4.6 - The Reactivity Series
The reactivity series
- Setting metals out in order of how readily they react gives the reactivity series: most reactive at the top, least reactive at the bottom.
- The order is the same whichever reaction is used to find it, so the series predicts what any of these metals will do.
The order to know
Why the order comes out this way
- When a metal reacts, its atoms lose electrons to form positive ions (cations).
- How reactive a metal is depends on how easily its atoms lose those outer electrons.
- Potassium gives up its outer electron very easily, so it reacts vigorously.
- Gold holds on to its electrons so tightly that it's found in the Earth unreacted as the native metal.
- The same electron loss happens in every reaction, which is why a metal's position stays the same across different reactions.
Carbon and hydrogen are the two markers
- Carbon and hydrogen aren't metals - they're put into the series as two fixed markers:
- Hydrogen marks the line for dilute acids - A metal above hydrogen reacts with a dilute acid and displaces the hydrogen (e.g. magnesium, zinc and iron all fizz in dilute hydrochloric acid). A metal below hydrogen does not react with dilute acids (copper, silver and gold sit in acid unchanged).
- Carbon marks the line for extracting a metal from its ore - A metal below carbon can be extracted from its ore by heating with carbon. A metal above carbon cannot, and must be extracted using electrolysis instead.
Metals with water, steam and dilute acid
With a dilute acid
- A metal that reacts with a dilute acid produces a salt and hydrogen gas:
- metal + dilute acid → salt + hydrogen
- magnesium + hydrochloric acid → magnesium chloride + hydrogen
- Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
- The hydrogen comes off as bubbles, and the rate of bubbling indicates how fast the reaction is going.
| Metal | Reaction with cold dilute acid |
|---|---|
| Potassium, sodium, calcium | React explosively - dangerously fast |
| Magnesium | Reacts vigorously, producing a steady stream of bubbles |
| Zinc | Reacts slowly in the cold, faster if warmed |
| Iron | Reacts very slowly, giving only a few bubbles |
| Copper, silver, gold | No reaction |
With cold water
- A reactive metal with cold water produces a metal hydroxide and hydrogen:
- metal + water → metal hydroxide + hydrogen
- calcium + water → calcium hydroxide + hydrogen
- Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
- Observations with cold water:
- Potassium floats, fizzes rapidly and ignites with a lilac flame.
- Sodium melts into a ball, fizzes and skates across the surface.
- Calcium sinks then rises with bubbles, leaving a cloudy alkaline solution.
- Magnesium reacts extremely slowly, forming only a few bubbles over days.
- Zinc, iron and copper do not react with cold water.
With steam
- Less reactive metals that do not react with cold water may react with steam to produce a metal oxide and hydrogen:
- metal + steam → metal oxide + hydrogen
- magnesium + steam → magnesium oxide + hydrogen
- Mg(s) + H2O(g) → MgO(s) + H2(g)
- Magnesium burns with a bright white light, leaving a white solid.
- Zinc glows and forms zinc oxide (yellow when hot, white when cold).
- Iron glows red-hot and turns into black iron oxide.
- Copper does not react with steam.
Collecting the hydrogen
- Steam reaction - Damp mineral wool is heated to generate steam, which passes over the heated metal. The hydrogen gas is collected over water. To prevent suck-back of cold water that could crack the hot tube, remove the delivery tube from the water before turning off the Bunsen burner.
- Cold water reaction - An inverted funnel over the metal directs gas bubbles into an inverted water-filled test tube.
Aluminium's oxide layer
- Aluminium appears unreactive because it forms a thin, protective aluminium oxide (Al2O3) layer that prevents water or acid from reaching the metal underneath.
- Once this layer is removed or damaged, aluminium reacts rapidly.
Making it a fair comparison
Control the surface area, mass of metal, and temperature/concentration of reactants to make valid comparisons.
Elements reacting with oxygen
When elements burn in oxygen, they form oxides.
