Deduce an order of reactivity from a given set of experimental results

Metals – Reactivity Series (IGCSE Chemistry 0620)

Learning objective

From a set of experimental results, deduce the correct order of reactivity for the metals involved and use the series to predict the products of displacement reactions, corrosion behaviour and practical applications.

1. Reactivity series – what the syllabus requires

1.1 Core series (required for the syllabus)

The core series must be memorised and understood. It lists the metals (and hydrogen) in order of decreasing reactivity:

1.2 Extended series (useful for deeper understanding)

Some examinations include tin (Sn) and lead (Pb). They sit between iron and hydrogen:

1.3 Why hydrogen is placed in the series

• Metals **above** H liberate hydrogen gas when they react with dilute acids (e.g., Mg + 2HCl → MgCl₂ + H₂).
• Metals **below** H do **not** react with dilute acids under normal conditions (e.g., Cu + HCl → no reaction).
• Thus H provides a reference point that separates “acid‑reactive” from “non‑acid‑reactive” metals.

2. Key concepts

• Oxidation tendency: A more reactive metal loses electrons more readily, forming a cation.
• Displacement reactions: A metal higher in the series will displace a metal lower in the series from its aqueous salt.
• Reaction with dilute acids: Metals above H give H₂; those below H give no observable reaction.
• Reaction with cold water: Only the very reactive alkali and alkaline‑earth metals (K, Na, Ca) react visibly with liquid water.
• Reaction with steam (or hot water): Mg, Al and Zn react with steam, producing the metal oxide and hydrogen.
• Surface passivation (Aluminium): A thin Al₂O₃ layer forms instantly on Al, preventing further reaction unless the oxide is removed or the metal is placed in a strong acid/alkali.
• Corrosion & protection: Metals below H (Cu, Ag, Au) are naturally resistant. Metals above H corrode unless protected by coating, alloying, or cathodic protection.

3. Typical experimental observations

1. Colour change on a metal strip when immersed in a solution of another metal’s salt – indicates a displacement reaction.
2. Evolution of bubbles (hydrogen) when a metal is added to dilute acid.
3. Formation of a coloured precipitate when a metal ion is reduced.
4. No visible change – either the metal is below hydrogen in the series or the reaction is too slow to see.

4. Experimental data – full set of core metals

Metal	Reaction with dilute HCl	Displaces from salt of …	Reaction with cold water	Reaction with steam (or hot water)	Passivation observation
K (potassium)	Very vigorous bubbling; H₂ + KCl	All other metals listed	K + 2H₂O → 2KOH + H₂ (explosive)	Rapid reaction; K₂O + H₂	–
Na (sodium)	Vigorous bubbling; H₂ + NaCl	All other metals listed	Na + 2H₂O → 2NaOH + H₂ (explosive)	Rapid reaction; Na₂O + H₂	–
Ca (calcium)	Rapid bubbling; H₂ + CaCl₂	All metals below Ca (Mg, Al, …, Au)	Ca + 2H₂O → Ca(OH)₂ + H₂ (moderately vigorous)	Ca + H₂O(g) → CaO + H₂ (slow)	–
Mg (magnesium)	Rapid bubbling; H₂ + MgCl₂	Zn, Fe, Cu, Ag, Au	No visible reaction	Mg + H₂O(g) → MgO + H₂ (vigorous)	–
Al (aluminium)	Very slow/no bubbling unless oxide removed; AlCl₃ + H₂	Zn, Fe, Cu, Ag, Au (if oxide stripped)	No visible reaction (oxide layer)	Al + H₂O(g) → Al₂O₃ + H₂ (slow, surface‑passivated)	Fresh Al strip in HCl shows no reaction until surface is scratched – illustrates passivation.
Zn (zinc)	Moderate bubbling; H₂ + ZnCl₂	Fe, Cu, Ag, Au	No visible reaction	Zn + H₂O(g) → ZnO + H₂ (slow)	–
Fe (iron)	Slow bubbling; H₂ + FeCl₂	Cu, Ag, Au	No visible reaction	Fe + H₂O(g) → Fe₃O₄ + H₂ (very slow)	–
Cu (copper)	No bubbling	None	No visible reaction	No reaction	–
Ag (silver)	No bubbling	None	No visible reaction	No reaction	–
Au (gold)	No bubbling	None	No visible reaction	No reaction	–

5. How to deduce the reactivity order

1. Acid test: Metals that evolve H₂ with dilute HCl are above hydrogen; those that do not are below.
2. Displacement test: If metal A displaces metal B from its salt, then A > B.
3. Cold‑water test: Observe which metals react with liquid water – they occupy the top of the series (K, Na, Ca).
4. Steam test: Metals that react with steam (Mg, Al, Zn) are placed between the alkali/alkaline‑earth metals and the less reactive transition metals.
5. Passivation check: A metal that appears unreactive in acid but reacts when its surface is disturbed (Al) is still placed above hydrogen.
6. Combine all evidence to arrange the metals from most to least reactive.

