Hydroxides

There are two main methods for preparing metal hydroxides:

A) direct method of preparation

The hydroxides of metals that can be prepared by the direct method are typically those composed of strongly electropositive metals, such as LiOH, NaOH, and Ca(OH)₂.

Example: The direct preparation of NaOH:

2Na + 2H₂O → 2NaOH + H₂

B) indirect method of preparation

In this method, the metal hydroxide is prepared through the following reactions:

i) the action of water on metal oxides

The metal oxide reacts with water to form a metal hydroxide. This method is typically used for metals like calcium and magnesium.

MgO + H₂O → Mg(OH)₂

Note: The metal hydroxides prepared by this method are soluble in water.

ii) action of calcium hydroxide (milk of lime) on solutions of carbonates

By reacting calcium hydroxide with metal carbonate solutions, metal hydroxides are precipitated.

Ca(OH)₂ + Na₂CO₃ → CaCO₃ + 2NaOH

iii) precipitation by adding ammonia or sodium hydroxide

When ammonia solution or sodium hydroxide is added to a solution of a metal salt, a metal hydroxide is formed by precipitation.

ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄

iv) electrolysis of metal chloride solution

When a metal chloride is electrolyzed, hydroxides are produced at the cathode due to the preferential discharge of hydroxyl ions.

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

1. alkali metal hydroxides (MOH)

Physical properties:

1. White crystalline solids.
2. They melt at moderate temperatures without decomposition (except LiOH).
3. They are deliquescent solids.
4. They are highly soluble in water, forming alkaline solutions.

Chemical properties:

1. The basic strength increases down the group. Example: Calcium hydroxide is the strongest base.
2. Cold and dilute alkalis react with chlorine to form metal chloride and hydrochlorite. 2NaOH + Cl₂ → NaCl + NaOCl + H₂O
3. Hot and concentrated alkalis react with chlorine to form metal chloride and chlorate. 3NaOH + Cl₂ → NaCl + NaClO₃ + H₂O
4. Alkali hydroxides react with acids to form salts and water.

Uses of NaOH and KOH:

1. Absorption of acidic gases like CO₂.
2. Neutralization reactions (e.g., NaOH in the manufacture of soap, paper, and silk).
3. Used in precipitation reactions (e.g., to remove metals from their salts).

2. alkaline earth metal hydroxides (M(OH)₂)

Physical properties:

1. White crystalline solids.
2. Solubility increases down the group from beryllium to barium hydroxide. Beryllium hydroxide is insoluble in water.
3. Calcium hydroxide solubility decreases with temperature, while solubility increases for magnesium, strontium, and barium hydroxides.

Chemical properties:

1. Alkaline earth hydroxides react with acids to form salts and water. Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
2. Alkaline earth hydroxides react with carbon dioxide to form carbonates. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
3. Beryllium hydroxide is amphoteric, reacting with both acids and bases. Be(OH)₂ + NaOH → Na₂BeO₂ + 2H₂O

3. reaction with ammonium salts

All hydroxides except Be(OH)₂ react with ammonium salts to release ammonia gas, which turns litmus paper alkaline.

Ca(OH)₂ + NH₄Cl → CaCl₂ + NH₃ + H₂O

4. action with sulphur dioxide

Sulphur dioxide turns lime water milky due to the formation of calcium sulphite. When excess SO₂ is added, the milky color disappears due to the formation of calcium bisulphite.

Ca(OH)₂ + SO₂ → CaSO₃ + H₂O

1. Lime water (Ca(OH)₂) is used to test for carbon dioxide.
2. Magnesium hydroxide (milk of magnesium) is used as an antacid.
3. Ca(OH)₂ is used in making builders mortar (mixture of slaked lime, sand, and water).
4. Used in making bleaching powder.
5. Used for neutralizing acids in the soil.
6. Ca(OH)₂ and water mixture (whitewash) is used for coating walls and ceilings.
7. Used in water softening.
8. Ca(OH)₂ is used in sugar refining.