ACIDS AND BASES –
Strength of acids and bases I
Peter Chimtali
1M HCl(aq)
1M CH3COOH(aq)
Reaction of zinc with a strong and a
weak acid.
1
Strength of acids and bases
•
Strength refers to how much an acid or base ionizes in
solution
Strong
Weak
• Strong electrolytes
• Weak electrolytes
• Ionize completely (~100%)
• Ionize partially (usually <5%)
• An equilibrium reaction
2
The extent of dissociation for strong acids.
Strong acids are assumed to dissociate completely in
solution.
Reactant
favoured or
product
favoured?
product
Strong acid: HA(g or l) + H2O(l)
H3O+(aq) + A-(aq)
The extent of dissociation for weak acids.
Weak acids are assumed to dissociate only slightly (less than 5%) in
solution.
Before
dissociation
Reactant
favored or
product
favored?
After dissociation, at
equilibrium
Reactant
Solutions of weak acids and bases contain ionized and non-ionized
species.
Weak acid: HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Strong Acid (HCl)
Weak Acid (HF)
HF (aq) + H2O (l)
HCl (aq) + H2O (l)
H3O+ (aq) + F- (aq)
H3O+ (aq) + Cl- (aq)
5
Concentration vs- Strength
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
A stronger acid can donate H+ more readily
than a weaker acid.
Concentration vs- Strength
1M HCl(aq)
1M CH3COOH(aq)
Reaction of zinc with a strong and a weak acid.
The Acid-Dissociation Constant (Ka)
Strong acids dissociate completely into ions in water:
HA(g or l) + H2O(l)
H3O+(aq) + A-(aq)
In a dilute solution of a strong acid, almost no HA molecules
exist: [H3O+] = [HA]init or [HA]eq = 0
+][A-]
[H
O
3
Qc =
[HA][H2O]
at equilibrium, Qc = Kc >> 1
Nitric acid is an example: HNO3 (l) + H2O(l)
H3O+(aq) + NO3-(aq)
Weak acids dissociate very slightly into ions in water:
HA(aq) + H2O(aq)
H3O+(aq) + A-(aq)
In a dilute solution of a weak acid, the great majority of HA
molecules are undissociated: [H3O+] << [HA]init or [HA]eq = [HA]init
[H3O+][A-]
Qc =
[HA][H2O]
at equilibrium, Qc = Kc << 1
The Meaning of Ka, the Acid Dissociation Constant
For the ionization of an acid, HA:
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Since the concentration of water is
[H3O+] [A-]
Kc =
high, and does not change significantly
[HA] [H2O]
during the reaction, it’s value is absorbed
Therefore:
into the constant.
[H3O+] [A-] The stronger the acid, the higher the [H3O+]
Kc =
at equilibrium, and the larger the Ka:
[HA]
Stronger acid
higher [H3O+]
larger Ka
For a weak acid with a relative high Ka (~10-2 ), a 1 M solution
has ~10% of the HA molecules dissociated.
For a weak acid with a moderate Ka (~10-5 ), a 1 M solution
has ~ 0.3% of the HA molecules dissociated.
For a weak acid with a relatively low Ka (~10-10 ), a 1 M solution
has ~ 0.001% of the HA molecules dissociated.
ACID STRENGTH
Strong bases dissociate completely into ions in water
NaOH (s)
H 2O
Na+
(aq) +
OH-
Kc >> 1
(aq)
Weak bases dissociate very slightly into ions in water
NH3 (aq) + H2O (l)
B (aq) + H2O (l)
Kc << 1
NH4+ (aq) + OH- (aq)
BH+ (aq) + OH- (aq)
[OH-][BH+]
[OH-][BH+]
Kc =
Kc [H2O] = Kb =
[H2O][B]
[B]
stronger base →
higher [OH-]
→larger Kb
Kb is the base ionization constant
Kb is the equilibrium constant for
ionization of the Brønsted base
Kb
weak base
strength
12
BASE STRENGTH
Strength of conjugate acid-base pairs
Conjugate Pair
A-(aq) + H3O+(aq)
conjugate conjugate
base
acid
HA(aq) + H2O(l)
acid
base
Ka =
[H3O+][A-]
[HA]
Conjugate Pair
Acid Is
strong
weak
[H3O+]
high
low
Ka
large
small
Equil.
right
left
Conjugate base is
weak
strong
The weaker the acid, the stronger its conjugate base.
The stronger the acid, the weaker its conjugate base.
Strength of conjugate acid-base pairs
Conjugate Pair
BH+ (aq) + OH- (aq)
B (aq) + H2O (l)
base
acid
conjugate
acid
conjugate
base
Kb =
[OH-][BH+]
[B]
Conjugate Pair
Base Is
strong
Weak
[OH-]
high
low
Kb
large
small
Equil.
right
left
Conjugate acid is
weak
strong
The weaker the base, the stronger its conjugate acid.
