Question

Which among the following does not undergo hydrolysis? (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4}\)

Short answer

Answer: (b) Acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\))

Step-by-step solution

Identify the type of compounds given

In this exercise, we have 4 compounds: (a) Sodium carbonate (\(\mathrm{Na}_{2}\mathrm{CO}_{3}\)) - a salt (b) Acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)) - a carboxylic acid (c) Ammonium acetate (\(\mathrm{CH}_{3}\mathrm{COONH}_{4}\)) - a salt (carrying both acidic and basic ions) (d) Potassium sulfate (\(\mathrm{K}_{2}\mathrm{SO}_{4}\)) - a salt

Check for hydrolysis in salts

Salts are formed as a result of neutralization reactions between an acid and a base. When salts dissolve in water, they undergo hydrolysis, which is the breakdown of the salt due to the interaction with water molecules. Sodium carbonate and potassium sulfate are both salts, which, when dissolved in water, will undergo hydrolysis. So, both (a) and (d) can be eliminated as options for the compound that does not undergo hydrolysis.

Check for hydrolysis in carboxylic acids

Carboxylic acids, such as acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)), are polar compounds and can form hydrogen bonds with water molecules. However, in the case of acetic acid, hydrolysis is considered as the breakdown of the compound into smaller fragments due to the reaction with water. Since acetic acid does not undergo such a reaction, option (b) can be considered as not undergoing hydrolysis.

Check for hydrolysis in ammonium acetate

Ammonium acetate (\(\mathrm{CH}_{3}\mathrm{COONH}_{4}\)), being a salt (formed by the reaction between acetic acid and ammonia), will undergo hydrolysis when dissolved in water. This can be eliminated as well.

Select the correct option

Based on the analysis in Steps 2-4, the compound that does not undergo hydrolysis among the given options is (b) Acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)).

Key concepts

Salt Hydrolysis

Understanding the concept of salt hydrolysis is crucial for grasping the behavior of salts in aqueous solutions. It refers to the reaction where the salt that forms from an acid-base neutralization reaction further reacts with water, breaking down into its constituent ions and altering the pH of the solution.

For instance, when sodium carbonate (\textbf{Na}\(_2\)CO\(_3\)) dissolves in water, it dissociates into sodium ions (\textbf{Na}\(^+\)) and carbonate ions (CO\(_3$$^{2-}\)). These ions interact with water, leading to a slight change in the concentration of hydrogen (\textbf{H}\(^+\)) or hydroxide (\textbf{OH}\(^-\)) ions in the solution. This can make the solution acidic or basic, depending on the strength of the original acid and base from which the salt was formed.

Salts from strong acids and strong bases, like potassium sulfate (\textbf{K}\(_2\)SO\(_4\)), don't significantly change the pH of the water solution as both the acid and the base are fully ionized, leading to a neutral pH. Conversely, salts from a strong base and a weak acid, or a strong acid and a weak base, will exhibit basic or acidic characteristics, respectively, due to incomplete ionization and the dominance of one type of ion in the water.

Carboxylic Acids

Carboxylic acids, such as acetic acid (\textbf{CH}\(_3\)\textbf{COOH}), have a distinct structure featuring a carboxyl group that is composed of a carbon atom double-bonded to an oxygen atom and singly bonded to a hydroxyl group (\textbf{OH}). This functional group is key to the behavior of carboxylic acids, influencing their ability to engage in hydrogen bonding and their characteristic acidic properties.

Carboxylic acids can donate a hydrogen ion (\textbf{H}\(^+\)) to water, a typical acid behavior, leading to the formation of their conjugate base and the hydronium ion (\textbf{H}\(_3\)O\(^+\)). However, it's important to note that this ionization is not equivalent to hydrolysis, which implies a breakdown into smaller fragments. Instead, acetic acid remains largely intact, with its acid dissociation being a reversible process. Therefore, carboxylic acids like acetic acid don't undergo hydrolysis in the same sense as salts do; they ionize by transferring a proton to water.

Neutralization Reactions

Neutralization reactions are the cornerstone of understanding how salts are formed and their behavior in water. In a neutralization reaction, an acid reacts with a base to produce a salt and water. This reaction is exothermic, releasing energy in the form of heat.

The classic example is the reaction between hydrochloric acid (\textbf{HCl}) and sodium hydroxide (\textbf{NaOH}), which produces common table salt (\textbf{NaCl}) and water (\textbf{H}\(_2\)O). It's exemplified by the equation: \textbf{HCl} + \textbf{NaOH} → \textbf{NaCl} + \textbf{H}\(_2\)\textbf{O}. This reaction type highlights the interchange of ions where the hydrogen ions (\textbf{H}\(^+\)) from the acid combine with the hydroxide ions (\textbf{OH}\(^-\)) from the base to form water.

The produced salt, depending on the acid and base it originated from, may then undergo hydrolysis when dissolved in water. Understanding neutralization reactions is not only fundamental in chemistry but also paramount in various real-world applications, such as treating acidic or basic waste streams and antacid medications, where it provides relief from excess stomach acid.