9.4 Reactivity Series Revision Notes
1. Overview
The reactivity series is a ranking of metals based on their chemical reactivity, specifically their ability to lose electrons and form positive ions. Understanding this order allows us to predict how metals will behave in chemical reactions, how they are extracted from their ores, and their suitability for various real-world applications.
Key Definitions
- Reactivity Series: A list of metals (and some non-metals like carbon and hydrogen) arranged in order of their chemical reactivity, from most reactive to least reactive.
- Displacement Reaction: A reaction in which a more reactive element takes the place of a less reactive element in a compound.
- Oxidation: The gain of oxygen (Core) or the loss of electrons (Supplement).
- Reduction: The loss of oxygen (Core) or the gain of electrons (Supplement).
Core Content
The Order of the Reactivity Series
You must memorize this order (from most to least reactive):
- Potassium (K)
- Sodium (Na)
- Calcium (Ca)
- Magnesium (Mg)
- Aluminium (Al)
- Carbon (C) - Non-metal reference
- Zinc (Zn)
- Iron (Fe)
- Hydrogen (H) - Non-metal reference
- Copper (Cu)
- Silver (Ag)
- Gold (Au)
(a) Reactions with Cold Water
Only the most reactive metals react with cold water to produce a metal hydroxide and hydrogen gas.
- Potassium: Reacts violently; produces a lilac flame.
- Sodium: Reacts vigorously; fizzes and melts into a ball.
- Calcium: Reacts steadily; the water becomes cloudy as calcium hydroxide forms.
General Equation: Metal(s) + Water(l) → Metal Hydroxide(aq) + Hydrogen(g) Example (Sodium): Sodium + Water → Sodium hydroxide + Hydrogen 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
(b) Magnesium with Steam
Magnesium reacts very slowly with cold water but reacts vigorously with steam to produce a metal oxide and hydrogen. Equation: Magnesium + Water (steam) → Magnesium oxide + Hydrogen Mg(s) + H₂O(g) → MgO(s) + H₂(g)
(c) Reactions with Dilute Hydrochloric Acid
Metals above hydrogen in the series react with acid to produce a salt and hydrogen gas.
- Magnesium: Fast reaction, lots of bubbles.
- Zinc & Iron: Slower reactions.
- Copper, Silver, Gold: No reaction (they are below hydrogen).
General Equation: Metal(s) + Hydrochloric Acid(aq) → Metal Chloride(aq) + Hydrogen(g) Example (Zinc): Zinc + Hydrochloric acid → Zinc chloride + Hydrogen Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Deducing Order from Experimental Results
If an "Unknown Metal X" reacts with acid but "Unknown Metal Y" does not, Metal X is higher in the reactivity series than Metal Y. We use the rate of bubble production (hydrogen) or temperature change to determine the specific rank.
Extended Content (Extended curriculum only)
Tendency to Form Positive Ions
Reactivity is defined by how easily a metal atom loses its outer shell electrons to form a positive ion (cation).
- Potassium (High reactivity): Loses its electron very easily to become K⁺.
- Copper (Low reactivity): Does not lose electrons easily.
Displacement Reactions
A more reactive metal will displace a less reactive metal from its aqueous salt solution. Example: Zinc is more reactive than Copper. Zinc + Copper(II) sulfate → Zinc sulfate + Copper Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Ionic Equation (showing electron transfer): Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
- The Zinc atom is oxidized (loses 2 electrons).
- The Copper ion is reduced (gains 2 electrons).
The Apparent Unreactivity of Aluminium
Aluminium is high in the reactivity series, yet it does not react with water and reacts very slowly with acids.
- Reason: Aluminium reacts rapidly with oxygen in the air to form a very thin, tough, and unreactive layer of Aluminium Oxide (Al₂O₃).
- This oxide layer sticks strongly to the metal surface, acting as a protective barrier that prevents water or acids from reaching the metal underneath.
Key Equations
| Reaction Type | Word Equation | Balanced Symbol Equation |
|---|---|---|
| Metal + Water | Metal + Water → Hydroxide + Hydrogen | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) |
| Metal + Steam | Metal + Steam → Oxide + Hydrogen | Mg(s) + H₂O(g) → MgO(s) + H₂(g) |
| Metal + Acid | Metal + Acid → Salt + Hydrogen | Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) |
| Displacement | Metal A + Metal B Salt → Metal A Salt + Metal B | Mg(s) + FeSO₄(aq) → MgSO₄(aq) + Fe(s) |
Symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous/dissolved in water.
Common Mistakes to Avoid
- ❌ Wrong: Writing that Magnesium + Water makes Magnesium Hydroxide in the steam reaction.
- ✓ Right: Magnesium + Steam makes Magnesium Oxide (MgO). Hydroxides are usually formed with cold water.
- ❌ Wrong: Thinking Aluminium is at the bottom of the series because it doesn't react.
- ✓ Right: Aluminium is very reactive, but it is protected by an oxide layer.
- ❌ Wrong: Forgetting state symbols in equations.
- ✓ Right: Always include (s), (l), (g), or (aq) as requested by IGCSE guidelines.
Exam Tips
- Command Words: When asked to "State," provide a brief fact (e.g., "State the gas produced: Hydrogen"). When asked to "Explain," give reasons using the reactivity series (e.g., "Copper does not react because it is below hydrogen in the reactivity series").
- Unknown Metals: Exams often give you "Metal X, Y, and Z" and their reactions. Use a "knock-out" system to rank them from most to least reactive.
- Contexts: Laboratory observations are common. If you see "effervescence," the gas is likely hydrogen (test with a lighted splint for a "squeaky pop").
- Typical Values: In thermochemistry questions related to reactivity, a temperature rise of 3.1°C might be used to compare the energy released by different metals in displacement reactions. Higher temperature rise = more reactive metal.