Introduction to
Ka for Weak
Acids
Strong acids ionize completely in aqueous solutions, but weak acids ionize only partially.
We’ll consider the weak acid
CH3COOH.
It is called ethanoic or acetic acid.
We’ll consider a 0.10 M solution of the weak acid, CH3COOH. This is called ethanoic acid
or more commonly, acetic acid.
O
H
H
O
O
H
H
H
H
H
O
H
H
O
H C C
H
O H
H
H
O
H
O
O
H
O
H
O
H
H
H
H
H
Here is a diagram showing models of a CH3COOH molecule in a water solution.
O
H C C
H
O H
H
O
H
H
In this diagram, we’ll show just one of the water molecules.
O
H C C
H
O H
H
O
H
H
Almost all of the collisions between CH3COOH and water molecules (click) result in no
reaction. They just bounce off of each other.
Almost all of the
CH3COOH molecules in
0.10 M CH3COOH
remain as neutral
molecules
O
H
H
O
H C C
H
O H
H
So almost all of the acetic acid molecules in 0.10 M acetic acid remain as neutral
CH3COOH molecules.
O
H C C
H
O H
H
O
H
H
However, in rare cases, the collision will be successful.
O
H C C
H
O H
H
O
H
H
A water molecule will collide with the hydrogen atom (click) attached to the oxygen atom
in CH3COOH.
O
H C C
H
O H
H
–
+
O
H
H
And it will remove that hydrogen atom, forming a hydronium ion and a CH3COO minus
ion.
CH3COO–
ion
O
H C C
H
O
H
–
We show the formulas here,
+
H
O
H
H
H3O+ ion
Hydronium Ion
CH3COO–
ion
+
H
O
H C C
H
O
H
–
Of course, H3O+ is called the hydronium ion
O
H
H
H3O+ ion
Hydronium Ion
Ethanoate Ion
CH3COO–
ion
+
H
O
H C C
H
O
H
–
CH3COO minus is called the ethanoate ion,
O
H
H
H3O+ ion
Acetate Ion
Hydronium Ion
Ethanoate Ion
CH3COO–
ion
O
H C C
H
O
H
–
Or more commonly, the acetate ion.
+
H
O
H
H
H3O+ ion
The Weak Acid CH3COOH is much less
than 100% ionized in aqueous solution.
Because only a very small percentage of the acetic acid molecules form hydronium and
acetate ions, we can say that the weak acid, acetic acid is much less than 100% ionized.
The Weak Acid CH3COOH is much less
than 100% ionized in aqueous solution.
CH 3COOH(aq)  H 2O( l )
Acetic Acid

H 3O(aq )
Hydronium ion
Here’s the equation for the ionization of acetic acid.


CH 3COO(aq)
Acetate ion
The Weak Acid CH3COOH is much less
than 100% ionized in aqueous solution.
CH 3COOH(aq)  H 2O( l )

H 3O(aq)


CH 3COO(aq)
An
Equilibrium
This does not go to completion. An equilibrium is established between the neutral acetic
acid molecule and the ions it forms.
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
The molecular form
is favoured
And in this equilibrium (click), the molecular form of CH3COOH is highly favoured.
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)
CH 3COOH
If we add enough acetic acid (click) to water to form a 0.10 M solution…
CH 3COOH(aq)  H 2O( l )


H 3O(aq)
 CH 3COO(aq)
CH 3COOH
CH 3COOH
CH 3COOH

CH 3COOH
H 3O

CH
3COOH
CH
CH 3COOH
3COO CH COOH
3
CH 3COOH
A small number of the acetic acid molecules ionize, and the bulb glows dimly. Low
conductivity is a property of weak acid solutions.
COOH(aq)  H 2O( l )
CH 3COOH
K eq 
[ ][
[


H 3O(aq)
 CH 3COO(aq)
]
]
In the equilibrium constant or Keq expression for this reaction, concentrations of the products, hydronium
and acetate ions (click) go on top and the concentration of the reactant acetic acid (click) goes on the
CH 3COOH(aq)  H 2O( l )
K eq 
[ ][
H 3O 
]
CH 3COO


