Question

\(\mathrm{Al}(\mathrm{OH})_{3}\) is insoluble in water. It dissolves in concentrated NaOH solution. Write a balanced ionic equation for this reaction. What type of reaction is this?

Short answer

\(\mathrm{Al(OH)_3 + OH^- \rightarrow [Al(OH)_4]^-}\); dissolution (acid-base) reaction.

Step-by-step solution

Understand the compounds involved

We're dealing with aluminum hydroxide, \( \mathrm{Al(OH)_3} \), which is generally insoluble in water. It can, however, react with concentrated sodium hydroxide solution, \( \mathrm{NaOH} \). The resulting ionic equation will show aluminum hydroxide reacting with the hydroxide ions from sodium hydroxide to form a soluble complex.

Identify the reaction products

When \( \mathrm{Al(OH)_3} \) reacts with \( \mathrm{OH^-} \) ions from \( \mathrm{NaOH} \), the product is a soluble complex \( \mathrm{[Al(OH)_4]^-} \), known as the aluminate ion. The presence of excess \( \mathrm{OH^-} \) ions allows the aluminum hydroxide to dissolve. Thus, \( \mathrm{Na^+} \) ions are spectator ions in this reaction.

Write the balanced ionic equation

In the balanced ionic equation, we'll see \( \mathrm{Al(OH)_3} \) reacting with four hydroxide ions to form the aluminate ion, reflecting the solubility change. The ionic equation is:\[ \mathrm{Al(OH)_3 (s) + OH^- (aq) \rightarrow [Al(OH)_4]^- (aq)} \]

Classify the reaction type

This reaction is a dissolution reaction where an insoluble compound becomes soluble in the presence of excess hydroxide ions. It can also be seen as an acid-base reaction, where aluminum hydroxide acts as an amphoteric substance, reacting with the base \( \mathrm{NaOH} \).

Key concepts

Understanding Aluminum Hydroxide

Aluminum hydroxide, denoted as \( \mathrm{Al(OH)_3} \), is a fascinating compound with unique solubility characteristics. It is primarily known for being insoluble in water, which means it does not dissolve easily when mixed with water.

This compound tends to remain in its solid form because water molecules cannot sufficiently separate the aluminum ions from the hydroxide ions. However, its behavior changes in the presence of concentrated bases.

In this context, aluminum hydroxide becomes interesting due to its reaction with substances like sodium hydroxide, where it transforms into a soluble form.

Role of Sodium Hydroxide

Sodium hydroxide, represented chemically as \( \mathrm{NaOH} \), is a strong base commonly used in various chemical reactions. In this particular reaction, sodium hydroxide plays an essential role by providing hydroxide ions \( \mathrm{OH^-} \) that interact with aluminum hydroxide.

When aluminum hydroxide comes into contact with concentrated \( \mathrm{NaOH} \), the hydroxide ions facilitate the formation of a soluble complex. This reaction exemplifies \( \mathrm{NaOH} \)'s potency as a base, as it can dissolve aluminum hydroxide by converting it into the aluminate ion \( \mathrm{[Al(OH)_4]^-} \).

To note, \( \mathrm{Na^+} \) ions do not partake in the primary chemical transformation and are considered spectator ions in the reaction.

Balancing the Ionic Equation

Balancing chemical equations is a critical skill in chemistry, ensuring that the same number of each type of atom appears on both sides of the equation. In the context of this reaction, the ionic equation needs to be balanced.

Let's consider the essential players: the insoluble \( \mathrm{Al(OH)_3} \) and the hydroxide ions \( \mathrm{OH^-} \).

• First, make sure the aluminum atoms are balanced on each side.
• Check that the hydroxide ions are equally accounted for.

The balanced ionic equation will look like this:\[\mathrm{Al(OH)_3 (s) + OH^- (aq) \rightarrow [Al(OH)_4]^- (aq)}\]
This balanced equation represents the transition from a solid state to a dissolved state, showcasing the chemical change.

Amphoteric Nature of Substances

An amphoteric substance can act as both an acid and a base, reacting differently depending on the surrounding chemical environment.

Aluminum hydroxide \( \mathrm{Al(OH)_3} \) is a classic example of an amphoteric compound. In acidic environments, it acts like a base, and in basic environments, it behaves like an acid. This dual capability is why \( \mathrm{Al(OH)_3} \) can dissolve in concentrated sodium hydroxide.

During the reaction with \( \mathrm{NaOH} \), aluminum hydroxide accepts hydroxide ions acting as an acid, leading to the formation of the soluble aluminate ion. This flexibility is a defining feature of amphoteric substances, making them vital in various chemical applications.