Strong acids ionize completely in aqueous solutions, but weak acids ionize only partially. Introduction to K a for Weak Acids.

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