Hydrolysis of Salts
• when salts dissociate in water, the resultant hydrated ions may react further with Water
• the pH of the sol’n may change.
• this reaction with water is called hydrolysis of salts.
For the following 0.100 M solutions:
Na2CO3 predicted pH? < 7 7 > 7
Na2CO3(aq) ® 2 Na+(aq) + CO32- (aq)
then,
Na+ + 2 H2O ® NaOH + H3O+ ; Ka = very small
and
CO32- + H2O ® HCO31- + OH- ; Kb = 2.1 x 10-4
\ Kb > Kw > Ka \ pH > 7
Actual pH:
NH4Cl predicted pH? < 7 7 > 7
NH4C1(aq) ® NH4+(aq) + Cl- (aq)
then,
NH4+ + H2O ® NH3 + H3O+ ; Ka = 5.6 x 10-10
and
Cl1- + H2O ® HCl + OH- ; Kb = very small
\ Ka > Kw > Kb \ pH < 7
Actual pH:
NaC2H3O2 predicted pH? < 7 7 > 7
NaC2H3O2(aq) ® Na+(aq) + C2H3O21- (aq)
then,
Na+ + 2 H2O ® NaOH + H3O+ ; Ka = very small
and
C2H3O21- + H2O ® HC2H3O2+ OH-;Kb = 5.6 x 10-10
\ Kb > Kw > Ka \ pH > 7
Actual pH:
NH4C2H3O2 predicted pH? < 7 7 > 7
NH4C1(aq) ® NH4+(aq) + C2H3O21- (aq)
then,
NH4+ + H2O ® NH3 + H3O+ ; Ka = 5.6 x 10-10
and
C2H3O21- + H2O ® HC2H3O2+ OH-;Kb = 5.6 x 10-10
\ Ka = Kb neither dominates so Kw wins \ pH = 7
Or, NH4+ pH from above was 5.14 and C2H3O21- pH was 8.87 which adds to 14 and averages to 7.
Na2C2O4 predicted pH? < 7 7 > 7
Na2C2O4(aq) ® 2 Na+(aq) + C2O42- (aq)
then,
Na+ + 2 H2O ® NaOH + H3O+ ; Ka = very small
and
C2O42- + H2O ® HC2O41- + OH- ; Kb2 = 1.5 x 10-10
\ Kb2 > Kw > Ka \ pH > 7
Actual pH:
NaHCO3 predicted pH? < 7 7 > 7
NaHCO3(aq) ® Na+(aq) + HCO31- (aq)
then, (Ka = very small as before)
HCO31- + H2O ® H2CO3 + OH- ; Kb1 = 2.3 x 10-8
Or
HCO31- + H2O ® CO32- + H3O+ ; Ka2 = 4.7 x 10-11
\ Kb1 > Ka2 > Kw > Ka \ pH > 7
Actual pH:
Al(NO3)3 predicted pH? < 7 7 > 7
Al(NO3)3 + 6 H2O ® Al(H2O)63+ + 3 NO31-
then,
Al(H2O)63+ + H2O ® Al(H2O)5(OH)3+ + H3O+
Ka = 9.8 x 10-6
NO31- + H2O ® HNO3 + OH- ; Kb = very small
\ Ka > Kw > Kb \ pH < 7
Actual pH:
Lewis Acid/Base Theory
• this is a more general model that includes and extends the Brønsted-Lowry theory.
• Brønsted-Lowry acids are Lewis acids and Brønsted-Lowry bases are Lewis bases, but the opposite is not always the case.
Lewis Acids:
• is an electron pair acceptor ( B-L: proton donor)
• must have an empty valence orbital to accept (share) a pair of non-bonding electrons.
Lewis Bases:
• is an electron pair donor ( B-L: proton acceptor)
• must possess a pair non-bonding valence electrons