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Mobile App Chapter 6: Problem 41
Which of the following during hydrolysis gives an acidic solution in water? (a) \(\mathrm{KNO}_{3}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{KCl}\)
Short Answer
Expert verified
Answer: (b) \(\mathrm{NH}_{4} \mathrm{Cl}\)
Step by step solution
01
Understand the concept of hydrolysis
Hydrolysis is a chemical reaction that occurs when a substance reacts with water, causing the water molecule to split into its constituent ions (hydrogen ions, \(\mathrm{H}^{+}\), and hydroxide ions, \(\mathrm{OH}^{-}\)). In the case of salts, hydrolysis can lead to either acidic, basic, or neutral solutions depending on the degree to which the cation or the anion of the salt reacts with \(\mathrm{H}^{+}\) or \(\mathrm{OH}^{-}\) ions in water.
02
Analyze each option
(a) \(\mathrm{KNO}_{3}\): \(\mathrm{K}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions do not hydrolyze in water. Both ions are considered neutral since they do not react with \(\mathrm{H}^{+}\) or \(\mathrm{OH}^{-}\) ions. Therefore, \(\mathrm{KNO}_{3}\) yields a neutral solution.
(b) \(\mathrm{NH}_{4} \mathrm{Cl}\): This salt is composed of \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{Cl}^{-}\) ions. The \(\mathrm{NH}_{4}^{+}\) ion will hydrolyze in water according to the following reaction, donating an \(\mathrm{H}^{+}\) ion:
\[\mathrm{NH}_{4}^{+} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3}\mathrm{O}^{+}\]
On the other hand, \(\mathrm{Cl}^{-}\) does not hydrolyze in water since it is derived from a strong acid (\(\mathrm{HCl}\)). So, due to hydrolysis of the ammonium ion, the solution will have a higher concentration of \(\mathrm{H}^{+}\) ions, making it acidic.
(c) \((\mathrm{NH}_{4})_{2} \mathrm{CO}_{3}\): This salt is composed of \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{CO}_{3}^{2-}\) ions. Like in option (b), the \(\mathrm{NH}_{4}^{+}\) ion will hydrolyze and donate an \(\mathrm{H}^{+}\) ion. The carbonate ion, \(\mathrm{CO}_{3}^{2-}\), will also hydrolyze in water, accepting an \(\mathrm{H}^{+}\) ion:
\[\mathrm{CO}_{3}^{2-} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-}\]
The concentration of \(\mathrm{H}^{+}\) ions from the ammonium ion will be approximately equal to the concentration of \(\mathrm{OH}^{-}\) ions from the carbonate ion, leading to a neutral or slightly basic solution.
(d) \(\mathrm{KCl}\): This salt is composed of \(\mathrm{K}^{+}\) and \(\mathrm{Cl}^{-}\) ions. Both ions do not hydrolyze in water, so \(\mathrm{KCl}\) yields a neutral solution.
03
Determine the answer
Among the given options, only \(\mathrm{NH}_{4} \mathrm{Cl}\) produces an acidic solution during hydrolysis due to the formation of \(\mathrm{H}^{+}\) ions from the hydrolysis of the ammonium ion. Therefore, the correct answer is (b) \(\mathrm{NH}_{4} \mathrm{Cl}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Acidic and Basic Solutions
Understanding the nature of acidic and basic solutions is essential when studying hydrolysis of salts. Acidic solutions are characterized by a higher concentration of hydrogen ions \( \mathrm{H}^{+} \) than hydroxide ions \( \mathrm{OH}^{-} \), while basic solutions have more hydroxide ions than hydrogen ions. The pH scale is commonly used to quantify acidity or basicity, with values lower than 7 indicating acidity and values higher than 7 indicating basicity, while a pH of 7 is considered neutral.
The behavior of salts in water determines whether the resulting solution will be acidic, basic, or neutral. Salts derived from a strong acid and a strong base do not undergo significant hydrolysis and, consequently, yield neutral solutions. If a salt is formed from a strong acid and a weak base, the solution tends to be acidic as the cation of the weak base (often a positively charged metal or ammonium ion) can donate hydrogen ions after reacting with water. Conversely, salts from a weak acid and a strong base tend to create basic solutions as the anion of the weak acid can accept hydrogen ions, forming hydroxide ions in the process.
