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
Science
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
The ratio is larger than allowed by the equilibrium law, and the
acetate ions will combine with the hydrogen ions to form acetic acid
more rapidly than the ions are formed from it, until equilibrium is
reëstablished. In a molar solution of acetic acid, 0.42% of the acid
is ionized, and we have
[CH_{3}CO_{2}^{−}] × [H^{+}] / [CH_{3}CO_{2}H] =
0.0042 × 0.0042 / 0.9958 = 1.8E−5.
If an equivalent quantity of sodium acetate should be added, ‹i.e.›
one mole or 82 grams of the salt per liter, the salt being ionized to
the extent of 53%, we would have
0.534 × 0.0042 / 0.9958 > 1.8E−5.
The equilibrium constant will be satisfied (and the velocity of
ionization of the acetic acid will become equal to the velocity of its
[p113] formation from its ions) when[217] [CH_{3}CO_{2}^{−}] = 0.53,
[H^{+}] = 0.000,034 and [CH_{3}CO_{2}H] = 0.999,966. If we use two
characteristic places in the decimals, we have
(0.53 × 0.000034) / 1 = 1.8E−5.
The most significant fact in the new condition of equilibrium is the
‹extremely small concentration of hydrogen-ion› in the solution.
Since the acid properties of acetic acid are due to its forming
hydrogen-ion, we would conclude that such properties of acetic
acid are ‹very much weakened› by the presence of its own salts.
This conclusion has been fully verified by careful quantitative
measurements and can be demonstrated as follows[218]:
EXP. To two of three portions of a dilute solution of methyl
orange equal quantities of acetic acid are added (‹e.g.› 0.5 c.c.
molar acid), the third portion being reserved to show the color
of the neutral solution of the indicator. Now, if into one of the
solutions, to which acetic acid has been added, a few crystals of
sodium acetate are gradually dropped, the color reverts gradually
to the color of the original neutral solution; the concentration of
hydrogen-ion becomes so small, that it does not visibly affect this
indicator, which is not very sensitive to acids, (H^{+}).[219]
The solution, according to the views expressed, ‹should still be
very slightly acid›, and by the use of an indicator (litmus paper)
which is much more sensitive to the hydrogen-ion than is methyl
orange, no difficulty is found in recognizing this fact also. As a
matter of experiment, then, an acid like acetic acid ‹is very much
weaker in the presence of its own salts than in their absence›, and
the equilibrium constant and the concentrations of the components
used determine the extent to which the hydrogen-ion is suppressed.
The same must be true for all weak acids and similar relations must
[p114] hold for all weak bases[220]—in general, ‹weak acids and weak
bases are very much weakened by the addition of their own salts›.
The importance of recognizing such changes, in considering analytical
reactions, may be illustrated as follows:
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