Alkali Metals: Properties, Reactions & Uses

What are Alkali Metals?

Alkali metals are the Group 1 elements in the periodic table. They include:

• Lithium (Li)
• Sodium (Na)
• Potassium (K)
• Rubidium (Rb)
• Caesium (Cs)
• Francium (Fr)

Key Properties:

1. Softness: Can be cut with a knife.
2. Shiny: Freshly cut surfaces are silvery.
3. Low density: Lithium, sodium, and potassium float on water.
4. Low melting/boiling points: Compared to other metals.
5. Highly reactive: React vigorously with water and air.

Trends Down the Group

As we move down Group 1, the following trends are observed:

Why do these trends occur?

• Increasing atomic radius: More electron shells are added.
• Decreasing melting/boiling points: Metallic bonds weaken as the size of the atoms increases.
• Increasing reactivity: The outer electron is lost more easily because it is farther from the nucleus.

Reactions with Air and Water

Reaction with Air (Oxidation)

Alkali metals react with oxygen to form oxides. For example:

• Lithium forms lithium oxide (Li₂O).
• Sodium forms sodium peroxide (Na₂O₂).
• Potassium, rubidium, and caesium form superoxides (KO₂, RbO₂, CsO₂).

Reaction with Water

Alkali metals react vigorously with water to form hydroxides and hydrogen gas. The reaction becomes more violent as we go down the group.

General equation:

2M + 2H₂O → 2MOH + H₂

where M is an alkali metal.

Example: Sodium and Water

• Sodium floats on water.
• It melts into a silvery ball.
• The reaction produces heat, causing the hydrogen gas to catch fire.

Example: Potassium and Water

• Potassium reacts more vigorously than sodium.
• The hydrogen gas produced ignites immediately.
• The reaction is explosive if potassium is in small pieces.

Reactions with Acids

Alkali metals react with acids (like hydrochloric acid) to form salts and hydrogen gas.

General equation:

2M + 2HCl → 2MCl + H₂

where M is an alkali metal.

Example: Sodium and Hydrochloric Acid

• Sodium dissolves in the acid.
• Hydrogen gas is evolved.
• The reaction is exothermic (releases heat).

Uses of Alkali Metals

Extraction of Alkali Metals

Alkali metals are extracted from their ores using electrolysis. For example:

• Sodium is extracted from rock salt (NaCl) using the Downs cell.
• Potassium is extracted from carnallite (KCl·MgCl₂·6H₂O).

Electrolysis of Molten Sodium Chloride

flowchart TD
    A["Molten NaCl"] -->|"Electrolysis"| B["Na⁺ + Cl⁻"]
    B --> C["Cathode: 2Na⁺ + 2e⁻ → 2Na"]
    B --> D["Anode: 2Cl⁻ → Cl₂ + 2e⁻"]
    C --> E["Sodium metal"]
    D --> F["Chlorine gas"]
    E & F --> G["Products: Sodium and Chlorine"]

Comparison with Alkaline Earth Metals

Solved Examples

Example 1: Writing the Reaction of Potassium with Water

Question: Write the balanced chemical equation for the reaction of potassium with water.

Solution: Potassium (K) reacts with water (H₂O) to form potassium hydroxide (KOH) and hydrogen gas (H₂).

The unbalanced equation is:

K + H₂O → KOH + H₂

To balance it:

• There are 2 hydrogen atoms on the left (in H₂O) and 2 hydrogen atoms on the right (in H₂).
• There is 1 potassium atom on the left and 1 potassium atom on the right.
• There is 1 oxygen atom on the left and 1 oxygen atom on the right.
• However, we need 2 potassium atoms to balance the hydroxide ions.

The balanced equation is:

2K + 2H₂O → 2KOH + H₂

Example 2: Identifying the Product of Sodium with Oxygen

Question: What is the product formed when sodium reacts with excess oxygen?

Solution: When sodium reacts with limited oxygen, it forms sodium oxide (Na₂O). When sodium reacts with excess oxygen, it forms sodium peroxide (Na₂O₂).

The reaction with excess oxygen is:

2Na + O₂ → Na₂O₂

NEB Board-Style Questions

Short Answer Questions

1. Why do alkali metals have low melting and boiling points?
2. Write the reaction of lithium with water.
3. Name two uses of sodium.
4. Why is potassium more reactive than sodium?
5. What is the trend in atomic radius down Group 1?

Long Answer Questions

1. Explain the trend in reactivity of alkali metals down the group. Give reasons for the trend.
2. Describe the reaction of sodium with water. What safety precautions should be taken while performing this reaction in the laboratory?
3. How are alkali metals extracted from their ores? Explain with the example of sodium extraction.
4. Compare the properties and uses of lithium, sodium, and potassium.

Exam Tip

1. Memorize trends: Know the trends in atomic radius, melting point, boiling point, density, and reactivity down Group 1.
2. Reaction equations: Be able to write balanced equations for reactions with water, air, and acids.
3. Uses: Remember key uses of lithium, sodium, and potassium.
4. Safety: Always mention safety precautions (e.g., keeping alkali metals in kerosene) in practical-based questions.
5. Comparisons: Be ready to compare alkali metals with alkaline earth metals or other groups.
6. Diagrams: Draw labeled diagrams for reactions (e.g., sodium floating on water with hydrogen gas evolving).
7. Applications: Link properties to uses (e.g., low density of lithium makes it useful in batteries).