Reactivity Series
What is the Reactivity Series?
The reactivity series is a list of metals arranged in order of decreasing reactivity. It also includes hydrogen and carbon (non-metals), which are useful reference points.
Order of reactivity (most to least reactive):
| Metal/Element | Notes |
|---|---|
| Potassium (K) | Reacts violently with cold water |
| Sodium (Na) | Reacts vigorously with cold water |
| Lithium (Li) | Reacts steadily with cold water |
| Calcium (Ca) | Reacts with cold water |
| Magnesium (Mg) | Reacts slowly with cold water; rapidly with steam/acid |
| Aluminium (Al) | Protected by oxide layer |
| (Carbon) | Reference point for extraction |
| Zinc (Zn) | Reacts with dilute acids |
| Iron (Fe) | Reacts with dilute acids |
| (Hydrogen) | Reference point for acid reactions |
| Copper (Cu) | Does not react with dilute acids |
| Silver (Ag) | Does not react with water or acids |
| Gold (Au) | Extremely unreactive; found uncombined |
Evidence for the Reactivity Series
Reactions with Water
Metals that are more reactive react with water (cold water or steam) to produce a metal hydroxide or oxide and hydrogen gas.
| Reaction | Equation |
|---|---|
| Potassium + water | 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) |
| Sodium + water | 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) |
| Lithium + water | 2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g) |
| Calcium + water | Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) |
| Magnesium + steam | Mg(s) + H2O(g) → MgO(s) + H2(g) |
Metals below magnesium (aluminium, zinc, iron, etc.) do not react with cold water. Copper, silver and gold do not react with water at all.
Reactions with Dilute Acid
Metals above hydrogen in the reactivity series react with dilute hydrochloric or sulfuric acid:
metal + acid → salt + hydrogen
| Metal | With HCl | With H2SO4 |
|---|---|---|
| Magnesium | Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) | Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g) |
| Zinc | Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) | Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) |
| Iron | Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) | Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g) |
Copper and metals below it do not react with dilute HCl or H2SO4. (Note: nitric acid is NOT used with metals in this specification as it behaves differently.)
Displacement Reactions
A more reactive metal can displace a less reactive metal from a solution of its salt.
Example: Iron displaces copper from copper(II) sulfate solution: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Observation: The iron surface becomes coated with pink-brown copper metal; the blue colour of the copper sulfate solution fades.
A more reactive metal can also displace a less reactive metal from its oxide when heated:
Example: 2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g) (iron extraction by carbon)
Rusting of Iron
Rust is hydrated iron(III) oxide, formed when iron corrodes.
Conditions required for rusting: both oxygen (from air) AND water must be present. Neither alone is sufficient.
Evidence: Iron in dry air does not rust. Iron in water that has been boiled (to remove dissolved oxygen) and sealed does not rust. Only iron exposed to both water and air rusts.
Prevention of Rusting
Barrier methods: prevent oxygen and water from reaching the iron surface.
- Painting: used on bridges, cars, railings
- Plastic coating: used on bicycle chains, outdoor furniture
- Grease or oil: used on machinery and tools
- Tin plating: used on food cans (note: if the tin is scratched, the iron underneath still rusts)
Galvanising: coating iron with zinc.
- The zinc layer acts as a physical barrier.
- Even if the zinc layer is scratched, the zinc sacrificially protects the iron (because zinc is more reactive than iron).
Sacrificial protection (sacrificial anode): attaching a more reactive metal (such as zinc or magnesium) to the iron object.
- The more reactive metal preferentially oxidises (rusts) instead of the iron.
- Used for underground pipes, ship hulls and oil rigs.
Redox
Oxidation and reduction often occur simultaneously in a redox reaction.
| Term | Definition 1 (oxygen) | Definition 2 (electrons) |
|---|---|---|
| Oxidation | Gain of oxygen | Loss of electrons |
| Reduction | Loss of oxygen | Gain of electrons |
| Oxidising agent | Causes oxidation of another substance; is itself reduced | Accepts electrons from another substance |
| Reducing agent | Causes reduction of another substance; is itself oxidised | Donates electrons to another substance |
Memory aid: OIL RIG (Oxidation Is Loss of electrons; Reduction Is Gain of electrons).
Example: In the reaction 2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g):
- Iron oxide is reduced (loses oxygen); carbon is the reducing agent.
- Carbon is oxidised (gains oxygen); iron oxide is the oxidising agent.
Example: In Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s):
- Mg loses electrons (is oxidised); Mg is the reducing agent.
- Cu2+ gains electrons (is reduced); CuSO4 is the oxidising agent.
Practical: Reactions of Metals with Acids
Aim: Investigate reactions between dilute HCl and H2SO4 with metals (e.g. magnesium, zinc and iron).
Method: Add a small piece of each metal to dilute acid in a test tube. Record observations.
Results:
- Magnesium reacts vigorously, producing many bubbles of hydrogen rapidly; the metal dissolves quickly.
- Zinc reacts moderately, producing bubbles less quickly.
- Iron reacts slowly, producing few bubbles.
- Copper produces no bubbles; no reaction.
This order of reactivity matches the reactivity series.
Exam Questions
Q: Explain why calcium reacts with cold water but copper does not.
Calcium is above hydrogen in the reactivity series and is reactive enough to displace hydrogen from water. Copper is below hydrogen and is not reactive enough to react with cold water.
Q: State the conditions required for iron to rust.
Iron requires both oxygen (from air) and water to rust. Neither alone is sufficient.
Q: Describe how galvanising prevents iron from rusting.
Galvanising involves coating iron with a layer of zinc. The zinc layer acts as a physical barrier, preventing oxygen and water from reaching the iron. Even if the zinc layer is scratched, the zinc acts as a sacrificial metal: zinc is more reactive than iron so it preferentially oxidises, protecting the iron underneath.
Q: In the reaction Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s), identify the oxidising agent and explain your choice.
The oxidising agent is CuSO4 (copper(II) sulfate). The Cu2+ ions accept electrons from iron (Fe → Fe2+ + 2e-), so the copper ions are reduced and the CuSO4 is the oxidising agent.
Exam Tip
Learn the reactivity series order. A common mnemonic is: Please Send Lewis Chainsaws, Making Aluminium Zinc Instead He Can’t See Gold (Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, Zinc, Iron, Hydrogen, Copper, Silver, Gold).