Slow and fast rate of Chemical Reaction

RATE OF CHEMICAL REACTIONS


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ACID AND BASE



       The word acid is derived from Greek word which means ‘sour’. These have generally sour taste and they can change blue litmus into red. On the other hand the bases are the substance which have bitter taste and which can convert red litmus into blue. When bases are dissolved in water these are given a special name alkali.

Concepts of acids and bases

       Following are the three concepts of acids and bases-

    1. Arrhenius concept
    2. Bronsted- Lowry concept
    3. Lewis concept

1. Arrhenius concept

Acid
       According to Arrhenius an acid is a substance which dissociates in an aqueous solution to give hydrogen ions (H+). For example, HCl, HNO3, CH3COOH, H2SO4 etc. out of these acids some acids are strong acids while others are weak. The strength of an acid depends upon its degree of dissociation in water. The acids which can completely dissociate into ions on adding with water thus produce a large number of H+ ions are called strong acids e.g. HCl, HNO3 and H2SO4 are strong acids because they dissociate into ions completely. On the other hand the acids which partially dissociate into ions on addition with water and give small number of H+ ions are called weak acids e.g. CH3COOH is a weak acid. The dissociation of acids in water can be shown by following equations:

Strong acids:

HNO3          H+ + NO3-

H2SO4          2H+ + SO42-

HCl          H+ + Cl-

Weak acid:

CH3COOH           H+ + CH3COO-

Base

       According to Arrhenius a base is a substance which dissociate in an aqueous solution to give hydroxyl ions (OH-). For example, NaOH, KOH, Ca(OH)2 are bases because these can dissociate in water to give hydroxyl ions as given below-

Strong bases:

       The bases which completely dissociate into water and give a large number of hydroxyl ions (OH-) are called as strong base. For example, NaOH and KOH.

NaOH          Na+ + OH-

KOH          K+ + OH-

Weak bases:

       The bases which partially dissociate into ions on addition of water and give a small number of hydroxyl ions (OH-) are called weak bases. For example, Ca(OH)2.

Ca(OH)2           Ca2+ + 2OH-

2. Bronsted-Lowry concept

       According to Bronsted Lowry concept an acid is defined as a substance which has a tendency to donate a proton (H+) to any other substance and a base is a substance which has a tendency to accept proton (H+) from any other substance. In other words, an acid is proton donor and a base is proton acceptor. In terms of this definition, acids and bases are inter-related with each other as shown below:

Acid           Base + proton

Examples:

HCl + NH3           NH4+ + Cl -
Acid   base                                      

CH3COOH + NH3           NH4+ + CH3COO -
Acid       base                                          


In these examples HCl and CH3COOH lose a proton so these act as acids while NH3 accepts a proton so it acts as a base.

3. Lewis concept

       According to Lewis concept an acid is a substance which can accept a pair of electrons from other substances while a base is a substance which can donate a pair of electrons to other substances. In other words an acid is an electron pair acceptor while a base is an electron pair donor. For example,

NH3 + BF3           NH3-BF3
base                                  acid   




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