Question

Specify which of the following salts will undergo hydrolysis: \(\mathrm{KF}, \mathrm{NaNO}_{3}, \mathrm{NH}_{4} \mathrm{NO}_{2}, \mathrm{MgSO}_{4}, \mathrm{KCN},\) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COONa}, \mathrm{RbI}, \mathrm{Na}_{2} \mathrm{CO}_{3}, \mathrm{CaCl}_{2}, \mathrm{HCOOK}\)

Short answer

NH₄NO₂, KCN, C₆H₅COONa, Na₂CO₃, and HCOOK will undergo hydrolysis.

Step-by-step solution

Understanding Hydrolysis

Hydrolysis of a salt occurs when the salt consists of an acidic cation or a basic anion. In other words, it's more likely when one of the ions formed upon dissolution has a tendency to react with water to form either an acidic or basic solution.

Identify Salts with Basic Anions

Determine if the anion of the salt is derived from a weak acid. If yes, the anion will undergo hydrolysis. Example anions that cause basic hydrolysis include CN⁻ from KCN and \( ext{CO}_3^{2-} \) from \( ext{Na}_2 ext{CO}_3 \).

Identify Salts with Acidic Cations

Check if the cation is derived from a weak base. An example of this is NH₄⁺ from NH₄NO₂, which can hydrolyze because NH₄⁺ comes from the weak base NH₃.

Evaluate Each Salt

Evaluate each salt to classify if it will undergo hydrolysis: - KF: Formed from a strong base (KOH) and a strong acid (HF); does not hydrolyze. - NaNO₃: Formed from a strong base (NaOH) and a strong acid (HNO₃); does not hydrolyze. - NH₄NO₂: Formed from a weak base (NH₃), hydrolyzes. - MgSO₄: Strong base (Mg(OH)₂) and strong acid (H₂SO₄), limited hydrolysis due to Mg²⁺. - KCN: Formed from a strong base (KOH) and weak acid (HCN); undergoes hydrolysis. - C₆H₅COONa (sodium benzoate): Weak acid anion (C₆H₅COO⁻); undergoes hydrolysis. - RbI: Formed from strong base (RbOH) and strong acid (HI); does not hydrolyze. - Na₂CO₃: Weak acid anion (CO₃²⁻); undergoes hydrolysis. - CaCl₂: Strong base (Ca(OH)₂) and strong acid (HCl); does not hydrolyze. - HCOOK: Formed from strong base (KOH) and weak acid (HCOOH); undergoes hydrolysis.

Conclusion

From the evaluation in Step 4, the salts that will undergo hydrolysis are NH₄NO₂, KCN, C₆H₅COONa, Na₂CO₃, and HCOOK.

Key concepts

Weak Acid Anions

When discussing hydrolysis in salt solutions, weak acid anions often play a critical role. An anion is derived from an acid, and if this acid is weak, its conjugate base (the anion) is relatively strong. This strength allows it to react with water, leading to hydrolysis. During this reaction, the anion accepts a proton from water, forming hydroxide ions and increasing the solution's pH.

Consider carbonate ion \(\text{CO}_3^{2-}\) from \(\text{Na}_2\text{CO}_3\). This ion arises from carbonic acid, a weak acid. Therefore, \(\text{Na}_2\text{CO}_3\) undergoes hydrolysis, resulting in a basic solution. Understanding how these weak acid anions behave in aqueous solutions helps in predicting the outcome or pH level of the solution. Other examples include benzoate \(\text{C}_6\text{H}_5\text{COO}^-\) in sodium benzoate \(\text{C}_6\text{H}_5\text{COONa}\). When these anions react with water, they increase the hydroxide ion concentration, creating a basic environment.

Acidic Cations

Acidic cations are cations that can cause a salt to hydrolyze in water, resulting in an acidic solution. These cations usually come from weak bases. When such a cation dissolves in water, it tends to donate protons to the water molecules.

Take the example of \(\text{NH}_4^+\), which is derived from the weak base ammonia (\(\text{NH}_3\)). When this ammonium ion is in solution, it reacts with water to form ammonium hydroxide and hydronium ions, making the solution acidic.

In any hydrolysis analysis, identifying whether the salt's cation originates from a weak base is key. These acidic cations react with water in a way that increases the concentration of hydronium ions, thus decreasing the solution's pH.

Basic Anions

Basic anions are formed when a salt dissolves in water, and they largely influence the salt’s ability to undergo hydrolysis and become basic. These anions typically stem from the dissociation of weak acids.

One such example is the cyanide ion \(\text{CN}^-\) from potassium cyanide (\(\text{KCN}\)). Here, \(\text{CN}^-\) comes from hydrogen cyanide, a weak acid. The weak acid's conjugate base, \(\text{CN}^-\), is strong enough to react with water, consuming protons and releasing hydroxide ions.

As these basic anions react with water, they move the equilibrium towards the production of hydroxide ions, elevating the pH and making the solution basic. The presence of basic anions is a strong indicator that hydrolysis will occur, resulting in an increased pH level.

Salt Solution Chemistry

Salt solution chemistry deals with the behavior of salts when they dissolve in water, particularly focusing on whether they undergo hydrolysis. The composition of the salt determines this.

In general, salts formed from strong acids and bases will not hydrolyze, and the solution remains neutral. However, if a salt forms from a weak acid or a weak base, hydrolysis is possible.

For example:

• Salts like \(\text{NaCl}\), from strong acid and bases, do not affect the pH significantly.
• Salts, such as \(\text{NH}_4\text{NO}_2\), where the cation comes from a weak base, tend to make the solution acidic.
• Salts like \(\text{Na}_2\text{CO}_3\), with anions from a weak acid, produce a basic solution.

Understanding the origin of the ions can help predict the resulting pH when they interact in water. It's this balance—or imbalance—between the acid and base components of the salt that dictates whether hydrolysis will occur and the nature of the resulting solution.