The stronger the base, the weaker its conjugate acid.
Classifying the relative strength of acids and bases
• Strong acids
1. The hydrohalic acids HCl, HBr and HI
2. Oxoacids in which the number of O atoms exceeds
the number of ionizable protons by two or more,
such as HNO3, H2SO4 and HClO4
• Weak acids
1. The hydrohalic acid HF
2. Acids in which H is not bonded to O or to a halogen,
such HCN
3. Oxoacids in which number of O atoms equals or
exceeds by one the number of ionizable protons such
as HClO, HNO2 and H3PO4
4. Carboxylic acids (RCOOH)
• Strong bases: water-soluble compounds containing O2-
and OH- . Cations usually those of the most active metals.
• Weak bases
1.
Ammonia (NH3)
2.
Amines (RNH2, R2NH, R3N)
Classify each of the following compounds as a
strong acid, weak acid, strong base, or weak base:
a)
H2SeO4
b) (CH3)2CHCOOH
c) KOH
d) (CH3)2CHNH2
•
The conjugate base of a strong acid has no measurable strength.
The
stronger the
acid …
… the
weaker the
conjugate
acid.
… the
weaker the
conjugate
base.
And the
stronger the
Ionization Constants of Conjugate Acid-Base Pairs
H3O+(aq) + A-(aq)
HA(aq) + H2O(l)
A-(aq) + H2O (l)
HA (aq) + OH- (aq)
H2O(l) + H2O(l)
H3O+(aq) + OH-(aq)
KaKb = Kw
[H3O+][A-]
Ka =
[HA]
[OH-][HA]
Kb =
[A-]
Kw = [H3O+][OH-]
= 1.0 x 10-14
Weak Acid and Its Conjugate Base
Kw
Ka =
Kb
Kw
Kb =
Ka
Ionization Constants of Conjugate Acid-Base Pairs
NH4+ (aq) + H2O(l)
H3O+(aq) + NH3 (aq)
NH3(aq) + H2O (l)
NH4+ (aq) + OH- (aq)
H2O(l) + H2O(l)
H3O+(aq) + OH-(aq)
KaKb = Kw
[H3O+][NH3]
Ka =
[NH4+]
[OH-][NH4+]
Kb =
[NH3]
Kw = [H3O+][OH-]
= 1.0 x 10-14
Weak Acid and Its Conjugate Base
Kw
Ka =
Kb
Kw
Kb =
Ka
Direction of acid-base reactions
The stronger an acid, the weaker is its conjugate base.
The stronger a base, the weaker is its conjugate acid
Conjugate Pair
stronger
acid
stronger
+ base
weaker
base
+
weaker
acid
Conjugate Pair
An acid–base reaction is favoured in the direction
from the stronger member to the weaker member of
each conjugate acid–base pair.
In any acid-base reaction, the equilibrium will favour the
reaction that moves the proton to the stronger base.
Relative Acid-Base Strength and Reaction Direction
General Rule: An acid-base reaction proceeds to the greater extent
in the direction in which a stronger acid and stronger base form a
weaker acid and a weaker base.
a
b
b
a
NO3- + H3O+
HNO3 + H2O
Kc > 1
A competition for the proton between the two bases!
a
b
HF + H2O
18-23
b
a
F- + H3O+
Kc < 1
Predict the net direction and whether Kc is greater or less than 1 for
each of the following reactions (assume equal initial concentrations
of all species):
(a) H2PO4-(aq) + NH3(aq)
HPO42-(aq) + NH4+(aq)
(b) H2O(l) + HS-(aq)
OH-(aq) + H2S(aq)
PLAN: Identify the conjugate acid-base pairs. Determine the relative
strength of each (from a Ka table). The stronger the species, the
more preponderant its conjugate.
SOLUTION:
(a) H2PO4-(aq) + NH3(aq)
stronger
stronger
acid
base
HPO42-(aq) + NH4+(aq)
weaker
weaker acid
base
Net direction is to the right with Kc > 1.
(b) H2O(l) + HS-(aq)
weaker
weaker
acid
base
OH-(aq) + H2S(aq)
stronger base stronger acid
Net direction is to the left with Kc < 1.
