HYDROLYSIS

 HYDROLYSIS OF SALT :

WHEN A SALT IS DISSOLVED IN WATER , IT UNDERGOES IONIZATION . THE IONS OF SALT INTERACT WITH OPPOSITE IONS OF WATER TO FORM ACIDIC , BASIC, OR NEUTRAL SOLUTIONS. THIS PROCESS IS CALLED HYDROLYSIS .

HYDROLYSIS MAY BE DEFINED AS THE INTERACTION OF IONS OF A SALT WITH OPPOSITELY CHARGED IONS OF WATER TO GIVE ACIDIC OR BASIC SOLUTION.

FOR  EXAMPLE  GENERAL SALT  XY

                           XY  +  WATER(H₂O)   à   X⁺   +   Y^-

                            H₂O   --->     H⁺    +   OH^-

NEXT ,       RESULT =    XOH   +    HY

NOTE  HYDROLYSIS IS REVERSE OF NEUTRALISATION .

NOTE   THE EQUILIBRIUM CONSTANT OF HYDROLYSIS  REACTION IS CALLED HYDROLYSIS CONSTANT AND REPRESENTED BY K_h

 

DEGREE OF HYDROLYSIS

THE FRACTION OF THE TOTAL SALT THAT GETS HYDROLYSED AT EQULIBRIUM IS CALLED DEGREE OF HYDROLYSIS.

IT IS REPRESENTED BY h.

TYPES OF SALT :

     (a)    SALT OF STRONG ACID AND STRONG BASE 

     (b)   SALT OF WEAK ACIDS AND STRONG BASES

    (c)    SALT OF STRONG ACIDS AND WEAK BASES

    (d)   SALT OF WEAK ACIDS AND WEAK BASES

 

      

 

(    1)     SALT OF STRONG ACID AND STRONG BASE  :-

       EXAMPLES OF SALT ARE NACL , KNO₃ , KCL, NA₂SO₄ , NaNo₃

        Consider    NACL,

         NACL  ----->   NA⁺   +   Cl^-

         H₂O     <------>     H⁺   +  OH^-

NEITHER  NA⁺   IONS  NOR  Cl^-  IONS COMBINE WITH  OH^-  IONS OR H⁺   IONS.

SO,   [H⁺]     =       [OH^-]

HENCE, THE SOLUTION IS REMAINS NEUTRAL AND THE PH = 07 .

SO, SALT OF STRONG ACID AND STRONG  BASE IS NOT HYDROLYSED.

(     2)    SALT OF STRONG ACID AND WEAK BASE :-

EXAMPLES OF SALT ARE NH₄Cl

Consider     NH₄Cl

NH₄Cl       ------>     NH₄      +     Cl

 H₂O     <------>     H⁺   +  OH^-

HERE ,  THE OH^-  IONS OF WATER ATTACKS THE NH₄⁺  IONS OF THE SALT TO FORM NH₄OH  WHICH IS POORLY IONISED.  AS OH^- IONS ARE TAKEN UP , MORE  OF WATER IONISE TO MAINTAIN THE CONSTANT VALUES OF K_W .  DUE TO THIS CONCENTRATION OF H⁺ IONS IN SOLUTION BECOMES MORE THAN OH^- IONS CONCENTRATION.

SO, PH   <  7

     (3)    SALT OF STRONG BASE AND WEAK ACID :-

EXAMPLE OF SALT ARE CH₃CooNa

Consider    CH₃CooNa

CH₃CooNa    ------>       CH₃Coo^-   +  Na ⁺

H₂O     <------>     H⁺   +  OH^-

HERE , THE H⁺  IONS OF WATER ATTACK THE WEAK PART OF SALT  i.e.  THE CH₃Coo^-    ION WHICH IS POORLY IONISED. AS H⁺ IONS ARE TAKEN UP ,MORE OF WATER IONISES TO MAINTAIN THE CONSTANT VALUE OF K_W .  DUE TO THIS THE CONCENTRATION OF OH^- IONS IN SOLUTION BECOMES MORE THAN THE H⁺ ION CONCENTRATION.

SO , SOLUTION IS BASIC IN NATURE AND PH >  7

      (4)    SALT OF WEAK  ACID AND WEAK  BASE :-

EXAMPLE OF SALT ARE CH₃COONH₄

CONSIDER   CH₃COONH₄

CH₃COONH₄      -------->     CH₃COO^-    +   NH₄⁺

H₂O     <------>     H⁺   +  OH^-

HERE,  THE CH₃COO^-  IONS ATTACKS THE H⁺  IONS TO FORM CH₃COOH  (ACETIC ACID) . THE NH₄⁺   IONS  ATTACKS  THE OH^-  IONS TO FORM NH₄OH . SO, H⁺  ION CONCENTRATION IS EQUAL TO OH^-  ION CONCENTRATION BEFORE AND AFTER ADDITION .

SO, SOLUTION REMAINS NEUTRAL (MAY OR MAY NOT).