Problem 33 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct statement(s) about the oxoacids, \(\mathrm{HClO}_{4}\) and \(\mathrm{HClO}\), is (are) [Adv. 2017] (a) The central atom in both \(\mathrm{HClO}_{4}\) and \(\mathrm{HClO}\) is \(s p^{3}\) hybridized (b) \(\mathrm{HClO}_{4}\) is more acidic than HClO because of the resonance stabilization of its anion (c) \(\mathrm{HClO}_{4}\) is formed in the reaction between \(\mathrm{Cl}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) (d) The conjugate base of \(\mathrm{HClO}_{4}\) is weaker base than \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Statements (a), (b), and (d) are correct.

Step 1: Determine the Hybridization of the Central Atom

For both \(\text{HClO}_4\) and \(\text{HClO}\), the central atom is chlorine (Cl). In \(\text{HClO}_4\), Cl is bonded to four oxygen atoms and has no lone pairs, which makes it \(sp^3\) hybridized. In \(\text{HClO}\), Cl is bonded to one oxygen atom and has two lone pairs, which also results in \(sp^3\) hybridization. Thus, statement (a) is correct.

Step 2: Compare Acidic Strengths and Resonance

Perchloric acid (\(\text{HClO}_4\)) is a stronger acid compared to hypochlorous acid (\(\text{HClO}\)) due to the resonance stabilization of the \(\text{ClO}_4^-\) anion. The \(\text{ClO}_4^-\) anion has resonance structures that distribute the negative charge over four oxygen atoms. In contrast, the \(\text{ClO}^-\) anion does not have such resonance stabilization. Therefore, statement (b) is correct.

Step 3: Evaluate Formation Reaction

\(\text{HClO}_4\) is not directly formed by the reaction between \(\text{Cl}_2\) and \(\text{H}_2\text{O}\). This reaction typically forms hydrochloric acid (\(\text{HCl}\)) and hypochlorous acid (\(\text{HClO}\)). Hence, statement (c) is incorrect.

Step 4: Determine Relative Strength of Conjugate Bases

The conjugate base of \(\text{HClO}_4\) is \(\text{ClO}_4^-\). This ion is highly stable due to resonance and is a very weak base because \(\text{HClO}_4\) is a strong acid. In comparison, \(\text{H}_2\text{O}\) is a weak acid, and \(\text{OH}^-\) is a stronger base than \(\text{ClO}_4^-\). Therefore, statement (d) is correct.

Key Concepts

Acidic Strength

Understanding acidic strength is fundamental in chemistry, especially when comparing acids like \(\text{HClO}_4\) and \(\text{HClO}\). Acidic strength refers to an acid's ability to donate protons (\(\text{H}^+\)) in a solution. A stronger acid dissociates more in water, increasing the concentration of protons. - **\(\text{HClO}_4\):** Known as perchloric acid, it is a very strong acid, almost completely ionizing in water.- **\(\text{HClO}\):** Known as hypochlorous acid, it is weaker, less ionized in solution.The difference in acidic strength can significantly impact chemical reactions. Strong acids like \(\text{HClO}_4\) are more reactive, while weaker acids like \(\text{HClO}\) are less reactive in acid-base reactions. This is crucial in predicting the outcomes of reactions involving these acids.

Resonance Stabilization

Resonance stabilization plays a key role in determining the stability of anions formed after proton donation, which in turn affects acidic strength. - **Resonance structures** exist when there are multiple ways to distribute electrons in a molecule. This increases the stability because the true structure is a hybrid of all possible resonance forms. Perchlorate ion (\(\text{ClO}_4^-\)) has resonance. The negative charge from the ion is spread out across four oxygen atoms, thus stabilizing the ion through resonance. The hypochlorite ion (\(\text{ClO}^-\)), does not have such resonance stabilization. The negative charge is localized on a single oxygen atom, making it less stable compared to the resonating structure of the perchlorate ion. Thus, \(\text{HClO}_4\) is more acidic than \(\text{HClO}\) since its conjugate base is significantly more stable.

Oxoacids

Oxoacids are a group of acids that contain hydrogen, oxygen, and another element (typically a non-metal). In our case, chlorine is the central atom in both \(\text{HClO}_4\) and \(\text{HClO}\). Each oxoacid's strength is influenced by:- **Number of Oxygen Atoms:** More oxygen atoms mean greater acidic strength due to increased electron-withdrawing power, which stabilizes the conjugate base.- **Electronegative Central Atom:** Chlorine, being electronegative, helps draw electron density away from the hydrogen-oxygen bond, allowing easier release of the proton.Thus, \(\text{HClO}_4\) is much stronger because of its additional oxygen atoms, which optimally stabilize the resulting conjugate base through resonance.

Conjugate Base

The concept of a conjugate base is critical when discussing acidity. When an acid donates a proton, the remainder of the molecule is the conjugate base. - **Conjugate Bases:** \(\text{ClO}_4^-\) (from \(\text{HClO}_4\)) vs. \(\text{ClO}^-\) (from \(\text{HClO}\)). A strong acid has a weak conjugate base. This rule helps explain why the \(\text{ClO}_4^-\) ion is such a weak base compared to \(\text{OH}^-\). The strong acidity of \(\text{HClO}_4\) leaves a very stable ion behind, making it less likely to re-accept a proton.Oxoacids like \(\text{HClO}_4\) with a highly stable conjugate base due to resonance, show minimal basic characteristics. Understanding conjugate bases is essential to grasp the reactivity and properties of various acids.