Problem 91 - Chemistry Matter and Change

Question

Write formula equations and net ionic equations for the hydrolysis of each salt in water. \begin{equation} \text { a. sodium carbonate }\quad\text { b. ammonium bromide } \end{equation}

Step-by-Step Solution

Answer

Sodium carbonate forms hydroxide ions; ammonium bromide forms hydronium ions through hydrolysis.

Step 1: Determine Ion Components of the Salt for Sodium Carbonate

Sodium carbonate (Na_2CO_3) is composed of sodium ions (Na^+) and carbonate ions (CO_3^{2-}). In water, it dissociates into these ions.

Step 2: Hydrolysis Reaction for Sodium Carbonate

Carbonate ions (CO_3^{2-}) undergo hydrolysis in water. The reaction is CO_3^{2-} + H_2O ightarrow HCO_3^- + OH^-. Sodium ions (Na^+) do not undergo hydrolysis and remain spectator ions.

Step 3: Write Net Ionic Equation for Sodium Carbonate

The net ionic equation for the hydrolysis of sodium carbonate is simply the hydrolysis of the carbonate ion: CO_3^{2-} + H_2O ightarrow HCO_3^- + OH^-.

Step 4: Determine Ion Components of the Salt for Ammonium Bromide

Ammonium bromide (NH_4Br) is composed of ammonium ions (NH_4^+) and bromide ions (Br^-). In water, it dissociates into these ions.

Step 5: Hydrolysis Reaction for Ammonium Bromide

Ammonium ions (NH_4^+) can undergo hydrolysis. The reaction is NH_4^+ + H_2O ightarrow NH_3 + H_3O^+. Bromide ions (Br^-) do not undergo hydrolysis and remain spectator ions.

Step 6: Write Net Ionic Equation for Ammonium Bromide

The net ionic equation for the hydrolysis of ammonium bromide is the hydrolysis of the ammonium ion: NH_4^+ + H_2O ightarrow NH_3 + H_3O^+.

Key Concepts

Salt Hydrolysis

Salt hydrolysis is a chemical process where a salt reacts with water, resulting in the formation of an acidic or basic solution. Not all salts are involved in hydrolysis, but when they do react, the ions of the salt interact with water molecules to either donate or accept protons.

- When a salt derived from a strong base and weak acid dissolves in water, the solution tends to be basic. This is because the anion from the weak acid undergoes hydrolysis, increasing the concentration of hydroxide ions \(OH^-\).- Conversely, a salt derived from a weak base and strong acid forms an acidic solution because the cation can donate protons to water, increasing the concentration of hydronium ions \(H_3O^+\).

In the case of sodium carbonate (a), the carbonate ion is derived from carbonic acid, a weak acid, and reacts with water to produce hydroxide ions, resulting in a basic solution. In contrast, ammonium bromide (b) possesses ammonium ions that come from ammonia, a weak base, and upon hydrolysis, form acidic solutions.

Net Ionic Equations

Net ionic equations are simplified chemical equations that show only the particles that participate in the chemical reaction, leaving out the spectator ions.

To write a net ionic equation, follow these steps:

• Identify all the ions present in the reaction mixture.
• Determine which ions actually participate in the reaction.
• Exclude the ions that do not undergo any change, known as spectator ions.

In the scenario of salt hydrolysis, the net ionic equations focus on the ions from the salt that change during the reaction. For sodium carbonate, the net ionic equation \(CO_3^{2-} + H_2O \rightarrow HCO_3^- + OH^-\) showcases only the carbonate ion reacting with water. For ammonium bromide, the net ionic equation \(NH_4^+ + H_2O \rightarrow NH_3 + H_3O^+\) features ammonium ion hydrolysis.

Ion Components

Ion components are crucial in understanding the behavior of salts in water. Each salt breaks down into specific ions when dissolved, which play different roles in chemical reactions.

- In sodium carbonate (\(Na_2CO_3\)), the ion components are sodium ions \(Na^+\) and carbonate ions \(CO_3^{2-}\).- In ammonium bromide (\(NH_4Br\)), the ion components are ammonium ions \(NH_4^+\) and bromide ions \(Br^-\).

While both of these salts dissociate into their ions in water, only certain ions undergo reactions specific to hydrolysis. Understanding ion components helps determine which ions will interact with water molecules and participate in the hydrolysis process.

Spectator Ions

Spectator ions are ions present in the solution that do not participate in the actual chemical reaction. They remain unchanged throughout the process.

- For sodium carbonate \(Na_2CO_3\), the sodium ions \(Na^+\) are spectator ions, as they do not react with water during hydrolysis.- For ammonium bromide \(NH_4Br\), the bromide ions \(Br^-\) act as spectator ions, remaining in the solution unaltered.

Identifying spectator ions is crucial in writing net ionic equations because they can be omitted. This simplifies the equation, highlighting only the substances that are actively involved in the chemical change, thus providing a clearer picture of the reaction occurring.