The behavior of salts in water determines whether the resulting solution will be acidic, basic, or neutral. Salts derived from a strong acid and a strong base do not undergo significant hydrolysis and, consequently, yield neutral solutions. If a salt is formed from a strong acid and a weak base, the solution tends to be acidic as the cation of the weak base (often a positively charged metal or ammonium ion) can donate hydrogen ions after reacting with water. Conversely, salts from a weak acid and a strong base tend to create basic solutions as the anion of the weak acid can accept hydrogen ions, forming hydroxide ions in the process.
Role of the pH Scale
The pH scale serves as a quick reference to predict the acidity or basicity of a solution that results from salt hydrolysis. By understanding the parent acid or base of a salt's ions, scientists and students can anticipate the pH of the solution without having to measure it directly every time.Ammonium Chloride Hydrolysis
Ammonium chloride \( \mathrm{NH}_{4} \mathrm{Cl} \) is an example of a salt that, when dissolved in water, undergoes hydrolysis to form an acidic solution. It contains ammonium ions \( \mathrm{NH}_{4}^{+} \) and chloride ions \( \mathrm{Cl}^{-} \). The chloride ion is the conjugate base of hydrochloric acid, a strong acid. It does not react with water because it is already in its lowest energy state. Therefore, no significant hydrolysis occurs on the chloride ion's part.
On the other hand, the ammonium ion is the conjugate acid of ammonia, a weak base. It tends to donate a hydrogen ion to water, as illustrated in the reaction:
\[\mathrm{NH}_{4}^{+} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3}\mathrm{O}^{+}\]
This reaction shows that ammonium chloride hydrolysis increases the concentration of hydronium ions \(\mathrm{H}_{3}\mathrm{O}^{+}\) in the solution, which lowers the pH and leads to acidic conditions. It's important to note that while both ions are capable of interacting with water, it's the reaction of the ammonium ion that defines the acidity of the solution.
On the other hand, the ammonium ion is the conjugate acid of ammonia, a weak base. It tends to donate a hydrogen ion to water, as illustrated in the reaction:
\[\mathrm{NH}_{4}^{+} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3}\mathrm{O}^{+}\]
This reaction shows that ammonium chloride hydrolysis increases the concentration of hydronium ions \(\mathrm{H}_{3}\mathrm{O}^{+}\) in the solution, which lowers the pH and leads to acidic conditions. It's important to note that while both ions are capable of interacting with water, it's the reaction of the ammonium ion that defines the acidity of the solution.
Importance of Hydrolysis Reactions
Understanding ammonium chloride hydrolysis is significant, as it demonstrates how the ionic components of a salt can independently affect the pH of a solution. It's especially relevant in biological and environmental systems where controlling pH levels is crucial.Salt Reactivity in Water
When salts dissolve in water, they dissociate into their respective ions. The reaction of these ions with water, known as hydrolysis, can alter the pH of the solution. The reactivity of each ion in water is governed by their origin - whether they come from a strong or weak acid or base.
Salts from strong acids and bases, like potassium chloride \( \mathrm{KCl} \) and sodium nitrate \( \mathrm{NaNO}_{3} \) usually do not undergo hydrolysis because their ions are relatively stable in water. Owing to this stability, such salts produce neutral solutions. On the contrary, salts from weak bases and strong acids, or weak acids and strong bases, tend to react with water, shifting the pH away from neutrality.
The degree of reactivity also depends on the solubility of the salt; highly soluble salts dissociate completely, leading to more significant pH changes. Keeping track of salt solubility and the acid/base strength of the parent compounds allows students and chemists to predict how a salt will behave in water, whether it will form an acidic, basic, or neutral solution.
Salts from strong acids and bases, like potassium chloride \( \mathrm{KCl} \) and sodium nitrate \( \mathrm{NaNO}_{3} \) usually do not undergo hydrolysis because their ions are relatively stable in water. Owing to this stability, such salts produce neutral solutions. On the contrary, salts from weak bases and strong acids, or weak acids and strong bases, tend to react with water, shifting the pH away from neutrality.
The degree of reactivity also depends on the solubility of the salt; highly soluble salts dissociate completely, leading to more significant pH changes. Keeping track of salt solubility and the acid/base strength of the parent compounds allows students and chemists to predict how a salt will behave in water, whether it will form an acidic, basic, or neutral solution.
