Problem 20 - Chemistry and Chemical Reactivity

Question

Dissolving ammonium bromide in water gives an acidic solution. Write a balanced equation showing how this can occur.

Step-by-Step Solution

Answer

Ammonium bromide dissociates into \( NH_4^+ \) and \( Br^- \), then \( NH_4^+ \) reacts with water to form \( H_3O^+ \), making the solution acidic.

Step 1: Identify Chemical Formulas

The formula for ammonium bromide is \( NH_4Br \). When it dissolves in water, it dissociates into its ions, ammonium \( NH_4^+ \) and bromide \( Br^- \). This is the first reaction we investigate.

Step 2: Dissociation in Water

When \( NH_4Br \) dissolves in water, it dissociates as follows: \[ NH_4Br (s) \rightarrow NH_4^+ (aq) + Br^- (aq) \]. This shows the separation of ammonium bromide into ammonium and bromide ions in aqueous solution.

Step 3: Reaction of Ammonium Ion

The ammonium ion \( NH_4^+ \) can react with water. The equation for this is: \[ NH_4^+ (aq) + H_2O (l) \rightarrow NH_3 (aq) + H_3O^+ (aq) \]. This reaction shows that \( NH_4^+ \) donates a proton to water, forming hydronium ions \( H_3O^+ \) which makes the solution acidic.

Key Concepts

Acidic Solutions

When a substance dissolves in water and increases the concentration of hydrogen ions ( H^+ ), the resulting mixture is known as an acidic solution. This happens when a compound releases or facilitates the formation of H^+ in the solution.

Important characteristics of acidic solutions include:

• A sour taste.
• The ability to conduct electricity due to the presence of ions.
• A pH value less than 7.

For the case of ammonium bromide dissolving in water, H_4^+ ions are responsible for the solution's acidic nature. The presence of H_3O^+ (hydronium ions) in the solution is a direct indication of its acidic quality. Hydronium ions increase the H^+ concentration significantly, lowering the pH and affirming the acidic property of the solution.

Ion Dissociation

Ion dissociation is the process through which compounds split into ions when dissolved in a solvent like water. It is a crucial step that leads to various chemical reactions in aqueous solutions.

During this process:

• The ionic compound separates into its constituent ions.
• The ions become surrounded by water molecules, aiding in their dispersion through the solution.

For example, when ammonium bromide ( NH_4Br ) dissolves in water, it dissociates into ammonium ( NH_4^+ ) and bromide ( Br^- ) ions. This dissociation is key as it provides free ions that can participate in further reactions, such as the ones involving the formation of hydronium ions. Without dissociation, the chemical species wouldn't be able to interact readily within the solution.

Hydronium Ion Formation

The formation of hydronium ions (H_3O^+) is a hallmark of creating an acidic solution. This process involves the transfer of protons (H^+) from one molecule to another, specifically from an acid to water.

Let's explore how this occurs with ammonium ions:

• The ammonium ion (NH_4^+) acts as a proton donor.
• It transfers a proton to a nearby water molecule (H_2O), which accepts it.
• This donation transforms the water molecule into a hydronium ion (H_3O^+).

The equation representing this interaction is: \[ NH_4^+ (aq) + H_2O (l) \rightarrow NH_3 (aq) + H_3O^+ (aq) \]

Formation of hydronium ions indicates an increase in acidity due to the rise in H^+ ion concentration. This process is crucial in understanding why solutions like that of ammonium bromide in water become acidic.