Ionized acid concentration at equilibrium
percent ionization =
x 100%
Initial concentration of acid
For a monoprotic acid, HA:
HA(aq) + H2O(l)
[H3O+]
x 100%
Percent ionization =
[HA]0
[HA]0 = initial concentration
%ionization
[HA]0
H3O+(aq) + A-(aq)
weak acid
strength
%ionization
25
p (anything) = -log (anything)
Other important relationships
pKa = -log Ka
Because
then
and
pKb = -log Kb
KaKb = Kw = 1.0 x
10-14
For a conjugate
acid–base pair
-log Ka – log Kb = -log Kw = 14.00
pKa + pKb = pKw = 14.00
NOW [H+],[OH-],pH, Ka, Kb, pKa, pKb pOH are all
related
26
Acid Name (Formula)
Ka at 250C
pKa
Hydrogen sulfate ion (HSO4-)
1.02x10-2
1.991
Nitrous acid (HNO2)
7.1x10-4
3.15
Acetic acid (CH3COOH)
1.8x10-5
4.74
Hypobromous acid (HBrO)
2.3x10-9
8.64
Phenol (C6H5OH)
1.0x10-10
10.00
Ka
pKa
ACID STRENGTH
The Relationship Between Ka and pKa
Diprotic and Triprotic Acids
• May yield more than one hydrogen ion per molecule.
• Ionize in a stepwise manner; that is, they lose one proton at
a time. Conjugate base of first step is acid of second step.
•An ionization constant expression can be written for each
ionization stage.
28
Monoprotic acids
HCl
H+ + Cl-
HNO3
H+ + NO3-
H+ + CH3COO-
CH3COOH
Strong electrolyte, strong acid
Strong electrolyte, strong acid
Weak electrolyte, weak acid
Diprotic acids
H2SO4
H+ + HSO4-
Strong electrolyte, strong acid
HSO4-
H+ + SO42-
Weak electrolyte, weak acid
Triprotic acids
H3PO4
H2PO4HPO42-
H+ + H2PO4H+ + HPO42H+ + PO43-
Weak electrolyte, weak acid
Weak electrolyte, weak acid
Weak electrolyte, weak acid
29
Polyprotic acids
acids with more than one ionizable proton
Percent HA dissociation =
H3PO4(aq) + H2O(l)
H2PO4
-(aq)
+ H2O(l)
HPO42-(aq) + H2O(l)
Ka1 > Ka2 > Ka3
H2PO4
HPO4
-(aq)
2-(aq)
[HA]dissociated
[HA]initial
+ H3
+ H3
O+(aq)
O+(aq)
PO43-(aq) + H3O+(aq)
Ka1 =
Ka2 =
Ka3 =
x 100
[H3O+][H2PO4-]
[H3PO4]
[H3O+][HPO42-]
= 7.2x10-3
= 6.3x10-8
[H2PO4-]
[H3O+][PO43-]
= 4.2x10-13
[HPO42-]
The first H+ comes off much easier than the second.
Ionization constants decrease as protons are removed
ACID STRENGTH
Summary – acid strength
32
Concept check
Consider an acid acting in water?
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
(a) What is the equilibrium constant expression?
[H3O+][A-]
Ka =
[HA]
(b) If the equilibrium lies to the right, is the value for Ka
large (or >1) or small (or <1)?
large (or >1)
(c) If water is a better base than A–, do products or reactants
dominate at equilibrium?
Products
(d) Does this mean that HA is a weak or strong acid?
Strong
Concept check
Consider a solution of 1 M HCl. Arrange the
following from strongest to weakest base
H2O(l); A–(aq) (from weak acid HA); Cl–(aq)
A–, H2O, Cl–
Acetic acid (CH3COOH) and HCN are both weak acids.
Acetic acid is stronger than HCN. Arrange these bases
from weakest to strongest
H2O(l); Cl–(aq) (from strong acid HCl); CN–(aq) ;
CH3COOCl–, H2O, CH3CO2–, CN–
Concept check
Consider a solution of NaA where A– is the anion from
weak acid HA:
A–(aq) + H2O(l)
(a) Which way will equilibrium lie?
HA(aq) + OH-(aq)
left
(b) Is the value for Kb large (or >1) or small (or <1)?
<1
(c) Does this mean that A– is a weak or strong base?
weak
Concept check
Consider: F–(aq) + HCN(aq)
HF(aq) + CN-(aq)
(Ka for HCN is 6.2×10–10; Ka for HF is 7.2×10–4.)
(a) Is the value for K large (or >1), small (or <1), or 1?
(b) Calculate the value of K
<1
8.6 x 10-7
Concept check
Compare solutions of 4.00 M formic acid and 8.00 M
formic acid
(a) Is the value of Ka (4 M formic acid) greater,
smaller or the same as that of Ka (8 M formic acid)
same
(b) Is the %ionization of 4 M formic acid higher or lower
or the same as that of 8 M formic acid?
higher
(c) Is the pH of 4 M formic acid higher or lower or the
same as that of 8 M formic acid?
higher
(d) Calculate the %ionization of 4 M formic acid?
0.67%