Question

Account for the fact that when aqueous solution of KCN is added to a solution of aluminium sulfate, a precipitate of \(\mathrm{Al}(\mathrm{OH})_{3}\) forms.

Short answer

The precipitate \(\mathrm{Al}( ext{OH})_3\) forms due to the reaction between \(\mathrm{KCN}\) and \(\mathrm{Al}_2(\mathrm{SO}_4)_3\), leading to hydroxide ions which cause \(\mathrm{Al}^{3+}\) ions to precipitate as \(\mathrm{Al}( ext{OH})_3\).

Step-by-step solution

Identify the compounds involved

When an aqueous solution of KCN is added to a solution of aluminium sulfate, the compounds involved are potassium cyanide (KCN) and aluminium sulfate (\(\mathrm{Al}_2(\mathrm{SO}_4)_3\)).

Determine the reaction type

This is a double displacement (or metathesis) reaction, where the cation from one compound exchanges with the anion from the other compound, leading to the formation of different products.

Write the chemical equations

The reaction between \(\mathrm{KCN}\) and \(\mathrm{Al}_2(\mathrm{SO}_4)_3\) can be primarily broken down into the formation of \(\mathrm{Al}^{3+}\), \(\mathrm{SO}_4^{2-}\), \(\mathrm{K}^+\), and \(\mathrm{CN}^-\) ions in solution. The formation of \(\mathrm{Al}^3+\) and \(\mathrm{OH}^-\) ions leads to the precipitation of \(\mathrm{Al}( ext{OH})_3\).

Write the net ionic equation

The net ionic equation for the precipitation of aluminium hydroxide is:\[\mathrm{Al}^{3+} + 3\mathrm{OH}^- \rightarrow \mathrm{Al}( ext{OH})_3 \]Here, \(\mathrm{OH}^-\) ions are produced in solution via the hydrolysis reaction of \(\mathrm{CN}^-\), which reacts with water to increase the hydroxide concentration.

Explain the precipitate formation

The hydroxide ions produced from the reaction between \(\mathrm{CN}^-\) and water leads to an increase in the pH of the solution, encouraging the precipitation of \(\mathrm{Al}( ext{OH})_3\) which is not soluble in water.

Key concepts

Double Displacement Reactions

In chemistry, a double displacement reaction, also known as a metathesis reaction, plays a significant role in various chemical processes. Imagine a dance where partners are swapped. In these reactions, the cations and anions of two different compounds exchange partners to create two new compounds.

Here's how it generally looks:

• Compound AX + Compound BY → Compound AY + Compound BX

In our case, when potassium cyanide (KCN) and aluminum sulfate (\(\mathrm{Al}_2(\mathrm{SO}_4)_3\)) react, the ions rearrange to form potassium sulfate and aluminum cyanide. Sometimes, these reactions yield a precipitate, a solid that separates out of the solution, due to the low solubility of one of the products in water. That's what's happening in this reaction with the formation of aluminum hydroxide (\(\mathrm{Al}(\mathrm{OH})_3\)).

Understanding double displacement reactions is essential because they are common in various processes, including industrial applications and biological systems. They can lead to precipitation, neutralization, and other significant chemical processes.

Net Ionic Equations

Net ionic equations are an essential tool for simplifying chemical reaction equations. They allow you to focus on the actual chemical species that undergo a change, discarding the spectator ions that do not participate.

For instance, consider the reaction given between the aqueous solutions of potassium cyanide and aluminum sulfate, leading to the formation of aluminum hydroxide. The balanced chemical equation might seem complex; however, the net ionic equation zeroes in on the key players. This is:

• \(\mathrm{Al}^{3+} + 3\mathrm{OH}^- \rightarrow \mathrm{Al}(\text{OH})_3 \)

This simplification is powerful in highlighting which ions are responsible for the precipitate. Instead of tracking all ions through the reaction, net ionic equations allow chemists to focus only on the interactions that form new substances. This approach proves valuable in understanding reactions such as precipitation, as it directly addresses the chemical changes at play.

Hydroxide Ion Formation

Hydroxide ions (\(\mathrm{OH}^-\)) play a crucial role in the precipitation of aluminum hydroxide during this reaction. But how do these ions form in the solution, especially when not directly added?

The secret lies in a process called hydrolysis. In the presence of water, certain ions, like cyanide ions (\(\mathrm{CN}^-\)), can react to produce hydroxide ions. Here's a simplified view:

• \(\mathrm{CN}^- + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HCN} + \mathrm{OH}^- \)

This reaction leads to an increase in hydroxide ion concentration, which helps form aluminum hydroxide. The hydroxide ions increase the solution's pH, making conditions favorable for aluminum ions to precipitate as aluminum hydroxide, a compound that readily forms solids under basic conditions.

Recognizing the formation of hydroxide ions through such indirect means is vital in understanding many reactions. It helps explain the resulting solution behavior, such as precipitation or pH changes, which are common in laboratory and natural processes.