Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

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Chemistry : Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

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Salt Hydrolysis:

1.  Hydrolysis of salt of [SA – SB]:

  • Solution is neutral in nature (pH = pOH = 7)

  • Ex NaCl, Na2SO4, KNO3 etc.

2.  Hydrolysis of [WA - SB] 

  • In this type of salt hydrolysis anion reacts with water therefore called as anionic hydrolysis.

  • Solution is basic in nature as [OH] increases.

  • pH of solution is greater than 7.

  1. Relation between kh, kw, ka

CN + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev HCN + OH

                        Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                                              …1

For weak acid, HCN Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev H+ + CN

                        Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                                                  … 2

For water,    H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev  H+ + OH

                        Kw = [H+] [OH]                                            … 3    

eq (1) x eq. (2) = eq (3)

kb × ka= kw

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

b)  Degree of hydrolysis 

                        CN + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev HCN + OH

                         C                         0         0

                        C- Ch                Ch     Ch

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Since h <<< 1

1 – h ≈ 1

Hence, kh = Ch2

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

c)   pH of the solution

            [OH]  = Ch

            [OH] =  Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

            [OH] = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

            Kw = [OH] [H+]

            [H+] = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

            [H+] = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

On taking log both sides

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev           

 

3) Hydrolysis of strong Acid – Weak Base [SA - WB] 

 

                        NH4Cl, (NH4)2 SO4 ZnCl2 etc.

In this type of salt hydrolysis, cation reacts with H2O, therefore called as cationic hydrolysis.

Solution is acidic in nature as [H+] is increased.

pH of the solution is less than 7.

a)   Relation between kh, kw and kb

            NH4+ + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4OH + H+

Hydrolysis constant, kh = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                    …1

For weak base, NH4OH Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev  NH4+ + OH

Kb = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                         … 2

For water, H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev H+ + OH

Kw = [H+] [OH]                                                        … 3

Multiply eq. (1) x (2) = eq. 3

i.e. kh x kb = kw

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

b) Degree of hydrolysis – represented by h

                        NH4+ + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4OH + H+

                        C                           0            0

                        C – Ch                  Ch         Ch

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Since h <<< 1, then (1 – h) ≈ 1

Kh = Ch2

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

c)   pH of the solution

pH = - log [H+]

[H+] = Ch = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

On taking – log on both sides

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

4. Hydrolysis of [WA – WB]

ex: NH4CN, (NH4)2 CO3 etc.

Solution may be neutral, acidic or basic depending upon the nature of acid & base.

a) Relation between Kn1Kw1Ka and Kb

            NH4+ + CN + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4OH + HCN

            Kb =  Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                                                                              … 1

For weak base, NH4OH  Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4++OH

            Kb = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev                                                                                                      …2

For weak acid, HCN Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev H+ + CN

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

For water, H2O   Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev H+ + OH

Kw = [OH] [H+]

Multiply eq. (1) ×eq (2) ×eq (3) = eq (4)

Kh x kb x ka = kw

a) Degree of Hydrolysis 

                                    NH4+ + CN + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4OH + HCN

                                     C        C                            0               0

At equilibrium             C – Ch C – Ch                   Ch              Ch

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Since, h <<< 1, then (1 - h) ≈ 1

Kh = h2

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

b) pH of the solution 

for eq. (3)

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

[H+] = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Since, h <<< 1, then (1-h)  ≈ 1

[H+] = ka× h = ka× Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Taking log both sides

- log [H+] =Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Case I: If pka = pkb

Then pH = 7, the solution is neutral

Case II: If pKa>pkb

Then pH > 7, then solution is Basic

Case III: pKa<pKb

Then, pH < 7, solution is acidic

Example: what is the pH of 0.4 M aqueous solution of NaCN, given that the pkb (CN) is .70.

Solution:         CN + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev  HCN + OH

                        C                        O         O

                        C – Ch               Ch       Ch

pKb(CN) is given means equilibrium constant of this reaction is given. Because this reaction its express the basicity of CN ion.

pkb = - log kb

4.7 = - log kb

Kb = 1.9 × 10–5

now, kbSalt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

since h <<< 1, the (1 – h) ≈ 1

kb = Ch2

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

h = 6.89 × 103

[OH] = Ch = 0.4 × 6.89 × 103= 2.76 × 103

pOH = - log [2.76 × 103]

pOH = 2.55

pH = 11.45

Example : Find out the kh of a centimolar solution of NH4Cl. If the dissociation constant for NH4OH is 106 and kw = 1014. Find the degree of hydrolysis and pH of solution.

Solution: NH4Cl is a salt of strong acid and weak base.

                        NH4+ + H2O Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev NH4OH + H+

                        C                            0            0

                        C- Ch                   Ch        Ch

Equilibrium constant

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Kb = 106

Kh = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev  =108

Kh = Ch2

108 = Ch2

[H+] = Ch = 102× 103 = 105

pH = 5

Dissociation of Polyprotic Acid

H3A     Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev    H+        +          H2A'                            ka1

C- x                 x+y+z              x – y

H2A    Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev    HA2–    +          H+                                          ka2

x-y                   y – z                x+y+z

HA2–    Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev     A3–       +          H+                                          ka3

y-z                   z                      x+y+z 

assumption:     ka1>> ka2>>> ka3

hence, x + y + z ≈ x

x – y≈ x

y - z≈ y

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

So, Ka1 = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Ka2 = y

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

  • From eq1., you can calculate the value of x.

  • By putting value of x you will get the value of y from eq. 2

  • By putting values of x and y, you will get the value of 2 from eq. 3.

Hydrolysis of amphiprotic Anion:

This is one of the case of Anion hydrolysis, cations are not hydrolysed in this case.

NaHCO3etc. can undergo ionisation to form H+ ion and can undergo hydrolysis to form OH (Na+ ion is not hydrolysed)

                                   Ionization

a)  1)   HCO3 + H2O   Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev     CO32 +H3O+

                                                                    acid

                                      Hydrolysis

              HCO3 + H2O   Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev    H2CO+ OH

                                                                   Base

                        pH (HCO3)    = Salt Hydrolysis - Ionic Equilibrium Chemistry Notes | EduRev

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