SALT- II

SALT II

 Water of Crystalization

There are two kinds of salts.

They are hydrated salts and anhydrous salts.

Hydrated salts: are salts that contain water of crystallization. Anhydrous salts: are salts that contain no water of crystallization.

Water of crystallization: is a definite or fixed amount of water associated with salts as they come out of solution.

Although all hydrated salts (hydrates) are crystalline, not all crystalline salts are hydrates, eg NaCl, KMnO₄.

Examples of hydrated salts are CuSO₄ 5H₂O Copper(ii)tetraoxosulohate(vi) pentahydrate.

FeSO4. 7H2O Iron(ii)tetraoxosulphate(vi) heptahydrate

Na₂CO₃. 10H₂O Sodium trioxocarbonate(iv)decahydrate

Other hydrates are; Zn(NO₃)₂. 6H₂O; MgSO₄. 7H₂O; ZnSO₄. 7H₂O; Cu(NO₃)₂. 3H₂O

When the hydrated salts loose their water of crystallization as a result of heating, they become anhydrous or amorphous (shapeless).

Some crystalline salts which are not hydrates are; NaCl, KNO₃, (NH₄)₂SO₄, KMnO₄ etc.

 Efflorescent; Deliquescent and Hygroscopic Substances or Salts;

Certain crystals (salts) containing water of crystallization gradually loose all or part of it, when they are exposed to the air and become anhydrous salts. Such salts are said to be efflorescent and the phenomenon is known as efflorescence. Examples of efflorescent salts are: Na₂CO₃. 10H₂O Sodium troxocarbonate (iv) decahydate. ; Na₂SO₄. 10H₂O Sodium tetraoxosulphate (vi) decahydate.

Some substances or compounds, when exposed to the air absorb enough water from the air to form solution. Such substances are called the deliquescent substances and the phenomenon is known as deliquescence. Examples of deliquescent compounds are:

NaOH (Sodium hydroxide) ; KOH ( Potassium hydroxide) ; CaCl₂  (Calcium chloride)

FeCl₃ (Iron(iii) chloride) ; MgCl₂ (Magnesium chloride) ; P₂O₅ ( Phosphorus(v)oxide)

Some substances when exposed to the air can absorb water but without dissolving in it. Such substances are said to be hydroscopic. Examples of hydroscopic substances are: CaO, CuO, NaNO₃ and Conc. H₂SO₄. Concentrated H₂SO₄ absorbs water and becomes diluted.

 Uses Of Hydroscopic and Deliquescent Substances

They are used as drying agents in the desiccators and in the laboratory for drying gases.

Conc. H2SO4 cannot be used in drying NH3 gas because the gas can react with it.

2NH3(g) + H2SO4(aq) (NH4)2SO4(aq)

Hydrolysis of Salts

When salts dissolve in water they are hydrolyzed. The reaction between a salt and water to give either acidic or basic solution is known as hydrolysis.

Hydrolysis involves the split of water molecules into its ions, H⁺ and OH^-. These ions then get attracted to the opposite ions of the salt. The degree of attraction determines which ion, i.e. H⁺ or OH^- will be more in solution, thereby resulting in the solution being acidic (more H⁺ in solution) or basic (more OH^- in solution) or neutral (equal conc. of H⁺ and OH^- in solution).

The nature of the resultant solutions depends on the nature of the salts:

Salts formed from strong acids and strong bases (example, NaCl – HCl + NaOH)

These salts give neutral solutions (the H⁺ and OH^- an attracted to the opposite ions of the salt in the same degree). Example, for the hydrolysis of NaCl, H⁺ and OH^- ions are attracted at equal ease to Cl^- and Na⁺ respectively to from HCl and NaOH.

Therefore, the solution contains equal concentration of H⁺ and OH^- ions.

Salts formed from strong acids and weak bases (example, NH₄Cl – HCl + NH₃)

Solutions of these salts are acidic (i.e. conc. of H⁺ is more than that of OH^-). The OH^- ions are attracted more to the positive ions of the salts than the H⁺ ions are attracted to the negative ions.

Example, for the hydrolysis of NH₄Cl, OH^- ions are attracted more to NH₄⁺ than H⁺ ions attracted to Cl^- ions. Hence, there is more concentration of H⁺ ions in solution, which results in the solution being acidic.

Salts formed from weak acids and strong bases (e.g. NaHCO₃ – NaOH + H₂CO₃ ; Na₂CO₃ – NaOH + H₂CO₃)

The solutions are basic (i.e. the concentration of OH^- ions is more than that of H⁺ ions). This is because H⁺ ions are more attracted to the negative ions of the salt than OH^- ions attracted to the positive ions of the salt.

Notice that the solution of NaHCO₃ is basic, even though the salt is an acid salt. The name acid salt only means that there is or are still hydrogen atom(s) from an acid in the salt which can still be replaced by metallic ions. lt does not in all cases mean the solution of the salt is acidic.

lt will be acidic if the acid salt is formed from a strong acid, example, NaHSO₄ (the hydrogen atom will be furnished in solution as the only positive ion).

Salts formed from weak acids and weak bases (e.g. (NH₄)₂CO₃) The solutions of these salts are neutral (equal attraction between the positive ions of the salt and OH^- ions of water; and the negative ions of salt and H⁺ ions of water. Hence equal concentration of H⁺ and OH^- are in solution).

Note: hydrolysis also occurs in certain compounds which are not salts (examples are carbohydrates, fats and proteins). Therefore, hydrolysis can be defined generally as the reaction between the ions of water (i.e. H⁺ and OH^-) and the opposite ions or poles of a substance formed with the loss of water molecules, to produce the reactants from which it was initially formed.

Example, when NaCl is hydrolysed, it produces HCl and NaOH - from which it was initially produced with the elimination of water molecules.