The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2.: With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.Stieglitz, Julius
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The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2.: With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.
Stieglitz, Julius
Chemistry, Analytic -- Qualitative
From this experimental result, the equilibrium constant, which is
called the ‹ionization constant› of the acid, is found to have the
value 18.2E−6. If the ratio [H^{+}] × [CH_{3}CO_{2}^{−}] /
[CH_{3}CO_{2}H] really is a constant, the same value, within
the limits of experimental errors, should be obtained from
acetic acid in other concentrations. Now, if the above solution
is diluted to ten times its volume, the concentration of the
acid is made 0.01 mole ‹per liter›, the conductivity [p099] is
found to have increased to 14.5 reciprocal ohms, and the percentage
of ionized acid is then 14.5 / 347, or 4.17. Here, [H^{+}]
and [CH_{3}CO_{2}^{−}] = 0.01 × 0.0417 and [CH_{3}CO_{2}H] =
0.01 × 0.9583. Inserting these values in our general equation
and calculating the result, we obtain 18.1E−6 as the value
of the constant. In the following table[180] are given the
molar conductivities, Λ (column 2), of acetic acid of varying
concentrations, ‹m› (column 1). The degrees of ionization, α, and
the ionization constant, calculated according to the equilibrium
equation, are given in columns 3 and 4.
IONIZATION OF ACETIC ACID. Λ_{∞} = 347.
m. Λ. 100 α. K.
0.1 4.67 1.34 18.2E−6
0.08 5.22 1.50 18.3E−6
0.03 8.50 2.45 18.5E−6
0.01 14.50 4.17 18.1E−6
It is evident, that a constant value is found for the ratio
[H^{+}] × [CH_{3}CO_{2}^{−}] / [CH_{3}CO_{2}H] and that the
ionization of acetic acid, in these dilute solutions, obeys the law
of chemical equilibrium.[181] The equilibrium constant expresses in
definite, quantitative terms the tendency of acetic acid to ionize in
dilute solution. Examination of other acids shows that there is an
enormous range in the values found for their respective ionization
constants. The constants are the best ‹measure› of the ‹strength›
of the ‹acids› as acids. Obviously, the more readily acids in
equivalent solutions ionize, the greater will be the concentration
of the hydrogen-ion to which the characteristic acid properties are
due, and the more pronounced (stronger) will be the exhibition of
these properties. From the ionization constants one may calculate,
for instance, the proportion in which two competing acids will
neutralize a base, when the latter is used in quantity insufficient
to neutralize both acids. [p100]
Inspection of the equation for acetic acid, which is the typical
equilibrium equation for all ‹monobasic› acids, shows that the
greater the degrees of ionization of acids are in equivalent
solutions, ‹i.e.› the greater the concentrations of the hydrogen-ion
which their ionization produces in equivalent solutions, the larger
will be the values of their ionization constants. The acids with the
‹larger› constants are, then, the ‹stronger› acids.
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