4.1 - Comparing the reactivity of metals
Metals do not all react in the same way. Some react quickly with water, some react only with dilute acids, and some can push another metal out of a salt solution. In this lesson, the key skill is to use observations as evidence and deduce a relative order of reactivity.
Relative reactivity evidence
Relative reactivity means how reactive one metal is compared with another. Use reaction evidence to decide which metal is more reactive.
Relative reactivity
Relative reactivity is the order of metals from more reactive to less reactive, based on how readily they react under the same conditions.
The command word Deduce means use the information given to reach a conclusion. A good deduction names the evidence and then links it to the conclusion:
Magnesium produced bubbles faster than zinc in the same dilute acid, so magnesium is more reactive than zinc.
For a fair comparison, the conditions should be the same where possible: similar metal surface area, clean metal surfaces, same temperature, same volume and concentration of acid or salt solution, and the same observation time. If the conditions are not the same, the comparison is weaker.
Safety also matters. Very reactive metals such as sodium and potassium are not suitable for student acid tests because their reactions can be dangerously vigorous. School evidence normally uses teacher-approved metals and dilute solutions.
Using reactions with water
Reaction with water is strong evidence of high reactivity. A metal that reacts quickly with cold water is more reactive than a metal that shows little or no reaction with water under the same conditions.
For metals that react with cold water, the general word equation is:
metal + water -> metal hydroxide + hydrogen
Useful observations include:
| Observation with water | What it suggests |
|---|---|
| rapid fizzing or bubbling | hydrogen is being produced quickly |
| metal moves around or disappears | the metal is reacting |
| solution becomes alkaline | a metal hydroxide has formed |
| little or no visible change | the metal is less reactive than a metal that reacts quickly |
Example deductions:
| Metal | Observation in cold water | Deduction from this test |
|---|---|---|
| calcium | bubbles form steadily | calcium is quite reactive |
| magnesium | little or no visible change in cold water | magnesium is less reactive than calcium in this test |
| copper | no visible change | copper is less reactive than calcium in this test |
This water test does not fully order every metal. If two metals both show no visible reaction with cold water, you need extra evidence from dilute acids or salt solutions to compare them.
Using reactions with dilute acids
Many metals that do not react with cold water will still react with dilute hydrochloric acid or dilute sulfuric acid. The acid rules here do not apply to oxidising acids such as nitric acid. The general reaction pattern is:
metal + acid -> salt + hydrogen
Hydrogen is observed as bubbles. If the same acid conditions are used, a faster rate of bubbling usually means the metal is more reactive.
Example word and symbol equations:
magnesium + hydrochloric acid -> magnesium chloride + hydrogen
Mg + 2HCl -> MgCl2 + H2
zinc + sulfuric acid -> zinc sulfate + hydrogen
Zn + H2SO4 -> ZnSO4 + H2
A typical set of acid observations might be:
| Metal | Observation in the same dilute acid | Relative conclusion |
|---|---|---|
| magnesium | rapid fizzing | most reactive of these metals |
| zinc | steady fizzing | less reactive than magnesium |
| iron | slow fizzing | less reactive than zinc |
| copper | no visible reaction | least reactive of these metals |
The key is not the exact number of bubbles; it is the comparison. If magnesium and zinc are tested with the same acid concentration, volume and temperature, faster fizzing from magnesium is evidence that magnesium is more reactive.
Using salt solution displacement
Salt solutions let you compare metals directly. A more reactive metal can displace a less reactive metal from a solution of its salt.
Displacement reaction
In this lesson, a displacement reaction is a reaction in which a more reactive metal takes the place of a less reactive metal in a salt solution.
The general pattern is:
more reactive metal + less reactive metal salt -> more reactive metal salt + less reactive metal
Continue this lesson.
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