class 12

1. Salt hydrolysis

2. Salt hydrolysis
It is defined as the reaction of the cation or anion
of the salt with water to produce either acidic or
basic solution.
On the basis of relative strength of acid and base
produced due to hydrolysis, salts are classified into four
classes

3. a. Salts of strong acid and strong base:
NaCl, NaNO3, Na2SO4, KCl, KNO3, K2SO4, etc. are the examples of salts of
strong acid and strong base. Being these salts as strong electrolyte,
they give cation and anion when dissolved in water. These ions do not
hydrolyse i.e., these ions don’t interact with water. Consequently, there
is no change in concentration of H+ and OH- ions in the solution. Hence
the resulting solution is neutral.
Example:…………….in board
Hence it can be generalised that the salts of strong acids and strong
bases don’t undergo hydrolysis and the resulting solution is neutral.

4. b. Salts of weak acid and strong base:
CH3COONa, Na2CO3, K2CO3, Na3PO4, etc. are the examples salts of weak
acid and strong base. On treating these salts with water produce weak
acid and strong base.
Example………..in board
As it produces OH- ions, the solution of such salt is alkaline in nature.

5. c. Salts of strong acid and weak base:
NH4Cl, CuSO4, NH4NO3, AlCl3, CaCl2, FeCl3 etc. are the examples of salts
of strong acid and weak base. On treating these salts with water
produces strong acid and weak base.
Examples……………..in board
As it produces H+ ions, the solution of such a salt is acidic in nature

6. d. Salts of weak acid and weak base:
(NH4)2CO3, AlPO4, CH3COONH4 etc. are the examples of the salts of
weak acid and weak base. On treating these salts with water produces
parent weak acid and weak base.
Examples……………in board
Thus, it involves both cationic and anionic hydrolysis. The resulting
solution may neutral or slightly acidic or basic depending upon the
relative degrees of ionisation of the weak acid and the weak base
produced.

7. Solubility product:
Salts like AgCl, CaSO4, PbSO4, PbCl2 etc. are considered as insoluble
salts. But no salts is obsoletely insoluble in water i.e. they dissolve
slightly in water. So they are called sparingly soluble salts.
If a sparingly soluble salt like AgCl is stirred in water, it dissolves in
water to a very small extent and forms the saturated solution at a given
temperature. At saturated condition, the ions in the solution collide
and reproduce solid phase. Ultimately, a dynamic equilibrium is
established between the solute and ions in the solution.
Examples…………in board.

8. Thus in general the solubility product(Ksp) of a sparingly soluble salt at a
given temperature is defined as the product of molar concentration of
ions in saturated solution in which each concentration terms being
raised to the power equal to stoichiometric coefficient of ions in
balanced equation.

9. Depending upon the value of ionic product, a solution may be
unsaturated, saturated or super saturated.
1. If ionic product (IP)< Ksp, the solution is said to be unsaturated. It
can dissolve more solute at a given temperature.
2. If ionic product (IP)= Ksp, the solution is said to be saturated.
3. If ionic product (IP) > Ksp, the solution is said to be supersaturated.
Hence, precipitation of the salt takes place.

10. By comparing the value of ionic product with solubility product, it is
possible to predict whether precipitation will occur or not when two
solutions containing common ions are mixed together. If ionic product
is greater than the solubility product then precipitation will take place
otherwise not. This principle is called theory of precipitation or
solubility product principle

11. Relation between solubility and solubility
product:( numerical basis)
• For a salt of the type AB:…………in board
• For a salt of the type AB2……………..in board
• For a salt of the type AB3………………..IN BOARD
• For a salt of the type A2B3………..CLASS WORK

12. Relation between solubility and solubility
product (theoretical basis):
The solubility product of sparingly soluble salt is related with solubility.
Solubility is defined as the amount of solute in gram dissolved in 100g
of solvent to prepare saturated solution at a given temperature.
The solubility is applicable to all the solutes i.e. electrolytes as well as
non electrolytes. But solubility product is applied only for sparingly
soluble electrolytes. The magnitude of solubility for a given electrolyte
decreases on addition of other electrolyte having common ion at a
given temperature but solubility product remains constant even on the
addition of electrolyte having common ion. It is known that solubility
as well as solubility product increases with increase in temperature.

