Acid, Bases and Salts : Bases

 

BASES

According Arrhenius theory, any compound that can donate hydroxide ions (OH) when dissolved in water, is called as a base.

Substance + Water → Metal ion + OH

Example:

NaOH(aq)  → Na+(aq) + OH(aq)

In general, hydroxides of alkali metals (LiOH, NaOH, KOH, RbOH, CsOH) and alkaline earth metals (Mg(OH)2 , Ca(OH)2 , Sr(OH)2 , Ba(OH)2)  are considered as alkalies. Alkalies are water soluble bases.

Example:

KOH(aq) → K+(aq) + OH(aq)

Ca(OH)2(aq) → Ca2+(aq) + OH(aq)

If base is dissolved in water , pH of water becomes more than 7, at 25ºC. 

Note: What is pH ? 

We can measure a solution is acidic or basic by using a scale called pH. 

Technically pH = – log (H+

At 25ºC ; If any solution has pH<7, solution is acidic

If solution pH > 7, solution is basic

If pH = 7, solution is neutral

 

Later Chemists found some compounds whose solution in water is basic but their molecules do not contain OH ions. for example : Ammonia (NH3), Pyridine (C5H6N ), Methyl amine (CH3NH2). As you can see their molecular formula does not contain hydroxyl ions (OH), but if these compounds are dissolved in water;    pH >7

Therefore , there are some bases , they do not contain hydroxide ion in their molecular formula.

Ammonia when dissolved in water forms ammonium hydroxide which is a weak base.

NH3 + H2O → NH4OH (aq)

Oxides, carbonates (along with hydroxides) of alkali metals and alkaline earth metals are also considered as basic in nature.

Example: CaO, MgO, Na2O, K2O etc

 

Classification of bases:

Classification I : On the basis of strength:

1.Strong bases:

The bases which undergoes complete ionisation in aquesous solution are called as strong bases.

Example: NaOH, KOH, CsOH etc

 

2. Weak bases:

The bases which undergoes partial ionisation in aqueous solution are called weak bases.

Example: NH4OH, CH3NH2 , pyridine etc.

 

Classification II : on the basis of acidity:

Based on acidity bases are:

1. Mono acidic base

2. Di acidic base

3. Tri acidic base

 

1.Mono acidic bases:

Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases.

Example: NaOH, KOH etc

2. Di acidic bases: 

Bases which produces two hydroxide ions in aqueous solutions are called di acidic bases.

Example: Ca(OH)2, Mg(OH)etc

3. Tri acidic bases:

Bases which produces three hydroxide ions in aqueous solutions are called tri acidic bases.

Example: Al(OH)3, Fe(OH)etc

Classification III : on the basis of their nature : 

  1. Orgnic Bases : They contain carbon in their molecular formula. Amines of organic chemistry are organic bases. Examples : Methyl amine CH3NH2 , Ethyl amine C2H5NH2 etc. They are organic molecules which act as base. 
  2. Inorganic Bases: They are inorganic molecules which act as base . examples : NaOH, Ca(OH)2 etc

Physical Properties of Bases:

  • Bases are bitter to taste, soapy to touch.
  • Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.
  • Bases liberates heat on dilution.

 

Chemical Properties of Bases:

  • Reaction with active metals:

Bases react with metals to liberate hydrogen gas

Example: Sodium hydroxide react with zinc and liberate hydrogen and sodium zincate.

NaOH + Zn → Na2ZnO2 + H2

 

  • Reaction with non-metal oxides:

Bases react with non-metallic oxides to form salt and water. This is similar to a neutralization reaction between an acid and a base.           

Example: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water

Ca(OH)2 + CO2 → CaCO3 + H2O

From this reaction, it can be concluded that non-metallic oxides are acidic in nature.

 

  • Reaction with acids:

Bases reacts with acids to form salts and water.

Example:

Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water.

KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)

 

Indicators in presence of Bases:

Bases turns red litmus to blue.

Phenolphthalein turns pink in presence of bases.

Methyl orange turns to yellow in presence of bases.

 

Uses of Bases:

Sodium hydroxide (NaOH)
  • In the manufacture of soaps, paper and synthetic fibres like rayon.
Calcium hydroxide (Ca(OH)2)
  • In the manufacture of bleaching powder
  • It is used as a disinfectant
Magnesium hydroxide (Mg(OH)2)
  • As an antacid to neutralize the acid in the stomach (doctor gives milk of magnesia to neutralize acidity of stomach)
Ammonium hydroxide (NH4OH)
  • In the preparation of fertilizers like ammonium phosphate and ammonium sulphate

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Acid, Bases and Salts : Salts

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