Name: pH of a Solution
Uploaded: 2019-07-19
Duration: 4 min 31 s
Description: This video discusses the pH scale and how it is useful to determine the behaviour of an acidic or basic solution. Does it also tell what the pH of a neutral...

Question

Describe the various postulates of Arrhenius theory of acids and bases. What are the limitations of this theory?

Accepted Answer

The various postulates of Arrhenius theory of acids and bases are given below:

(i) According to this concept, acids are substances which produce hydrogen ions when dissolved in water.

Example: $HCl \overset{}{\to} H^{+} + {Cl}^{-}$

(ii) Bases are substances which produce hydroxyl ions when dissolved in water.

Example: $NaOH \overset{}{\to} {Na}^{+} + {OH}^{+}$

(iii) Neutralisation of an acid and a base is based on the reaction between $H^{+}$ ions and ${OH}^{-}$ ions to form undissociated water molecules.

$H^{+} + {OH}^{-} \overset{}{\to} H_{2} O$

(iv) During electrolysis of an aqueous solution of an acid, $H^{+}$ ions are produced in the cathode and negative ions are produced in anode.

Thus, $HCl (aq) \overset{}{\to} H^{+} (aq) + {Cl}^{-} (aq)$

At cathode: $2 H^{+} (aq) + 2 e^{-} \overset{}{\to} H_{2} (g)$

At anode: $2 {Cl}^{-} (aq) - 2 e^{-} \overset{}{\to} {Cl}_{2} (g)$

Limitations of Arrhenius theory:

(i) This theory explains the acidic and the basic characters of the substances in aqueous solution. But, it fails to explain the existence of ions even in the solid state as in sodium hydroxide ( ${Na}^{+} {OH}^{-}$ ).

(ii) According to this theory, an acid releases $H^{+}$ ions in aqueous medium, but it cannot exist independently in aqueous state. It accepts a pair of electrons from water molecule to form hydrated proton or hydronium ion.

(ii) There are a number of substances which do not provide $H^{+}$ ions and ${OH}^{-}$ ions , but still behave as acids and bases. Example: ${CO}_{2}$ is an acid and ${NH}_{3}$ is a base.

Describe the various postulates of Arrhenius theory of acids and bases. What are the limitations of this theory?

Important Questions on Equilibria

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