydrogen and oxygen.
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, Water joins the competition.. Dissolve an ionic compound in water and the solution contains extra ions from the water: hydrogen ions and hydroxide ions. 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: Concentrated brine | Switch the simulator to sodium chloride solution. 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: Dilute sulfuric acid and copper(II) sulfate | Sulfate ions are never discharged at the anode, so oxygen forms there instead. | Pairs |
| 21 to 30 min | Teach and check: Half-equations for solutions | Extended. | Connect-Extend-Challenge |
| 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, 17 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 Half-equations for solutions, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Sodium is never produced at the cathode from an aqueous solution: it is more reactive than hydrogen, so hydrogen gas is released instead. | Sodium metal only forms when molten sodium chloride is electrolysed. |
Electrolysis of aqueous solutions: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Concentrated aqueous sodium chloride is electrolysed using inert electrodes. Name the product at each electrode, give a test for each, and name the substance left in the solution.
Explain why hydrogen, and not sodium, is formed at the cathode in the question above.
Dilute sulfuric acid is electrolysed. Name the gas at each electrode and explain why the volume of gas at the cathode is twice that at the anode.
(Extended) Predict the product at the anode when (a) concentrated aqueous potassium iodide and (b) dilute aqueous potassium iodide are electrolysed with inert electrodes.
(Extended) Aqueous copper(II) sulfate is electrolysed using carbon electrodes. Describe what is seen at each electrode and in the solution, and write the half-equation at the cathode.
(Extended) Write the half-equations for the reactions at the cathode and the anode during the electrolysis of concentrated hydrochloric acid.
Answer key and marking notes
- Cathode: hydrogen; a lighted splint gives a squeaky pop.
- Anode: chlorine; it bleaches damp litmus paper.
- Sodium hydroxide is left in solution.
- The solution contains Na+ ions and H+ ions (from water).
- Sodium is more reactive than hydrogen, so hydrogen ions are discharged instead.
- Cathode: hydrogen. Anode: oxygen.
- The overall change is the splitting of water, 2H2O → 2H2 + O2.
- Two molecules of hydrogen form for every molecule of oxygen, so the volume of hydrogen is twice that of oxygen.
- (a) iodine (a concentrated halide solution gives the halogen).
- (b) oxygen (a dilute halide solution gives mainly oxygen).
- Cathode: a pink-brown layer of copper forms.
- Anode: bubbles of a colourless gas, oxygen.
- The blue colour of the solution fades as Cu2+ ions are removed.
- Cu2+ + 2e− → Cu.
- Cathode: 2H+ + 2e− → H2.
- Anode: 2Cl− → Cl2 + 2e−.
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.
Which ions are present in concentrated aqueous sodium chloride?
- ANa+, Cl−, H+ and OH−
- BNa+ and Cl− only
- CNa+, Cl−, H+ and O2−
- DNa+, Cl− and H2O+
Concentrated aqueous sodium chloride is electrolysed for some time. A few drops of universal indicator are then added to the solution. What is seen, and why?
- AIt turns purple, because sodium hydroxide is left in the solution
- BIt turns red, because hydrogen ions are left in the solution
- CIt stays green, because only sodium chloride remains
- DIt turns red, because sodium metal reacts with the water
The diagram shows dilute sulfuric acid being electrolysed with inert electrodes. Tube P has collected twice the volume of gas collected in tube Q. Which row identifies the gas in tube P and the electrode it formed at?
- Ahydrogen, at the cathode
- Boxygen, at the anode
- Chydrogen, at the anode
- Doxygen, at the cathode
Which test confirms the gas produced at the anode when dilute sulfuric acid is electrolysed?
- Ait relights a glowing splint
- Bit gives a squeaky pop with a lighted splint
- Cit bleaches damp litmus paper
- Dit turns limewater milky
A student electrolyses aqueous sodium chloride but wants to produce sodium metal at the cathode. Which change would work?
- AElectrolyse molten sodium chloride instead of a solution
- BUse a more concentrated solution of sodium chloride
- CUse platinum electrodes instead of graphite
- DIncrease the voltage of the power supply
Aqueous silver nitrate, AgNO3, is electrolysed using inert electrodes. What are the products?
- Asilver at the cathode and oxygen at the anode
- Bhydrogen at the cathode and oxygen at the anode
- Csilver at the cathode and nitrogen dioxide at the anode
- Dhydrogen at the cathode and nitrogen at the anode
Concentrated aqueous copper(II) chloride is electrolysed using carbon electrodes. What are the products?
