Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

The most acidic oxide is (a) \(\mathrm{MgO}\) (b) \(\mathrm{CaO}\) (c) \(\mathrm{Na}_{2} \mathrm{O}\) (d) \(\mathrm{Al}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The most acidic oxide is \(\text{Al}_{2}\text{O}_{3}\).

Step 1: Understand the Concept of Acidic Oxides

Acidic oxides are oxides that react with water to form an acid or with a base to form a salt. Typically, acidic oxides are non-metal oxides. An example is carbon dioxide which forms carbonic acid when reacted with water.

Step 2: Identify Each Option's Nature

Analyze each given oxide option to determine if it's a metallic or non-metallic oxide: - \(\text{MgO}\), \(\text{CaO}\), and \(\text{Na}_{2}\text{O}\) are basic oxides, formed from metals. - \(\text{Al}_{2}\text{O}_{3}\) is amphoteric, which means it can react as an acid or a base.

Step 3: Evaluate Which Option is Most Acidic

Between acidic and amphoteric oxides, the more non-metallic in character, the more acidic it tends to be. Since none of the options is a simple nonmetal oxide, we look for amphoteric ones like \(\text{Al}_{2}\text{O}_{3}\), which can act as an acid with bases due to its particular position along the periodic table's metal-nonmetal divide.

Step 4: Conclusion

The most acidic option among the given is \(\text{Al}_{2}\text{O}_{3}\), being amphoteric, it behaves more closely to acidic oxides when compared to purely basic metal oxides such as \(\text{MgO}\), \(\text{CaO}\), and \(\text{Na}_{2}\text{O}\).

Key Concepts

Amphoteric Oxides

Amphoteric oxides are a unique class of compounds that have dual personality traits. They can behave as either acids or bases depending on the surrounding chemical environment. This dual ability comes from their chemical structure, which allows them to participate in acid-base reactions from both sides.

For instance, Aluminum oxide (Al₂O₃) is a classic example of an amphoteric oxide. It can react with strong acids to form salts and water, acting like a base. Conversely, it can also react with strong bases to form salts and water, taking on an acidic role. The behavior of amphoteric oxides makes them particularly interesting as they exhibit adaptability based on their environment.

• React with acids to produce salts typical of a basic oxide.
• React with bases to produce salts typical of an acidic oxide.

In practical applications, this ability allows amphoteric oxides to be used in numerous industrial processes where variable pH conditions are required.

Metallic Oxides

Metallic oxides are compounds formed between metals and oxygen. They are known for their basic nature, meaning they tend to react with acids to produce salts and water. Common examples of metallic oxides include magnesium oxide (MgO), calcium oxide (CaO), and sodium oxide (Na₂O).

These oxides usually exhibit the following characteristics:

• They are typically solid at room temperature.
• They have high melting and boiling points due to strong ionic bonds.
• Their reactions with acids are exothermic, releasing a significant amount of heat.

Most metallic oxides are insoluble in water, but the ones that do dissolve form alkaline solutions, also known as bases. These properties make them useful in a wide range of applications, from construction to agriculture, highlighting their importance in both everyday life and industrial processes.

Periodic Table Trends

The periodic table is a powerful tool in understanding chemical behaviors and trends. One crucial trend is how the acidity and basicity of oxides change across periods and down groups.

As you move from left to right across a period in the periodic table, the nature of oxides changes from basic to acidic. This shift results from increasing electronegativity and decreasing metallic character. On the left, you find metallic oxides, which are generally basic. As you move right, toward the nonmetals, acidic oxides become more prevalent.

• Metals on the left usually form basic oxides.
• Metalloids and nonmetals on the right form acidic oxides.

When moving down a group, the basicity of oxides tends to increase. However, the behavior of metalloids like aluminum (found along the stair-step line separating metals from nonmetals) is particularly enlightening. Elements like aluminum exhibit amphoteric behavior due to their intermediate positioning within periodic trends. Understanding these trends is key in predicting the chemical behavior of elements and their compounds.