Download Salt Hydrolysis

Salt Hydrolysis
Salts
 Ionic compound made up of CATION and ANION
 Has acidic and basic properties
 Based on ions produced when salts dissociate
 No acid/base properties—group I/II cations
(ex. Na+, Li+, K+, Ca+2)
 No basic properties—conjugate bases from monoprotic
acids (ex. Cl-, Br-, NO3-)
 Ex. NaCl, CaBr2
Salt Hydrolysis
 Acid-base reactions between ions and water
 What do we mean by acidic, basic, or neutral salts?
 Think about how salt is formed……
1. Salt Formation from Strong
Base and Weak Acid
 Salt forms a BASIC solution.
 Conjugate base ion reacts with water to give hydroxide
(OH-) ions.
 Ex. Potassium fluoride (KF)
 KF  K+ + F F- + H2O  HF + OH-
2. Salt Formation from a
Strong Acid and Weak Base
 Salt forms an ACIDIC solution
 Conjugate acid reactions with water to give hydronium
ion (H3O+)
 Ex. Ammonium nitrate (NH4NO3)
3. Salt Formation from Strong
Acid and Strong Base
 Salt forms a NEUTRAL solution
 Conjugate base resulting from salt dissociation is weak
 Ex. Sodium chloride (NaCl)
Example 1:
 Calculate the concentration of HOAc, OAc- and OH- at
equilibrium in a 0.10M NaOAc solution (Ka for HOAc =
1.8 x 10-5).
Types of Acid-Base Reactions
1) Strong Acid/Strong Base
2) Weak Acid/Strong Base
3) Strong Acid/Weak Base
4) Weak Base/Weak Acid
1) Strong Acid/Strong Base
 Reaction goes to completion, K = 1x1014, pH = 7
 H3O+ + OH-  H2O(l) + H2O(l)
 Always this net ionic equation
 H+ and OH- form H2O
Example 2: HCl + NaOH 
2) Weak Acid/Strong Base
 Produces H2O and the conjugate base from weak acid
 Reaction goes MOSTLY to completion
 Solution’s pH is more dependent on CONJUGATE Base (A)’s reaction with water
 Find equilibrium constant (K) by combining known
dissociation reactions to get the correct overall equation and
solve for K
Example 3:
 HNO2 + OH-
H2O + NO2-
 HNO2 + H2O(l)  H3O+ + NO2- Ka = 5.1 x 10-4
 H3O+ + OH-  2H2O(l)
K = 1 x 1014
3. Strong Acid/Weak Base
 Weak bases tend to contain nitrogen-based
compounds such as NH3
 Weak bases accept protons from acid
 Combine known dissociation reactions to get the
correct overall equation and solve for K
Example 4:
 CH3NH2 + H3O+
CH3NH3+ + H2O(l)
 CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.8x10-4
 H3O+ + OH-  2H2O
K = 1.0x1014
4) Weak Base/Weak Acid
 Reaction direction is dependent on equilibrium constant
value.
 Most reactions do NOT go to completion
 Use this format and find Ka and Kb values for acid and
base dissociations.
 HA + H2O(l)  H3O+ + A B + H2O(l)  BH+ + OH H3O+ + OH-  2H2O(l)
Ka
Kb
K = 1.0 x 1014
Example 5:
 CH3NH2 + HNO2  CH3NH2+ + NO2-
 Krxn = (Ka) (Kb)(1x1014) = 2.5x107
Homework
 pp. 670 #63-64, 67-70