Salt hydrolysis

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, NaNO 3 , Na 2 SO 4 , KCl , KNO 3 , K 2 SO 4 , 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: CH 3 COONa, Na 2 CO 3 , K 2 CO 3 , Na 3 PO 4 , 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: NH 4 Cl, CuSO 4 , NH 4 NO 3 , AlCl 3 , CaCl 2 , FeCl 3 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: (NH 4 ) 2 CO 3 , AlPO 4 , CH 3 COONH 4 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 , CaSO 4 , PbSO 4 , PbCl 2 etc. are considered as insoluble salts. B ut 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( K sp ) 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. If ionic product (IP)< K sp , the solution is said to be unsaturated. It can dissolve more solute at a given temperature. If ionic product (IP)= K sp , the solution is said to be saturated. If ionic product (IP) > K sp , 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 AB 2 ……………..in board For a salt of the type AB 3 ………………..IN BOARD For a salt of the type A 2 B 3 ………..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 s olubility 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 The product of molar concentration of ions in the solution is called ionic product. Ionic product denotes the product of concentration terms in all types of solution whether saturated or unsaturated. Define Solubility product is the ionic product in the saturated solution.

14. Common ion effect: I t 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: 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: Precipitation of sulphides of group II metal ions (basic radicals) : In group II of qualitative analysis, when H 2 S 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 H 2 S is suppressed due to the presence of common hydrogen ion produced by HCl. H 2 S ⇋ 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 NH 4 OH is added to the original solution containing NH 4 Cl then the metal ions such as Fe +++ , Al +++ and Cr +++ are precipitated as their hydroxides. The role of NH 4 Cl in this case is to suppress the degree of ionisation of NH 4 OH due to common ion effect. NH 4 OH ⇋ NH 4 + + OH - NH 4 Cl ⇢ NH 4 + + 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 H 2 S gas is passed into the original solution containing NH 4 Cl and NH 4 OH, then the metal ions such as Ni ++ , Mn ++ , Zn ++ and Co ++ are precipitated in the form of their sulphides. …………..In board The role of NH 4 Cl is to suppress the degree of ionisation of NH 4 OH 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 H 2 S as OH - consumes H + to form H 2 O. ………in board This leads to an increase in concentration of S -- so that ionic product exceeds solubility product a nd 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 + H 2 S Pb ++ , Bi ++ , Cd ++ , As +++ , Sb ++ , Sn ++ , Ag + , Hg ++ ,Cu ++ As sulphides III A NH 4 Cl + NH 4 OH Fe +++ , Al +++ , Cr +++ As hydroxides IIIB NH 4 Cl + NH 4 OH + H 2 S Zn ++ , Mn ++ , Co ++ , Ni ++ As sulphides IV NH 4 Cl + NH 4 OH +(NH 4 ) 2 CO 3 Ca ++ , Sr ++ , Ba ++ As carbonates