Acids and Bases: Arrhenius concept of acids and bases

One of the properties that acids and bases have in common is that they are electrolytes--they form ions when they dissolve in water.The Arrhenius definition of acids and bases is one of the oldest.

A modern statement of the Arrhenius concept of acids and bases is as follows:An acid is a substance that,when dissolved in water,increases the concentration of hydrogen ion, H +(aq ).A base is a substance that,when dissolved in water,increases the concentration of hydroxide ion, OH- (aq).

This definition is very useful when we talk about acids and bases as being electrolytes in solution.

In Arrhenious theory,a strong acid is a substance that completely ionizes in aqueous solution to given H3O+ (aq) and an anion. An example is perchloric acid,HCLO4 .

HCLO4 (aq) + H2O ---------> H3O+ (aq) + ClO4  (aq)

Other examples of strong acids are H2SO4,HI,HCl and HNO3.

A strong base completely ionizes in aqueous solution to give OH- and a cation . Sodium hydroxide is an example of strong base.

 NaOH (s) ----------> Na + (aq) + OH- (aq)

A strong base are hydroxide of group IA elements and group IIA elements (except Be).

Most of the other acids and bases you encounter are weak .They are not completely ionized in solution and exist in reversible reactionn with the corresponding ions. Consider acetic acids, HC2H3O2.The reaction is

HC2H3O2 (aq) + H2O <-------------------> H30+ + C2H3O2- (aq) 

Acids and bases neutralize one another because the hydrogen ion and the hydroxide ion combine with one another to form water.

 

Neutralization:

H+ + OH- ------>  H2O

HCl + NaOH -----> H2O + NaCl

   The equation shown here is the net-ionic version of acid-base neutralization.In general, however, it will show that an acid and a base neutralize one another to form water and a salt. If the salt is soluble, it won't show up in the net ionic equation, but the ions are there in solution as spectator ions.

  

 

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