Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The nature of an aqueous solution of \(\mathrm{FeCl}_{2}\) is (a) basic (b) acidic (c) amphoteric (d) neutral

Step-by-Step Solution

Answer

(b) acidic

Step 1: Identify the Hydrolysis

When \(fFeCl_{2}\) is dissolved in water, it dissociates into \(fFe^{2+}\) and \(fCl^{-}\) ions. To determine the nature of the solution, we consider the possibility of hydrolysis of these ions in water.

Step 2: Analyze Anion Hydrolysis

The \(\dacu fCl^{-}\) ion comes from hydrochloric acid (HCl), which is a strong acid. Strong acid anions do not undergo hydrolysis and do not affect the pH of the solution.

Step 3: Analyze Cation Hydrolysis

The \(\dfc fFe^{2+}\) ion can undergo hydrolysis in water. \(qFe^{2+} + 2H_2O ightarrow Fe(OH)_2 + 2H^+\). This reaction produces hydrogen ions (H\(^{+}\)), making the solution acidic.

Step 4: Conclusion about the Solution

Since the hydrolysis of \(fFe^{2+}\) releases \(\dacu H^+\) ions into the solution, it results in an increase in acidity. Therefore, the solution is acidic.

Key Concepts

Cation Hydrolysis

Cation hydrolysis occurs when a metal ion interacts with water, transforming into a hydroxide and releasing hydrogen ions. This process often happens with cations from weak bases. In the case of iron(II) chloride (\(\text{FeCl}_2\)), when it dissolves in water, it separates into \(\text{Fe}^{2+}\) and \(\text{Cl}^-\) ions. The \(\text{Fe}^{2+}\) cation can undergo hydrolysis. When \(\text{Fe}^{2+}\) undergoes this process, it forms \(\text{Fe(OH)}_2\) along with releasing \(\text{H}^+\) ions: \[ \text{Fe}^{2+} + 2\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_2 + 2\text{H}^+ \]The hydrogen ions make the solution acidic. This is typical for metal cations originating from transition metals, as they can create slightly acidic solutions. Always remember, when considering cation hydrolysis, the origin of the metal ion is crucial for predicting the pH changes involved.

Anion Hydrolysis

Anion hydrolysis involves anions reacting with water, sometimes affecting the pH of the solution. However, not all anions undergo hydrolysis. For anions derived from strong acids, like hydrochloric acid, hydrolysis does not occur, avoiding any pH change in the solution. For instance, the chloride ion (\(\text{Cl}^-\)) from \(\text{FeCl}_2\) originates from a strong acid—hydrochloric acid (HCl). This is important because ions from strong acids typically do not interact with water to alter the pH:- **Strong acid anions**: Usually do not hydrolyze- **Anions from weak acids**: More likely to hydrolyze and alter pHIn the hydrolysis consideration of \(\text{FeCl}_2\), the \(\text{Cl}^-\) anion does not undergo hydrolysis and therefore does not affect the acidity or basicity of the solution. Only the metal ion's behavior needs assessment for understanding pH changes.

pH of Solutions

The pH of a solution is a measure of its acidity or basicity. It is determined by the concentration of hydrogen ions (\(\text{H}^+\)) in the solution. Solutions can be classified based on their pH:- **Acidic solutions**: Have a pH less than 7, with higher \(\text{H}^+\) concentration- **Neutral solutions**: Have a pH equal to 7- **Basic solutions**: Have a pH greater than 7In the context of \(\text{FeCl}_2\), cation hydrolysis leads to an acidic pH due to the release of \(\text{H}^+\) ions:- **Cation Hydrolysis** \(\text{Fe}^{2+} + 2\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_2 + 2\text{H}^+\), results in increased acidity.Understanding the sources of ions—even those that do not affect the pH like chloride anion—helps one comprehend how different substances affect the pH when dissolved in water. This knowledge helps predict the behavior of various compounds in aqueous solutions, crucial for chemistry and many real-world applications.