Weak
Acids
Acids and bases can be classified as either strong or weak. Here we’ll deal with
weak acids
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
A Weak Acid is an acid that is less than 100% ionized in aqueous solution.
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
Acetic Acid
An example of a weak acid is acetic acid, CH3COOH
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH 3COOH( aq)  H 2O( l )
Acetic Acid
It reacts with water


H 3O(aq)
 CH 3COO(a
q)
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH3COOH(aq)  H2O( l )
Acetic Acid
To produce


H3O(aq)
 CH3COO(aq)
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH 3COOH(aq)  H 2O( l )
Acetic Acid
A hydronium ion.
H 3O(aq )  CH 3COO(aq )
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH 3COOH(aq)  H 2O( l )
Acetic Acid
And an acetate or ethanoate ion.


H 3O(aq)
 CH 3COO(aq)
Acetate ion
A Weak Acid is an acid that is
less than 100% ionized in aqueous
solution.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
Acetic Acid
Acetate ion
An
Equilibrium
Unlike strong acids, which ionize to completion, weak acids exist as equilibrium
mixtures, as shown by the double arrow.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
The molecular form
is favoured
For most of the weak acids dealt with in Chemistry 12 (click), the molecular form is
highly favoured at equilibrium
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
The concentrations of
the ions are very low
And the concentrations of the ions are very low compared to that of the molecules.
CH 3COOH(aq)  H 2O( l )
CH 3COOH CH COOH
CH 3COOH
3
H 3O 
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH CH COO


H 3O(aq)
 CH 3COO(aq)
A solution of acetic
acid is mostly neutral
CH3COOH molecules
3
So a solution of acetic acid consists mostly of neutral CH3COOH molecules.
CH 3COOH(aq)  H 2O( l )
CH 3COOH CH COOH
CH 3COOH
3
H 3O 
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH CH COO
3


H 3O(aq)
 CH 3COO(aq)
A solution of acetic
acid is mostly neutral
CH3COOH molecules
The concentrations of
the ions are very low
And the concentrations of the ions in this solution are very low.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
If we insert a conductivity apparatus into pure water, it does not conduct enough
to make the bulb glow.
CH 3COOH(aq)  H 2O( l )
CH 3COOH
Now, we’ll add some acetic acid (click) to the water


