Acid, Bases and Salts : Salts

SALTS

 

The compounds formed by the reaction between an acids and a bases are known as a salts.

Acid + Base → Salt + Water

Salts are ionic compounds which contain positively charged cations and negatively charged anions. During salt formation cation is coming from base and anion is coming from acid.

Example: In Sodium chloride (NaCl) formation cation sodium is coming from sodium hydroxide and anion chlorine is coming from hydrochloric acid.

NaOH + HCl → NaCl + H2O

Classification of salts:

Based on nature of the salts, they are classified into different types. 

  1. Neutral salts
  2. Acidic salts
  3. Basic salts
  4. Double salts
  5. Complex salts

Neutral Salts:

These salts are formed by the reaction between strong base and strong acid.

NaCl is neutral salt formed by the reaction of HCl with NaOH.

HCl + NaOH → NaCl + H2O

NaCl is neutral salt , if it is dissolved in water ; solution remains neutral, pH = 7 at 25°C.

Acidic salts:

Salts which are formed by the combination of strong acids and weak bases are called acidic salts.

Example:

 

NH4OH + HCl → NH4Cl + H2

NH4Cl is acidic salt; if it is dissolved in water solution will be acidic.

 

Basic salts:

Salts which are formed by the reaction of Strong base and weak acid  are called Basic salts.

Example:

NaOH + CH3COOH→  CH3COONa + H2

CH3COONa is a basic salt; if it is dissolved in water ,solution turns basic.

Ca(OH)Cl is formed by the partial replacement of hydroxide group from Ca(OH)2 by chloride ions of acid.

Ca(OH)2 + HCl → Ca(OH)Cl + H2O

 

Table below giving neutral, acidic and basic salts.

 

Type of Salt Type of Acid Type of Base Example of salt 
Neutral salt

pH = 7

Strong Acids: 

HCl

H2SO4

Strong Bases:

NaOH

KOH

NaCl

K2SO4

Acidic salt

pH < 7 

Strong Acids

HCl

HNO3

Strong Bases

NH4OH

Mg(OH)2

NH4Cl

Mg(NO3)

Basic Salt

pH > 7

Weak Acids

H2CO3

CH3COOH

Strong Bases

NaOH

KOH

Na2CO3

CH3COOK

 

 

 

Complex salts:

Some salts contain two different cations or two different anions or both. These salts are called Complex Salts. They can be further classified into 2 types :

1. Double salts  2. Coordination compounds 

Double salts:

Salts that are formed by mixing of two simple salts which are obtained crystallization. In other words, these salts on dissolving in water loose all the ions. 

Example:

Potash alum – K2SO4 Al2 (SO4)3 .24H2O

Dolomite – CaCO3.MgCO3

Coordination Compounds : These salts do not loose all the ions they consists.

Example:

[Ag(NH3)2]Cl  ⇄  [Ag ( NH3 )2 ]+ + Cl

K4[Fe(CN)6] ⇄ 4K+ + [Fe(CN)6]4-

Salts in our daily life:

Baking soda-

Chemical name: Sodium hydrogen carbonate or Sodium bicarbonate

Molecular formula: NaHCO3

Sodium hydrogen carbonate is commenly called as baking soda.

Sodium hydrogen carbonate is used in the baking industry.

It is used in preparation of soda acid.

It is also used in foam type fire extinguishers.

 

Washing soda:

Molecular formula:  Na2CO3.10H2O

Chemical name : Sodium carbonate decahydrate

Adding water to sodium carbonate and this allowing this mixture to cool to forms decahydrated sodium carbonate. This is commenly called as washing soda.

Na2CO3 + 10H2O → Na2CO3.10H2O

In general sodium carbonate is prepared by passing CO2 gas through concentrated NaOH.

2NaOH + CO2 → Na2CO3 + H2O

 

Properties:

It is a white crystalline solid. It exists as decahydrate of sodium carbonate.

When exposed to dry air and heating it loses water molecules to change into anhydrous form.

Na2CO3.10H2O + Exposure to open dry air → Na2CO3.H2O + 9H2O

Na2CO3.H2O + Heating → Na2CO3

It is soluble in water and during dilution heat will releases out.

On reaction with acids sodium carbonate releases carbon dioxide along with the formation of sodium salts and water.

Na2CO3 + HCl → 2NaCl + H2O + CO2

Sodium carbonate is used to manufacture of glass, cleansing agents, soap, glass and paper.

 

Bleaching powder (CaOCl2):

Bleaching powder chemically known as calcium oxy chloride.

It is prepared by the reaction between chlorine and slaked lime at about 40ºC.

Ca(OH)2 + Cl2 → Ca(OCl)Cl + H2O + Cl2

Ca(OH)2 + H2SO4 → CaSO4 + H2O + Cl2

It acts a strong oxidising agent to bleach substances.

CaOCl2 + KNO2 → CaCl2 + KNO3

CaOCl2 + H2S → CaCl2 + H2O + S

It is used to bleach cotton, linen textiles and wood pulp.

Coloured matter + Bleaching powder → Colourless product

It is also used to disinfect drinking water.

 

Hydrated salts:

The molecules of salts which contain fixed number of water molecules in them are called hydrated salts.

In general they exists as dry in pure form.

These salts on heating loses water molecules in them and forms anhydrous salts.

Example:

Ferrous sulphateheptahydrate (FeSO4.7H2O) on heating loses water molecules in it.

          FeSO4.7H2O (on heating) → FeSO4 + 7H2O

Some of the hydrated salts along with their chemical formula.

 

Name of the salt Chemical formula
Sodium carbonate decahydrate Na2CO3.10 H2O
Zinc Sulphate heptahydrate or White vitriol ZnSO4.7H2O
Magnesium sulphate heptahydrate or Epsom salt MgSO4.7H2O
Potash alum K2SO4 Al2 (SO4)3 .24H2O
Copper (II) sulphate pentahydrate or Blue vitriol CuSO4.5H2O
Calcium sulphate dihydrate or Gypsum CaSO4.2H2O

 

Plaster of paris (CaSO4. 1/2 H2O):

Plaster of paris which is chemically called calcium sulphate hemihydrate.

Since it is brought to use from paris, called as “plaster of paris”.

It is prepared by heating of gypsum at 373K.

CaSO4.2H2O    373 K → CaSO4  1 2 H2O    +    1 1 2 H2O   

  Gypsum                         Plaster of Paris            Water

 

Uses of plaster of Paris: 

Plaster of Paris is used to make fine statues

  • It is used as a bandage, proofing material, sealing agent.
  • It is used for making statues, toys and decorative articles.
  • It is also used for smoothing  wall surfaces. 

 

 

 

 

 

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Acid, Bases and Salts : Neutralization Reaction

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