Transcript Acid-Base Properties of Salts - Ms. Witt's Site

Acid-Base Properties of Salts
• Salt = ionic compound
• Dissolving in water will break it into its
ions which can behave like acids or
bases
Claudia Witt:
Salts Producing Neutral Solutions
List strong
bases (circle
cations) and
strong acids
(circle anions)
to show salts
formed
• Salts that consist of the cations of strong
bases and the anions of strong acids have
no effect on [H+] when dissolved in water
– They will make a neutral solution (pH = 7)
Salts Producing Basic Solutions
• Start by determining which major species has
the largest effect on the pH
– Example: aqueous solution of sodium acetate
(NaC2H3O2)
• Ka X Kb = Kw
• The Ka value of the parent weak acid (acetic
acid above) can be used to determine the Kb
• For any salt whose cation has neutral
properties and whose anion is the conjugate
base of a weak acid, the aqueous solution will
be basic.
Practice with Kb
• Using the data, calculate Ka or Kb and
write the reaction with water for each of
the following aqueous ions:
– NO2-(Ka for HNO2 = 4.0 X 10-4)
– F- (Ka for HF = 7.2 X 10-4)
– C6H5NH3+ (Kb for aniline, C6H5NH2, equals
3.8 X 10-10)
Example
• Calculate the pH of a 0.30 M NaF solution.
The Ka value for HF is 7.2 X 10-4.
– Answer: 8.31, basic solution
Salts Producing Acidic Solution
• Salts in which the anion is not a base
and the cation is the conjugate acid of a
weak base will produce acidic solutions
• Example: Calculate the pH of a 0.10 M
NH4Cl solution. The Kb value for NH3 is
1.8 X 10-5.
– pH = 5.13
Salt Producing Acidic Solution
• Second Type: salt that produces an acidic
solution that has a highly charged metal ion like
AlCl3…Al cation has a 3+ charge
• A higher charged metal ion has a stronger
acidity in the hydrated ion
• Ex: Calculate the pH of a 0.010 M AlCl3 solution
when the Ka value for Al(H2O)63+ is 1.4 X 10-5
• Answer: pH = 3.43
Predicting Acidity Using Ka
• If Ka value for the acidic
ion is greater than Kb
value for the basic ion,
the solution will be acidic.
• If the Kb value is larger
than Ka, the solution will
be basic
• If Ka = Kb, the solution will
be neutral
Example
• Predict whether an aqueous solution of
each of the following salts will be acidic,
basic, or neutral
– NH4C2H3O2
– NH4CN
– Al2(SO4)3
Summary
Structure’s Effect
• Bond Strength and Polarity will determine the likelihood of it to
donate/accept a hydrogen ion
– Bond Strength: electronegativity comes into play. Higher
electronegativity = more polar = dissociates more completely in water
• Oxyacids (H - O - X): acid strength increases with more oxygen
atoms attached to the central atom
– Oxygen (highly electronegative) pulls the electrons closer to one side
and is more likely to produce a hydrogen ion
• Attached ions: greater charge of ion = more acidic
Acid Vs. Base with O-X Bonds
• H - O - X bond will produce an acid in water if
the O-X bond is strong and covalent (X has a
high electronegativity). When dissolved in
water, the O-X bond will not break (the H-O
bond will instead). These are called acidic
oxides.
• If O-X bond is ionic (X has a low
electronegativity), the bond will break in water
and a base will be produced. These can
consist of group 1A and 2A metals and are
called basic oxides.