H 3O(aq)
 CH 3COO(aq)
Because H2O is a
liquid, it is NOT
included in the Keq
expression.
[ CH 3COOH ]
Because water is a liquid in this equation, it is Not included in the Keq expression.
CH 3COOH(aq)  H 2O( l )
Weak
acid
K eq 
[ ][
H 3O 
]
CH 3COO
[ CH 3COOH ]
Because CH3COOH is a weak acid,


H 3O(aq)
 CH 3COO(aq)
CH 3COOH(aq)  H 2O( l )
Weak
a
acid
K eq 
[ ][
H 3O 
]
CH 3COO
[ CH 3COOH ]
The equilibrium constant is called (click) K a


H 3O(aq)
 CH 3COO(aq)
CH 3COOH(aq)  H 2O( l )
K eq
a 
[ ][
H 3O 
]
CH 3COO


H 3O(aq)
 CH 3COO(aq)
The numerator
is small
[ CH 3COOH ]
Because weak acids have a low degree of ionization, the numerator of the Ka
expression is small.
CH 3COOH(aq)  H 2O( l )
K eq
a 
[ ][
H 3O 
]
CH 3COO
[ CH 3COOH ]
And the denominator is large


H 3O(aq)
 CH 3COO(aq)
The numerator
is small
The denominator
is large
CH 3COOH(aq)  H 2O( l )
K eq
a 
The Value of Ka
is small
[ ][
H 3O 
]
CH 3COO
[ CH 3COOH ]


H 3O(aq)
 CH 3COO(aq)
The numerator
is small
The denominator
is large
So the values of Ka for weak acids are typically quite small.
CH 3COOH(aq)  H 2O( l )
K eq
a 
[ ][
H 3O 


H 3O(aq)
 CH 3COO(aq)
]
CH 3COO
[ CH 3COOH ]
The extent of any ionization depends on the temperature
The extent of
ionization
depends on the
temperature
CH 3COOH(aq)  H 2O( l )
K eq
a 
[ ][
H 3O 
]
CH 3COO
[ CH 3COOH ]
The value of Ka
depends on the
temperature
Therefore, the value of Ka depends on temperature.


H 3O(aq)
 CH 3COO(aq)
The extent of
ionization
depends on the
temperature
The values of Ka
on the Acid Table
The values of Ka on the Acid Table
The values of Ka
on the Acid Table
Are all measured at room temperature or 25°C
CH 3COOH(aq)  H 2O( l )
K a K eq
a 
 CH 3COOH 
[ ][
H 3O 
]
CH 3COO


H 3O(aq)
 CH 3COO(aq)
= 1.8 × 10–5
[ CH 3COOH ]
Looking on the table, we see that the value of Ka for CH3COOH, which is called
ethanoic or acetic acid, at 25°C is 1.8 × 10-5
We see that as we move down the table, the values of Ka progressively get smaller
from 1.7 × 10-1 for iodic acid (click) to 1.0 × 10-14 for water.
The relative strengths of different acids can be compared using their Ka values. For
example, benzoic acid, with a Ka of 6.5 × 10-5
stronger
weaker
Is stronger than boric acid, with a Ka of 7.3 × 10-10
Strong
Acids
Notice that the Ka values for the 6 strong acids on the top of the table are all listed
as “very large”
K a(HClO4 )
 H 3O   ClO4 

  very large

 HClO4 
For example, if we look at perchloric acid HClO4,
100%
ionization
K a(HClO4 )
 H 3O   ClO4 

  very large

 HClO4 
The single arrow pointing to the right means that perchloric acid is essentially
100% ionized in aqueous solution at 25°C.
100%
ionization
K a(HClO4 )
 H 3O   ClO4 
High





 very large
 HClO4 
So the concentrations of hydronium and perchlorate ions are high
100%
ionization
K a(HClO4 )
 H 3O   ClO4 

  very large

 HClO4  Close to zero
And the concentration of neutral HClO4 molecules is close to zero
100%
ionization
Large
K a(HClO4 )
 H 3O   ClO4 

  very large

 HClO4 
Close to
zero
This means that the ratio of the concentrations of ions to the concentration of
neutral HClO4 molecules is very large.
All Strong Acids are
100% ionized, so their
Ka values are listed as
“Very Large”
All Strong Acids are 100% ionized, so their Ka values are listed as “Very Large”
Weak
Acids
Ka values
for Weak
Acids
In Chemistry 12, we use actual Ka values, only when dealing with Weak Acids.