Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of relative acidity is (a) \(\mathrm{HClO}>\mathrm{HClO}_{2}>\mathrm{HClO}_{3}>\mathrm{HClO}_{4}\) (b) \(\mathrm{HClO}_{4}>\mathrm{HClO}_{3}>\mathrm{HClO}_{2}>\mathrm{HClO}\) (c) \(\mathrm{HClO}>\mathrm{HClO}_{4}>\mathrm{HClO}_{2}>\mathrm{HClO}_{3}\) (d) \(\mathrm{HClO}_{3}>\mathrm{HClO}_{2}>\mathrm{HClO}_{4}>\mathrm{HClO}\)

Step-by-Step Solution

Answer

The correct order is (b) \( \mathrm{HClO}_4 > \mathrm{HClO}_3 > \mathrm{HClO}_2 > \mathrm{HClO} \).

Step 1: Identify the Chemical Species

We need to compare the relative acidity of four oxoacids of chlorine: Hypochlorous acid (\( \mathrm{HClO} \)), Chlorous acid (\( \mathrm{HClO}_2 \)), Chloric acid (\( \mathrm{HClO}_3 \)), and Perchloric acid (\( \mathrm{HClO}_4 \)).

Step 2: Understand the Effect of Oxygen in Acidity

In oxoacids of chlorine, the acidity increases with the increase in the number of oxygen atoms. This is because oxygen atoms increase the acid's ability to stabilize the negative charge of the conjugate base via resonance.

Step 3: Order the Acids by Number of Oxygen Atoms

Order the acids by increasing number of oxygen atoms: \( \mathrm{HClO} \) (1 oxygen), \( \mathrm{HClO}_2 \) (2 oxygens), \( \mathrm{HClO}_3 \) (3 oxygens), \( \mathrm{HClO}_4 \) (4 oxygens).

Step 4: Establish the Relationship Between Oxygen Atoms and Acidity

The more oxygen atoms present, the higher the acidity. Thus, \( \mathrm{HClO}_4 \) is the strongest acid, and \( \mathrm{HClO} \) is the weakest.

Step 5: Select the Correct Answer Based on Acidity Trend

According to the trend, the order of acidity from strongest to weakest is \( \mathrm{HClO}_4 > \mathrm{HClO}_3 > \mathrm{HClO}_2 > \mathrm{HClO} \). This matches option (b).

Key Concepts

Chlorine Oxoacids

Chlorine oxoacids are a family of acids derived from chlorine combined with oxygen. This family includes several key members, such as Hypochlorous acid (\( \mathrm{HClO} \)), Chlorous acid (\( \mathrm{HClO}_2 \)), Chloric acid (\( \mathrm{HClO}_3 \)), and Perchloric acid (\( \mathrm{HClO}_4 \)). Each of these acids contains chlorine atoms bonded to varying numbers of oxygen atoms. As the number of oxygen atoms increases, so does the oxidation state of chlorine.
Chlorine oxoacids are important in chemistry due to their unique properties and varying strengths. They tend to be more acidic compared to other acids with similar molecular weights. This behavior mainly results from the presence of highly electronegative oxygen atoms, which influences their overall stability and acidity.
Understanding the relationship between the structure of chlorine oxoacids and their acidity is crucial in both academic studies and practical applications. This knowledge helps in predicting reactivities and guiding chemical synthesis.

Effect of Oxygen on Acidity

The number of oxygen atoms plays a critical role in determining the acidity of chlorine oxoacids. Oxygen atoms are highly electronegative, which allows them to pull electron density away from the other atoms in the molecule. This ability helps stabilize the negative charge that forms on the conjugate base after the acid donates a proton (\( H^+ \)).
When we compare acids, those with more oxygen atoms often form more stable conjugate bases. The extra oxygen atoms can spread out the negative charge more effectively. This stabilization happens through resonance, where the electrons can be "delocalized" or spread out across several atoms.
Therefore, the more oxygen atoms there are, the stronger the acid typically will be. Perchloric acid (\( \mathrm{HClO}_4 \)), having four oxygen atoms, is the strongest among chlorine oxoacids. Its conjugate base is most stabilized, making it more willing to donate a proton. This property illustrates why more oxygen within an acid molecule usually correlates with increased acidity.

Conjugate Base Resonance

The concept of resonance plays a vital role in the strength of acids, especially in chlorine oxoacids. When an oxoacid like \( \mathrm{HClO}_4 \) dissociates, it forms a conjugate base (\( \mathrm{ClO}_4^- \)) that enjoys great stabilization due to resonance.
Resonance involves the electron cloud being distributed over more than one atom. In the conjugate base of perchloric acid, the negative charge can be spread across all four oxygen atoms. This dispersion minimizes any single location carrying the entire negative charge, which significantly stabilizes the anion.
This stability makes the initial acid more likely to lose a proton, increasing its strength. In simple terms, if the conjugate base is stable due to resonance, the parent acid is typically stronger. Hence, this is why perchloric acid, with its well-resonated conjugate base, is a very strong acid. Conjugate base resonance is a core concept that links structure to acidity, especially in a series like the chlorine oxoacids.

Relative Acidity Order

Understanding the order of acidity among chlorine oxoacids is essential. Generally, the more oxygen atoms an oxoacid has, the higher its acidity level. The correct order of acidity from strongest to weakest in these acids is:

• Perchloric acid (\( \mathrm{HClO}_4 \))
• Chloric acid (\( \mathrm{HClO}_3 \))
• Chlorous acid (\( \mathrm{HClO}_2 \))
• Hypochlorous acid (\( \mathrm{HClO} \))

This order is not coincidental. It stems from the stabilizing effect additional oxygen atoms provide through resonance, which aids in the even distribution of the negative charge in the conjugate base.
By understanding this pattern, students can predict acid strength more confidently. This knowledge further aids in various applications, from chemical synthesis to environmental science, where controlling acidity levels is crucial. Hence, mastering the concept of relative acidity order not only boosts academic understanding but also enhances practical chemical application skills.