Hydroxides of the Period 3 Elements

When studying the hydroxides of Period 3 elements from sodium to chlorine, you encounter a variety of chemical behaviors. This section explores the structures, properties, and reactions of these hydroxides.

Sodium and Magnesium Hydroxides: Basic Hydroxides

Sodium hydroxide (NaOH) and magnesium hydroxide (Mg(OH)₂) are simple, ionic compounds. They are characterized as basic hydroxides due to their ability to react with acids to form salts. For example, when these hydroxides react with dilute hydrochloric acid (HCl), they produce colorless solutions of sodium chloride (NaCl) and magnesium chloride (MgCl₂), respectively:

These reactions illustrate the strong basic nature of sodium and magnesium hydroxides.

Aluminium Hydroxide: An Amphoteric Compound

Aluminium hydroxide (Al(OH)₃) exhibits amphoteric behavior, meaning it can act as both an acid and a base. This dual nature is demonstrated through its reactions with both acids and bases.

When reacting with dilute hydrochloric acid, aluminium hydroxide forms a colorless solution of aluminium chloride (AlCl₃):

However, aluminium hydroxide can also react with a strong base like sodium hydroxide (NaOH), producing a colorless solution of sodium tetrahydroxoaluminate (NaAl(OH)₄):

This ability to react with both acids and bases highlights aluminium hydroxide’s amphoteric nature.

Hydroxides of Silicon, Phosphorus, Sulfur, and Chlorine: Covalent and Acidic

The hydroxides of silicon (Si), phosphorus (P), sulfur (S), and Chlorine (Cl) contain -OH groups covalently bonded to the central atoms. These hydroxides are all acidic, though their strength varies significantly. For instance, silicic acids, such as orthosilicic acid (H₄SiO₄), are very weak acids, whereas phosphoric acid (H₃PO₄) is a stronger, yet still weak, acid in comparison to strong mineral acids.

Sulfuric acid (H₂SO₄) and perchloric acid (HClO₄) are examples of very strong acids formed by sulfur and chlorine, respectively, in their highest oxidation states.

Here are some key hydroxides and their structures:

• Orthosilicic Acid: Weakly acidic.
• Phosphoric (V) Acid: A weak acid, stronger than simple organic acids like ethanoic acid.
• Sulfuric Acid: A very strong acid.
• Perchloric Acid: A very strong acid with chlorine in its highest oxidation state.

Summary

The hydroxides of Period 3 elements display a range of chemical behaviors from basic to amphoteric to highly acidic. Sodium and magnesium hydroxides are quintessential bases, reacting readily with acids. Aluminium hydroxide’s amphoteric nature allows it to react with both acids and bases, while the hydroxides of silicon, phosphorus, and sulfur demonstrate varying degrees of acidity. Understanding these properties and reactions not only provides insight into the elements themselves but also lays the groundwork for more advanced studies in inorganic chemistry.