10.2 Alkali Metals

The elements in Group IA of the periodic table, other than hydrogen, are called the alkali metals. They earn this name because they dissolve in water, giving alkali.

Element	Symbol (Z)	Full Electron configuration	Shorthand Electron configuration
Lithium	Li (3)	1s2 2s1	[He] 2s1
Sodium	Na (11)	1s22s22p63s1	[Ne] 3s1
Potassium	K (19)		[Ar] 4s1
Rubidium	Rb (37)		[Kr] 5s1
Caesium	Cs (55)		[Xe] 6s1
Francium	Fr (87)		[Rn] 7s1

• Due to one electron in the valence shell, their valency is 1, and the oxidation number is +1 in the combined state.
• Highly electropositive and extremely reactive elements.
• Low ionisation potential and low electronegativity.
• Typical metals possess metallic lustre, malleability and ductility.
• Silvery white when freshly cut but tarnish in the air.
• Good conductors of electricity and heat.
• Relatively soft solids that can be cut with a knife. These have relatively low densities.
• Low melting and boiling points. And the m. pt. and b. pt. decrease down the group.
• Powerful reducing agents.

Occurrence

Sodium is far too reactive to exist freely in nature — you always find it locked inside compounds. Its main minerals are as follows:

1. Sodium chloride: The most abundant mineral and ore of Sodium. Found in seawater, various lakes and as rock salt. Sea water normally contains 1.5% NaCl. The Dead Sea contains 8% NaCl.
2. Chile Saltpetre (NaNO₃)
3. Glauber’s Salt (Na₂SO₄.10H₂O)
4. Borax (Na₂B₄O₇.10H₂O) etc.

Extraction from Down’s process

Sodium is generally extracted from the most common, abundant and cheap mineral (ore), Sodium chloride.

However, it cannot be extracted by a chemical (Carbon or aluminium) reduction process because

1. Sodium chloride cannot be easily roasted to get Sodium oxide. Besides, Sodium oxide cannot be reduced by Carbon, itself being a stronger reducing agent.
2. Carbon reduction has to be carried out at a high temperature where sodium forms sodium carbide rather than free sodium metal.

Even electrolytic reduction encountered the following difficulties.

1. NaCl melts at 802⁰C. The boiling point of Na is 883⁰C. Therefore, if sodium metal is produced at such a temperature, Na metal forms a metallic fog with NaCl. Such fog can corrode and short-circuit the cell, and it is difficult to handle and separate.
2. Both sodium and chlorine produced are highly reactive, so they can corrode the cell.

To overcome these difficulties, JC Down mixed sodium chloride with calcium chloride in a ratio of 2:3, which lowers the melting point to about 600⁰C. At this temperature, neither sodium nor chlorine corrodes the cell nor is metallic fog formed. Even less energy is enough to maintain the temperature.

The Downs cell is a cylindrical iron tank lined inside with refractory material. The Graphite anode is kept at the centre, and a cylindrical iron cathode surrounds it. The anode and cathode are separated by an iron gauze, which keeps the products of electrolysis separated.

On passing electricity, these reactions occur,

At anode,

At cathode,

The net cell reaction is:

The sodium metal liberated at the cathode rises through the molten sodium chloride because it is lighter. Chlorine gas liberated at the anode escapes. As the reaction progresses, the concentration of NaCl decreases, and more NaCl can be added to maintain the plant’s operation. Calcium is not liberated instead of sodium because it requires a higher voltage.

Advantages:

• Highly pure Sodium metal is obtained (99.8%).
• Chlorine is formed as a byproduct, which is a very useful industrial chemical.
• Impure salt can be directly used.

Physical properties

• Soft silvery white metal with metallic lustre; soft enough to slice with a knife.
• Less dense than water (it actually floats — briefly — before reacting!).
• M. pt. 97⁰C, B pt. 883⁰C.

