Building the Reactivity Series from Observations
- We can compare how different metals react with water, acids and oxygen.
- From these observations, we can arrange metals from most reactive to least reactive.
- This list is called the reactivity series.
Metal Reactions with Water
Metals show very different behaviours with water:
- Very reactive metals (e.g. potassium, sodium, lithium, calcium):General equation (for Group 1 metals): $$2 \mathrm{M}(\mathrm{~s})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{MOH}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})$$
- React vigorously with cold water.
- Produce hydrogen gas and a metal hydroxide.
- Often produce heat and may ignite the hydrogen.
- Moderately reactive metals (e.g. magnesium, zinc, iron):
- Little or no reaction with cold water.
- React with steam to form a metal oxide and hydrogen gas.
- Unreactive metals (e.g. copper, silver, gold):
- Do not react with water or steam.
Magnesium with steam: $$\operatorname{Mg}(\mathrm{s})+\mathrm{H}_2 \mathrm{O}(\mathrm{~g}) \rightarrow \mathrm{MgO}(\mathrm{~s})+\mathrm{H}_2(\mathrm{~g})$$
Observation pattern: The more vigorous and easier the reaction with water, the more reactive the metal.
Metal Reactions with Acids
- Metals can also be compared by how they react with dilute acids like HCl or H₂SO₄.
- Metals above hydrogen in the reactivity series react with dilute acids to produce salt and hydrogen gas.
- General equation: $$\text { Metal }+ \text { Acid } \rightarrow \text { Salt }+\mathrm{H}_2(\mathrm{~g})$$
- Metals below hydrogen (e.g. copper, silver, gold) do not react with dilute acids at room temperature.
Magnesium and hydrochloric acid:
$$\mathrm{Mg}(\mathrm{~s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_2(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})$$
Observation pattern: The more vigorous the reaction with acids, the higher the metal is placed in the reactivity series.
Metal Reactions with Oxygen
Most metals react with oxygen to form metal oxides, but at very different rates.
- Highly reactive metals (e.g. potassium, sodium):
- React rapidly with oxygen in air.
- May need to be stored under oil to stop them reacting.
- Moderately reactive metals (e.g. magnesium, iron):
- Burn in air when heated strongly, forming a metal oxide.
- Iron slowly reacts with oxygen and water over time → rust (iron oxide).
- Unreactive metals (e.g. copper, silver, gold, platinum):
- React only very slowly or hardly at all.
- Gold and platinum are described as noble metals because they resist oxidation.
Constructing the Reactivity Series
- By comparing all these reactions, we can order metals from most reactive to least reactive.
- A simplified reactivity series (with hydrogen included for reference) is illustrated below:
Based on its vigorous reaction with cold water, where would you place lithium in the reactivity series relative to sodium and potassium?
Displacement Reactions and the Reactivity Series
What Is a Displacement Reaction?
Displacement reaction
A displacement reaction happens when a more reactive metal displaces a less reactive metal from one of its compounds (usually a solution of a salt).
Rule: A metal higher in the reactivity series will displace a metal lower in the series from its compound. A metal lower in the series cannot displace a more reactive metal.
General pattern: $$\text { More reactive metal + metal salt solution → new metal salt + less reactive metal }$$
Magnesium and Copper(II) Sulfate
Magnesium is above copper in the reactivity series.
Word equation:
magnesium + copper(II) sulfate → magnesium sulfate + copper
Balanced symbol equation:
$$\mathrm{Mg}(\mathrm{~s})+\mathrm{CuSO}_4(\mathrm{aq}) \rightarrow \mathrm{MgSO}_4(\mathrm{aq})+\mathrm{Cu}(\mathrm{~s})$$
What you observe:
- The blue colour of CuSO₄ solution fades as Cu²⁺ ions are replaced.
- Brown-pink copper metal deposits on the magnesium.
- The magnesium strip may get darker and eventually disappear.
What would happen if copper metal were added to a solution of magnesium sulfate? (Hint: check their positions in the reactivity series.)
The Thermite Reaction (Displacing Iron from Its Oxide)
Aluminium is higher than iron in the reactivity series, so it can displace iron from iron(III) oxide.
