Problem 148 - A Complete Resource Book in Chemistry for JEE Main

Question

Among \(\mathrm{Al}_{2} \mathrm{O}_{3}, \mathrm{SiO}_{2}, \mathrm{P}_{2} \mathrm{O}_{3}\) and \(\mathrm{SO}_{2}\) the correct order of acidic strength is [2004] (a) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SO}_{2}\) (b) \(\mathrm{SO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{Al}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{SiO}_{2}<\mathrm{SO}_{2}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{P}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{SO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The correct order is (a) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SO}_{2}\).

Step 1: Understand Oxide Nature

Metal oxides are generally basic or amphoteric, while non-metal oxides are acidic. Identify each compound: \(\mathrm{Al}_{2} \mathrm{O}_{3}\) is amphoteric, \(\mathrm{SiO}_{2}\), \(\mathrm{P}_{2} \mathrm{O}_{3}\), and \(\mathrm{SO}_{2}\) are acidic as they are non-metal oxides.

Step 2: Review Acidic Strength Trend

In non-metal oxides, acidic strength increases with the non-metal's electronegativity. In the sequence, \(\mathrm{P}_{2} \mathrm{O}_{3}\) is less acidic than \(\mathrm{SO}_{2}\) due to phosphorus being less electronegative than sulfur.

Step 3: Consider Electronegativity of Non-metals

Sulfur has higher electronegativity compared to phosphorus and silicon, thus \(\mathrm{SO}_{2}\) is more acidic than \(\mathrm{P}_{2} \mathrm{O}_{3}\) and \(\mathrm{SiO}_{2}\).

Step 4: Compare Amphoteric Oxide

\(\mathrm{Al}_{2} \mathrm{O}_{3}\) is amphoteric and generally less acidic than non-metal oxides like \(\mathrm{SO}_{2}\), \(\mathrm{P}_{2} \mathrm{O}_{3}\), and \(\mathrm{SiO}_{2}\).

Step 5: Determine Correct Order

From steps above, order them from least to most acidic: \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SO}_{2}\). This matches option (a).

Key Concepts

Metal and Non-Metal Oxides

Oxides are compounds that contain oxygen and another element. Metal oxides usually form when metals react with oxygen. They are typically solid at room temperature and often exhibit basic properties, meaning they can neutralize acids.
For instance, sodium oxide and calcium oxide react with water to produce alkaline solutions.

• Metal oxides often form ionic compounds.
• They tend to be basic or amphoteric, where the latter can react with both acids and bases.

On the other hand, non-metal oxides are formed when non-metals combine with oxygen. These oxides usually exhibit acidic properties and react with water to produce acidic solutions.

• Examples include carbon dioxide and sulfur dioxide.
• Non-metal oxides either form covalent bonds or polymeric structures.

Recognizing whether an oxide is from a metal or non-metal helps us predict its chemical behavior.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons in a bond. Within the periodic table, electronegativity increases across a period and decreases down a group.
Sulfur, found in the same period as phosphorus, is more electronegative. This higher electronegativity means sulfur can more effectively pull electrons towards itself when bonded to oxygen, forming a stronger acidic oxide, like sulfur dioxide (\( \text{SO}_2 \)).
A few key points to remember about electronegativity:

• The higher the electronegativity of the non-metal, the stronger its oxide's acidic properties.
• Fluorine is the most electronegative element.

This concept can directly influence the acidic strength of non-metal oxides because the more electronegative a non-metal is, the more acidic its oxide might be.

Amphoteric Oxides

Amphoteric oxides are special because they can react with both acids and bases to form salts and water. This dual behavior makes them unique compared to purely acidic or basic oxides.
Aluminum oxide (\( \text{Al}_2\text{O}_3 \)) is a common example of an amphoteric oxide. It can react with hydrochloric acid to form aluminum chloride and with sodium hydroxide to form sodium aluminate.
Characterizing features of amphoteric oxides include:

• Not a strong acid or base itself but capable of reacting as either.
• Found mostly in oxides of metalloid or some metal elements.

Knowing an oxide is amphoteric helps to predict its reactions depending on the reactant's nature—acidic or basic.

Acidic Strength Comparison

When comparing the acidic strength of different oxides, it is important to consider two primary factors: the element's electronegativity and whether the oxide is metal, non-metal, or amphoteric.
For instance, in the sequence provided: \( \text{Al}_2\text{O}_3 \), \( \text{SiO}_2 \), \( \text{P}_2\text{O}_3 \), and \( \text{SO}_2 \), silicium, phosphorus, and sulfur are non-metals, leading to more acidic oxides compared to \( \text{Al}_2\text{O}_3 \), an amphoteric oxide.
The order from least to most acidic is:

• \( \text{Al}_2\text{O}_3 \) (amphoteric)
• \( \text{SiO}_2 \) (less acidic non-metal oxide)
• \( \text{P}_2\text{O}_3 \) (moderate acidic non-metal oxide)
• \( \text{SO}_2 \) (most acidic due to high electronegativity of sulfur)

Understanding these factors helps to determine how different substances will behave in a chemical environment in terms of acidity.