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Module 1: Acid, Bases and Salts

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Types of Acid and Bases, Ionization and Dissociation Strength

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Arrhenius concept of acids and bases
According to Arrhenius an acid is a substance that when dissolved in water produces hydronium ions by reacting with water molecules.Hydrochloric acid produces hydronium ion and chloride anion.A hydronium ion is a hydrated proton.Since it is further hydrated by more than one water molecule a hydronium ion is usually represented by the electrolytic dissociation equation.Thus the formation of hydronium ions is shown by the electrolytic dissociation equation.The dissociation of HCl in a water solution is complete.Therefore hydrochloric acid is a strong acid.According to Arrhenius a base is a substance that produces hydroxide ions when dissolved in water.Sodium hydroxide is a typical base since it dissociates in water solution to produce hydroxide ions.Sodium hydroxide dissociates completely in a water solutiontherefore it is considered a strong base.
Bronsteds acids and bases
All acids dissociate in water solution to produce hydronium ions.They obey the Bronsted Lowry definition of acid since they donate a proton to a water molecule.Group I and II hydroxides dissociate in water solution to produce hydroxide ions.Hydroxide ions are Bronsted bases since they can accept a proton to form a water molecule.The ammonia molecule is a Bronsted base too.In water solution it accepts a proton from water molecule to form an ammonium ion.The reaction of HCl and NH3 in the gaseous phase involves transfer of a proton.HCl is the donor of the proton.NH3 is the acceptor.Thus HCl is the acid and NH3 is the base in terms of the Bronsted Lowry theory.This theory explains why neutralization reactions can occur in the absence of water.