Problem 25 - Chemistry and Chemical Reactivity

Question

If each of the salts listed here were dissolved in water to give a 0.10 M solution, which solution would have the highest \(\mathrm{pH}\) ? Which would have the lowest pH? (a) \(\mathrm{Na}_{2} \mathrm{S}\) (b) \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) (d) NaF (e) \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\) (f) \(\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

Highest pH: (a) \(\mathrm{Na}_{2}\mathrm{S}\), Lowest pH: (f) \(\mathrm{AlCl}_{3}\).

Step 1: Understanding the Problem

We need to determine which salt, when dissolved in water to form a 0.10 M solution, will result in the highest and lowest pH. This involves understanding how each salt behaves in solution—whether it produces acidic or basic solutions.

Step 2: Analyzing Basic Salts

Basic salts come from the salts of weak acids. For (a) \(\mathrm{Na}_{2}\mathrm{S}\), which forms \(\mathrm{S}^{2-}\), sulphide ions (abla) react with water to form hydroxide ions. For (b) \(\mathrm{Na}_{3} \mathrm{PO}_{4}\), it is the salt of a weak acid (\( \mathrm{H}_{3} \mathrm{PO}_{4} \)), so it will make the solution basic. Similarly, (d) \(\mathrm{NaF}\) and (e) \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\) will be basic because they are salts of weak acids like \( \mathrm{HF} \) and \( \mathrm{CH}_{3}\mathrm{COOH} \).

Step 3: Analyzing Acidic Salts

Acidic salts come from the salts of strong acids and weak bases. Option (f) \(\mathrm{AlCl}_{3}\) is the chloride of a trivalent aluminum ion. It can react with water to form \(\mathrm{HCl} \), resulting in an acidic solution. Also, (c) \(\mathrm{NaH}_{2}\mathrm{PO}_{4}\) can release \(\mathrm{H}^{+}\) ions, making the solution slightly acidic.

Step 4: Determining Highest pH Solution

The strongest basic salt will produce the highest pH. Among options (a), (b), (d), and (e), (a) \(\mathrm{Na}_{2}\mathrm{S}\) provides \(\mathrm{S}^{2-}\), which strongly reacts with water to form \(\mathrm{OH}^{-}\) ions, resulting in a high pH.

Step 5: Determining Lowest pH Solution

The salt that highly tends towards acid will produce the lowest pH. In this case, (f) \(\mathrm{AlCl}_{3}\) is a strong Lewis acid which hydrolyzes in water, creating an acidic environment and thus the lowest pH.

Key Concepts

Acidic and Basic Salts

When salts dissolve in water, they can produce acidic or basic solutions depending on their origins. Basic salts typically come from strong bases reacting with weak acids. Take, for instance, sodium sulfide (\(\mathrm{Na}_2\mathrm{S}\)). It originates from sodium hydroxide, a strong base, and hydrogen sulfide, a weak acid. In water, this salt dissociates to form sulphide ions (\(\mathrm{S}^{2-}\)), which can further react with water to form hydroxide ions (\(\mathrm{OH}^-\)), resulting in a basic solution.

• Basic salts - derived from strong base + weak acid
• Acidic salts - derived from strong acid + weak base

For acidic salts, the scenario is reversed. An example is aluminum chloride (\(\mathrm{AlCl}_3\)), which comes from hydrochloric acid, a strong acid, reacting with aluminum hydroxide, a weak base. When dissolved in water, it can release hydrogen ions (\(\mathrm{H}^+\)), making the solution acidic.

Salt Hydrolysis

Salt hydrolysis is a reaction where the salt interacts with water to alter the pH of the solution. Through this process, parts of the salt react with water molecules, often affecting the concentration of hydroxide or hydrogen ions in the solution.
Sodium phosphate (\(\mathrm{Na}_3\mathrm{PO}_4\)) and sodium acetate (\(\mathrm{NaCH}_3\mathrm{CO}_2\)) are examples of salts that undergo hydrolysis to make basic solutions. Phosphate ions and acetate ions react with water to generate hydroxide ions, elevating the pH.

• Salt interacts with water
• Formation of hydroxide or hydrogen ions
• Affects solution's pH

On the other hand, aluminum chloride (\(\mathrm{AlCl}_3\)) will hydrolyze to produce an acidic solutions. It forms hydrochloric acid, creating more hydrogen ions that lower the pH, demonstrating how hydrolysis can result in acidic conditions.

Weak Acids and Bases

Understanding weak acids and bases is crucial when predicting the behavior of salts in solutions. They only partially dissociate in water, resulting in an equilibrium state. This weak dissociation contributes to the hydrolysis of their respective salts.
Consider hydrofluoric acid (\(\mathrm{HF}\)). It's a weak acid, meaning that its salt, sodium fluoride (\(\mathrm{NaF}\)), will have a tendency to dissociate slightly in water, forming hydroxide ions, which increase pH, making the solution basic.

• Weak acids/bases show partial dissociation
• Equilibrium state in water
• Weak acid salt forms a basic solution

Similarly, acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) is another weak acid, meaning sodium acetate will generate hydroxide ions upon hydrolysis, again making the aqueous solution basic.

Lewis Acids in Solution

Lewis acids are classified differently than the traditional Bronsted-Lowry acids. Rather than donating protons, they accept electron pairs. This concept can help explain the behavior of certain salts when dissolved in water.
Aluminum chloride (\(\mathrm{AlCl}_3\)) is a good example. Here, the aluminum ion acts as a powerful Lewis acid. In water, it can attract and stabilize electron pairs, leading to the release of hydrogen ions and resulting in an acidic solution.

• Lewis acids accept electron pairs
• Explains behavior in forming acidic solutions

Because of this, \(\mathrm{AlCl}_3\) has one of the lowest pH values when in solution at the same concentration. This unique mechanism distinguishes Lewis acids from other categories of acids, emphasizing electron pair interactions.