Word equation:
aluminium + iron(III) oxide → aluminium oxide + iron
Balanced symbol equation:
$$2 \mathrm{Al}(\mathrm{~s})+\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s}) \rightarrow \mathrm{Al}_2 \mathrm{O}_3(\mathrm{~s})+2 \mathrm{Fe}(\mathrm{l})$$
- This reaction is highly exothermic (releases lots of heat).
- It produces molten iron, used in railway track welding and other repairs.
Think of the reactivity series as a league table:
- A “higher-ranked” metal can take the place of a “lower-ranked” metal in its compound.
- If a reaction seems unlikely, check the positions of the metals in the series.
Real-World Uses of the Reactivity Series
The reactivity series is not just a school concept; it helps us understand and solve real problems in the world around us.
Corrosion and Rusting
Corrosion
Corrosion is a slow, continuous deterioration of metals caused by reactions with substances in the environment, such as oxygen, water, acids, salts (electrolytes).
- Iron is relatively reactive → it rusts easily to form hydrated iron(III) oxide.
- Zinc is more reactive than iron → it can be used to protect iron.
- Gold and platinum are very unreactive → they do not corrode easily.
Why does the reactivity series matter?
- Metals higher in the series corrode more readily.
- Metals lower (like gold) resist corrosion and are used where durability is important.
Preventing Corrosion
We can use the reactivity series to slow down corrosion:
- Protective coatings – paint, oil, plastic, or plating create a barrier between metal and environment.
- Galvanisation – coating iron or steel with zinc.
- Zinc is more reactive than iron.
- If the coating is scratched, the zinc acts as a sacrificial metal, reacting first and protecting the iron.
- Sacrificial anodes – blocks of a more reactive metal (e.g. zinc or magnesium) attached to steel structures such as pipelines or ship hulls.
Metal Extraction from Ores
Many metals are found in nature as compounds in ores, not as pure metals. The reactivity series helps decide how to extract them.
- Very reactive metals (e.g. K, Na, Ca, Mg, Al):
- Are high in the series.
- Their compounds are very stable.
- Extracted by electrolysis of molten salts (e.g. aluminium from Al₂O₃).
- Moderately reactive metals (e.g. Zn, Fe):
- Extracted by chemical reduction, often using carbon or carbon monoxide.
- Carbon is placed above zinc and iron in the series, so it can remove oxygen from their oxides.
- Unreactive metals (e.g. Au, Ag):
- Low in the reactivity series.
- Can often be found in their native (elemental) form.
- Require little or no chemical extraction.
Extraction of iron in a blast furnace: $$\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})+3 \mathrm{CO}(\mathrm{~g}) \rightarrow 2 \mathrm{Fe}(\mathrm{l})+3 \mathrm{CO}_2(\mathrm{~g})$$
Key idea: The more reactive the metal, the more energy is needed to extract it, so its extraction method is usually more complex and expensive.
Other Real-World Applications
- Galvanisation and construction
- Steel bridges, ships and cars are often protected by zinc coatings.
- The reactivity series predicts that zinc will corrode instead of the iron.
- Batteries and electrochemical cells
- In a simple cell, the metal higher in the reactivity series acts as the negative electrode (anode) and tends to lose electrons more easily.
- The potential difference (voltage) depends on the difference in reactivity between the two metals used.
- Jewellery and electronics
- Gold and silver are at the bottom of the reactivity series and resist corrosion.
- This makes them ideal for jewellery and for electrical contacts in electronics (where corrosion would cause poor connections).
- What observations about metal reactions with water, acids and oxygen would help you decide where to place an unknown metal in the reactivity series?
- Will a strip of zinc react with a solution of copper(II) sulfate?
- Will a strip of copper react with a solution of zinc sulfate? Explain using the reactivity series.
- Why does iron rust more easily than copper?
- How does galvanisation use the reactivity series to protect iron?
- Why must aluminium be extracted by electrolysis, while iron can be extracted by reduction with carbon?
- Give one example of how knowledge of the reactivity series is important in everyday life or industry, and explain why.
