The shape of the lesson
By the end of the lesson, learners can
- Setting up electroplating: In the simulator, watch copper ions pick up electrons and become copper atoms on a metal surface.
- Copper electrodes: Extended.
- Refining copper: Extended.
- Cathode: object to be plated (or pure copper sheet).
- Anode: plating metal (or impure copper), which dissolves.
- Electrolyte: contains ions of the plating metal.
- Extended: copper electrodes keep the blue colour constant.
One picture every idea returns to
In electroplating the object to be plated is the cathode, the plating metal is the anode, and the electrolyte is a solution containing ions of the plating metal. Metal ions gain electrons at the cathode and coat the object, while the anode dissolves to replace them. Metals are electroplated to improve appearance and to resist corrosion. Extended: copper is refined the same way, with an impure copper anode and a pure copper cathode in copper(II) sulfate solution.
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, Moving metal atom by atom.. Swap the inert anode for a lump of metal and electrolysis stops making gases. 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: Setting up electroplating | In the simulator, watch copper ions pick up electrons and become copper atoms on a metal surface. Project tab 2 of the lab (Electrochemistry lab, electron transfer) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Copper electrodes | Extended. | Pairs |
| 21 to 30 min | Teach and check: Refining copper | Extended. | Anticipation / Reaction Guide |
| 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 Refining copper, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| The object being plated must be the cathode. | Metal ions are positive, so they are attracted to the negative electrode. Connect the object to the positive terminal and it dissolves instead of being coated. |
Electroplating and refining copper: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
A student wants to nickel-plate a steel paperclip. State what should be used as (a) the cathode, (b) the anode and (c) the electrolyte.
Give two reasons why metal objects are electroplated.
Explain why the object to be plated must be connected to the negative terminal of the power supply.
(Extended) Compare the electrolysis of aqueous copper(II) sulfate using carbon electrodes with the electrolysis using copper electrodes. State what happens at each anode and to the colour of the solution in each case.
(Extended) Describe how impure copper is purified by electrolysis. Include the electrodes, the electrolyte and what happens to the impurities.
(Extended) During refining, an impure copper anode loses 120 g and the cathode gains 114 g. Explain the difference and calculate the percentage purity of the copper that dissolved from the anode.
Answer key and marking notes
- (a) the steel paperclip.
- (b) a piece of nickel.
- (c) a solution containing nickel ions, for example nickel sulfate solution.
- To improve their appearance.
- To protect them from corrosion.
- The metal ions in the electrolyte are positive.
- They are attracted to the negative electrode (cathode), where they gain electrons and form a layer of metal on the object.
- Carbon electrodes: oxygen is given off at the anode.
- Carbon electrodes: the blue colour fades as copper ions are removed.
- Copper electrodes: the copper anode dissolves, forming Cu2+ ions.
- Copper electrodes: the blue colour stays the same, because copper ions are replaced as fast as they are removed.
- The anode is impure copper; the cathode is a thin sheet of pure copper.
- The electrolyte is copper(II) sulfate solution.
- Copper dissolves from the anode as Cu2+ ions and is deposited as pure copper on the cathode.
- The impurities fall to the bottom of the cell as anode sludge.
- Only the copper is deposited on the cathode; 6 g of impurities fell to the bottom as sludge.
- Percentage purity = 114 ÷ 120 × 100 = 95%.
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.
A steel food can is to be coated with a thin layer of tin by electroplating. Which electrolyte should be used?
- Aa solution of a tin compound, such as tin(II) sulfate
- Ba solution of an iron compound, such as iron(II) sulfate
- Cdilute sulfuric acid
- Dconcentrated aqueous sodium chloride
The diagram shows a cell that will be used to silver-plate a steel spoon. Where should the spoon go, and what should the other electrode be made of?
- Athe spoon at Q, with a bar of silver at P
- Bthe spoon at P, with a bar of silver at Q
- Cthe spoon at Q, with a bar of copper at P
- Dthe spoon at P, with a bar of copper at Q
What happens to the silver anode while an object is being silver-plated?
- AIt dissolves and gets smaller
- BIt gets thicker as silver is deposited on it
- CBubbles of oxygen form on it and it stays the same size
- DIt does not change, because silver is unreactive
Many forks and spoons are made of steel and then silver-plated, rather than being made of solid silver. What is the main reason?
