Reactions of metals with dilute acids
2 marks · HardA student reacts equal moles of aluminium and iron separately with excess dilute hydrochloric acid at room temperature. The aluminium sample shows little to no fizzing at first but then reacts quickly when warmed. The iron sample reacts slowly, producing gentle fizzing throughout.
Use the observations to explain which metal is more reactive with dilute hydrochloric acid, and give a reason for aluminium’s unusual reaction pattern.
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Also accept: Aluminium is more reactive than iron because, after warming, it reacts vigorously, while iron only fizzes slowly. The initial lack of reaction in aluminium is due to its protective oxide layer, which prevents the acid from reaching the metal until it is removed by heating.; Aluminium is more reactive: it only reacts slowly at first because of its oxide layer, but once this is removed by warming, it reacts rapidly, whereas iron shows only slow fizzing throughout.
Common mistakes: Saying iron is more reactive because it reacts first; Not mentioning the oxide layer as the cause of slow reaction
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Marking points
M1 · 1 mark · Correctly identifies aluminium as more reactive than iron, based on observations.
M2 · 1 mark · Explains that aluminium reacts slowly at first because of its thin oxide layer, but once warmed and the layer is removed, it reacts quickly (reason required).
Worked solution
Aluminium is more reactive with dilute hydrochloric acid than iron. This is indicated by the fact that, after warming, aluminium reacts vigorously, while iron only reacts slowly with gentle fizzing.
The reason aluminium did not react much at first is that it has a thin, strong oxide layer on its surface that prevents the acid from reaching the metal. When the sample is warmed, the acid can remove this oxide layer, allowing the reaction to proceed quickly.