SS1 Chemistry Lesson Note on Salts: Definition; Types; Hydrolysis and Uses of Salts

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SALTS

1. definition and characteristics of salts
2. preparation of salts and
3. types of salts
4. uses and hydrolysis of salt in water

PERIOD 1: SALTS

Definition of salts

A salt is formed when all or part of the ionizable hydrogen of an acid is replaced by metallic or ammonium ions. It consists of all aggregation of positively charged metallic ion and negatively charged acid ions.

 Characteristics of salts

1. Water of crystallization: This can be defined as the definite amount of water that some substances chemically combine with when they form salt and such salts are referred to as hydrated salts. When heated, they give off their respective water of crystallization and by so doing become anhydrous salt. When hydrated, they are crystalline in shape but when heated and the water of crystallization is released, there’s a loss of crystalline shape. Examples of salt with water of crystallization are sodium trioxosulphate (IV) decahydrate, Na₂SO₃ .10H₂O, Iron (II) tetraoxosulphate (VI) heptahydrate, FeSO₄.7H₂O, sodium trioxocarbonate (IV) decahydrate, Na₂CO₃.10H₂O etc.
2. Efflorescence: This is the loss of part or all of the water of crystallization from a crystalline salt when exposed to the atmosphere to form a lower dehydrate or the anhydrous salt; e.g. Na₂CO₃ 10H₂O (washing soda).
3. Deliquescence: This is a phenomenon in which a substance absorbed moisture (water) from the atmosphere when exposed and turn into solution; e.g. CaCl₂ , FeCl₃, , CuCl_2, ZnCl₂ etc.
4. Hygroscopic: This is a phenomenon in which a substance absorbed moisture (water) from the atmosphere when exposed and does not turn into solution but merely become sticky or moist; e.g., sodium trioxonitrate(V), potassium trioxonitrate(V). Hygroscopic substances are commonly employed as drying agents or desiccants.

Drying agents or desiccants are substances that have strong tendency to absorb moisture or water. They are used to dry gases in the laboratory. A drying agent cannot be used if it reacts with the substance to be dried; that is why H₂SO₄ cannot be used to dry ammonia gas because they will react to form ammonium tetraoxosulphate (vi).

       2NH₃­­­ +   H₂SO₄      →   (NH₄)₂SO₄

EVALUATION:

State three properties of salts with two examples each.

PERIOD 2: PREPARATION OF SALTS

The method chosen to prepare a salt depends largely on (a) its solubility in water (b) its stability to heat.

Preparation of soluble salts

1. Reaction of acid and metal: Here, the more reactive metal (Ca, Mg, Zn, Fe), directly displaces the hydrogen ion in the acid; e.g.

Zn  +  2HCl      →    ZnCl₂   +  H₂

2. Reaction of alkaline and acid: Salts are formed when acid reacts with alkaline; e.g.

KOH  +  HNO₃    →      KNO₃ + H₂O

3. Dilute acid and insoluble base: A salt is formed when a dilute acid is heated and the insoluble base is added to the acid until no more bases can dissolve in the acid. E.g 2HCl +   CuO    →   CuCl₂  +   H₂O

Preparation of insoluble salts

3. Double decomposition: This is done by missing two compounds, one containing the metallic radical and the other the acidic radical of the required insoluble salt; e.g. H₂SO₄ + CuO  →   CuSO₄  +  H₂O
4. Combination of constituent elements: This involves the combination of the elements that make up the insoluble salt; e.g.

Fe  +  S    →      FeS

2Fe + 3Cl₂    →    2FeCl₃

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