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Reactivity Series and Metal Extraction: Question 7

Syllabus 9.4

Multiple choice Extended 1 mark

Aluminium is above zinc and iron in the reactivity series. Yet when a clean strip of aluminium foil is placed in cold water, or in dilute hydrochloric acid, it appears to react barely at all, far more slowly than its position in the reactivity series would suggest. If the same strip is first rubbed with steel wool, or coated with a mercury salt, it then reacts rapidly with the acid.

Which statement correctly explains this apparent unreactivity of aluminium?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Note the apparent contradiction

Aluminium sits above zinc and iron in the reactivity series, so on reactivity alone it should react with water and dilute acid faster and more vigorously than either of them. In practice, an untreated strip of aluminium appears almost unreactive. This apparent contradiction is the effect to be explained.

Step 2: Consider what removing the surface does

The key clue is what happens when the surface is disturbed: rubbing with steel wool or coating with a mercury salt (which disrupts the surface layer) causes the same piece of aluminium to react rapidly. This shows the metal itself has not changed. Only its surface has.

Step 3: Identify the correct explanation

Aluminium reacts with oxygen in the air essentially instantly, forming a thin, continuous, tightly adhering layer of aluminium oxide on its surface. This layer acts as a physical barrier, stopping water molecules or acid particles from reaching the reactive aluminium metal underneath. Crucially, this layer is self-healing: if it is scratched or removed, fresh aluminium is exposed and immediately reacts with oxygen to reform the protective layer. Only when the layer is removed and prevented from reforming (e.g. by an amalgam of mercury) does the aluminium react at the rapid rate consistent with its true position in the reactivity series.

Why the other options are wrong

  • A: Aluminium’s true reactivity, once its oxide layer is removed, is faster than zinc’s or iron’s, consistent with its usual high position. Its reactivity series position is not wrong.
  • C: The reactions of reactive metals with water and dilute acid are exothermic, not endothermic; heat absorption is not the reason for the slow apparent reaction.
  • D: This is contradicted directly by the scenario. Once the oxide layer is disrupted, aluminium reacts readily with cold, dilute acid.

Final answer

  • The apparent unreactivity is explained by aluminium’s protective, self-healing oxide layer, option B.