Acid-Base Equilibria
Topic 16 in red book
Chapter 14 in textbook
Arrhenius
• acids increase [H+] when dissolved in water
• acids can be classified as monoprotic, diprotic
or triprotic
• bases increase [OH-] when dissolved in water
• bases can be classified as monobasic, dibasic,
or tribasic
A-B Strength
• strong acids & bases ionize completely and are
strong electrolytes
• 7 acids & 8 bases (memorize them!!!)
• diprotic acids & dibasic bases do NOT ionize
completely, only their first H+ or OH- ionizes
completely.
• Strong acid + strong base = neutral salt
Bronsted-Lowry A-B
• restricts definition to H+
– acids donate H+
– bases accept H+
• allows the classification of less traditional A-B
• conjugate A-B pairs = 2 formulas in an
equation whose formulas differ by a H+
• strong A-B are not equilibrium expressions,
but all other A-B are reversible
What is the acid, base, and the
conjugates?
HClO +H2O
H3O+ + ClO-
CO32- + H2O
OH- + HCO3-
Wait, water can go both ways?
• amphoteric substances can behave as either
an acid or base depending on what they react
with.
• water and anions with protons (H+) attached
are most common amphoterics
Weak A-B
• only partially ionize in water and are weak
electrolytes
• can be written as equilibrium expressions with a
Ka or Kb value
• K value indicates how much the acid or base will
ionize (high K = higher ionization)
• larger K values indicate a stronger acid or base
• For di- and tri- protic/basic, there will be 2 K
values (one for the first ionization and one for the
second)
Autoionization of Water
•
•
•
•
H2O + H2O
OH- + H3O+
reversible equilibrium where water can
donate a proton to itself
Kw = 1.0 x 10-14 at room temp.
For any conjugate A-B pair, Kw = Ka x Kb
What is the Ka value for NH4+?
Example
Is an aqueous solution of Na2HPO4 acidic or
basic?
Lewis A-B Theory
• very general definition of A-B using electron
pairs
– acid = electron pair acceptor
– base = electron pair donor
• In the following equation, which species acts
as a Lewis acid and which acts as a base?
BF3 + NH3
BF3NH3
pH scale
pH = -log[H+]
• works for pH ranges from 2-12 and
approximates pH outside that window
• The exponent on the [H+] is an indicator of
approximate pH.
Strong A-B
• no equilibrium b/c all acid/base ionizes
• use original acid concentration to calculate pH
• Calculate the [H+] and pH in a solution of
0.37M hydrochloric acid.
• Calculate the [OH-] and pH in a 0.58M solution
of NaOH
Weak Acids
• use RICE to find equilibrium concentrations:
R HA + H2O
H3O+ + AI I
0
0
C -x
+x
+x
E I-x
x
x
• b/c Ka for most weak acids is less than 10-3, I-x
is about equal to I, so Ka = x2/I
Example
What is the pH of a 0.20M solution of acetic acid
(CH3COOH)?
Weak Bases
• similar calculations as acids (replace H3O+ with
OH-)
R B + H2O
OH- + HB+
I I
0
0
C -y
+y
+y
E I-y
y
y
• b/c Kb for most weak bases is less than 10-3, I-y
is about equal to I, so Kb = y2/I
Example
What is the pH of a 0.68M solution of aqueous
ammonia?
Classify the following as
weak/strong acids/bases:
•
•
•
•
•
chloric acid
ammonium chloride
calcium hydroxide
ethyl amine
sodium cyanide
Example
Measurements show that the pH of a 0.10M
solution of acetic acid is 2.87. What is the Kb
of potassium acetate?
A-B properties of salt solutions
• for the most part anions are slightly basic (because they
attract protons) and cations are slightly acidic (because
they can donate protons)
• ions from strong A-B are the only neutral ions
• To determine if a salt is acidic or basic, look at the ions it
forms:
–
–
–
–
–
ignore any neutral ions
if anion is left, salt is basic
if cation is left, salt is acidic
if both cation & anion are neutral, salt is neutral
if both cation & anion are not neutral, the A-B-ness can’t be
determined from the formula
Classify the following salts as
acidic, basic, or neutral:
•
•
•
•
•
NaNO2
CH3NH3Cl
NaCl
MgSO4
Al2(SO3)3
A-B-ness & chemical structure
3 factors affect attraction of electrons (acidity
increases with stronger attraction of
electrons)
1. ionic charge – when comparing similar atoms,
more positive ions are stronger acids
2. oxidation # on central atom – when comparing
similar formulas with the same central atom, the
higher the ox#, the stronger the acid
3. electronegativity – when comparing similar
formulas with different central atoms, the higher
the EN, the stronger the acid
Common Base Reactions
• Strong bases also include hydrides (H-),
nitrides (N3-), and carbides (C22-)
– NaH + H2O  H2 + Na+ + OH– Mg3N2 + 6H2O  2NH3 + 3Mg2+ + 6OH– Ca2C2 + 2H2O  C2H2 + Ca2+ + 2OH-
• strong bases also include oxides of groups 1&2
metals
– Li2O + H2O  2Li+ + 2OH– CaO + H2O  Ca2+ + 2OH-
Common Acid Reactions
• nonmetal oxides (aka. acid anhydrides) turn
into acids when placed in water
– SO2 + H2O
– CO2 + H2O
– Cl2O7 + H2O 
H2SO3
H2CO3
2H+ + 2ClO4-