Chemistry: Ionic Compounds: Relative Strength of Acids and Bases (For CBSE, ICSE, IAS, NET, NRA 2022)

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Relative Strength of Acids and Bases

  • Various acids and bases have different strength which depends on their nature.
  • According to Arrhenius concept, in aqueous solutions strong electrolytes completely dissociates and produce H + ions and they are called as strong acids. Some examples of strong acids are sulphuric acid, nitric acid etc. E. g. HCl (g) + H2O H3O + (aq) + Cl- (aq)
  • The weak acids have weak electrolytes which ionize incompletely because of the reversible process and produce H + ions. E. g. CH3COOH CH3COOH (aq) + H2O H + (aq) + CH3COO-
  • The strong and weak bases are defined as the electrolyte which undergoes complete and incomplete ionization, respectively. NaOH is strong bases and NH3 is weak bases.
  • The strong bases are hydroxides of all elements in Group 1 and 2 except beryllium.
  • Most of the other bases are weak bases.
  • According to Bronsted-Lowry concept, an acid is a species that donates protons whereas bases are which accepts the protons, this depends on the species which is accepting the protons. E. g. 1: CH3COOH + NH2 CH3COO- + NH3 +-
  • The relative strength of different acids and bases are compared in same solvent such as water.
  • The relative strength of an acid is defined as the relative tendency to lose or donate a proton to water.
  • The strong acid such as HCl lose protons almost entirely in water as shown in equation below: HCl (aq) + H2O (aq) H3O + (aq) + Cl- (aq)
  • The weak acid acetic acid donates protons to water to the extent of only 3 % and the equation is given below: CH3COOH + H2O H3O ++ CH3COO-

Quantitative Aspects of Strength of Acids and Bases:

Ionization of Weak Acids

The ionization of weak acid, HA, can be represented as HA (aq) + H2O (I) H3O + (aq) + A- (aq)

The equilibrium constant is given as follow:

Keq = [H3O + ] [A-] [H2O] [HA]

(or) Keq = [H3O + ] [A-] = Ka [HA]

  • Where, Ka = acid dissociation constant or ionization constant of the acid
  • The ionization is complete in strong acids or 100 % , so the sign of equilibrium is represented by single arrow as shown in equation HA (aq) H + (aq) + A- (aq)
  • The strength of the acid is measured by the magnitude of the equilibrium constant. Usually, higher the value of equilibrium constant the stronger is the acid.
  • The constant Ka gives the extent to which the acids are dissociated in water and it depends on the temperature.

Ionization of Weak Bases

The ionization of the weak bases is represented as BOH (aq) B + (aq) + OH- (aq)

The equilibrium constant of the above equation is as follows:

Kb = [B + ] [OH-] = [BOH]

where Kb = Dissociation constant of base, B = Base

BH + = protonated base

OH = Hydroxide ion

  • The Ka and Kb value gives the relative strength of weak bases.
  • Higher the value of Kb stronger is the base

Polyprotic Acids

  • Polyprotic acids are acids which have more than one ionizable protons.
  • The acids are called as diprotic of it has 2 ionizable protons/molecule. E. g. H2SO4, H2CO3
  • The acid with 3 ionizable protons/molecule is called as triprotic acid. E. g. H3PO4

Degree of Ionization and Dissociation

  • The degree of ionization id defined as fraction of total amount of weak acod or base that exists in the ionized form and it is denoted by ‘a’ .
  • Consider a weak acid HA which partially dissociates in its aqueous solutions and The following equilibrium is established HA (aq) + H2O (I) H3O + (aq) + A- (aq)
Degree of Ionization and Dissociation
HAH2OH3O +A-
Initial concentration (moles)c00
Equilibrium concentrationsc (1-α)

The equilibrium constant expression can be written as

Since HA is weak acid so α ≪ 1, a is 1

  • So, Ka = cα2 or α = √ Ka/c
  • Similarly, for weak base, the degree of dissociation is given as .

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