Question

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

Short answer

(d) \(\mathrm{Al}_2 \mathrm{O}_3\) is the most acidic oxide.

Step-by-step solution

Understanding Acidic Oxides

Acidic oxides are formed from nonmetals; they react with water to create acids and with bases to form salts. Many metal oxides, particularly those of alkali and alkaline earth metals, tend to form basic oxides, while oxides of nonmetals are often acidic.

Categorizing the Given Options

Let's review each option:- (a) \(\text{MgO}\) is the oxide of magnesium, an alkaline earth metal, which is basic.- (b) \(\text{CaO}\) is the oxide of calcium, an alkaline earth metal, also basic.- (c) \(\text{Na}_2\text{O}\) is the oxide of sodium, an alkali metal, and basic.- (d) \(\text{Al}_2\text{O}_3\) is the oxide of aluminum, which is amphoteric, meaning it has both acidic and basic properties.

Identifying the Most Acidic Oxide

Since \(\text{MgO}\), \(\text{CaO}\), and \(\text{Na}_2\text{O}\) are basic oxides, they do not qualify as the most acidic. The amphoteric oxide \(\text{Al}_2\text{O}_3\) can behave as an acid when reacting with bases. Therefore, among the given choices, \(\text{Al}_2\text{O}_3\) is the most acidic.

Key concepts

Amphoteric Oxides

Amphoteric oxides are fascinating because they display dual properties, behaving both as acids and as bases depending on their reacting partner. These oxides can react with acids to form salt and water, and they can also react with bases under certain conditions. One prominent example is aluminum oxide,

• This oxide, represented by the formula \(\text{Al}_2\text{O}_3\), can neutralize an acid or a base.
• When it reacts with a strong base, such as sodium hydroxide (\(\text{NaOH}\)), it forms sodium aluminate (\(\text{NaAlO}_2\)) and water.
• Conversely, when it reacts with a strong acid like hydrochloric acid (\(\text{HCl}\)), it produces aluminum chloride (\(\text{AlCl}_3\)) and water.

These dual characteristics make amphoteric oxides very adaptable and versatile in various chemical reactions.
They are not only important in industrial chemistry but also play a significant role in natural processes, such as neutralizing acidic soils.

Basic Oxides

Basic oxides are formed predominantly from metals, especially those in the alkali and alkaline earth metal groups. These oxides readily react with water to form hydroxides, which are basic in nature. Let's explore some key features:

• Common examples include magnesium oxide (\(\text{MgO}\)) and calcium oxide (\(\text{CaO}\)).
• These oxides dissolve in water to produce alkaline solutions such as \(\text{Mg(OH)}_2\) and \(\text{Ca(OH)}_2\).
• They react with acids to form salts and water, undergoing a neutralization process.

Consider magnesium oxide; it neutralizes acids like sulfuric acid to produce magnesium sulfate and water. This characteristic is why basic oxides are often used in neutralizing acidic industrial wastes and in treating acidic soils in agriculture.

Nonmetal Oxides

Nonmetal oxides are typically acidic, as they often form acids when reacted with water. These oxides are composed primarily of elements located to the right of the periodic table. Here's how they function:

• Common examples are carbon dioxide (\(\text{CO}_2\)) and sulfur trioxide (\(\text{SO}_3\)).
• When these oxides react with water, they yield acids like carbonic acid (\(\text{H}_2\text{CO}_3\)) and sulfuric acid (\(\text{H}_2\text{SO}_4\)).
• They react with bases to produce salts and water.

For instance, sulfur trioxide combines with water to produce sulfuric acid. When reacting with sodium hydroxide, it forms sodium sulfate and water.
Understanding these reactions is critical, especially in environmental science, as they play a major role in phenomena such as acid rain formation.