Question

(a) Aluminium is a more active metal than iron, but suffers less corrosion. Why?
(b) Explain and give reasons why aluminium vessels should not be cleaned with powders containing alkalis.
(c) Why is the food containing iron salts should not be cooked in aluminium utensils?

Answer 1

(a) In moist air, aluminium undergoes oxidation and a thin oxide layer is formed on its surface. This layer makes the metal inert. It acts as a protective barrier and stops further corrosion.

$$\begin{gathered} 4\mathrm{Al}+3{\mathrm{O}}_2+2\mathrm{H}{}_2\mathrm{O}\stackrel{}{\to }2{\mathrm{Al}}_2{\mathrm{O}}_3.{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Aluminium}(\mathrm{In}\mathrm{moist}\mathrm{air})\mathrm{Aluminium}\mathrm{oxide}\left(\mathrm{Hydrated}\right) \end{gathered}$$

(b) Because aluminium reacts with alkalis, it should not be cleaned with powders containing alkalis. This causes the discolouration of aluminium.

$$\begin{gathered} 2\mathrm{Al}+2\mathrm{NaOH}+2{\mathrm{H}}_2\mathrm{O}\stackrel{}{\to }2{\mathrm{NaAlO}}_2+3{\mathrm{H}}_2 \\ \mathrm{Aluminium}\mathrm{Alkali}\mathrm{Sodium}\mathrm{aluminate} \end{gathered}$$

(c) As aluminium is more reactive than iron, it can displace iron from salts present in the food . This results in the conversion of iron salts into aluminium salts, which can be harmful to health. Hence, cooking food (containing iron salts) in aluminium vessels should be avoided.