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The Acidity Constant, Ka

 The strength of an acid is represented by its ionization constant (acidity constant), Ka

Ka= product of concentrations of ionized species concentration of intact acid

Ka =

The Acidity Constant, Ka  The Ka implies the concentrations of the

acid and the ions Ka > 1 Ionized products greater than intact acid.

Ka < 1 Ionized products less than intact acid.

Ka >> 1 Ionization goes to completion (strong acid). (e.g., > 103)

Ka << 1 Ionization does not occur to an appreciable amount. (e.g., < 10–3)

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pKa = – log (Ka)

The Acidity Constant, Ka  Since the Ka values for various acids have

such a wide range, a more manageable way to discuss this measure of acidity is to use

Compare pKa and Ka Values

pKa 14 12 10 8 6 4 2 0

strong acids weak acids

Ka 10-14 10-10 10-6 10-2

The smaller the value of the pKa the stronger the acid.

-2

102

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The Henderson-Hasselbalch Equation •  pH in aqueous solutions is a

function of the concentrations of acids and bases within them –  equation defines pH as a

function of the strength of an acid (pKa) and the concentration of the acid and conjugate base

•  Allows calculation of pH given a certain conc. of acid

•  Allows calculation of amount of acid/base needed to obtain a desired pH

–  when pH = pKa, the [HA] = [A-]

pH = -log[H3O+]

Ka =[HA]

[H3O+ ][A- ]HA + H2O H3O+ + A-

[A-][H3O+ ] =[HA]

[A-]pH =

[HA]-log = -logKa - log

[A-][HA]

pH = pKa + log[HA][A-]

Henderson-Hasselbalch Equation

pKa = -logKa

Ka

Ka

Acid Strength HA + H2O H3O+ + A-

HA

A-

A-

Has a strong conj. base (higher energy)

WEAK ACID

STRONG ACID

E N E R G Y

ionization easier

 The difference between a strong acid and a weak acid can be described by the stability of the conjugate base.

Has a weak conj. base (lower energy)

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Acid Strength  A more stable conjugate base means a

stronger acid.

HA

stabilization

E N E R G Y

A-

A-

Acid Strength  Factors that influence stability of the

conjugate base include: •  Resonance •  Electronegativity •  Atomic Size •  Hybridization •  Inductive Effects

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Resonance Effects  More or better resonance structures of the

conjugate base lead to a stronger acid.

Resonance Effects

18

10

5

45

30

25

20

9

28

25

15

pKa Values increasing quality of resonance

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Resonance Effects  The Acetate Ion

acetate ion

acetic acid

Resonance Stabilized Equivalent structures (charges on oxygens)

Resonance Effects

-

More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen)

 The Phenolate Ion

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Electronegativity  Placing the negative charge on a more

electronegative element (from the same period) in the conjugate base leads to a stronger acid.

Electronegativity pKa Values increasing

electronegativity

20

15

5

CH4

NH3

H2O

HF

>45

34

16

3.5

RCH3

RNH2

ROH

45

35

18

Consider the conjugate bases

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Electronegativity pKa Values

Consider the conjugate bases

increasing electronegativity

20

15

5

CH4

NH3

H2O

HF

> 45

34

16

3.5

RCH3

RNH2

ROH

45

35

18

Atomic Size  Placing the negative charge on a larger

atom (from the same group) in the conjugate base leads to a stronger acid.

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Atomic Size pKa Values increasing

size

H2O

H2S

H2Se

H2Te

16

7

4

3

Consider the ionic radii

HF

HCl

HBr

HI

3.5

– 7

– 9

– 10

F–

Cl–

I–

Br–

Electronegativity pKa Values

Consider the ionic radii

increasing size

HF

HCl

HBr

HI

3.5

–7

–9

–10

H2O

H2S

H2Se

H2Te

16

7

4

3

1.36 Å

1.81 Å

1.95 Å

2.16 Å

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Hybridization •  More s character in the orbital bearing the

negative charge in the conjugate base leads to a stronger acid.

Hybridization

sp3

sp2

sp

> 45

35

25

As electrons in hybrid orbitals become closer to the nucleus, they are lower in energy

-1.74

-7

:

:

: pKa

pKa

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Inductive Effects  Electron-withdrawing effects due to

differences in electronegativity pull electron density away from the negatively charged end of the conjugate base, lowering the energy and stabilizing the conjugate base, making the acid stronger.

Inductive Effects  Electron-donating effects due to

differences in electronegativity push electron density toward the negatively charged end of the conjugate base, increasing the energy and destabilizing the conjugate base, making the acid weaker.

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Inductive Effects Electron-withdrawing Groups

F, Cl, Br, O, N R, CH3, B, Si electronegative elements pull electron density away from carbon

alkyl groups and elements less electronegative than carbon push electron density toward carbon

Remember, the electron-withdrawing and -donating groups work through the s bond system, while resonance groups work through the p system.

Electron-donating Groups

Inductive Effects Chlorine helps to stabilize – CO2

– by withdrawing electrons

This effect diminishes with distance—it extends for about 3 bonds

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Inductive Effects

3.13

2.87

2.81

2.66

4.75

2.81

1.29

0.65

pKa Values

increasing electronegativity

increasing substitution

Inductive Effects

2.81 4.75 pKa:

 Increasing substitution

0.65