Organic Chemistry

Section 1.17 Organic Acids and Bases; pKaand pH 41

Whether a reversible reaction favors reactants or products at equilibrium is indicat-
ed by the equilibrium constantof the reaction, Remember that brackets are used
to indicate concentration in moles/liter [i.e., molarity (M)].

The degree to which an acid (HA) dissociates is normally determined in a dilute so-
lution, so the concentration of water remains nearly constant. The equilibrium expres-
sion, therefore, can be rewritten using a new constant called the acid dissociation
constant,

The acid dissociation constant is the equilibrium constant multiplied by the molar
concentration of water (55.5 M).
The larger the acid dissociation constant, the stronger is the acid—that is, the
greater is its tendency to give up a proton. Hydrogen chloride, with an acid dissocia-
tion constant of is a stronger acid than acetic acid, with an acid dissociation con-
stant of only For convenience, the strength of an acid is generally
indicated by its value rather than its value, where

The of hydrogen chloride is and the of acetic acid, a much weaker acid, is
4.76. Notice that the smaller the the stronger is the acid.

very strong acids

moderately strong acids

weak acids

extremely weak acids

Unless otherwise stated, the values given in this text indicate the strength of the
acid in water. Later (in Section 10.10), you will see how the of an acid is affected
when the solvent is changed.
The pHof a solution indicates the concentration of positively charged hydrogen ions
in the solution. The concentration can be indicated as or, because a hydrogen ion
in water is solvated, as The lower the pH, the more acidic is the solution.

Acidic solutions have pH values less than 7; basic solutions have pH values greater
than 7. The pH values of some commonly encountered solutions are shown in the mar-
gin. The pH of a solution can be changed simply by adding acid or base to the solution.

pH=-log [H 3 O+]

[H 3 O+].

[H+]

pKa

pKa

pKa 715

pKa= 5 – 15

pKa= 1 – 5

pKa 61

pKa,

pKa - 7 pKa

pKa=-log Ka

pKa Ka

1.74* 10 -^5.

107 ,

Ka=

[H 3 O+][A-]

[HA]

= Keq[H 2 O]

Ka.

Keq=

[H 3 O+][A-]

[H 2 O][HA]

HA+H 2 O÷H 3 O++A-

Keq.

HCl +

hydrogen

chloride

H 2 O H 3 O+ + Cl−

H 3 O+ +

acetic acid

H 2 O

O

+ −

CH 3

C

OH

O

CH 3

C

O

The stronger the acid, the smaller is its

pKa.

NaOH, 0.1M

Household bleach

Household ammonia

Borax

Tomatoes

Wine

Coffee

Solution pH

Cola, vinegar

Lemon juice

Gastric juice

Baking soda

Egg white, seawater

Human blood, tears

Milk

14

13

12

11

10

9 8 7 6 5 4 3 2 1 0

Milk of magnesia

Saliva

Rain