Aluminium reacts with warm dilute hydrochloric acid to give aluminium chloride solution and hydrogen. Explain why the reaction of dilute hydrochloric acid with aluminium foil is fairly slow whereas the reaction with the same mass of aluminium powder can be extremely vigorous.
The correct answer and explanation is :
The reaction of dilute hydrochloric acid with aluminium foil is fairly slow, whereas with aluminium powder it is extremely vigorous, primarily due to differences in surface area and the presence of an oxide layer on the aluminium.
Aluminium is a reactive metal, but it is protected by a thin, tough oxide layer (Al₂O₃) that forms rapidly on its surface when exposed to air. This layer acts as a barrier, preventing the metal beneath from reacting readily with dilute hydrochloric acid (HCl). In the case of aluminium foil, the surface area is relatively small and most of it is covered with this oxide layer, which slows down the reaction. Even though aluminium is thermodynamically capable of reacting with HCl, the oxide layer must first be broken down or dissolved before the acid can access the reactive metal beneath.
In contrast, aluminium powder has a much greater surface area relative to its mass. Even though each tiny particle of powder is still coated with the oxide layer, the enormous cumulative surface area means that more acid can simultaneously interact with the aluminium. Also, once the reaction starts and generates hydrogen gas (H₂), it creates turbulence that helps disrupt the oxide layer further, accelerating the reaction. The larger surface area also allows more efficient heat generation and retention, which speeds up the reaction even more.
Once the oxide barrier is breached, the reaction proceeds rapidly:
$$
2Al(s) + 6HCl(aq) \rightarrow 2AlCl₃(aq) + 3H₂(g)
$$
In powdered form, this reaction can become vigorous or even explosive due to the fast production of hydrogen gas and heat. Therefore, the key factors contributing to the difference in reaction rates are the surface area available for reaction and the effectiveness of the protective oxide layer on aluminium.