BASES



BASES

A base is a substance that dissociates in water to produce hydroxide ions as the only negatively charged ions.

A base according to J. N. Bronsted from Denmark and J. M. Lawry from England can also be defined as a proton acceptor.

For example, when dry calcium hydroxide is placed on a dry litmus paper there is no observable change.

However, when calcium hydroxide is dissolved in water, the resulting solution turns red litmus paper blue.

This is because in solution calcium hydroxide dissociates into to calcium ions and hydroxide ions.

Ca(OH)2                                                                                                     Ca2+ (aq)  + OH – (aq)

The hydroxide ions (OH-) are responsible for alkalinity properties.

BASE STRENGTH

The higher the number of hydroxide ions, the higher the PH value, the greater the conductivity and the stronger the base.

For example

Sodium hydroxide (NaOH) solution has a higher PH than ammonium solution; this is because when NaOH is dissolved in water, it completely ionizes to form Na+ and OH- ions.

NaOH                                                                                              Na+   +    OH-

 

On the other hand when ammonia gas is dissolved in water, some of the ammonia molecules react with water molecules to form ammonium hydroxide.

NH3 (g)       + H2O (l)                                                                                       NH4OH (aq)

The ammonium hydroxide partially ionizes to form ammonium ions and hydroxide ions and thus contains fewer hydroxide ions compared to sodium hydroxide.

NH4OH                                                                                           NH4+       +            OH-

The hydroxide ions are responsible for the alkaline properties. The higher the number of hydroxide ions a solution contains the higher the Ph value and the greater the conductivity.

This makes sodium hydroxide a stronger alkali than ammonium hydroxide solution.

ACID BASE REACTIONS

Acids react with bases to form a salt and water. The salt formed is dependent on the acid and the cat ion present.

These reactions are known as neutralization.

For example

Sodium hydroxide + hydrochloric acid                                               sodium chloride +    water

NaOH     +   H2SO4                                                                                        Na2SO4        +      H2O

Copper oxide  +   sulphuric VI acid                                                        copper sulphate  +  water

CuO      +      H2SO4                                                                                               CuSO4    +    H2O

Calcium hydroxide + nitric V acid                                                          calcium nitrate + water

Ca (OH) 2    +      HNO3                                                                                      Ca(NO3)2   +   HNO3

During these reactions, the hydrogen ions from the acid combine with hydroxide ions from the base to form water.



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