S3.1.5 Metallic and non-metallic oxides Notes

Metal Oxides, Non-Metal Oxides, and Amphoteric Oxides

Basic Oxides: Metal Oxides Reacting with Water

1. Metal oxides, commonly formed by metals in Groups 1 and 2 of the periodic table, are referred to as basic oxides.
2. They earn this name because their reaction with water produces hydroxides, which are alkaline in nature, increasing the pH of the solution.

Reaction of Metal Oxides with Water

1. When a basic oxide reacts with water, it forms a hydroxide.
2. For example, sodium oxide $ \text{Na}_2\text{O} $ reacts with water to produce sodium hydroxide:$$
   \text{Na}_2\text{O} (s) + \text{H}_2\text{O} (l) \rightarrow 2\text{NaOH} (aq)
   $$
3. This reaction releases a significant amount of heat (exothermic) and results in a strongly basic solution due to the dissociation of sodium hydroxide into sodium ions $ \text{Na}^+ $ and hydroxide ions $ \text{OH}^- $.

Group Trends in Basic Oxides

1. Group 1 Oxides (Alkali Metals): Oxides such as $\text{Li}_2\text{O} $, $ \text{Na}_2\text{O} $, and ($\text{K}_2\text{O} $ react readily with water to form strong bases.
2. Group 2 Oxides (Alkaline Earth Metals): Oxides like $ \text{MgO} $ and $ \text{CaO} $ also form hydroxides, though their reactions with water are less vigorous compared to Group 1 oxides.

Acidic Oxides: Non-Metal Oxides Reacting with Water

1. On the opposite end of the spectrum, non-metal oxides react with water to form acidic solutions.
2. These oxides are typically found among elements on the right-hand side of the periodic table (e.g., Groups 14–17).

Reaction of Non-Metal Oxides with Water

1. Non-metal oxides react with water to produce acids.
2. For instance, carbon dioxide $ \text{CO}_2 $ reacts with water to form carbonic acid:$$
   \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{H}_2\text{CO}_3 (aq)
   $$
3. Similarly, sulfur dioxide $ \text{SO}_2 $ reacts with water to form sulfurous acid:$$
   \text{SO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{H}_2\text{SO}_3 (aq)
   $$

Group Trends in Acidic Oxides

1. Group 14 Oxides: Carbon dioxide $ \text{CO}_2 $ is acidic, while silicon dioxide $ \text{SiO}_2 $ is also acidic but does not dissolve readily in water.
2. Group 16 Oxides: Sulfur oxides $ \text{SO}_2 $, $ \text{SO}_3 $ form acids such as sulfurous acid $ \text{H}_2\text{SO}_3 $ and sulfuric acid $ \text{H}_2\text{SO}_4 $.

Amphoteric Oxides: Both Acidic and Basic Behavior

1. Some oxides, such as aluminum oxide $ \text{Al}_2\text{O}_3 $, can behave as both acidic and basic.
2. These are called amphoteric oxides because they react with both acids and bases, depending on the conditions.

Reaction with Acids

1. When reacting with an acid, an amphoteric oxide behaves as a base.
2. For example, aluminum oxide reacts with hydrochloric acid to produce aluminum chloride and water: $$
   \text{Al}_2\text{O}_3 (s) + 6\text{HCl} (aq) \rightarrow 2\text{AlCl}_3 (aq) + 3\text{H}_2\text{O} (l)
   $$

Reaction with Bases

1. When reacting with a base, an amphoteric oxide behaves as an acid.
2. For instance, aluminum oxide reacts with sodium hydroxide to form sodium aluminate: $$
   \text{Al}_2\text{O}_3 (s) + 2\text{NaOH} (aq) + 3\text{H}_2\text{O} (l) \rightarrow 2\text{Na[Al(OH)}_4\text{]} (aq)
   $$

Periodic Trends in Oxide Behavior

The acid-base nature of oxides follows predictable trends across the periodic table:

1. Across a Period:
   1. Oxides become less basic and more acidic as you move from left to right across a period.
   2. For example, in Period 3:
      1. Sodium oxide $ \text{Na}_2\text{O} $ is basic.