- AA thin silver layer gives the attractive appearance of silver at a much lower cost
- BSolid silver does not conduct electricity
- CSilver is too reactive to be used on its own
- DSilver-plating makes the cutlery lighter than solid steel
A key is being copper-plated with a constant current. Its mass increases by 0.20 g in the first 10 minutes. What total increase in mass would you expect after 30 minutes with the same current?
- A0.60 g
- B0.20 g
- C0.30 g
- D6.0 g
Why is copper used for electrical wiring purified by electrolysis?
- AImpurities increase the electrical resistance of copper
- BImpurities make copper react with the air and burn
- CPure copper is cheaper than impure copper
- DImpure copper does not conduct electricity at all
In a copper refinery, an impure copper anode containing 97.5% copper by mass loses 80.0 g. Assuming all the copper that leaves the anode is deposited, what mass does the pure cathode gain?
- A78.0 g
- B80.0 g
- C2.0 g
- D82.0 g
Aqueous copper(II) sulfate is electrolysed using two pure copper electrodes. How do the masses of the electrodes change?
- AThe anode loses mass and the cathode gains the same mass
- BThe anode gains mass and the cathode loses the same mass
- CBoth electrodes gain mass
- DNeither electrode changes, but oxygen forms at the anode
Which statement describes the electron transfer during copper-plating with a copper anode?
- ACopper atoms at the anode are oxidised; copper ions at the cathode are reduced
- BCopper atoms at the anode are reduced; copper ions at the cathode are oxidised
- CCopper ions at the anode gain electrons; copper atoms at the cathode lose electrons
- DNo electrons are transferred, because the same metal is at both electrodes
Quiz key
The electrolyte must contain ions of the plating metal, here tin, so a solution of a tin compound is used; Sn2+ ions gain electrons at the steel cathode and form the tin layer. Iron ions would deposit iron, which is what the can is already made of. Dilute sulfuric acid and brine contain no tin ions, so they would produce hydrogen at the cathode instead of a tin coating.
Q is wired to the short, thick plate, the negative terminal, so Q is the cathode. The object being plated must be the cathode, because positive Ag+ ions are attracted there and gain electrons to form silver. The anode, P, should be silver, so that it dissolves and replaces the silver ions used up. With the spoon at P it would be the anode and would not be coated. A copper anode would put Cu2+ ions into the solution, which would then be deposited along with the silver.
Silver atoms in the anode lose electrons and go into solution as Ag+ ions, so the anode dissolves and gets smaller. This replaces the silver ions that are deposited on the object at the cathode. Metal builds up on the cathode, not the anode. Oxygen forms only at an inert anode. Even an unreactive metal like silver is oxidised when it is made the anode of the cell.
Silver is expensive, so a thin plated layer gives the shiny look of silver (and protects the steel from corrosion) while most of the object is cheap, strong steel. Silver is an excellent conductor, and it is unreactive, which is exactly why it makes a good surface coating. A thin coating adds a little mass; it does not make the object lighter.
A constant current deposits copper at a constant rate, so the mass gained is proportional to the time. 30 minutes is 3 × 10 minutes, so the increase is 3 × 0.20 = 0.60 g. 0.20 g assumes plating stops after the first 10 minutes; 0.30 g adds only half as much again; 6.0 g multiplies by 30 instead of by 3.
A wire needs as low a resistance as possible, and even small amounts of impurities increase the resistance of copper, so wiring copper is refined to very high purity. Impure copper still conducts, just less well. Refining costs energy, so pure copper is not cheaper, and impurities do not make copper burn in air.
Mass of copper = 97.5 ÷ 100 × 80.0 = 78.0 g, and this is what is deposited on the cathode. The other 80.0 − 78.0 = 2.0 g is impurity that falls to the bottom as anode sludge. 80.0 g ignores the impurities, 2.0 g is the mass of sludge, and the cathode cannot gain more mass than the anode loses.
The copper anode dissolves: Cu → Cu2+ + 2e−, so it loses mass. At the cathode Cu2+ + 2e− → Cu, so it gains mass. The same electrons pass through both, so the mass lost equals the mass gained (with pure electrodes there is no sludge). Copper ions go to the negative cathode, not the anode, and oxygen only forms when the anode is inert, such as carbon.
At the anode, Cu → Cu2+ + 2e−: copper atoms lose electrons, which is oxidation. At the cathode, Cu2+ + 2e− → Cu: copper ions gain electrons, which is reduction. The second and third options reverse the processes. Electrons are certainly transferred; the power supply pushes them from the anode round the circuit to 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.