Question

When aluminum foil is placed in hydrochloric acid, nothing happens for the first 30 seconds or so. This is followed by vigorous bubbling and the eventual disappearance of the foil. Explain these observations.

Short answer

In summary, when aluminum foil is placed in hydrochloric acid, there is an initial delay in the reaction due to the protective aluminum oxide layer on the foil's surface. Once this layer dissolves, aluminum reacts vigorously with the acid, forming aluminum chloride and hydrogen gas, which results in bubbling. As the aluminum is converted to aluminum chloride, the foil eventually disappears.

Step-by-step solution

Understanding the reaction between aluminum and hydrochloric acid

To explain the observations, we first need to understand the chemical reaction that takes place when aluminum reacts with hydrochloric acid. The balanced chemical equation for this reaction is given below: \[2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)\] In this reaction, aluminum reacts with hydrochloric acid to form aluminum chloride and hydrogen gas. The production of hydrogen gas is responsible for the vigorous bubbling observed.

Discussing the initial delay in the reaction

The initial delay in the reaction is due to the formation of a thin layer of aluminum oxide (Al2O3) on the surface of the aluminum foil. This oxide layer protects the aluminum from reacting with the hydrochloric acid immediately. It is important to note that aluminum is a highly reactive metal but does not corrode easily in the atmosphere due to the presence of this protective oxide layer.

Explaining the onset of the vigorous bubbling and the disappearance of the foil

After the initial delay, the protective aluminum oxide layer starts to dissolve in the hydrochloric acid, allowing the aluminum to come into direct contact with the acid. This leads to the vigorous reaction between aluminum and hydrochloric acid, producing aluminum chloride and hydrogen gas. The hydrogen gas forms bubbles, leading to the vigorous bubbling observed. As the reaction progresses, the aluminum foil is converted into aluminum chloride, which dissolves in the acid solution. Eventually, there is no more solid aluminum left, resulting in the disappearance of the foil. In conclusion, the initial delay in the reaction between aluminum and hydrochloric acid is due to the protective aluminum oxide layer on the surface of the aluminum foil. Once this layer is dissolved in the acid, the vigorous reaction begins, producing hydrogen gas bubbles and causing the foil to disappear as it is converted into aluminum chloride.