Question

Write a net ionic equation for the reaction with \(\mathrm{Al}^{3+}\) by which (a) a complex ion forms when it reacts with \(\mathrm{OH}^{-}\). (b) a precipitate forms when it reacts with the phosphate ion. (c) the precipitate formed with \(\mathrm{OH}^{-}\) is dissolved by a strong acid.

Short answer

Question: Write the net ionic equations for the following reactions involving the aluminum ion, Al³⁺: (a) formation of a complex ion with hydroxide ions, (b) formation of a precipitate with phosphate ions, and (c) dissolving the precipitate formed with hydroxide ions by a strong acid. Answer: (a) Al³⁺ + 3 OH⁻ → Al(OH)₃ (b) Al³⁺ + PO₄³⁻ → AlPO₄ (c) Al(OH)₃ + 3 H⁺ → Al³⁺ + 3 H₂O

Step-by-step solution

(a) Formation of a complex ion with hydroxide ions

Step 1: Write the balanced molecular equation: \(\mathrm{Al}^{3+} + 3\,\mathrm{OH}^{-} \rightarrow \mathrm{Al(OH)}_{3}\) Step 2: Separate soluble species into individual ions (none in this case). Step 3: Remove spectator ions (none in this case). Step 4: Write the net ionic equation: \(\mathrm{Al}^{3+} + 3\,\mathrm{OH}^{-} \rightarrow \mathrm{Al(OH)}_{3}\)

(b) Formation of a precipitate with phosphate ions

Step 1: Write the balanced molecular equation: \(\mathrm{Al}^{3+} + \mathrm{PO}_{4}^{3-} \rightarrow \mathrm{AlPO}_{4}\) Step 2: Separate soluble species into individual ions (none in this case). Step 3: Remove spectator ions (none in this case). Step 4: Write the net ionic equation: \(\mathrm{Al}^{3+} + \mathrm{PO}_{4}^{3-} \rightarrow \mathrm{AlPO}_{4}\)

(c) Dissolving the precipitate formed with hydroxide ions by a strong acid

Step 1: Write the balanced molecular equation: \(\mathrm{Al(OH)}_{3} + 3\,\mathrm{H}^{+} \rightarrow \mathrm{Al}^{3+} + 3\,\mathrm{H}_{2}\mathrm{O}\) Step 2: Separate soluble species into individual ions (none in this case). Step 3: Remove spectator ions (none in this case). Step 4: Write the net ionic equation: \(\mathrm{Al(OH)}_{3} + 3\,\mathrm{H}^{+} \rightarrow \mathrm{Al}^{3+} + 3\,\mathrm{H}_{2}\mathrm{O}\)

Key concepts

Complex Ion Formation

When we talk about complex ion formation, we typically refer to the union of a central metal atom with multiple ligands. Ligands are atoms, ions, or molecules that can donate a pair of electrons to the metal, creating a coordination complex. This process is fundamental in many areas, including biochemistry and metallurgy.

In the provided exercise, aluminum ions (\texttt{Al^{3+}}) react with hydroxide ions (\texttt{OH^-}) to form a complex ion. The reaction can be described as follows:\[\mathrm{Al}^{3+} + 3\,\mathrm{OH}^{-} \rightarrow \mathrm{Al(OH)}_{3}\]In this scenario, the hydroxide ions act as ligands, each donating a lone pair of electrons to the aluminum ion, resulting in the formation of the aluminum hydroxide complex. The equation presented doesn't immediately reveal the nature of a complex ion, but it underscores the initial interaction that leads to such complex formation in certain contexts.

It's interesting to note that complex ions often exhibit distinct properties compared to their individual components, including changes in solubility, reactivity, and color. Understanding their formation is a key aspect of predicting product formation in reactions involving metals and ligands.

Precipitate Reactions

Precipitate reactions are a type of chemical reaction where an insoluble solid, called a precipitate, forms when two aqueous solutions are mixed together. The formation of a precipitate is fundamentally important in the fields of chemistry, environmental science, and industrial processes.

In the net ionic equation from the exercise involving aluminum ions (\texttt{Al^{3+}}) and phosphate ions (\texttt{PO4^{3-}}), a precipitate of aluminum phosphate is formed:\[\mathrm{Al}^{3+} + \mathrm{PO}_{4}^{3-} \rightarrow \mathrm{AlPO}_{4}\]The aluminum and phosphate ions combine in a fixed ratio to produce the insoluble compound aluminum phosphate (\texttt{AlPO4}). This is a classic example of a precipitate reaction, where the driving force is the formation of an insoluble product from the reaction of ions in solution.

From an educational perspective, when learning about precipitate reactions, students should be particularly aware of solubility rules. Knowing which combinations of ions typically form a precipitate can greatly enhance the prediction and understanding of these types of reactions.

Acid-Base Reactions

Acid-base reactions are among the most fundamental and widely studied types of chemical reactions in chemistry. These reactions involve the transfer of protons (H+ ions) from an acid to a base. In an aqueous solution, acids are typically defined as proton donors, while bases are proton acceptors.

The dissolution of aluminum hydroxide (\texttt{Al(OH)3}) precipitate in the presence of a strong acid, as described in part (c) of the exercise, is an example of an acid-base reaction:\[\mathrm{Al(OH)}_{3} + 3\,\mathrm{H}^{+} \rightarrow \mathrm{Al}^{3+} + 3\,\mathrm{H}_{2}\mathrm{O}\]In this net ionic equation, the strong acid (proton donor) reacts with the aluminum hydroxide precipitate (base), resulting in the formation of soluble aluminum ions and water molecules. This particular type of acid-base reaction is also referred to as a neutralization reaction, where the base (initially part of the precipitate) is neutralized by the acid, leading to the precipitate's dissolution.

Understanding acid-base reactions is crucial for many areas, including environmental science, medicine, and industry. Recognizing the interaction between acids and bases allows chemists to control reaction conditions, predict the outcomes of reactions, and manipulate processes in laboratory and industrial settings.