13. Difference between ionic product and
solubility product
Ionic product solubility product
1. The product of molar
concentration of ions in the
solution is called ionic product.
2. Ionic product denotes the
product of concentration terms
in all types of solution whether
saturated or unsaturated.
1. Define
2. Solubility product is the ionic
product in the saturated
solution.

14. Common ion effect:
It is defined as, “The suppression of degree of ionisation of weak
electrolyte in the presence of strong electrolyte having a common ion to
the weak electrolyte.”
For example:, addition of ammonium chloride to ammonium hydroxide
suppresses the degree of ionisation of ammonium hydroxide. This is
due to common ion.
……………in board

15. Application of common ion effect and
solubility product:
1. Predicting precipitation of salt on mixing the solution: The principle of
solubility product and common ion effect are applied in the precipitation
of soluble salts in pure state from their saturated solution.
For the precipitation of NaCl, its saturated solution is prepared by
dissolving in minimum quantity of water. Then HCl gas is passed through this
solution.
………….In board
Due to the presence of common chloride ion, the ionisation of NaCl is
suppressed, thus NaCl is precipitated.
Thus, when HCl gas is passed,[Cl-] increases, thus ionic product i.e.[Na+][Cl-]
exceeds the value of Ksp, so NaCl is precipitated.

16. 2. Precipitation of metal ions in qualitative
analysis:
a) Precipitation of sulphides of group II metal ions (basic radicals): In
group II of qualitative analysis, when H2S gas is passed into the O.S
containing HCl, then the metal ions such as Pb++, Cu++, Hg++, Cd++ etc., are
precipitated in the form of their sulphides. The ionisation of H2S is
suppressed due to the presence of common hydrogen ion produced by
HCl.
H2S ⇋ 2H+ + S--
HCl ⇢ H+ + Cl-
Thus, in group II, the concentration of S-- ions is decreased due to
common ion effect. In such a solution the ionic product of sulphide ions
and metal ions will be quite low, but their solubility product are quite low
than the ionic product. That is why metal ions of group II are precipitated
as sulphides

17. b) Precipitation of hydroxides of group III A metal ions(basic radicals): In
group IIIA of qualitative analysis when NH4OH is added to the original
solution containing NH4Cl then the metal ions such as Fe+++, Al+++and Cr+++
are precipitated as their hydroxides. The role of NH4Cl in this case is to
suppress the degree of ionisation of NH4OH due to common ion effect.
NH4OH ⇋ NH4
+ + OH-
NH4Cl ⇢ NH4
+ + Cl-
The solubility product of hydroxides of group III A metal ions is relatively
low. Therefore, with this suppressed concentration of OH- ion, their ionic
product exceed their solubility product and hence they get precipitated
as their hydroxides.

18. c) Precipitation of sulphides of group III B metal ions: In group III B of
qualitative analysis, when H2S gas is passed into the original solution
containing NH4Cl and NH4OH, then the metal ions such as Ni++, Mn++, Zn++
and Co++ are precipitated in the form of their sulphides.
…………..In board
The role of NH4Cl is to suppress the degree of ionisation of NH4OH in order
to maintain low concentration of OH- ions. This low concentration of OH- ion
prevent precipitation of hydroxides of group III B metal ions. Presence of OH-
ions further increase ionisation of H2S as OH- consumes H+ to form H2O.
………in board
This leads to an increase in concentration of S-- so that ionic product exceeds
solubility product and only group IIIB get precipitated

19. Home work
d) Precipitation of carbonates of group IV basic radicals:

20. Group Group reagents cations Form of
precipitates
II A Dilute HCl + H2S Pb++, Bi++,
Cd++, As+++,
Sb++, Sn++, Ag+,
Hg++,Cu++
As sulphides
III A NH4Cl + NH4OH Fe+++, Al+++,
Cr+++
As hydroxides
IIIB NH4Cl + NH4OH + H2S Zn++, Mn++,
Co++, Ni++
As sulphides
IV NH4Cl + NH4OH
+(NH4)2CO3
Ca++, Sr++, Ba++ As carbonates