Reactivity SeriesCambridge IGCSE Chemistry: Revision notes
Section 1
What is the reactivity series and how is it ordered?
The reactivity series is an ordered list of metals arranged by their tendency to lose electrons and form positive ions. The more readily a metal loses electrons, the more reactive it is.
The complete reactivity series (in order of decreasing reactivity):
| Position | Metal |
|---|---|
| Most reactive | Potassium (K) |
| Sodium (Na) | |
| Calcium (Ca) | |
| Magnesium (Mg) | |
| Aluminium (Al) | |
| Carbon (C) | |
| Zinc (Zn) | |
| Iron (Fe) | |
| Hydrogen (H) | |
| Copper (Cu) | |
| Silver (Ag) | |
| Least reactive | Gold (Au) |
Metals higher in the series are more reactive because they have a greater tendency to form positive ions by losing electrons. This is fundamental to predicting reaction outcomes and understanding displacement reactions.
Think of the reactivity series as a 'willingness to give' scale — metals at the top are eager to give away electrons, whilst those at the bottom hold onto them tightly.
Section 2
How do metals in the reactivity series react with water?
The vigour of reaction with water depends on the metal's position in the reactivity series. Different metals react differently with cold water or steam.
Reactions with cold water:
Potassium reacts vigorously with cold water:
- 2K + 2H₂O → 2KOH + H₂↑
- The metal melts into a ball and skitters across the surface
- Hydrogen gas is produced (burns with a lilac flame)
Sodium reacts readily with cold water:
- 2Na + 2H₂O → 2NaOH + H₂↑
- Similar vigorous reaction to potassium, but less intense
- Hydrogen gas is produced
Calcium reacts moderately with cold water:
- Ca + 2H₂O → Ca(OH)₂ + H₂↑
- Less vigorous than sodium; the metal sinks and bubbles
- Hydrogen gas is produced
Magnesium does not react with cold water but reacts readily with steam:
- Mg + H₂O(g) → MgO + H₂↑
- A bright white flame is observed
- This shows magnesium's moderate reactivity — it requires the higher temperature of steam to react
The pattern shows that reactivity with water decreases down the series from potassium to magnesium.
Examiners expect you to describe the observations you would see in these reactions (melting, vigour, colour of flame, gas produced) as well as write balanced equations. Always state what happens, not just the chemistry.
If asked to compare potassium and calcium reactions with water: potassium reacts more vigorously because it is higher in the reactivity series and has a greater tendency to lose electrons. Potassium's reaction is so vigorous the metal melts and skitters, whilst calcium sinks and reacts more slowly.
Section 3
How do metals react with dilute hydrochloric acid?
Metals above hydrogen in the reactivity series react with dilute hydrochloric acid to produce hydrogen gas. Metals below hydrogen do not react. The position in the series determines the vigour of the reaction.
Metals that react with dilute HCl (in order of reactivity):
Magnesium reacts vigorously:
- Mg + 2HCl → MgCl₂ + H₂↑
- Rapid bubbling, considerable heat released
Zinc reacts moderately:
- Zn + 2HCl → ZnCl₂ + H₂↑
- Steady bubbling, noticeable heat release
Iron reacts slowly:
- Fe + 2HCl → FeCl₂ + H₂↑
- Slow bubbling, minimal heat release
Copper does not react with dilute HCl:
- Copper is below hydrogen in the reactivity series
- No bubbling or gas evolution occurs
Silver and gold do not react with dilute HCl:
- Both are below hydrogen and are unreactive towards dilute acids
Explanation by position in reactivity series: Metals more easily lose electrons when they are higher in the series. A metal can only displace hydrogen from an acid if it is more reactive than hydrogen. The more reactive the metal, the faster and more vigorous the reaction.
Students often state that 'copper doesn't react because it's not reactive enough' — this is too vague. The correct explanation is: copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.
If comparing magnesium and iron with HCl: magnesium is higher in the reactivity series, so it loses electrons more readily. Magnesium reacts vigorously with rapid bubbling, whilst iron reacts slowly with minimal gas evolution.
Section 4
What are displacement reactions and how do they show reactivity?
A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its salt solution. This demonstrates the relative reactivities of metals in terms of their tendency to form positive ions.
Principle: A metal will displace another metal from solution if it is higher in the reactivity series.