- Acopper at the cathode and chlorine at the anode
- Bhydrogen at the cathode and chlorine at the anode
- Ccopper at the cathode and oxygen at the anode
- Dhydrogen at the cathode and oxygen at the anode
Aqueous copper(II) sulfate is electrolysed using carbon electrodes until the blue colour has almost gone. The remaining solution has a pH of about 1. Why?
- AH+ and SO42− ions are left behind, so the solution becomes sulfuric acid
- BCopper(II) hydroxide is left behind in the solution
- CSulfate ions are discharged at the anode, forming sulfur dioxide
- DHydrogen gas dissolves in the water and makes it acidic
Concentrated aqueous sodium chloride is electrolysed and 12 cm3 of hydrogen is collected at the cathode. Assuming no gas dissolves, what volume of chlorine is formed at the anode in the same time?
- A12 cm3
- B24 cm3
- C6 cm3
- D48 cm3
Quiz key
The salt provides Na+ and Cl− ions. Water supplies H+ and OH− ions, which is why aqueous electrolysis can produce hydrogen or oxygen. 'Na+ and Cl− only' describes molten sodium chloride. Water does not give free oxide ions, O2−, and H2O+ is not an ion found in water.
H+ ions are discharged at the cathode as hydrogen and Cl− ions at the anode as chlorine. The ions left in solution are Na+ and OH−, which is sodium hydroxide, an alkali, so universal indicator turns purple. The hydrogen ions are the ones removed, so the solution does not become acidic. The composition has changed, so it is not just sodium chloride. No sodium metal forms from an aqueous solution.
Electrolysis of dilute sulfuric acid effectively splits water: 2H2O → 2H2 + O2, so the volume of hydrogen is twice the volume of oxygen. Tube P, with the larger volume, holds hydrogen. Hydrogen comes from positive H+ ions, which are attracted to the negative cathode. Oxygen is the smaller volume (tube Q) and forms at the anode. Hydrogen cannot form at the anode and oxygen cannot form at the cathode.
Sulfate ions are never discharged, so oxygen forms at the anode, and oxygen relights a glowing splint. The squeaky pop is the test for hydrogen, which forms at the cathode. Bleaching damp litmus is the test for chlorine, which needs a concentrated chloride. Limewater tests for carbon dioxide, which is not produced.
In any aqueous solution H+ ions from water are present, and because sodium is more reactive than hydrogen, hydrogen is discharged instead of sodium. The only way to get sodium is to remove the water: electrolyse molten sodium chloride. A more concentrated solution changes the anode product (chlorine instead of oxygen), not the cathode product. Platinum is inert like graphite, and a higher voltage just makes more hydrogen faster.
Cathode: silver is less reactive than hydrogen, so Ag+ ions are discharged and silver forms. Anode: nitrate is not a halide, so oxygen forms from the water (4OH− → O2 + 2H2O + 4e−). Hydrogen at the cathode would be right only for a metal more reactive than hydrogen. Nitrate ions are not discharged, so no nitrogen or nitrogen dioxide is formed.
Copper is less reactive than hydrogen, so Cu2+ ions are discharged at the cathode and a pink-brown layer of copper forms. The solution is a concentrated chloride, so chlorine is formed at the anode. Hydrogen at the cathode is the result for reactive metals such as sodium. Oxygen at the anode would be expected from a dilute chloride solution or a sulfate, not a concentrated chloride.
The solution contains Cu2+, SO42−, H+ and OH−. Cu2+ is discharged at the cathode as copper and OH− at the anode as oxygen. The ions left are H+ and SO42−: sulfuric acid, which has a low pH. Copper(II) hydroxide would not be acidic and copper ions have been removed. Sulfate ions are not discharged. No hydrogen forms at the cathode here, because copper is discharged instead.
Cathode: 2H+ + 2e− → H2. Anode: 2Cl− → Cl2 + 2e−. Each molecule of either gas involves 2 electrons, so equal numbers of molecules, and therefore equal volumes, are formed: 12 cm3 of chlorine. 24 cm3 and 6 cm3 use the 2 : 1 ratio that belongs to hydrogen and oxygen from dilute sulfuric acid, where oxygen needs 4 electrons per molecule.
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.