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Acids and Bases
…and Airman 1st class HCl flew
back over the front lines, confident
that he had neutralized the
enemy’s strongest base.
Arrhenius acids and bases
• Substances that ionize in water to form H+
ions are acids.
• Substances that ionize in water to form
OH- ions are bases.
Bronsted-Lowry Definition
• Substances that donate a proton (H+ ion)
in a reaction are acids.
• Substances that accept a proton (H+ ion)
are bases.
Lewis Definition
• Substances that accept an electron pair in
a reaction are acids.
• Substances that donate an electron pair
are bases.
A horse is a horse
(of course, of course)
• A base is a base. If it acts as a base by
one definition, it will not be considered an
acid by another in the same reaction.
• The purpose of the other definitions is to
extend the concept, not contradict it.
Conjugates
• After an acid has donated a proton, the
rest of the species is the conjugate base.
HAA- + H+
• After a base has accepted a proton, the
resulting species is the conjugate acid.
B- + H+ HB
What is the conjugate base of…
•
•
•
•
•
•
•
HCl
CH3COOH
H2SO4
HSO4H2O
NH4+
NH3
What is the conjugate base of…
ACID (loses H+ to form) Conjugate base
• HCl

Cl• CH3COOH 
CH3COO• H2SO4

HSO4• HSO4
SO4-2
• H2O

OH• NH4+

NH3
• NH3

NH2-
What is the conjugate acid of…
•
•
•
•
•
•
NO3C2O4-2
HPO4-2
HSO4H2O
F-
What is the conjugate acid of…
Base (gains H+ to form) Conjugate acid
• NO3
HNO3
• C2O4-2

HC2O4• HPO4-2

H2PO4• HSO4
H2SO4
• H2O

H 3O +
• F
HF
Show the conjugate acid/base pairs
in the following reactions.
• C2O4-2 + H3O+

HC2O4- + H2O
• CH3COOH + NH2-  NH3 + CH3COO-
Show the conjugate acid/base pairs
in the following reactions.
Acid
• C2O4-2 + H3O+
Base
Conjugate
Base

HC2O4- + H2O
Conjugate
Acid
• CH3COOH + NH2-  NH3 + CH3COO-
Show the conjugate acid/base pairs
in the following reactions.
Acid
• C2O4-2 + H3O+
Conjugate
Base

