Problem 84 - Chemistry The Central Science

Question

Predict whether aqueous solutions of the following substances are acidic, basic, or neutral: (a) \(\mathrm{AlCl}_{3}\), (b) \(\mathrm{NaBr}\), (c) \(\mathrm{NaClO},(\mathrm{d})\) \(\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3},(\mathrm{e}) \mathrm{Na}_{2} \mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

The aqueous solutions of the given substances have the following pH nature: (a) \(\mathrm{AlCl}_{3}\) - acidic, (b) \(\mathrm{NaBr}\) - neutral, (c) \(\mathrm{NaClO}\) - basic, (d) \(\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3}\) - acidic, and (e) \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) - basic.

Step 1: Analyzing AlCl3

When \(\mathrm{AlCl}_{3}\) is dissolved in water, it forms \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^{-}\) ions. The \(\mathrm{Al}^{3+}\) ion can undergo hydrolysis to generate \(\mathrm{H}^{+}\) ions in the solution, which makes the solution acidic. Thus, an aqueous solution of \(\mathrm{AlCl}_{3}\) is acidic. #b)

Step 2: Analyzing NaBr

When \(\mathrm{NaBr}\) is dissolved in water, it forms \(\mathrm{Na}^{+}\) and \(\mathrm{Br}^{-}\) ions. Neither of these ions undergoes hydrolysis in water, so no \(\mathrm{H}^{+}\) or \(\mathrm{OH}^{-}\) ions are produced. As a result, an aqueous solution of \(\mathrm{NaBr}\) is neutral. #c)

Step 3: Analyzing NaClO

When \(\mathrm{NaClO}\) is dissolved in water, it forms \(\mathrm{Na}^{+}\) and \(\mathrm{ClO}^{-}\) ions. The \(\mathrm{ClO}^{-}\) ion can react with water to generate \(\mathrm{OH}^{-}\) ions, making the solution basic. Thus, an aqueous solution of \(\mathrm{NaClO}\) is basic. #d)

Step 4: Analyzing [CH3NH3]NO3

When \(\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3}\) is dissolved in water, it forms \(\mathrm{CH}_{3}\mathrm{NH}_{3}^+\) and \(\mathrm{NO}_{3}^-\) ions. The \(\mathrm{CH}_{3}\mathrm{NH}_{3}^{+}\) ion can release \(\mathrm{H}^{+}\) ions in the solution, making the solution acidic. Therefore, an aqueous solution of \(\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3}\) is acidic. #e)

Step 5: Analyzing Na2SO3

When \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) is dissolved in water, it forms \(\mathrm{Na}^{+}\) and \(\mathrm{SO}_{3}^{2-}\) ions. The \(\mathrm{SO}_{3}^{2-}\) ion can undergo hydrolysis to generate \(\mathrm{OH}^{-}\) ions, making the solution basic. Thus, an aqueous solution of \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) is basic.

Key Concepts

Hydrolysis

Hydrolysis is a fascinating reaction involving water molecules that can change the acidity or basicity of a solution. When certain salts dissolve in water, their ions may react with water to form either hydronium ions (\(\mathrm{H}^{+}\) ) or hydroxide ions (\(\mathrm{OH}^{-}\) ).
This process is called hydrolysis, and it's crucial to determining whether a solution becomes acidic or basic.
When an anion or cation from a dissolved salt interacts with water to form more \(\mathrm{H}^{+}\) ions, the resulting solution can become acidic. Conversely, if \(\mathrm{OH}^{-}\) ions are formed, the solution turns basic.

• For example, in the case of \(\mathrm{AlCl}_{3}\), the \(\mathrm{Al}^{3+}\) ions hydrolyze and increase \(\mathrm{H}^{+}\) concentration, making the solution acidic.
• Meanwhile, \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) hydrolyzes to form \(\mathrm{OH}^{-}\) ions, resulting in a basic solution.

Ion Formation

Ion formation occurs when ionic compounds dissolve in water, leading to the splitting of the compound into its respective ions. This is a fundamental process in aqueous chemistry.
Depending on the ions formed, the resultant solution can have different properties.
For instance, when a salt like \(\mathrm{NaBr}\) dissolves, it simplifies into \(\mathrm{Na}^{+}\) and \(\mathrm{Br}^{-}\) ions.

• Neither of these ions significantly reacts with water to alter acidity or basicity, thereby resulting in a neutral solution.
• Contrastingly, \(\mathrm{NaClO}\) disintegrates into \(\mathrm{Na}^{+}\) and \(\mathrm{ClO}^{-}\) ions. The \(\mathrm{ClO}^{-}\) can then react with water to produce \(\mathrm{OH}^{-}\) ions, turning the solution basic.

Understanding ion formation helps predict the behavior of substances when dissolved in water.

Acidic Solutions

Acidic solutions are solutions where the concentration of \(\mathrm{H}^{+}\) ions exceeds that of \(\mathrm{OH}^{-}\) ions. These solutions have a pH less than 7.
They often arise when a cation from a dissolved substance can donate hydrogen ions to the aqueous environment.
Consider the compound \(\mathrm{AlCl}_{3}\).

• When it dissolves, its \(\mathrm{Al}^{3+}\) ions undergo hydrolysis to release more \(\mathrm{H}^{+}\) ions, which lowers the pH and creates an acidic solution.
• Another example is \(\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3}\), where \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) releases \(\mathrm{H}^{+}\) ions, also leading to an acidic outcome.

Acidic solutions are crucial in many chemical reactions, and understanding their formation and properties is fundamental in chemistry.

Basic Solutions

Basic solutions feature a higher concentration of \(\mathrm{OH}^{-}\) ions than \(\mathrm{H}^{+}\) ions, giving them a pH above 7. These solutions typically arise when anions interact with water leading to the release of hydroxide ions.
For example, consider \(\mathrm{Na}_2\mathrm{SO}_3\).

• When this compound is dissolved in water, the \(\mathrm{SO}_{3}^{2-}\) ions undergo hydrolysis, thus producing \(\mathrm{OH}^{-}\) ions and raising the pH to make a basic solution.
• Similarly, \(\mathrm{NaClO}\) forms from \(\mathrm{ClO}^{-}\) ions reacting with water to generate \(\mathrm{OH}^{-}\) ions, and thus a basic solution is formed.

Basic solutions are common in cleaning agents and are essential for chemical reactions needing higher pH environments.