Ionic Theory

Physics Chemistry  Biology  Mathematics
Science > Chemistry > Ionic EquilibriaYou are Here

Electrolytes on the Basis of Ionic Theory: 

  • According to Arrhenius ionic theory, a substance (acid) base or salt, which when dissolved in water splits up spontaneously into positively and negatively charged ions and aqueous solution has electrical conductivity is called as an electrolyte.
  • e.g. NaCl

Types of Electrolytes on the Basis of Ionic Theory:

Strong electrolytes:

  • Substances which dissociate almost completely in their aqueous solutions even at moderate dilutions are called as strong electrolytes. Their dissociation reaction is irreversible.
  • Examples:
    • Strong acids like HCl, HNO3 H2SO4 etc.
    • Salts like NaCl, KCl,
    • Substances like H2S etc.


Characteristics of Strong Electrolytes:

  • Substances which dissociate almost completely in their aqueous solutions even at moderate dilutions are called as strong electrolytes.
  • Law of mass action is not applicable since dissociation is irreversible.
  • Their solution has high conductivity.

Weak electrolytes:

  • Substances which dissociate to a little (limited) extent in their aqueous solutions are called weak electrolytes.
  • Examples:
    • All weak acids like CH3COOH, HCOOH, all weak bases like NH4OH, salts like CH3 COONH4, CH3COOAg etc.

Characteristics of weak Electrolyte:

  • Substances which dissociate to a little (limited) extent in their aqueous solutions are called weak electrolytes.
  • Law of mass action is applicable since dissociation is reversible.
  • Their solution has low conductivity

Ionisation and Dissociation on the Basis of Ionic Theory:

Ionisation:

  • It is the formation of the ions from molecules which are not initially in the ionic state.
  • Example: In HCl molecule, H and Cl atoms are covalently bonded. But when dissolved in water forms H+ and Cl ions.

HCl(aq)    ⇌     H+(aq)  +    Cl(aq)

Characteristics of Ionisation:

  • It is the formation of the ions from molecules which are not initially in the ionic state.
  • The molecules undergoing ionisation do not contain ions of the elements forming the molecule


Dissociation:

  • The spontaneous splitting of a substance into positively and negatively charged ions in an aqueous solution is called as dissociation.
  • Example: In NaCl molecule, Na and Cl atoms are bonded with an ionic bond. They exist in the ionic state even after formation of the compound.

NaCl(aq)    ⇌     Na+(aq)  +    Cl(aq)

Characteristics of Dissociation:

  • The spontaneous splitting of a substance into positively and negatively charged ions in an aqueous solution is called as dissociation.
  • The molecules undergoing dissociation contain ions of the elements forming the molecule

Ionic Theory:

Degree of Dissociation (α):

  • The fraction of the total number of moles of an electrolyte that ionises (or dissociates)  into ions in an aqueous solution at equilibrium is called as the degree of dissociation.
  • It is denoted by ‘α’
  • Degree of dissociation

Ionic Theory 01

Percentage dissociation or ionisation  = α  × 100

Factors Affecting the Degree of Dissociation:

  • The degree of dissociation or ionisation depends on following factors.
    • The nature of solute
    • The nature of solvent
    • Concentration of solution
    • Temperature
    • It increases with dilution and also with temperature

Expression for the Dissociation Constant of an Acid (Ka):

  • Let ‘HA’ be a weak acid.  In an aqueous solution, it dissociates to a limited extent and equilibrium exists as,

HA   +    H2O   ⇌  H3O+(aq)  + A(aq)

By applying the law of mass action to above equilibrium we have

Ionic Theory 02

  • Now water is present in large excess as a solvent, its concentration can be assumed to be constant. Thus [H2O] = constant. Now the molar concentration of hydronium ion and hydrogen ion is the same. hence, [H3O+] = [H+]

Hence the equation (1) can be written as

Ionic Theory 03

Where “Ka “ is the dissociation constant of the acid.

  • The ratio of the product of molar concentration of ions formed to the molar concentration of unionised acid molecule at equilibrium is called as the dissociation constant of an acid
  • The value Kis expressed in terms of moles/dm3. The greater is Ka value the stronger is the acid.


