strong acids ionize completely in aqueous solutions, but weak acids ionize only partially....
TRANSCRIPT
Strong acids ionize completely in aqueous solutions, but weak acids ionize only partially.
Introduction to Ka for Weak
Acids
We’ll consider a 0.10 M solution of the weak acid, CH3COOH. This is called ethanoic acid or more commonly, acetic acid.
We’ll consider the weak acid CH3COOH. It is called ethanoic or acetic acid.
H
Here is a diagram showing models of a CH3COOH molecule in a water solution.
OH
HH
CO
H CO H
OH
H
OH
H
OH
HH
OH
HO
H
HO
HH
OH
HO
H
In this diagram, we’ll show just one of the water molecules.
H
H
CO
H CO H
HO
H
Almost all of the collisions between CH3COOH and water molecules (click) result in no reaction. They just bounce off of each other.
H
H
CO
H CO H
HO
H
So almost all of the acetic acid molecules in 0.10 M acetic acid remain as neutral CH3COOH molecules.
H
H
CO
H CO H
HO
H
Almost all of the CH3COOH molecules in
0.10 M CH3COOH remain as neutral
molecules
However, in rare cases, the collision will be successful.
H
H
CO
H CO H
HO
H
A water molecule will collide with the hydrogen atom (click) attached to the oxygen atom in CH3COOH.
H
H
CO
H CO H
HO
H
And it will remove that hydrogen atom, forming a hydronium ion and a CH3COO minus ion.
H
H
CO
H CO H
HO
H+
–
We show the formulas here,
H
H
CO
H CO
H
HO
H+
–
H3O+ ionCH3COO– ion
Hydronium Ion
Of course, H3O+ is called the hydronium ion
H
H
CO
H CO
H
HO
H+
–
H3O+ ionCH3COO– ion
Ethanoate Ion
CH3COO minus is called the ethanoate ion,
H
H
CO
H CO
H
HO
H+
–
H3O+ ionCH3COO– ion
Hydronium Ion
Ethanoate Ion
Or more commonly, the acetate ion.
H
H
CO
H CO
H
HO
H+
–
H3O+ ionCH3COO– ion
Acetate IonHydronium Ion
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.
Here’s the equation for the ionization of acetic acid.
The Weak Acid CH3COOH is much less than 100% ionized in aqueous solution.
3 (aq) 3 (aq2 ( ) 3 (aq))CH COOH H O CH COOH O l
Acetic Acid Acetate ionHydronium ion
This does not go to completion. An equilibrium is established between the neutral acetic acid molecule and the ions it forms.
The Weak Acid CH3COOH is much less than 100% ionized in aqueous solution.
3 (aq) 2 ( ) 3 (aq) 3 (aq)CH COOH H O H O CH COOl
An Equilibriu
m
And in this equilibrium (click), the molecular form of CH3COOH is highly favoured.
3 (aq) 2 ( )CH CO HO OH l
The molecular form is favoured
3 (aq) 3 (aq)H O CH COO
And the concentrations of the ions are very low compared to that of the molecules.
3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq) H O CH COO
The concentrations of the ions are very
low
So a solution of acetic acid consists mostly of neutral CH3COOH molecules.
3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq)H O CH COO
3CH COOH
3CH COOH
3CH COO
3CH COOH3CH COOH
3CH COOH
3CH COOH
3CH COOH
3CH COOH
3H O
A solution of acetic acid is mostly
neutral CH3COOH molecules
And the concentrations of the ions in this solution are very low.
3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq)H O CH COO
The concentrations of the ions are very
low
3CH COOH
3CH COOH
3CH COO
3CH COOH3CH COOH
3CH COOH
3CH COOH
3CH COOH
3CH COOH
3H O
A solution of acetic acid is mostly
neutral CH3COOH molecules
If we add enough acetic acid (click) to water to form a 0.10 M solution…
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
3CH COOH
A small number of the acetic acid molecules ionize, and the bulb glows dimly. Low conductivity is a property of weak acid solutions.
3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq)H O CH COO
3CH COOH
3CH COOH 3CH COO
3CH COOH3CH COOH
3CH COOH
3CH COOH
3CH COOH
3CH COOH3H O
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 bottom.
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO 3CH COO3H O
3CH COOH
eq
[ ][ ]
[ ]K
Because water is a liquid in this equation, it is Not included in the Keq expression.
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
Because H2O is a liquid, it is NOT
included in the Keq expression.
Because CH3COOH is a weak acid,
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
Weak
acid
The equilibrium constant is called (click) K a
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
Weak
acida
Because weak acids have a low degree of ionization, the numerator of the Ka expression is small.
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a
The numerator is small
And the denominator is large
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a
The numerator is small
The denominator is
large
So the values of Ka for weak acids are typically quite small.
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a
The numerator is small
The denominator is
largeThe Value of Ka
is small
The extent of any ionization depends on the temperature
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a
The extent of ionization
depends on the temperature
Therefore, the value of Ka depends on temperature.
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq
[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a
The extent of ionization
depends on the temperature
The value of Ka depends on the
temperature
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
The values of Ka on the Acid Table
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
3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO
eq[ ][ ]
[ ]K
3CH COOH
3H O3CH COO
a = 1.8 × 10–
5aK 3CH COOH
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
Is stronger than boric acid, with a Ka of 7.3 × 10-10
stronger
weaker
Notice that the Ka values for the 6 strong acids on the top of the table are all listed as “very large”
Strong Acids
For example, if we look at perchloric acid HClO4,
4
3 4a(HClO )
4
H O ClOvery large
HClOK
The single arrow pointing to the right means that perchloric acid is essentially 100% ionized in aqueous solution at 25°C.
4
3 4a(HClO )
4
H O ClOvery large
HClOK
100% ionization
So the concentrations of hydronium and perchlorate ions are high
4
3 4a(HClO )
4
H O ClOvery l
HClOargeK
100% ionization
High
And the concentration of neutral HClO4 molecules is close to zero
4
3 4a(HClO )
4
H O Clvery large
HC
O
lOK
100% ionization
Close to zero
This means that the ratio of the concentrations of ions to the concentration of neutral HClO4 molecules is very large.
4
3 4a(HClO )
4
H O Clvery l
O
HClOargeK
100% ionization
Large
Close to zero
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”
In Chemistry 12, we use actual Ka values, only when dealing with Weak Acids.
Ka values for Weak
Acids
WeakAcids