When anhydrous aluminium chloride reacts with water, solution Y is formed that contains a complex aluminium ion, Z, and chloride ions - AQA - A-Level Chemistry - Question 7 - 2018 - Paper 1

The complex ion Z, represented as [Al(H₂O)₆]³⁺, can act as a Brønsted–Lowry acid by donating a proton (H⁺) to water:

$[\text{Al}(\text{H}_2\text{O})_6]^{3+} + \text{H}_2\text{O} \rightarrow [\text{Al}(\text{H}_2\text{O})_5(\text{OH})]^{2+} + \text{H}_3\text{O}^+$

1. The formation of a white precipitate, which indicates the presence of a reaction product.
2. Effervescence or bubbling due to the formation of carbon dioxide gas when carbonate ions react with the complex ion.

In the two observations, the equation is:

$[\text{Al}(\text{H}_2\text{O})_6]^{3+} + 3\text{Na}_2\text{CO}_3 \rightarrow \text{Al}_2\text{(CO}_3\text{)}_3 \downarrow + 6\text{Na}^+ + 6\text{H}_2\text{O}$

1. The solution will turn to a colorless solution, indicating the complete reaction of the aluminium complex.
2. The formation of a white gelatinous precipitate may occur, representing the formation of aluminum hydroxide.

The equations for these observations are:

1. For the initial reaction:

$[\text{Al}(\text{H}_2\text{O})_6]^{3+} + 3\text{OH}^- \rightarrow [\text{Al(OH)}_3] \downarrow + 6\text{H}_2\text{O}$

2. When excess hydroxide is added:

$[\text{Al(OH)}_3] + \text{OH}^- \rightarrow [\text{Al(OH)}_4]^-$