HC2O4- + H2O
Conjugate
Acid
Base
Base
Conjugate
Acid
• CH3COOH + NH2-  NH3 + CH3COOAcid
Conjugate
Base
Water dissociates
2H2O H3O+ + OH(or)
H2O  H+ + OHThis makes an equilibrium for water where:
• Kw=[H+][OH-]=1 x 10-14 at 25oC
(endothermic or exothermic?)
(Does Kw increase or decrease at higher T?)
[H+] is inversely related to [OH-]
• When [H+] increases, [OH-] decreases in a
water solution, and vice versa.
(Why?)
pH
• The basic (and acidic) definitions are:
•
•
•
•
pH= -log [H+]
[H+]= 10-pH
pOH= -log [OH-] [OH-]=10 -pOH
Kw=[H+][OH-]=1 x 10 -14 (at 25oC)
pH + pOH = 14 (at 25oC)
Please recall:
• Strong acids and bases dissociate
completely in a water environment.
Weak acids and bases do not.
• Strong acids -nitric, hydrochloric, sulfuric,
hydrobromic, hydroiodic, perchloric
• Strong bases-Group 1 & 2 hydroxides—
(group 2 to the extent of their solubility)
Contents
pH [H+] [OH-] pOH Acidic
or Basic
1 .023 mol HCl /L
2 1.5g NaOH /L
3 ?mol LiOH/50ml 8.5
4 ?mol KOH/25ml
5 ?gHClO4/150ml
6 ?molBa(OH)2/L
2.5
.02
.007
[H+]
(M)
[OH-]
(M)
pOH
Acidic or
Basic
1 .023 mol HCl /L 1.64
.023
4.3 x
10-13
12.36
Acidic
2 1.5g NaOH /L
12.57
2.7
x10-13
.0375
1.43
Basic
3 1.6 x10-7 mol
LiOH/ 50ml
8.5
3.2
x10-9
3.2
x10-6
5.5
basic
4 7.9 x10 -5 mol
KOH/25ml
11.5
3.2
x10-13
3.2
x10-3
2.5
basic
5 .30 gHClO4
/150ml
1.70
.02
5.0
x10-13
12.30
acidic
6 .0035mol
Ba(OH)2/L
11.85
1.4
x10-12
.007
2.15
basic
Contents
pH
Strength of acids and bases.
• HCl is a strong acid, it dissociates
completely
• H2CO3 is a weak acid, it does not
dissociate completely.
• CH4 is so weak it’s pathetic, it does not
dissociate to any measurable extent.
• What about their conjugates?
Strength of acids and bases.
• Cl- is a pathetic base, it does not associate
with water to any measurable extent.
• HCO3- is a weak base, it does not
associate completely with water.
• CH3- is so strong a base, it associates
completely with water, leaving hydroxide.
• What about their conjugates?
Strength of acids and bases.
• The conjugate of a strong acid is a
pathetic base
• The conjugate of a weak acid is a weak
base—the stronger the acid, the weaker
the base and vice versa
• The conjugate of a pathetic acid is a
strong base
When comparing weak acids and
bases…
• For a weak acid,
HAH++AKa=[H+][A-]/[HA]
• For a weak base,
B-+H2OHB+OHKb=[HB][OH-]/[B-]
• The position of the equilibrium is the
strength of the acid or base.
For example:
Ka=1.8x10-5 for acetic acid
Ka=6.5x10-5 for benzoic acid
For example:
Ka=1.8x10-5 for acetic acid
Ka=6.5x10-5 for benzoic acid
• Benzoic acid is a stronger acid.
For example:
Ka=1.8x10-5 for acetic acid
Ka=6.5x10-5 for benzoic acid
• Benzoic acid is a stronger acid.
• .10M solutions of each would have a lower
pH for benzoic acid.
For example:
Ka=1.8x10-5 for acetic acid
Ka=6.5x10-5 for benzoic acid
• Benzoic acid is a stronger acid.
• .10M solutions of each would have a lower
pH for benzoic acid.
• An acetic acid solution could have a lower
pH, at a higher concentration.
Write the reaction and eq.
expression for:
• Ammonia associating with water
• Ammonium dissociating in water
Write the reaction and eq.
expression for:
• Ammonia associating with water
NH3(aq) + H2O (l)  NH4+ (aq) + OH- (aq)
Kb = [NH4+][OH-]/[NH3]
• Ammonium dissociating in water
NH4+ (aq)  H+ (aq) + NH3 (aq)
Ka = [H+ ][NH3]/[NH4+]
And…
Kb Ka = [NH4+][OH-][H+ ][NH3]/[NH4+][NH3]
And…
Kb Ka = [NH4+][OH-][H+ ][NH3]/[NH4+][NH3]
And…
Kb Ka = [NH4+][OH-][H+ ][NH3]/[NH4+][NH3]
= [OH-][H+ ]=Kw
This is true for any conjugate pair in a water
solution
The leveling effect
• You can’t have a stronger acid than H+
(same as H3O+, in water) or a stronger
base than OH- in a water environment.
• In liquid ammonia, acids and bases are
leveled to NH2- (a stronger base) and NH4+
(a weaker acid)
Where does the equilibrium lie?
• HC2O4- + H2O  C2O4-2 + H3O+
• CH3COOH + NH2-  NH3 + CH3COO-
Where does the equilibrium lie?
• HC2O4- + H2O  C2O4-2 + H3O+
(left, K<<1)
• CH3COOH + NH2-  NH3 + CH3COO
(right, K>>1)
Where does the equilibrium lie?
F-+ HNO2

