Synthetic Inorganic Chemistry

128 THE THEORY OF IONIZATION

in which k 2 is likewise a constant which depends on the chemical

affinities of C and D and naturally has a different numerical value

from k\.

Now a reversible reaction is at a point of equilibrium when

no further apparent change is taking place. The two opposing

reactions are without doubt taking place just the same, but they

exactly undo the effect of each other, making the total change

zero. Therefore

vel (-0 = vel (*-)

fc, [A] [B] = k 2 [C] [D]

[C] [D] _ K R

[ATTB]"^ = if

K is known as the equilibrium constant of the reaction. It is the

ratio of the two velocity constants k\ and k 2.

The number of components taking part in a reversible reaction

is not always four as in the equation involving ABCD. Thus in

the ionization of acetic acid, (HAc),

HAc ;=± H+ + Ac"

there is but one component on the left. Each molecule of HAc
has a definite tendency to ionize which is not dependent on any
other dissolved molecules or ion. Therefore the amount of acetic
acid which will ionize in unit time depends solely on the amount
present or
vel (-») = jfe, [HAc]

The speed of the opposing reaction is given by

vel («-) = k 2 [H+] [Ac"]

and the condition of equilibrium is given in the expression

[H+] X [Ac"]

[HAc]

K is the ionization constant of acetic acid and has a numerical
value of 0.000018 when concentrations are given in moles per
liter.
Another example of equilibrium is that between sulphur triox-