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

Question

Among \(\mathrm{A} 1_{2} \mathrm{O}_{3}, \mathrm{SiO}_{2}, \mathrm{P}_{2} \mathrm{O}_{3}\) and \(\mathrm{SO}_{2}\) the correct order of acidic strength is (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

Option (a) is correct: \( \mathrm{Al}_2 \mathrm{O}_3 < \mathrm{SiO}_2 < \mathrm{P}_2 \mathrm{O}_3 < \mathrm{SO}_2 \).

Step 1: Understanding Acidic Oxide Strength

Acidic oxides are typically oxides of nonmetals or oxides that can react with water to form an acid. The strength of an acidic oxide generally increases from left to right across a period in the periodic table.

Step 2: Analyze the given compounds

The compounds given are \( \mathrm{Al}_2 \mathrm{O}_3, \mathrm{SiO}_2, \mathrm{P}_2 \mathrm{O}_3, \) and \( \mathrm{SO}_2 \). \( \mathrm{Al}_2 \mathrm{O}_3 \) is an amphoteric oxide, \( \mathrm{SiO}_2 \) is a weak acidic oxide, \( \mathrm{P}_2 \mathrm{O}_3 \) is a moderately strong acidic oxide, and \( \mathrm{SO}_2 \) is a strong acidic oxide.

Step 3: Arrange the oxides by increasing acidic strength

From our periodic table knowledge, in general, the acidic strength increases from left to right across a period. Thus, the increasing order is: \( \mathrm{Al}_2 \mathrm{O}_3 < \mathrm{SiO}_2 < \mathrm{P}_2 \mathrm{O}_3 < \mathrm{SO}_2 \).

Step 4: Choose the correct answer

The correct order of increasing acidic strength corresponds with option (a): \( \mathrm{Al}_2 \mathrm{O}_3 < \mathrm{SiO}_2 < \mathrm{P}_2 \mathrm{O}_3 < \mathrm{SO}_2 \).

Key Concepts

Periodic Table Trends

Understanding the trends in the periodic table can greatly help in predicting the properties of elements and their compounds. As you move from left to right across a period in the periodic table, elements tend to become more electronegative. This means they have a greater tendency to attract electrons. This increase in electronegativity also correlates with the increase in acidic character of oxides.
In the context of oxides, non-metal oxides on the right side of a period tend to be acidic because these non-metals attract and hold onto electrons tightly, making them more effective at forming acids.
Conversely, metal oxides on the left side are often basic, as metals tend to lose electrons more easily.

• The acidic strength of oxides increases as you move from left to right across a period.
• The oxides of metals usually form basic solutions in water, whilst non-metal oxides tend to form acidic solutions.

Amphoteric Oxides

Amphoteric oxides are fascinating substances because they can react both as an acid and as a base. This dual nature makes them quite unique and gives them a specific place on the periodic table.
One common example is \(\mathrm{Al}_{2} \mathrm{O}_{3}\), which is an oxide of aluminum. Aluminum is located between the metals and non-metals on the periodic table, making it capable of forming an oxide that shows both acidic and basic properties.
An amphoteric oxide can neutralize both acids and bases leading to salt and water formation in both cases.

• This behavior is important particularly in understanding reactions in complex chemical processes.
• In industrial applications, amphoteric oxides like \(\mathrm{Al}_{2} \mathrm{O}_{3}\) are used because they can react in flexible ways depending on the requirements.

Acidic Oxides

Acidic oxides are the oxides of nonmetals and have a mark of forming acids when they react with water. These oxides are generally formed when nonmetals bind with oxygen, an electronegative atom that pulls electrons towards itself, enabling the oxide to accept electron pairs and hence act as an acid.
Silicon dioxide \(\mathrm{SiO}_{2}\) is a weak acidic oxide, while sulfur dioxide \(\mathrm{SO}_{2}\) is a strong acidic oxide. The difference in their strength helps in explaining their chemical behavior.

• Strong acidic oxides like \(\mathrm{SO}_{2}\) react more readily with water to form acids such as sulfurous acid (\(\mathrm{H}_{2}\mathrm{SO}_{3}\)).
• Weaker acidic oxides like \(\mathrm{SiO}_{2}\), on the other hand, do not react with water easily but might react under extreme conditions or with strong bases.

These properties are largely determined by the position of the parent element in the periodic table and their inherent electronegativities.