Expression for the Dissociation Constant of a Base (Kb):

  • Let ‘BOH’ be a weak base.  In an aqueous solution, it dissociates to a limited extent and equilibrium exists as,

BOH   +    H2O   ⇌  B+(aq)  + OH(aq)

By applying the law of mass action to above equilibrium we have

Ionic Theory 04

  • Now water is present in large excess as a solvent, its concentration can be assumed to be constant. Thus [H2O] = constant.

Hence the equation (1) can be written as

Ionic Theory 05

Where “Kb“ is the dissociation constant of the base.

  • The ratio of the product of molar concentration of ions formed to the molar concentration of unionised base molecules at equilibrium is called as the dissociation constant of a base.
  • The value of Kis expressed in terms of moles/dm 3.  The greater is Kb value the stronger is the base.

 Strength of Acids and Bases:

Strong Acids:

  • The acid which dissociates almost completely and produces a large number of H+ ions in aqueous solution is called as a strong acid.
  • Examples: HCl , HNO3 , H2SO4 , HCIO4 etc.

Characteristics of Strong Acids:

  • The concentration of H+ ions is more
  • pH of the solution in water is nearly zero.
  • They dissociate completely in water,  hence α = 1 or nearly equal to 1
  • They have high value of dissociation constant ka

Weak Acids :

  • The acid which dissociates to a small (limited) extent and produces a small number of H+ ions in an aqueous solution is called as a weak acid.
  • Examples: HCN, HCOOH, CH3COOH, H2CO3 etc.

Characteristics of Weak Acids:

  • The concentration of H+ ions is less.
  • pH of the solution in water is nearly seven.
  • They do not dissociate completely in water,  hence α is nearly equal to 0 and ( 1 – α) is nearly equal to 1.
  • They have low value of dissociation constant ka

Strong Bases:

  • A base which dissociates almost completely and produces a large number of OH ions in an aqueous solution is called as a strong base.
  • Examples: NaOH, KOH etc.

Characteristics of Strong Bases:

  • The concentration of OH ions is more.
  • pH of a solution in water is nearly 14.
  • They dissociate completely in water, hence α = 1 or nearly equal to 1
  • They have high value of dissociation constant kb


Weak Bases:

  • A base which dissociates to a small (limited) extent and produces a small number of OH ions in an aqueous solution is called as a weak base.
  • Examples: NH4OH, Ca(OH)2

Characteristics of Weak Bases:

  • The concentration of OH ions is less.
  • pH of the solution in water is nearly seven.
  • They do not dissociate completely in water,  hence α is nearly equal to 0 and  ( 1 – α) is nearly equal to 1.
  • They have low value of dissociation constant kb

Monobasic or Monoprotic Acid:

  • Acids like HCl, CH3COOH are called Monobasic or Monoprotic acids. One molecule of these acids produce one H+ ion hence they are called as Monobasic or Monoprotic acid.
  • For Monobasic acid,

Equivalent weight = Molecular weight

Normality = Molarity

 Dibasic or Diprotic Acid:

  • Acids like H2SO4 is called Dibasic or diprotic acids. One molecule of these acids produce two H+ ions hence they are called as dibasic or diprotic acid.
  • For dibasic acid

Equivalent weight = Molecular weight / 2

Monoacidic Base:

  • Bases like NaOH, KOH are called Monacidic bases. One molecule of these bases produces one    OH– ion hence they are called as Monacidic bases.
  • For Monoacidic base

Equivalent weight = Molecular weight

Normality = Molarity

Diacidic Base:

  • Acids like Ca(OH)2 is called Diacidic base. One molecule of these bases produces two OH ions. Hence they are called as diacidic base.
  • For diacidic base

Equivalent weight = Molecular weight / 2

Other Important Formulae Used in Numericals of Ionic Theory:

Ionic Theory 06

Ionic Theory 07



Important Terms Used in Ionic Theory:

Decimolar solution = 0.1 M solution

Semimolar solution = 0.5 M solution

Decinormal solution = 0.1 N soution

Seminormal solution = 0.5 N solution

M/5 solution = 1/5 M solution = 0.2 M solution

N/2 solution =  1/2 N solution = 0.5 N solution              

Science > Chemistry > Ionic EquilibriaYou are Here
Physics Chemistry  Biology  Mathematics

Leave a Comment

Your email address will not be published. Required fields are marked *