H 3O(aq)
 CH 3COO(aq)
CH 3COOH(aq)  H 2O( l )
CH 3COOH


H 3O(aq)
 CH 3COO(aq)
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
The acetic acid molecules spread out to fill the solution.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
CH 3COOH CH COOH
CH 3COOH
3
H 3O 
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH CH COO
3
A small number of the acetic acid molecules ionize, and the bulb glows dimly.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
A small number of ions are
present in the solution
CH 3COOH CH COOH
CH 3COOH
3
H 3O 
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH CH COO
3
Because there are a small number of ions present in the solution of a weak acid, the
conductivity is not zero, but it is low.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
Molecular Weak
Acids are Weak
Electrolytes
CH 3COOH CH COOH
CH 3COOH
3
H 3O 
CH 3COOH
CH 3COOH
CH 3COOH
CH 3COOH CH COO
3
So weak acids that start out as neutral molecules, like CH3COOH are weak
electrolytes.
Where do we find
Weak Acids?
So now that we know what weak acids are, where do we find them?
Strong Acids
You may recall that the 6 acids on the top left of the Acid table are classified as
strong acids. Remember these are 100% ionized in aqueous solution.
Strong Acids
Weak Acids
The species below hydronium on the left, all the way down to water can act as weak acids. Some of
these, including water are amphiprotic so they can also act as weak bases, as we’ll see later.
Strong Acids
Weak Acids
NOT Acids
Just a quick word about the two species on the very bottom of the left side,
hydroxide and ammonia. These cannot acts as acids in aqueous solution.
Strong Acids
Weak Acids
NOT Acids
They are found on the right side of the acid table and are classified as bases.
Strong Acids
Weak Acids
NOT Acids
The only reason they are written here,
Strong Acids
Weak Acids
NOT Acids
conjugate acids
bases
Is that they happen to be conjugate acids of the bases O2- and NH2–
NOT Acids
Single arrow
pointing
toward them
Notice they both have a single arrow pointing toward them, which is further verification
that these are NOT acids. These reactions go only in reverse, not forward.
G e t
w e a k e r
Looking at the weak acids, it is important to understand that (click) they get
progressively weaker as we go down the left side of the table, from HIO3 to H2O.
G e t
Degree
decreases
w e a k e r
of ionization
Because acids get weaker as we move down the table, we can also say that the degree of
ionization decreases.
get
smaller
decreases
w e a k e r
of ionization
Ka values
G e t
Degree
This is indicated by the fact that the values of their ionization constant, Ka, get smaller
as we move down the table. Take a look at these to verify this to yourself.
r
s t r o n g e
t
G e
Of course, we can also say that (click) weak acids progressively get stronger as we more
Up on the left side, from water at the bottom, to HIO3 at the top .
r
s t r o n g e
t
G e
on
of Ionizati
Degree
This means the degree of ionization increases as we move up.
Ka
values
get
larger
r
s t r o n g e
t
G e
on
of Ionizati
Degree
And again, this is reflected by the increase in Ka values as we move up.
Molecular
Weak
Acids
Molecular weak acids are indicated here by the red arrows. These are acids that are
neutral molecules before they ionize.
Degree
of ionization
Molecular
Weak
Acids
decreases
Because the degree of ionization decreases as we move down the table,
Degree
decreases
dissolved ions decreases
of ionization
Number of
Molecular
Weak
Acids
It means that the number of dissolved ions present in 1.0 Molar solutions of these
molecular acids will decrease as we move down.
Degree
decreases
dissolved ions decreases
of ionization
Number of
Molecular
Weak
Acids
Trend in
Conductivity
of Molecular
Acids?
So what do you think the trend in conductivity of molecular acids will be as we move
down the column?
Number of
dissolved ions decreases
electrical conductivity depends on the number of dissolved ions in solution, so as we
move down the column and the number of dissolved ions decreases,
dissolved ions decreases
Conductivity of molecular acids decreases
Number of
the conductivity of molecular acids also decreases.
Conductivity of molecular acids decreases
Higher conductivity
Lower conductivity
So if we were to compare the conductivity of phosphoric acid with that of boric acid, we
would predict that phosphoric acid has higher conductivity than boric acid.
Conductivity of molecular acids decreases
Higher conductivity
Higher Ka value
Lower conductivity
Lower Ka value
We can also see that higher conductivity correlates with a higher value for the ionization
constant Ka.
Ionic
Weak
Acids
Notice that many of the species that act as weak acids are ions to begin with.
G e t weaker
as acids
As expected, the ability of each of these to act as an acid (click) decreases as we move
down the left side of the table
Ka values
smaller
as acids
get
G e t weaker
This is reflected by a decrease in their Ka values as we move down.
NaHSO4  Na+ + HSO4–
Because these are ions, they do not occur as substances themselves in nature. If its an anion, it would
need to have an accompanying cation and if it’s a cation it would need to have an accompanying anion.
NaHSO4  Na+ + HSO4–
Spectator
cation
For example, the HSO4 minus ion could be accompanied by the spectator cation Na+.
NaHSO4  Na+ + HSO4–
The source of
HSO4– could be a
salt like NaHSO4
These two ions would result from the dissociation of the salt sodium bisulphate, NaH2SO4.
NaHSO4  Na+ + HSO4–
KHC2O4  K+ + HC2O4–
The source of HC2O4–
could be a salt like
KHC2O4
Similarly, the hydrogen oxalate ion HC2O4 minus could come from the dissociation of
the salt potassium hydrogen oxalate, KHC2O4
NaHSO4  Na+ + HSO4–
KHC2O4  K+ + HC2O4–
NH4Cl  NH4+ + Cl–
The source of NH4+
could be a salt like NH4Cl
And the positive ion NH4+ could come from the dissociation of an ammonium salt like
NH4Cl. The Cl minus ion is a spectator here.
Quite a Weak Acid
Notice that the ammonium ion, NH4+ is low on the left side of the table and its Ka value,
of 5.6 × 10–10 is quite small. This means NH4+ is quite a weak acid.
NH4Cl
water
Because NH4+ is quite a weak acid, we might expect its conductivity to be weak. So let’s
try it. We’ll set up a conductivity apparatus and (click) add enough of the salt NH4Cl…
NH4Cl
To produce a solution of 0.1 molar NH4Cl.
0.1 M
NH4 Cl 
0.1 M
Because NH4Cl is ionic, we know that it actually consists of a crystal lattice of NH4+ and
Cl minus ions. We show one of each ion here. Now we’ll see what happens.
NH4 Cl 
0.1 M
The salt NH4Cl will quickly and completely dissociate into free ammonium and chloride ions. And
notice the light bulb glows brightly to show that we now have HIGH conductivity.
NH4 
0.1 M
Cl 
0.1 M
As far as conductivity is concerned, it doesn’t matter that NH4+ is a weak acid. Because NH4Cl is a highly
soluble ionic salt, it dissociates completely into ammonium and chloride ions, both with a concentration of 0.1
Total ion concentration
= 0.1 M + 0.1 M = 0.2 M
NH4 
0.1 M
Cl 
0.1 M
So the total ion concentration in this solution is 0.1 plus 0.1
Total ion concentration
= 0.1 M + 0.1 M = 0.2 M
NH4 
0.1 M
Cl 
0.1 M
Which is 0.2 molar, high enough to account for the high conductivity.
NH4 
Cl 
0.1 M
Because NH4+ is quite a weak acid, most of it will remain as 0.1M NH4+ in solution.
NH4   H 2O
H 3O  NH 3
Cl 
And only a tiny fraction of it will ionize into hydronium ions and ammonia molecules. This occurs to a very
limited extent, so it will have no significant effect on already high total ion concentration in this solution.
So if we compare acetic acid with the ammonium ion.
NH4+ is a
weaker acid
than CH3COOH
We see that NH4+ is a much weaker acid than CH3COOH.
0.1 M NH4+ has a higher
conductivity than
0.10 M CH3COOH
However 0.10 M NH4+ would have a higher conductivity than CH3COOH.
0.10 M CH3COOH is a
molecular weak acid
This is because CH3COOH is a MOLECULAR weak acid. The only ions it produces in solution come from its
limited ionization as a weak acid. Because 0.1 M CH3COOH has few ions, it is a poor conductor or weak
0.10 M CH3COOH is a
molecular weak acid
0.10 M NH4+ is an
ionic weak acid
And although NH4+ is a weaker acid than CH3COOH, NH4+ is an IONIC weak acid. 0.1 M NH4+ ALREADY HAS a
high ion concentration of ions, even before it undergoes acid ionization to form hydronium. Because it has a