Alkali Metals: Sodium and Its Compounds

The elements of group 1 (IA) in the modern periodic table namely lithium(Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium are called alkali metals because all these metals react with water forming alkali.

Sodium:

Symbol: Na
Atomic mass: 23 amu
Atomic number: 11
Electronic configuration: 1s² 2s² 2p⁶ 3s¹
Oxidation state: +1

Occurrence:

Since alkali metals are highly reactive, they don’t occur in a free state but are widely distributed in a combined state. It is present in the earth’s crust at about 2.5 %. The common ores of sodium are:

• Rock salt or common salt: NaCl
• Caliche or chile salt petre: NaNO₃
• Borax: Na₂B₄O₇.10H₂O
• Glauber salt: Na₂SO₄.10H₂O

Extraction of sodium:

Sodium is a strong reducing agent and is very reactive. It cannot be extracted by the usual methods due to the following difficulties:

1. Sodium is a strong reducing agent. So, it cannot be extracted by reduction of its oxide with any other reducing agent.
2. Sodium cannot be extracted by electrolysis of its aqueous solution because it immediately reacts with water forming its hydroxide.
3. Sodium cannot be extracted by pyrometallurgy. At a very high temperature, it gets vaporized. Its vapor is deadly poisonous.

Sodium is extracted by electrolysis of sodium chloride in the molten state.The process of extraction of metal by the electrolysis of their molten ore is called electrometallurgy.For electrolysis, aqueous sodium chloride is not used because sodium reacts with water.However, there are following difficulties for the extraction of sodium by electrolysis:

1. For electrolysis, sodium chloride should be kept in the molten state. The melting point of sodium chloride is very high(802 °C). It is difficult to maintain a high temperature during electrolysis due to the high cost.
2. The boiling point of sodium (883 °C) is nearer to the melting point of sodium chloride. Hence, during electrolysis, sodium may vapourize with molten sodium chloride. Metallic fog is difficult to separate and short circuit the cell.
3. The electrolytic products (sodium and chlorine) are highly reactive at high temperatures and corrode the materials of the vessel.

These difficulties are removed in Down’s process (developed by American Chemist J.C.Down) in which sodium chloride is mixed with calcium chloride in the ratio of 2:3 to lower the melting point by about 600 °C. At this temperature, sodium and chlorine are less reactive and don’t corrode the materials of the vessel and sodium doesn’t form metallic fog.

Down’s process:

Theory:

Melting point of NaCl is lowered by adding CaCl₂.

When electricity is passed through the molten NaCl, the following reactions takes place:

Process:

The electrolytic cell used in this process is Down’s cell which is a cylindrical steel vessel lined with refractory fire bricks. Anode is graphite at the center of the cell arising from the bottom and cathode is cylindrical iron with a hood. Two electrodes are separated by iron wire gauze which keeps the products of electrolysis apart. On passing electricity through molten electrolyte, sodium is discharged at the cathode which being lighter than molten sodium chloride, rises up and collected and packed in steel drums in kerosene oil. At the anode, chlorine gas is liberated and escapes through hood.

Advantages:

• Sodium is obtained in pure form.
• Chlorine obtained as a byproduct has many industrial uses.
• Rock salt and impure salt can directly be used.

Physical properties:

1. Sodium is silvery-white in appearance with metallic lusture.
2. It is soft and easily cut by a knife.
3. It is lighter than water.
4. It is a good conductor of heat and electricity.
5. Its melting point is 97.5 °C and its boiling point is 883 °C.

Chemical properties:

1. Action with air: sodium tarnishes in air due to formation of oxide.

4Na + O₂ → 2Na₂O

The oxide absorbs moisture to give hydroxide which further absorbs carbon dioxide to form sodium carbonate.

Na₂O + H₂O → 2NaOH
2NaOH + CO₂ → Na₂CO₃ + H₂O

So sodium is stored in kerosene oil.
Sodium burns in the air to form a mixture of sodium oxide and sodium peroxide.

4Na + O₂ → 2Na₂O
2Na + O₂ → Na₂O₂

2. Action with water: Sodium violently reacts with water giving sodium hydroxide and water.

2Na + 2H₂O → 2NaOH + H₂ + heat

3. Action with ammonia:

2Na + 2NH₃ → 2NaNH₂(sodamide) + H₂

4. Action with acid:

2Na + 2HCl → 2NaCl + H₂

5. Action with non metals:

2Na + Cl₂ → 2NaCl
2Na + S → Na₂S (Sodium sulphide)
12Na + P₄ → 4Na₃P (Sodium phosphide)

Uses of sodium:

1. It is used in sodium vapor lamps to illuminate streets.
2. It is used in electricity.
3. It is used as a catalyst.
4. It is used as a reducing agent.
5. It is used to prepare many organic compounds.