Chemical properties – sodium is a busy reactor

Action with air

Exposed to moist air, sodium tarnishes rapidly through a three-step chain reaction:

First, Sodium metal absorbs oxygen, forming sodium oxide, which absorbs the moisture, changing it into sodium hydroxide. The Sodium hydroxide further absorbs carbon dioxide, forming sodium carbonate.

Therefore, sodium metal is stored dipped under kerosene.

[Hint: after the above sequence of reaction,

]

Action with water – dramatic and dangerous

When a piece of sodium is dropped into water, it reacts violently with water, forming caustic soda and hydrogen gas. This is a highly exothermic reaction. If a large amount of sodium is used, an explosion occurs.

Even the hydrogen gas formed is highly inflammable, making the reaction more dangerous.

Therefore, sodium fire cannot be extinguished by water.

Burning in Oxygen

Sodium burns in oxygen, forming mainly sodium peroxide and a small amount of sodium oxide.

Action with acids

It reacts with mineral acids, forming salt and displacing hydrogen gas.

Action with Hydrogen

When sodium is heated with hydrogen gas, Sodium hydride is formed, which is an unstable compound.

Metal hydrides are compounds where hydrogen has an oxidation number of −1.

Action with other non-metals

Sodium reacts with other nonmetals, forming generally binary compounds like

Action with ammonia

When sodium is heated with ammonia gas at about 300⁰C, Sodamide (sodium amide) is formed.

Sodium metal dissolves in liquid ammonia, forming a deep blue solution. Here, at first, sodium ionises as

Then both sodium ion and free electron are ammoniated as:

Uses of Sodium

• Sodium vapour lamp, which illuminates streets with yellow light, is distinctly visible in foggy weather as well.
• To prepare useful compounds like Sodamide, Sodium cyanide, Sodium peroxide, etc.
• Sodium amalgam is used as a reducing agent.
• As a lab reagent, especially for foreign element detection, Wurtz reaction, etc.

Physical properties

• White deliquescent solid, soapy in touch
• Highly soluble in water; the solution is highly corrosive to skin and vegetable matter, hence called “caustic soda”
• Strong alkali, turns litmus blue, phenolphthalein pink

Chemical properties

Reaction with salt solutions (precipitation reaction)

NaOH reacts with many soluble salts of metals, giving insoluble hydroxide.

Action with carbon monoxide

Concentrated sodium hydroxide, when heated with carbon monoxide gas at 50 atm pressure to 160⁰C, sodium formate is produced.

Uses

• Very important industrial alkali, used in the production of a large number of important compounds
• In the manufacture of Soap
  Oil + NaOH → Soap + Glycerol
• In the refining of oil
• In the paper and pulp industries
• Manufacturing of Rayon
• A common laboratory reagent, and in cosmetics and pharmaceuticals

Physical Properties

• Crystallises with 10 molecules of water: Na₂CO₃.10H₂O — commonly called washing soda.
• Efflorescent substance – loses water of crystallisation when exposed to the open atmosphere.

Chemical properties

1. Action with SO₂

Sodium carbonate reacts with Sulphur dioxide in the presence of Sulphur to give sodium thiosulphate.

2. Action with CO₂

When CO₂ is passed into an aqueous solution of sodium carbonate, sodium bicarbonate is produced.

Sodium bicarbonate is called Baking soda.

3. Formation of insoluble carbonates (precipitation reactions)

When sodium carbonate is treated with some soluble salts like CaCl₂, BaCl₂, AgNO₃, MgCl₂, etc., the carbonates of respective metals are precipitated out.

4. The action of water (Hydrolysis)

It is soluble in water due to hydrolysis into sodium hydroxide (strong base) and carbonic acid (weak acid). The solution is slightly basic (alkaline). This is an exothermic process.

Uses of Sodium carbonate (Washing soda)

1. Single largest use: manufacture of glass.
2. As washing soda in laundry
3. A water softening agent.
4. In making wood pulp, paper, etc.
5. In food processing, as a pH moderator.
6. A laboratory reagent.
7. As a component of the fire extinguisher along with H₂SO₄ and NaHCO₃.