HF + NO2-
HCN + C6H5NH2  CN- + C6H5NH3+
HCO2H + NH3  HCO2- + NH4+
?pH
• What is the pH of a .25 M acetic acid
solution?
?pH
• What is the pH of a .25 M acetic acid
solution?
• Use the ICE method.
CH3COOH  CH3COO- + H+
I)
.25 M
0M
0M
C)
-x
+x
+x
E) .25-xM
xM
xM
• Ka= [CH3COO-][H+] / [CH3COOH]
(This might be written Ka =[A-][H+] / [HA] or
Ka =[Ac-][H+] / [HAc])
=(xM)(xM)/(.25-xM)=1.8 x 10-5
The two sides are equal where
x=[H+]=.0021M, pH=2.68
For a weak acid solution…
• For a weak acid solution …
• For a WEAK acid solution…
• x is very small, the .25 M doesn’t change
very much. Try it.
• Ka=(x)(x)/(.25M)=1.8 x 10-5
The two sides are equal where
x=[H+]=.0021M, pH=2.67
?pH
• Various examples, using ICE method—
• Concentrate on
changes
moles
molarity&volumes
Henderson Hasselbach equation (later)
Diprotic acids
• Start with what you are given,
• Conclude the direction of change based
on K’s
• Calculate things
Overtitration
• Once you’ve neutralized a weak acid or
base, ignore it.
• The pH is based on the excess of strong
acid or base added after.
• Subtract what was used in the
neutralization, divide the excess by the
total volume
Buffer solutions
• Resist changes in pH
• Composed of significant amounts of a
weak acid and its conjugate base
• Can be a partly neutralized weak acid, or
mixed with the sodium salt as the base
• In the buffer range, the amount shifting is
insignificant—ignore it. At the edges, use
ICE
The Henderson-Hasselbach
equation
• Finding pH of a buffer solution
• pH=pKa + log([base]/[acid])
• pOH=pKb + log([acid]/[base])
• --applies poorly when fraction is too large
or small
Weak acid/base problems
1.
2.
3.
4.
5.
6.
7.
8.
9.
.30
4.40
10.54
11.18
2.67
1.99
9.22
2.35
4.74
10. 9.48
11. 11.48
12. 11.60
13. 4.78
14. 4.66
15. 9.26
16. 3.15
17. 3.55
18. 2.83
19. 10.95
20. 11.43
21. 4.44
22. 3.47
23. 5.05
24. 4.79
25. 2.70 x 10-9
26. 1.85 x 10-10
Salt hydrolysis
• Q: What happens to the pH of a solution
when you add a salt?
Salt hydrolysis
• Q: What happens to the pH of a solution
when you add a salt?
• A: It depends if it is an acidic salt or a
basic salt.
Salt hydrolysis
• Q: What happens to the pH of a solution
when you add a salt?
• A: It depends if it is an acidic salt or a
basic salt.
• “I don’t know. Are you a good witch or a
bad witch?”
Neutral salts
• If the cation is a group 1 metal, and the
anion is the conjugate base of a strong
acid, no effect.
If not…
• The cation will show some tendency to
associate itself with hydroxide—making
more H+ in solution
• The anion will show some tendency to
associate itself with H+, leaving more OHin solution
Generally-• Small, highly charged cations are more
acidic
• Kb’s are given for many anions
• If Ka>Kb, the solution is acidic, If Ka<Kb,
the solution is basic
Acidic or basic solutions?
•
•
•
•
•
•
•
•
Aqueous:
Na2SO4
KBr
NH4Cl
Al(NO3)3
CuCl2
Li3PO4
NH4C2H3O2
Solubility Equilibria
• When a minimally soluble salt dissolves in
water,
Solubility Equilibria
• Write the reaction for dissolving a
minimally soluble ionic compound
• Write the equilibrium expression
• Given a Ksp—calculate solubility
• Given solubility—calculate Ksp
• Calculate solubility in a solution with one
of the ions already.