Metal displacement and positive-ion formation
Explain the magnesium–zinc–iron–copper–silver comparisons through the more reactive metal's electron loss.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Identify oxidising and reducing agents in metal displacement.
- Use the reactivity series and balance ionic charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which of Mg, Zn, Fe, Cu and Ag is most reactive in the specified order?
What you will learn
- Predict displacement with the aqueous ions of magnesium, zinc, iron, copper and silver.
- Explain relative reactivity through the tendency to form positive ions and matched electron transfer.
Compare the solid metal with the metal belonging to the ion
For the five specified metals, descending reactivity is Mg>Zn>Fe>Cu>Ag. A more reactive metal can displace a less reactive metal from its aqueous ions under suitable stated conditions. Magnesium can displace Zn, Fe, Cu and Ag; zinc Fe, Cu and Ag; iron Cu and Ag; copper Ag; silver none of the other four. There is no net self-displacement when a metal meets its own ions in this model.
The more reactive metal has the greater tendency to lose electrons and form positive ions in these comparisons. Zn+Cu²⁺→Zn²⁺+Cu shows zinc oxidised while copper(II) is reduced. Zinc is the reducing agent and Cu²⁺ the oxidising agent. The label more reactive refers to the neutral-metal tendency in this series; it does not mean every positive ion of that metal is the strongest oxidising agent.
Balance the electron count, not just the metal names
Copper can displace silver through Cu+2Ag⁺→Cu²⁺+2Ag. Copper loses two electrons while two silver ions each gain one. For iron examples here, the supplied aqueous ion is Fe²⁺: Zn+Fe²⁺→Zn²⁺+Fe is a stated comparison. Different oxidation states or conditions need the actual equation; do not silently substitute Fe³⁺ and assume the same coefficients and products.
A visible coating of displaced metal and changes in dissolved species can support the reaction, but use the given species and observations rather than requiring one universal colour. No-displacement predictions for less reactive metals apply to this specified model, with suitable conditions. The reasoning rests on position, electron loss and conservation of charge together.
Pause and explain
In Zn+Cu²⁺→Zn²⁺+Cu, what happens to zinc?
Worked example
Predict copper with Ag⁺(aq), write the ionic equation and match electrons.
Show the worked solution
- Copper is above silver, so silver displacement is predicted in suitable conditions.
- Cu→Cu²⁺+2e⁻ supplies two electrons; 2Ag⁺+2e⁻→2Ag accepts them.
- Add to give Cu+2Ag⁺→Cu²⁺+2Ag with charge +2 on both sides.
Answer Cu+2Ag⁺→Cu²⁺+2Ag; copper is oxidised and silver ions reduced.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict zinc with Fe²⁺(aq), write the ionic equation and identify both redox changes.
Give me a hint
Zinc is above iron; both supplied positive ions are 2+.
Compare my reasoning
- Zn is more reactive and displaces iron in the stated model.
- Zn+Fe²⁺→Zn²⁺+Fe has charge +2 on both sides.
- Zinc loses two electrons and is oxidised; Fe²⁺ gains two and is reduced.
Zn+Fe²⁺→Zn²⁺+Fe; Zn oxidised, Fe²⁺ reduced.
Look for these in your work
- The ranking predicts the direction.
- Charge and electron changes agree.
Compare copper with Zn²⁺ and Ag⁺, using the specified series and balanced reacting equation.
Give me a hint
Copper lies below zinc but above silver.
Compare my reasoning
- Copper does not displace zinc from Zn²⁺ in this model.
- It does displace silver from Ag⁺.
- The reacting equation is Cu+2Ag⁺→Cu²⁺+2Ag.
Zn²⁺: no displacement. Ag⁺: Cu+2Ag⁺→Cu²⁺+2Ag.
Look for these in your work
- Both comparisons use the correct ion's metal.
- The silver coefficient balances electron transfer.
A student writes Mg+Ag⁺→Mg²⁺+Ag and claims displacement is impossible because the charges differ. Correct the equation and assess the reactivity prediction.
Give me a hint
Magnesium supplies two electrons, so two silver ions are needed.
Compare my reasoning
- Magnesium is above silver, so the displacement is supported in the suitable model.
- Mg→Mg²⁺+2e⁻ requires two Ag⁺ each gaining one electron.
- Correct to Mg+2Ag⁺→Mg²⁺+2Ag, charge +2 on both sides.
Mg+2Ag⁺→Mg²⁺+2Ag; balancing resolves the charge issue and displacement is predicted.
Look for these in your work
- The error is corrected rather than used as false reactivity evidence.
- Atoms, charge and electrons all match.
Common mistakes
- Comparing the metal with the counterion instead of the metal belonging to the aqueous ion.
- Writing one Ag⁺ for Cu forming Cu²⁺ and leaving charge unbalanced.
- Assuming Fe²⁺ and Fe³⁺ always give identical equations.
- Calling the more reactive solid metal the electron acceptor in these examples.
What can you explain now?
Will silver displace magnesium from Mg²⁺ in this model? Explain.
Compare with the explanation
No; silver is below magnesium in the series.
Silver has a lower tendency to lose electrons than magnesium in the stated metal comparison, so the proposed displacement is not predicted.
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Make it stick
Predict each specified metal with the others' ions, then balance one 2+/1+ displacement and explain its electron roles.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.