Salt hydrolysis

1. Salt Hydrolysis IB syllabus reference : 18.3

2. Salt Hydrolysis Salt Hydrolysis : reaction of the anions or cations in a salt with water to form an acidic or basic solution.

3. Salt + water = ? Neutral solution Basic solution Acidic solution

4. Salt + water = ? Neutral solution: Conjugate of strong Acid + conjugate of strong Base  neutral solution Things to consider: Metal hydroxides of group I and II except Be are strong bases All other metal hydroxides are weak bases

5. Salt + water = ? Neutral solution: Basic solution: Conj. of weak acid + Conj. of strong base  basic solution

6. Salt Hydrolysis Example Sodium Ethanoate (CH 3 COONa) The salt of a weak acid (CH 3 COOH) and a strong base (NaOH) Completely ionizes in solution: CH 3 COONa  CH 3 COO - + Na + CH 3 COO - acts as a base Will take H+ from water! CH 3 COO - + H 2 O CH 3 COOH + OH - Why is the resulting solution basic?

7. Salt + water = ? Neutral solution: Basic solution: Acidic solution: Conj. of weak base + Conj. of strong acid  acidic solution

8. Salt Hydrolysis Example Ammonium Chloride (NH 4 Cl) NH 4 Cl is the salt of a strong acid (HCl) and a weak base (NH 3 ) Completely ionizes in solution: NH 4 Cl  NH 4 + + Cl - NH 4 + acts as an acid in water NH 4 + + H 2 O H 3 O + + NH 3 Why is the resulting solution acidic?

9. Question Time Determine if the following aqueous solutions will be acidic, basic or neutral: NH 4 NO 3 KCl NaHCO 3 Na 2 SO 3

10. General Rules When the negative ion is from a weak acid then the salt is basic by hydrolysis When the positive ion is from a weak base then the salt is acidic by hydrolysis If the salt is formed from a strong acid and strong base then it is neutral If the salt is formed from a weak acid and weak base then its hydrolysis is determined by the relative Ka and Kb values

11. Charge Density This effect is typified in aluminium salts (the aluminium ion has a charge of 3+) which are very acidic in solution.