Metals heated in oxygen
- Metals react with oxygen to produce basic metal oxides:
- metal + oxygen → metal oxide
| Metal | Observation on heating | Equation |
|---|---|---|
| Potassium | Burns with a lilac flame, forming a white solid | 4K(s) + O2(g) → 2K2O(s) |
| Magnesium | Burns with a bright white flame, forming a white powder | 2Mg(s) + O2(g) → 2MgO(s) |
| Zinc | Glows to form a solid, yellow when hot and white when cold | 2Zn(s) + O2(g) → 2ZnO(s) |
| Iron | Filings burn with orange sparks, forming a black solid | 3Fe(s) + 2O2(g) → Fe3O4(s) |
| Copper | Does not burn; surface coats with black solid | 2Cu(s) + O2(g) → 2CuO(s) |
- Reactive metals also oxidise slowly at room temperature, gaining mass as oxygen is incorporated into the solid.
Non-metals burning in oxygen
- Non-metals react with oxygen to produce non-metal oxides, most of which are acidic (water and carbon monoxide are neutral):
- non-metal + oxygen → non-metal oxide
- Carbon - Glows orange and burns to form carbon dioxide:
- C(s) + O2(g) → CO2(g)
- In a limited supply of oxygen, incomplete combustion produces toxic carbon monoxide:
- 2C(s) + O2(g) → 2CO(g)
- Sulfur - Melts to a red liquid and burns with a blue flame, forming pungent sulfur dioxide:
- S(s) + O2(g) → SO2(g)
- Hydrogen - Burns with a pale blue flame to produce water:
- 2H2(g) + O2(g) → 2H2O(l)
Metal displacement
A displacement reaction occurs when a more reactive metal takes the place of a less reactive metal in a solution of its salt:
A more reactive metal will displace a less reactive metal from a solution of its salt.
Iron and copper(II) sulfate
- Iron is higher in the reactivity series than copper, so iron displaces copper:
- iron + copper(II) sulfate → iron(II) sulfate + copper
- Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
- Observations:
- The blue solution fades to pale green as Cu2+ ions are replaced by Fe2+ ions.
- A pink-brown solid (copper) coats the iron nail and falls to the bottom.
- The temperature rises because displacement reactions are exothermic.
Predicting displacement reactions
- A less reactive metal cannot displace a more reactive metal (e.g. copper added to iron(II) sulfate gives no reaction).
- The greater the difference in reactivity between the two metals, the larger the temperature rise during the reaction.
Deducing an order of reactivity
To determine an unknown reactivity series experimentally:
- React each metal with the same substance under identical conditions.
- Observe and record the rate of reaction (e.g. rate of bubbling, temperature change).
- Rank the metals - The metal producing the fastest reaction or highest temperature rise is the most reactive.
Fair testing
The following variables must be kept constant across all tests:
- Mass and surface area of the metal - Powders react faster than lumps of the same mass.
- Concentration and volume of acid or solution
- Starting temperature
Displacement tests
Adding each metal to salt solutions of the other metals reveals the full order: the metal that displaces the most other metals is highest in the series.
A tick means the metal in that row displaces the metal in that solution.
Worked example - An order from results
Four unidentified metals, A, B, C and D, were each added to dilute hydrochloric acid under the same conditions. Put the four metals in order of reactivity, most reactive first.
| Metal | Observations |
|---|---|
| A | A few bubbles form slowly |
| B | No reaction |
| C | Rapid stream of bubbles; metal disappears quickly |
| D | Steady bubbling, faster than A but slower than C |
Step 1 - Compare the rates of reaction
The rate of hydrogen gas production indicates the rate of reaction.
Step 2 - Rank the metals
- C produces bubbles fastest → most reactive.
- D bubbles faster than A but slower than C → second.
- A produces bubbles slowly → third.
- B shows no reaction → least reactive (sits below hydrogen in the reactivity series).
Order of reactivity: C > D > A > B
TermDefinition
Reactivity seriesA list of metals set out in order of how readily they react.
Native metalAn unreactive metal found in the Earth as the uncombined element, rather than in a compound.
Displacement reactionA reaction in which a more reactive element takes the place of a less reactive element in a compound or in a solution of its salt.
Key points to remember
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