Classwork Series and Exercises {Chemistry – SS1}: Bases and Alkalis

Week 2

Topic: Bases and Alkalis

Introduction

Bases are substances that, in aqueous solution, are slippery to the touch, taste bitter, change the colour of indicators (e.g., turn red litmus paper blue), react with acids to form salts, and promote certain chemical reactions (base catalysis). Bases are usually metallic oxides or metallic hydroxides.

Bases that are soluble in water are called alkalis.

 

In aqueous solution, alkali produces hydroxide ions (OH^–). In short, alkalis are substances that form hydroxide ions (OH^–(aq)) in water

Examples

Sodium hydroxide NaOH gives Na⁺(aq) and OH^–(aq) ions,

NaOH → Na⁺ + OH^–

Calcium hydroxide Ca(OH)₂ gives Ca²⁺(aq) and 2OH^–(aq) ions.

Ca(OH)₂ → Ca²⁺ + 2OH^–

Ammonia give NH₄⁺ and OH^–

NH₃ + H₂O → NH₄⁺ + OH^–

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In alkaline solution there are more OH^– ions than H⁺ ions

Strength of Alkalis

Similar to strength of acids, the strength of an alkali is defined by its ability to ionise and release hydroxide ions (OH^–) in the solution.

In a solution of strong alkali, all the alkali molecules are ionised in the water to produce hydroxide ions

In a solution of weak alkali, only small portion of the molecules are ionised to release hydroxide ions.

Table below shows some example of strong/weak alkalis.

Alkali
Strong	Weak
NaOH KOH LiOH	NH3

Physical Properties of Alkali

The following are the physical properties of alkali

1. Alkalis are bitter in taste.
2. Alkalis turn litmus from red to blue.
   Like acid, alkali can change the colour of litmus. In alkali solution, the colour of litmus turn blue.
3. Alkalis are soapy to touch.
4. Alkalis has pH value more than 7
   pH value is a measurement of the concentration of hydrogen ions in a solution. Alkali has very low concentration of solution hydrogen ion, even lower than water. Hence the pH value of alkali is higher than 7. (Note: The pH value of water is 7. The lower the concentration of hydrogen ions, the higher the pH value.
5. Alkalis can conduct electricity
   When a base dissolve in water, it will dissociate and form hydroxide ions. The present of the freely move ions make alkali an electrolyte.

Chemical Properties of Bases/Alkalis

Alkalis react with acids to form a salt and water — this is a neutralisation reaction:

Reaction between Alkalis and Acids

Acid + Alkali → Salt + Water

Example:

Potassium hydroxide + Nitric Acid → Potassium Nitrate + Water

KOH + HNO₃ → KNO₃ + H₂O

Alkali heat with Ammonium Salts

Alkalis, when warmed with ammonium salts, give off ammonia gas

Ammonium Salt + Alkali → Salt + Ammonia + Water

Example:
Ammonium Chloride + Sodium Hydroxide → Sodium chloride + Water + Ammonia

NH₄Cl  + NaOH → NaCl  +  H₂O  +  NH₃

More examples:

2NH₄Cl  +  Ca(OH)₂  → CaCl₂ + 2H₂O + 2NH₃

(NH₄)₂SO₄ +  2NaOH → Na₂SO₄ + 2H₂O + 2NH₃

Preparation of Bases and Alkalis

There are different ways of preparing bases and alkalis

• By burning metals in air or oxygen

2Mg (s) + O2(g) —–> 2MgO(s)

4Fe(s) + 3O2(g) —–> 2Fe2O3(s)

• Sodium hydroxide is prepared by

1. Heating slaked lime with dilute sodium trioxocarbonate (IV) solution

Ca(OH)₂(s) + Na₂CO_3(aq) —–> CaCO_3(s) + 2NaOH_(aq)

2. By electrolysis of brine

• Ammonium hydroxide (NH₄OH) is prepared by heating a mixture of ammonium salt and slaked lime to produce ammonia gas which is then dissolved in water

Ca(OH)_2(s) + (NH₄)₂SO_4(s) —–> CaSO_4(s) + 2H₂O_(g) + 2NH_3(g)

NH_3(g) + H₂O_(l) —–> NH₄OH_(aq)

• Insoluble bases are prepared by method of precipitation. This is done by addition of sodium hydroxide or potassium hydroxide to soluble salt solution

CuSO_4(aq) + 2KOH_(aq) —–> K₂SO_4(aq) + Cu(OH)_2(s)

Uses of Bases

• Sodium hydroxide (caustic soda) is used in the manufacture of soap. It is used in petroleum-refining; in making medicines, paper, pulp, etc. It is used in making rayon.
• Calcium hydroxide is also known as slaked lime. It is used to neutralize acid in water supplies; in the manufacture of bleaching powder; as a dressing material for acid burns; as an antidote for food poisoning; in the preparation of fungicides and in the mixture of whitewash. It is mixed with sand and water to make mortar which is used in the construction of buildings. It is also used by farmers on the fields to neutralize the harmful effects of acid rain.
• Ammonium hydroxide is used to remove ink spots from clothes and to remove grease from window-panes. It is used in the cosmetic industry.
• Alkalis are used in alkaline batteries. Generally, potassium hydroxide is used in such batteries.

Magnesium hydroxide is used in toothpaste to neutralise acid on teeth. It is also used in antacids to relieve indigestion

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