Edexcel IGCSE Chemistry · Spec 2.15-2.17
The Reactivity Series & Displacement
The reactivity series and how it predicts displacement reactions.
Chemistry revision video
The Reactivity Series & Displacement
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Explained
Ranking the metals, and using the ranking to predict
The reactivity series ranks metals by how readily they react, which really means how readily they lose electrons to form positive ions. Potassium, sodium, calcium and magnesium sit near the top; copper, silver and gold sit at the bottom.
Carbon and hydrogen are usually included even though they are not metals, because their positions decide how a metal is extracted and whether it reacts with acid.
Reactions with water and acid
Potassium, sodium and calcium react with cold water, giving a metal hydroxide and hydrogen. Potassium is violent enough to ignite the hydrogen.
Magnesium reacts only very slowly with cold water, but readily with steam, giving the oxide and hydrogen. Zinc and iron also react with steam.
Copper, silver and gold do not react with water at all.
With dilute acid, any metal above hydrogen reacts to give a salt and hydrogen, and the vigour of the fizzing follows the order of the series. Metals below hydrogen do not react, which is why copper coins survive and iron nails do not.
Displacement
A more reactive metal displaces a less reactive metal from its compound.
Put magnesium into copper sulfate solution and the magnesium takes the place of the copper. The blue solution fades, and a pink brown solid appears on the magnesium. Put copper into magnesium sulfate and nothing at all happens, because copper is lower in the series.
Those observations are the evidence. A colour change and a solid appearing are what tell you a displacement has occurred, and a question asking what you would see wants them named.
What the examiners and mark scheme say
A principal examiner report for Edexcel International GCSE Chemistry describes a question where no reaction occurred. Most learners identified that correctly, but far fewer explained it by comparing the reactivity of silver with that of copper. Many responses stated that silver was unreactive, or at the bottom of the reactivity series, without recognising the need to make a direct comparison.
The mark scheme for that question confirms it. The first mark is for saying silver is less reactive than copper, and the second, which depends on the first, is for concluding that there was therefore no reaction. Reactivity is a comparison, and a statement about one metal alone is not one.
The same report describes a halogen displacement question where a common error was writing that chlorine is more reactive than bromide, or that chlorine can displace bromide. Chlorine is an element and bromide is an ion, so those sentences compare two different kinds of thing. Compare chlorine with bromine, and say that chlorine displaces bromide ions from solution.
It also notes that many candidates omitted the colour change, which is what provides the evidence of a displacement reaction and of the trend in reactivity, and that few included an example of a reaction that would not occur. Showing where the pattern stops is part of showing you understand the pattern.
Learning the order
Potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
The two non metals are the ones to fix in place. Anything above carbon must be extracted by electrolysis; anything below it can be reduced with carbon. Anything above hydrogen reacts with dilute acid; anything below it does not. Those two boundaries answer most of the questions in this topic on their own.
Why it works
A metal high in the series loses its outer electrons easily, so it forms positive ions readily and reacts vigorously.
In a displacement reaction the more reactive metal gives up its electrons and becomes ions, while the less reactive metal's ions accept those electrons and become atoms. Describing it in terms of electrons transferred rather than places swapped is what turns a description into an explanation, and it is also why displacement reactions are redox reactions.
Spec 2.15-2.17
What you need to know
- Recall the order of the reactivity series
- Compare how metals react with water and acid
- Explain displacement reactions of metals
Active recall
Quick check
Answer each question before opening the answer.
What is a displacement reaction?
A more reactive metal displaces a less reactive metal from its compound.
How is a metal's reactivity linked to electrons?
More reactive metals lose their outer electrons more easily to form positive ions.
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