Problem 145 - General Chemistry

Question

A solution is prepared by dissolving ammonium nitrite in water. Predict whether the solution would be acidic or basic. If you want to make the solution have a neutral \(\mathrm{pH},\) which of the following could be added to achieve this result: \(\mathrm{HCl}, \mathrm{NaCl}\), or KOH? Justify your answer.

Step-by-Step Solution

Answer

The solution is acidic. Add KOH to neutralize it.

Step 1: Analyze the Components

Identify the ions that ammonium nitrite dissociates into when dissolved in water. Ammonium nitrite \((NH_4NO_2)\) dissociates into ammonium ions \((NH_4^+)\) and nitrite ions \((NO_2^-).\)

Step 2: Consider Acid-Base Reactions

Determine the acid-base nature of each ion. The \(NH_4^+\) ion is derived from \(NH_3\) and acts as a weak acid. The \(NO_2^-\) ion is the conjugate base of \(HNO_2\), and acts as a weak base.

Step 3: Compare Ionization Constants

Compare the strength of these ions' acid-base behavior by examining their ionization constants. The ammonium ion \((NH_4^+)\) tends to donate protons more readily than the nitrite ion \((NO_2^-\) accepts protons, implying the solution is slightly acidic.

Step 4: Predict Solution pH

Based on the ions present, determine the solution's pH. The presence of the \(NH_4^+\) weak acid makes the solution more likely to be acidic.

Step 5: Select a Neutralizing Agent

Propose which agent to add for achieving neutrality. To neutralize the acidity from \(NH_4^+\), a base like \(KOH\) (potassium hydroxide) should be added to increase the pH to neutral.

Step 6: Justify the Choice

Explain why \(KOH\) is suitable for neutralization. \(KOH\) is a strong base that will react with \(NH_4^+\) ions to form water and neutral ammonium hydroxide \((NH_4OH)\), effectively neutralizing the solution.

Key Concepts

Ammonium Nitrite

Ammonium nitrite is a chemical compound that is represented by the formula \(NH_4NO_2\). When dissolved in water, it undergoes dissociation. This means it breaks down into ammonium ions \((NH_4^+)\) and nitrite ions \((NO_2^-)\).

• The ammonium ion \((NH_4^+)\) is a crucial player in acid-base chemistry, acting as a weak acid. It originates from ammonia \((NH_3)\), which is known for accepting protons.
• On the other hand, the nitrite ion \((NO_2^-)\) functions as a weak base. It is the conjugate base of nitrous acid \((HNO_2)\), and is capable of accepting protons.

Knowing these characteristics is central to understanding the subsequent behavior of ammonium nitrite in solution. With both acidic and basic ions present, the interplay determines the overall pH of the solution.

Solution pH

The concept of pH is pivotal in analyzing the acidic or basic nature of any solution. When ammonium nitrite is added to water, we must evaluate the contributions of both \(NH_4^+\) and \(NO_2^-\) ions to determine the solution's pH. The pH scale typically ranges from 0 to 14:

• Values below 7 indicate an acidic solution.
• A pH of 7 denotes a neutral solution.
• Values above 7 reflect a basic solution.

In an ammonium nitrite solution, the ammonium ion acts as a weak acid. It tends to donate protons slightly more readily than the nitrite ion accepts them, which results in a slightly acidic solution. The pH will therefore be slightly less than 7, indicating its acidic nature. Understanding these nuances in pH can help predict the behavior of the solution in various chemical reactions.

Neutralization Reactions

When dealing with solutions that are not neutral, we often use neutralization reactions to adjust the pH to 7. To neutralize a slightly acidic solution, like one containing ammonium nitrite, it is necessary to add a base.

Consider the choices provided: \(HCl\), \(NaCl\), and \(KOH\):

• \(HCl\) is a strong acid, which would increase the acidity of the solution, not neutralize it.
• \(NaCl\) is a neutral salt and would not significantly affect the pH.
• \(KOH\) is a strong base and effectively counteracts the acid present in the solution.

Adding \(KOH\) to the ammonium nitrite solution results in a chemical reaction where \(KOH\) neutralizes \(NH_4^+\) ions, forming water and ammonium hydroxide \((NH_4OH)\). This process shifts the pH towards neutrality, stabilizing the solution at a more neutral pH level. Understanding these reactions is crucial for achieving desired pH levels in practical applications, like buffer solutions or industrial processes.