S3.1.5 Metallic and non-metallic oxides

Metal Oxides, Non-Metal Oxides, and Amphoteric Oxides

1. You're walking through a forest and come across a small pond.
2. You notice that when certain rocks are submerged in the water, some cause bubbling, some seem to dissolve and change the water's properties, while others remain unreactive.

This simple observation mirrors the chemical behavior of oxides—compounds formed by the reaction of elements with oxygen.

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

When a basic oxide reacts with water, it forms a hydroxide. For example, sodium oxide ${\text{Na}}_2\text{O}$ reacts with water to produce sodium hydroxide:

$${\text{Na}}_2\text{O}(s)+{\text{H}}_2\text{O}(l)\to 2\text{NaOH}(aq)$$

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

• 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.
• 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

Non-metal oxides react with water to produce acids. For instance, carbon dioxide ${\text{CO}}_2$ reacts with water to form carbonic acid:

$${\text{CO}}_2(g)+{\text{H}}_2\text{O}(l)\to {\text{H}}_2{\text{CO}}_3(aq)$$

Similarly, sulfur dioxide ${\text{SO}}_2$ reacts with water to form sulfurous acid:

$${\text{SO}}_2(g)+{\text{H}}_2\text{O}(l)\to {\text{H}}_2{\text{SO}}_3(aq)$$

Group Trends in Acidic Oxides

• 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.
• 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

When reacting with an acid, an amphoteric oxide behaves as a base. For example, aluminum oxide reacts with hydrochloric acid to produce aluminum chloride and water:

$${\text{Al}}_2{\text{O}}_3(s)+6\text{HCl}(aq)\to 2{\text{AlCl}}_3(aq)+3{\text{H}}_2\text{O}(l)$$

Reaction with Bases

When reacting with a base, an amphoteric oxide behaves as an acid. For instance, aluminum oxide reacts with sodium hydroxide to form sodium aluminate:

$${\text{Al}}_2{\text{O}}_3(s)+2\text{NaOH}(aq)+3{\text{H}}_2\text{O}(l)\to 2{\text{Na[Al(OH)}}_4\text{]}(aq)$$

Periodic Trends in Oxide Behavior

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

• Across a Period: Oxides become less basic and more acidic as you move from left to right across a period. For example, in Period 3:
  • Sodium oxide ${\text{Na}}_2\text{O}$ is basic.
  • Aluminum oxide ${\text{Al}}_2{\text{O}}_3$ is amphoteric.
  • Sulfur trioxide ${\text{SO}}_3$ is acidic.
• Down a Group: For metals, oxides tend to become more basic as you move down a group. For non-metals, oxides generally become less acidic.

Formation of Acid Rain and Ocean Acidification

Acid Rain Formation:

1. Acid rain forms when sulfur dioxide $(S{O}_2)$ and nitrogen oxides $(N{O}_x)$ are released into the atmosphere, primarily from the combustion of fossil fuels in power plants, vehicles, and industrial processes.
2. These gases react with water vapor, oxygen, and other chemicals in the atmosphere to form sulfuric acid $(H_{2}S{O}_4)$ and nitric acid $(HN{O}_3)$.
3. These acids dissolve in rainwater, lowering its pH and leading to acid rain.

Key Reactions:

• $S{O}_2+H_{2}O\to H_{2}S{O}_3$ (sulfurous acid)
• $S{O}_3+H_{2}O\to H_{2}S{O}_4$ (sulfuric acid)
• $2N{O}_2+H_{2}O\to HN{O}_2+HN{O}_3$ (nitrous and nitric acids)

Effects of Acid Rain:

• Environmental Impact:
  • Acid rain damages forests, harms aquatic life by lowering the pH of lakes and rivers, and accelerates the weathering of buildings and monuments made of carbonate-based materials like limestone and marble.

• Health Impact:
  • Fine particulate matter formed during acid rain reactions can worsen respiratory issues in humans.

Ocean Acidification:

1. Ocean acidification occurs when excess carbon dioxide $(C{O}_2)$ from human activities dissolves into seawater, forming carbonic acid $(H_{2}C{O}_3)$.
2. This acid dissociates into bicarbonate $(HC{O}_3^{-})$ and hydrogen ions $(H^{+})$, lowering the pH of the ocean and increasing its acidity.

Key Reactions:

• $C{O}_2+H_{2}O\to H_{2}C{O}_3$
• $H_{2}C{O}_3\rightleftharpoons HC{O}_3^{-}+H^{+}$
• $HC{O}_3^{-}\rightleftharpoons C{O}_3^{2-}+H^{+}$

Effects of Ocean Acidification:

• Marine Life Impact:
  • The increased acidity reduces the availability of carbonate ions $(C{O}_3^{2-})$, which are essential for organisms like corals, mollusks, and shellfish to build their calcium carbonate $(CaC{O}_3)$ shells and skeletons.
  • This weakens these organisms and disrupts the marine food chain.
• Ecosystem Impact:
  • Coral reefs, which support biodiversity and coastal protection, are particularly vulnerable to acidification, threatening entire ecosystems.

Reflection Questions