6. Worked example – using the data above

• Acid test: K, Na, Ca, Mg, Al (once oxide removed), Zn, Fe all give H₂ → they are above H. Cu, Ag, Au give no H₂ → below H.
• Displacement hierarchy (selected observations):
  • K displaces every other metal → top of the series.
  • Na displaces all except K → second.
  • Ca displaces all metals below Ca (Mg, Al, Zn, Fe, Cu, Ag, Au) → third.
  • Mg displaces Zn, Fe, Cu, Ag, Au → placed above them but below Ca.
  • Al (once oxide removed) displaces Zn, Fe, Cu, Ag, Au → sits between Mg and Zn.
  • Zn displaces Fe, Cu, Ag, Au → below Al.
  • Fe displaces Cu, Ag, Au → below Zn.
  • Cu, Ag, Au do not displace any metal listed → bottom of the series.

Resulting order (most → least reactive):

$$\text{K} \;>\; \text{Na} \;>\; \text{Ca} \;>\; \text{Mg} \;>\; \text{Al} \;>\; \text{Zn} \;>\; \text{Fe} \;>\; \text{H} \;>\; \text{Cu} \;>\; \text{Ag} \;>\; \text{Au}$$

7. Extending the series – an example with Sn and Pb

Both tin (Sn) and lead (Pb) lie between iron and hydrogen. A typical displacement observation:

• Sn + CuSO₄ → SnSO₄ + Cu (tin displaces copper, so Sn > Cu).
• Pb + FeSO₄ → PbSO₄ + Fe (lead does **not** displace iron, so Pb < Fe).

These results confirm the extended order: … Fe > Sn > Pb > H > Cu …

8. Corrosion, protection and practical applications

• Metals **below H** (Cu, Ag, Au) are naturally resistant to atmospheric corrosion – why they are used for jewellery, electrical conductors and coins.
• Metals **above H** corrode readily; they are protected by:
  • Coating with a less‑reactive metal or paint (e.g., galvanised iron).
  • Alloying (e.g., stainless steel – Fe mixed with Cr and Ni).
  • Cathodic protection (e.g., zinc anodes protecting ship hulls).
• Extremely reactive metals (K, Na) are unsuitable for most everyday objects because they react violently with moisture.

9. Common misconceptions

• All metals react with acids – only those above H do.
• Any colour change means a displacement reaction – some changes are due to surface oxidation or precipitation of hydroxides.
• More reactive = more useful – highly reactive metals are often too aggressive for practical use; moderate reactivity (e.g., Zn, Fe) is often ideal.

10. Practice questions

1. Question: Arrange the metals in order of reactivity (most → least) based on the observations:

   • Metal X reacts vigorously with cold water, producing H₂.
   • Metal Y displaces copper from CuSO₄ solution.
   • Metal Z does not react with dilute HCl.

   Answer: X > Y > H > Z. Example: Ca > Zn > H > Cu.

2. Question: Explain why magnesium can displace iron from FeSO₄ solution but iron cannot displace magnesium from MgSO₄ solution.

   Answer outline: Mg is higher in the series (more reactive) → it more readily loses electrons, so it reduces Fe²⁺ to Fe(s) while being oxidised to Mg²⁺. Fe, being lower, cannot oxidise Mg²⁺.

3. Question: Predict the products when aluminium metal is added to a solution of silver nitrate (AgNO₃). Write the balanced equation.

   Answer: 2 Al + 3 AgNO₃ → Al(NO₃)₃ + 3 Ag (solid). (Aluminium must be cleaned of its oxide layer for the reaction to be observed.)

4. Question: Tin is placed in a solution of copper(II) sulphate. Write the balanced equation and state what the observation tells you about the position of Sn in the series.

   Answer: Sn + CuSO₄ → SnSO₄ + Cu. Copper deposits as a reddish‑brown solid, showing Sn > Cu, so Sn lies above copper (and therefore above hydrogen).

5. Question: Why does iron react with dilute HCl but copper does not? Relate your answer to hydrogen’s position in the series.

   Answer: Iron is above hydrogen, so it can donate electrons to H⁺, liberating H₂. Copper is below hydrogen and therefore cannot reduce H⁺ under normal conditions.

11. Suggested diagram

12. Summary

By analysing reactions with dilute acids, cold water, steam, and the salts of other metals, students can construct a reliable reactivity series. The series not only predicts the outcomes of displacement reactions but also explains trends in corrosion, the need for protective measures, and the special behaviour of metals such as aluminium. Mastery of this topic enables accurate prediction of products, understanding of industrial applications, and successful performance in IGCSE examinations.