Question

Predict the \(\mathrm{pH}(>7,<7,\) or \(\approx 7)\) of aqueous solutions containing the following salts: (a) \(\mathrm{KBr}\) (b) \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3},\) (c) \(\mathrm{BaCl}_{2}\) (d) \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\).

Short answer

(a) ≈ 7, (b) < 7, (c) ≈ 7, (d) < 7.

Step-by-step solution

Understanding Salt Hydrolysis

Salts can affect the pH of a solution through a process called hydrolysis. The ions from the salt can react with water to form either hydroxide ions (increasing pH) or hydronium ions (decreasing pH). Thus, we need to identify the acid and base from which each salt is derived to determine the solution's pH.

Analyzing (a) KBr

Potassium bromide, KBr, is derived from a strong base (KOH) and a strong acid (HBr). Neither ion undergoes significant hydrolysis, so the pH of KBr solution will be approximately neutral, \( \mathrm{pH} \approx 7 \).

Analyzing (b) Al(NO₃)₃

Aluminum nitrate, \(\mathrm{Al(NO_3)_3}\), is derived from a weak base (Al(OH)₃) and a strong acid (HNO₃). The Al³⁺ ions can cause hydrolysis forming Al(OH)₃, resulting in acidic conditions, \( \mathrm{pH} < 7 \).

Analyzing (c) BaCl₂

Barium chloride, \(\mathrm{BaCl_2}\), is derived from a strong base (Ba(OH)₂) and a strong acid (HCl). Both ions do not significantly undergo hydrolysis, so the pH of BaCl₂ solution will be approximately neutral, \( \mathrm{pH} \approx 7 \).

Analyzing (d) Bi(NO₃)₃

Bismuth (III) nitrate, \(\mathrm{Bi(NO_3)_3}\), is derived from a weak base (Bi(OH)₃) and a strong acid (HNO₃). The Bi³⁺ ions can undergo hydrolysis, leading to the formation of Bi(OH)₃ and an acidic solution, \( \mathrm{pH} < 7 \).

Key concepts

Salt Hydrolysis

Salt hydrolysis is a fascinating process where the ions from dissolved salts interact with water. This interaction can significantly affect the pH of a solution. When a salt dissolves in water, it separates into its constituent ions. These ions can react with water in a way that affects whether the solution becomes more acidic or more basic.
For instance, if an ion reacts with water to create hydroxide ions (\( ext{OH}^- \)), the solution becomes basic (pH > 7). Conversely, if it leads to the formation of hydronium ions (\( ext{H}_3 ext{O}^+ \)), the solution becomes acidic (pH < 7). Understanding which ions are present in a salt helps us predict how they will behave in water and thus determine the solution's pH.
When dealing with salts such as \( ext{KBr} \) and \( ext{BaCl}_2 \), which are formed from strong acids and bases, you notice they don't affect the pH as much, leading to almost neutral solutions.

Acid-Base Reactions

Acid-base reactions play a crucial role in determining the behavior of salts in water. To understand these reactions, it's important to know about the strength of acids and bases involved in forming the salt.
Acids and bases can either be strong or weak, which directly impacts the resulting salt's capacity to cause hydrolysis. For example, potassium bromide (\( ext{KBr} \)) comes from a strong base (\( ext{KOH} \)) and a strong acid (\( ext{HBr} \)), leading to no significant change in pH upon dissolution.
In contrast, when a salt like bismuth nitrate (\( ext{Bi(NO}_3)_3 \)) comes from a weak base (\( ext{Bi(OH)}_3 \)) and a strong acid (\( ext{HNO}_3 \)), the resulting solution tends to be acidic. This difference arises due to the base's inability to fully neutralize the strong acid, allowing more hydronium ions to persist in solution.

Ion Hydrolysis

Ion hydrolysis involves the reaction of dissolved ions with water, resulting in changes to the solution’s pH. This process can result in either acidic or basic solutions depending on the ions.
When we consider a salt like \( ext{Al(NO}_3)_3 \), the aluminum ions undergo hydrolysis, reacting with water to form aluminum hydroxide (\( ext{Al(OH)}_3 \)) and releasing extra \( ext{H}^+ \) ions, increasing the acidity of the solution.
Meanwhile, some ions, like those in \( ext{KBr} \), do not participate in hydrolysis significantly, so they maintain a neutral pH. The extent of ion hydrolysis and the resulting pH change depends on the nature of the ions, particularly their tendency to release or absorb \( ext{H}^+ \) or \( ext{OH}^- \) ions.

Aqueous Solutions

When salts dissolve in water, they form aqueous solutions. These solutions consist of water molecules and charged ions. Understanding their behavior is key in predicting pH levels.
In a neutral aqueous solution, the concentration of hydronium and hydroxide ions are equal, leading to a pH of 7. However, the introduction of salts can disrupt this balance depending on the nature of the ions released.
For salts derived from strong acids and bases like \( ext{KBr} \) and \( ext{BaCl}_2 \), the solution remains neutral since the ions do not significantly alter the water dissociation equilibrium.
With salts such as \( ext{Al(NO}_3)_3 \), which derives from a weak base, the water dissociation is skewed toward an acidic environment because of ion hydrolysis. Recognizing these influences helps in anticipating whether an aqueous solution will be acidic, basic, or neutral.