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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Ionic Equilibria Page 1

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1. Introduction

Historically, acids are substances which are

  • sour in taste
  • turn blue litmus red.

Bases

  • have bitter taste
  • are soapy to touch
  • turn red litmus blue.

 

2. Arrhenius Theory of Acids and Bases

  • Acid is a substance that releases H+ ions (proton) in aqueous solution
    e.g. HCl(aq) H+(aq) + Cl
    -(aq)
    H2SO4(aq) H+(aq) + HSO4
    -(aq)
  • Base furnishes hydroxide ions (OH-) when dissolved in water
    e.g. NaOH(aq) Na+(aq) + OH- (aq)
    Ca(OH)2(aq) Ca+2(aq) + 2OH- (aq)

 

3. Limitations of Arrhenius Theory

  • The theory defines acids and bases in terms of their aqueous solutions rather than on the basis of the substances themselves.
  • It considers substances like HCl as acid only in water and not in non-aqueous solvents.
  • The theory does not explain basic nature of substances like pyridine, NH3, etc., which do not have OH- ions in their structure.
  • The theory does not explain an amphoteric behaviour of compounds like Zn(OH)2.
  • It does not explain acidic nature of salts like FeCl3 and basic nature of ammonia.

 

4. Lowry-Bronsted Theory

  1. Acid Substances which donate hydrogen ions (H+).
    Example HCl, H2SO4, CH3COOH.
  2. Base Substances which accept protons (H+) are called bases.
    Example NH3, H2O, OH
    -.

 

5. Conjugate Acid-base Pair

Pair of acid and base which differ only by a proton. Acid donates proton to form a base, while base accepts the proton. Thus, an acid loses a proton to form a conjugate base and a base accepts to form a conjugate acid.

Cl
- is a conjugate base of H-Cl
H3O+ is a conjugate acid of a base H2O.

Water is amphoteric according to Lowry and Bronsted theory. It functions as acid as well as base depending upon the nature of substance dissolved in it.

 

6. Lewis Acid-base Theory

Acid is a substance which can accept a lone pair of electrons.
Example H+, AlCl3, BF3.
BaseSubstance which can donate a lone pair of electrons.
Example Cl
-, H2O, OH-.

 

7. Ionisation

  • Ionisation: Formation of ions from substances which are not in the ionic state.
  • Dissociation: Formation of free ions capable of carrying electric current.

 

8. Degree of Dissociation

The fraction of total number of molecules of an electrolyte, which undergoes dissociation at equilibrium is called degree of dissociation (a).

 

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