Hydrolysis of Salts
December 2015
What we know so far….
Bronsted-Lowry Acids
Donate H+ (protons)
Bronsted-Lowry Bases
Accept H+ (protons)
Criteria to be an acid:
• Must have an H+ to give away
• Ka > 10-14
• Can be neutral (HNO2), positive (NH4+
- the conj acid of NH3 ) or negative
(HSO3- - the second ionization of
H2SO3)
Criteria to be a base:
• Must have a lone pair of electrons in
order to attract an H+
• Kb > 10-14 *see next slide
• Can be neutral (NH3) or negative (CN-)
but NEVER positive
Why > 10-14?
Consider Cl-:
Problem:
• can’t act as an base because HCl is a strong acid
• Kb is too small (Kb = ___); won’t make enough OH- to influence the pH
Consider the base B with Kb = 1x10-15
B(aq) + H2O  BH+(aq) + OH-(aq)
E 0.10-x
x
x
Calculate x, pOH and pH. What is unusual about your answer?
Salts
What are salts?
Recall the 2 products that are formed when an acid and a base react:
Acid + Base  ______ + ______
i.e.
KOH(aq) + HBr(aq)  KBr(aq) + H2O(l)
Salts:
• are ionic compounds that completely dissociate in water
• water surrounds all anions and cations
• Can these ions interact with the surrounding water like acids
and bases do?
Hydrolysis of Salts
Consider the following salt solutions:
NaCl(aq)
pH = 7
NaNO2(aq)
pH > 7
NH4Cl(aq)
pH < 7
Conclusion:
• something in the NaNO2 solution is accepting H+ from H2O to produce OH• something in the NH4Cl solution is donating H+ to H2O to produce H3O+
• nothing is donating/accepting H+ in the NaCl solution.
Consider the basic nature of NaNO2 and the
criteria to be a base.
• What part of NaNO2 is acting as a base?
• Write the ionization reaction that represents this.
• What is the Kb for NO2-?
• What is the pH of a 0.10M solution of NaNO2?
Consider the acidic nature of NH4Cl and the
criteria to be an acid.
• What part of NH4Cl is acting as an acid?
• Write the ionization reaction that represents this.
• What is Ka for NH4+?
• What is the pH of a 0.25M NH4Cl solution?
Consider the neutral nature of NaCl. Why are Na+
or Cl- unable to interact with water that surrounds
them?
Hydrolysis of Salts
When anions or cations or BOTH in a salt solution interact
with the water that surrounds them to produce acidic or
basic solutions.