General equation: A more reactive metal + salt solution of less reactive metal → salt of more reactive metal + less reactive metal
Examples using metals in the specification:
| More Reactive Metal | Less Reactive Metal Ion | Reaction Occurs? | Equation |
|---|---|---|---|
| Magnesium | Cu²⁺ solution | Yes | Mg + CuSO₄ → MgSO₄ + Cu |
| Magnesium | Zn²⁺ solution | Yes | Mg + ZnSO₄ → MgSO₄ + Zn |
| Zinc | Cu²⁺ solution | Yes | Zn + CuSO₄ → ZnSO₄ + Cu |
| Zinc | Fe²⁺ solution | Yes | Zn + FeSO₄ → ZnSO₄ + Fe |
| Iron | Cu²⁺ solution | Yes | Fe + CuSO₄ → FeSO₄ + Cu |
| Copper | Ag⁺ solution | Yes | Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag |
| Copper | Zn²⁺ solution | No | No reaction |
| Silver | Cu²⁺ solution | No | No reaction |
Why displacement reactions occur: The more reactive metal has a stronger tendency to form positive ions (lose electrons). It displaces the less reactive metal because it more readily donates electrons to the positive ions in solution.
Deducing reactivity from experimental results: If you observe a displacement reaction occurring, you can conclude the metal added is more reactive than the metal ion in solution.
When deducing reactivity order from experimental results, look for: (1) which reactions occurred and which didn't, (2) use this to order metals from the reactivity series logic — if metal A displaced metal B, then A is more reactive than B.
If zinc displaces copper from copper sulphate solution (Zn + CuSO₄ → ZnSO₄ + Cu), you know zinc is more reactive than copper. Zinc has a greater tendency to form positive ions, so it 'gives' electrons to Cu²⁺ ions, forcing the copper to become neutral metal.
Section 5
Why is aluminium apparently unreactive despite being quite reactive?
Aluminium is positioned in the upper-middle section of the reactivity series, suggesting it should be quite reactive. However, it appears unreactive in many situations. This apparent unreactivity is explained by the oxide layer that rapidly forms on its surface.
The oxide layer protection:
- When aluminium is exposed to air, it immediately reacts with oxygen to form aluminium oxide (Al₂O₃)
- This oxide layer is extremely thin but very tough and non-porous
- The oxide layer forms a protective barrier that prevents further reaction with oxygen, water, or dilute acids
- This process is called passivation
Evidence of aluminium's true reactivity:
- Aluminium does react with dilute hydrochloric acid if the oxide layer is removed (e.g. by rubbing with sandpaper or adding mercury)
- Freshly cut aluminium reacts with dilute HCl: 2Al + 6HCl → 2AlCl₃ + 3H₂↑
- Once the oxide layer reforms, the reaction stops
Why the oxide layer is protective:
- Al₂O₃ is a ceramic compound that is insoluble in water and resistant to dilute acids
- It adheres tightly to the metal surface and does not flake off
- This differs from the oxides of many other metals, which can be permeable or easily removed
Comparison: This explains why aluminium can be used in outdoor structures and aircraft (where reactivity would normally be a problem), whilst more reactive metals like iron rust readily.
The aluminium oxide layer is like a tough invisible shield — aluminium underneath is highly reactive, but the shield prevents it from reaching air, water, or acid, so it appears unreactive.
Examiners expect you to explain aluminium's apparent unreactivity in terms of its oxide layer, not the metal's position in the series. Always mention that the layer is tough, non-porous, and forms immediately.
Must Know
-
The reactivity series in order: K > Na > Ca > Mg > Al > C > Zn > Fe > H > Cu > Ag > Au. More reactive metals are more willing to lose electrons and form positive ions.
-
Water/steam reactions: Potassium and sodium react vigorously with cold water; calcium reacts moderately with cold water; magnesium does not react with cold water but reacts with steam. All produce hydrogen gas.
-
Dilute HCl reactions: Metals above hydrogen in the reactivity series react with dilute HCl producing hydrogen gas. Metals below hydrogen (Cu, Ag, Au) do not react. Vigour of reaction reflects position in series.
-
Displacement reactions prove reactivity: A more reactive metal displaces a less reactive metal from its salt solution. If a reaction occurs, the added metal is more reactive than the metal ion in solution.
-
Aluminium's apparent unreactivity is due to a protective oxide layer (Al₂O₃) that forms immediately on its surface. This layer is tough, non-porous, and insoluble in dilute acid, preventing further reaction — but aluminium is actually quite reactive beneath the layer.
-
Deducing reactivity from experiments: Observe which displacement reactions occur (✓) and which don't (✗). Metals that cause reactions are more reactive; those that don't are less reactive than the metal ion in solution.
That's the notes covered.
Carry on to the next subtopic.