Salt (O-level chemistry)

SALTS

A salt is a compound formed when the replaceable hydrogen atom(s) of acid is replaced by a metal or ammonium ion (NH₄⁺)

Types of salts

There two different types of salts

• Normal salt
• Acidic salt

Normal salt                           

It is the one in which all the replaceable hydrogen from the acid has been replaceable by the metal ion(s).

   Example

Sodium chloride                     NaCl

Sodium sulphate                     Na₂SO₄

Calcium carbonate                  CaCO₃

Calcium sulphate                    CaSO₄

Calcium chloride                     CaCl₂

Ammonium chloride               NH₄Cl

The formation of the normal salt is illustrated by the equation below;

2Na_{(s)  +}  H₂SO_{4(aq)          →}             Na₂SO_{4(aq)  +}  H_{2 (normal salt)}

Acidic salt

Is a salt in which part of replaceable hydrogen of the acid has been replaced by a metal

2Na_{(s)    +}2H₂SO_{4(aq)         →}    2NaHSO_{4(aq)  +}  2 H_{2(g) (acid salt)}

Examples include

• sodium hydrogen carbonate    (NaHCO₃)
• Sodium hydrogen sulphate     NaHSO₄
• Calcium hydrogen sulphate   Ca(HSO₄)₂
• Calcium hydrogen carbonate Ca (HCO₃)₂ etc.

Preparation of salts

1. From metal salt by direct combination of metals or oxide with acid e.g.

Na_(s)+ 2HCl_{(aq)   →}    2NaCl_(aq)  +  H_2(g)

The mixture is dried using a drying agent or evaporating to dryness

2. By neutralisation;

In this is the process by which a base reacts with an acid to form a salt and water only e.g. Sodium chloride can be prepared from sodium hydroxide and dilute hydrochloric acid.

2NaOH +  2 HCL _{(aq)       →}    2NaCl_(aq)    +  H₂O_(l)

3. Direct combination(synthesis);

 Aluminium chloride and iron (III)chloride can be prepared from the element directly

2Al_{(s) +}  3Cl_{2(g)        →}     2 AlCl_3(s)

2 Fe_(s)  +  3Cl_{2(g)       →}   2 FeCl_3(s)

4. Precipitation

Insoluble zinc carbonate, silver chloride (AgCl), and lead chloride can be prepared by precipitation e.g. silver chloride is prepared from a solution of silver nitrate and dilute hydrochloric acid where a precipitate of silver chloride is formed.

AgNO_{3(aq)  +}  HCl_{(aq)         →}    AgCl(s)   ₊ HNO_{3 (aq)}

The precipitate of AgCl is washed several times with distilled water.

NOTE:  insoluble salts can be prepared by either precipitation method or direct synthesis

Other examples of salts prepared by precipitation include;

• Lead chloride (PbCl₂)
• Lead sulphate (PbSO₄), barium sulphate (BaSO₄) etc_.

Preparation of zinc sulphate from zinc

•      Add dilute H₂SO_4(s) mixed with little copper sulphate to zinc in a beaker                
• Add more zinc to saturate the solution

Filter to remove the undissolved zinc

• Warm the filtrate to concentrate it
• Cool to room temperature
• Filter the crystals of zinc sulphate-7-water
• Wash the crystals with little cold distilled water
• Dry the crystals between filter papers

Zn_{(s)  +}  H₂SO_{4(l)  →} ZnSO_{4(aq)  +}  H₂

Preparation of iron (II) sulphate-7-water (FeSO_4.7H₂O)

• Add warm dilute sulphuric acid to iron filling in a beaker
• Add more iron fillings to saturate the solution
• Filter to remove the undissolved iron filings
• Warm the filtrate slightly to concentrate it
• Cool to room temperature
• Filter the crystals of iron (II) sulphate-7-water
• Wash the crystals with cold distilled water
• Dry the crystals between filter papers

   Fe_{(s)  +}  H₂SO_{4(aq)        →}            FeSO_{4(aq)  +}  H_2(g)

 Preparation of copper (II) sulphate-5-water from copper oxide

• Pour dilute sulphuric acid in a beaker, warm gently. (to increase on the solubility)
• Add copper oxide to the warm acid.
• Add more copper oxide to saturate the solution.
• Filter to remove the undissolved copper oxide
• Evaporate the filtrate concentrate it cool to room temperature
• Filter the crystals of copper (II) sulphate-5-water
• Wash the crystals with little cold distilled water
• Dry the crystals between the filter paper

CuO + H₂SO_{4(aq)       →}    CuSO_{4(aq)  +}  H₂O_(l)

N.B: Magnesium sulphate, zinc sulphate, and lead (II) nitrate can be prepared in the same way.

Mg(OH)_2(s) +H₂SO_{4(aq)   →}      MgSO_4(aq) + 2H₂O_(l)

PbO_(s) + 2HNO_{3(aq)   →}       Pb(NO₃)_2(aq) + H₂O_(l)

Preparation of sodium hydrogen sulphate.

• Add 20cm³ of 2M NaOH in a clean conical flask.
• Titrate with 2M H₂SO₄ from a burette using methyl orange indicator.
• The solution formed contains sodium sulphate.

2NaOH_(aq) + H₂SO_{4(aq)  →}  Na₂SO_{4 (aq)} +2H₂O_(l)

• Note the volume of H₂SO₄ used, say x cm³.
• Measure another 20 cm³ of 2M NaOH in a clean beaker.
• Add 2x cm³ of 2M H₂SO₄                   
• The solution formed contains sodium hydrogen sulphate.

NaOH_(aq) + H₂SO_{4(aq)      →}   NaHSO_4(aq) + H₂O_(l)   

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Compiled by Dr. Bbosa Science