Question

Explain what happens when Aluminium reacts with dilute Hydrochloric acid.

Answer 1

Hint: The reaction between Aluminium and dilute HCl results in a gaseous and an aqueous product. Now that you have this information, try and figure out what each of these products could be.

Step-by-Step Solution:
Let us look at the properties of Aluminium and its various chemical properties to help figure out the answer to this question.
Aluminium is a malleable, light, silvery-white metal. It is a good electrical conductor and is also amphoteric in nature– it can react with both acids and bases. Combining aluminium with an acid results in a typical single displacement reaction, forming aluminium salt and gaseous hydrogen.
Aluminium reacts with dilute hydrochloric acid at room temperature. The metal dissolves in hydrochloric acid, yielding aluminium chloride and colourless hydrogen gas. This reaction is irreversible, as the final products will not react with each other. The reaction between metallic aluminium and hydrochloric acid is what is known as an oxidation-reduction reaction.
Let us now go through how this reaction would proceed step-by-step.
Aluminium acts as the reducing agent, giving up electrons:
\[A{{l}^{0}}\text{ }-\text{ }3e\text{ }=~A{{l}^{3+}}\]
Cations of hydrochloric acid take these electrons and are reduced to molecular hydrogen:
\[2{{H}^{+}}\text{ }+\text{ }2e\text{ }=~H\uparrow \]
The complete ionic reaction equation reads:
\[2A{{l}^{0}}\text{ }+\text{ }6{{H}^{+}}\text{ }+\text{ }6C{{l}^{-}}\text{ }=\text{ }2A{{l}^{3+}}\text{ }+\text{ }6C{{l}^{-}}\text{ }+~3H\uparrow \]
Net-ionic form:
\[2A{{l}^{0}}\text{ }+\text{ }6{{H}^{+}}\text{ }=\text{ }2A{{l}^{3+}}\text{ }+~3{{H}_{2}}\uparrow \]
In molecular form, the reaction looks as follows:
\[2Al\text{ }+\text{ }6HCl\text{ }\to \text{ }2AlC{{l}_{3}}\text{ }+~3{{H}_{2}}\uparrow \]

Note: Keep in mind that this reaction will not take place as soon as you add the piece of aluminium to the hydrochloric acid solution.
That happens because the piece of aluminium is protected by a layer of aluminium oxide, \[A{{l}_{2}}{{O}_{3}}\], the same layer that protects aluminium from reacting with water.
The hydrochloric acid will take some time to eat through this protective layer, but once that happens, the reaction will proceed quite vigorously, i.e. hydrogen gas will start to bubble out of solution.