Part 26 (1/2)

It is a.s.sumed that the Na^{+} ion, for example, differs from the sodium atom in behavior because of the very considerable electrical charge which it carries and which, as just stated, must, in an electrically neutral solution, be balanced by a corresponding negative charge on some other ion. When an electric current is pa.s.sed through a solution of an electrolyte the ions move with and convey the current, and when the cations come into contact with the negatively charged cathode they lose their charges, and the resulting electrically neutral atoms (or radicals) are liberated as such, or else enter at once into chemical reaction with the components of the solution.

Two ions of identically the same composition but with different electrical charges may exhibit widely different properties. For example, the ion MnO_{4}^{-} from permanganates yields a purple-red solution and differs in its chemical behavior from the ion MnO_{4}^{--} from manganates, the solutions of which are green.

The chemical changes upon which the procedures of a.n.a.lytical chemistry depend are almost exclusively those in which the reacting substances are electrolytes, and a.n.a.lytical chemistry is, therefore, essentially the chemistry of the ions. The percentage dissociation of the same electrolyte tends to increase with increasing dilution of its solution, although not in direct proportion. The percentage dissociation of different electrolytes in solutions of equivalent concentrations (such, for example, as normal solutions) varies widely, as is indicated in the following tables, in which approximate figures are given for tenth-normal solutions at a temperature of about 18C.

ACIDS =========================================================================

SUBSTANCE

PERCENTAGE DISSOCIATION IN

0.1 EQUIVALENT SOLUTION _____________________________________________

___________________________

HCl, HBr, HI, HNO_{3}

90

HClO_{3}, HClO_{4}, HMnO_{4}

90

H_{2}SO_{4} <--> H^{+} + HSO_{4}^{-}

90

H_{2}C_{2}O_{4} <--> H^{+} + HC_{2}O_{4}^{-}

50

H_{2}SO_{3} <--> H^{+} + HSO{_}3^{-}

20

H_{3}PO_{4} <--> H^{+} + H_{2}PO_{4}^{-}

27

H_{2}PO_{4}^{-} <--> H^{+} + HPO_{4}^{--}

0.2

H_{3}AsO_{4} <--> H^{+} + H_{2}AsO_{4}^{-}

20

HF

9

HC_{2}H_{3}O_{2}

1.4

H_{2}CO_{3} <--> H^{+} + HCO_{3}^{-}

0.12

H_{2}S <--> H^{+} + HS^{-}

0.05

HCN

0.01

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BASES =========================================================================

SUBSTANCE

PERCENTAGE DISSOCIATION IN

0.1 EQUIVALENT SOLUTION _____________________________________________

___________________________

KOH, NaOH

86

Ba(OH)_{2}

75

NH_{4}OH

1.4

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SALTS =========================================================================

TYPE OF SALT

PERCENTAGE DISSOCIATION IN

0.1 EQUIVALENT SOLUTION _____________________________________________

___________________________

R^{+}R^{-}

86

R^{++}(R^{-})_{2}

72

(R^{+})_{2}R^{--}

72

R^{++}R^{--}

45

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The percentage dissociation is determined by studying the electrical conductivity of the solutions and by other physico-chemical methods, and the following general statements summarize the results:

!Salts!, as a cla.s.s, are largely dissociated in aqueous solution.

!Acids! yield H^{+} ions in water solution, and the comparative !strength!, that is, the activity, of acids is proportional to the concentration of the H^{+} ions and is measured by the percentage dissociation in solutions of equivalent concentration. The common mineral acids are largely dissociated and therefore give a relatively high concentration of H^{+} ions, and are commonly known as ”strong acids.” The organic acids, on the other hand, belong generally to the group of ”weak acids.”

!Bases! yield OH^{-} ions in water solution, and the comparative strength of the bases is measured by their relative dissociation in solutions of equivalent concentration. Ammonium hydroxide is a weak base, as shown in the table above, while the hydroxides of sodium and pota.s.sium exhibit strongly basic properties.

Ionic reactions are all, to a greater or less degree, !reversible reactions!. A typical example of an easily reversible reaction is that representing the changes in ionization which an electrolyte such as acetic acid undergoes on dilution or concentration of its solutions, !i.e.!, HC_{2}H_{3}O_{2} <--> H^{+} + C_{2}H_{3}O_{2}^{-}. As was stated above, the ionization increases with dilution, the reaction then proceeding from left to right, while concentration of the solution occasions a partial rea.s.sociation of the ions, and the reaction proceeds from right to left. To understand the principle underlying these changes it is necessary to consider first the conditions which prevail when a solution of acetic acid, which has been stirred until it is of uniform concentration throughout, has come to a constant temperature. A careful study of such solutions has shown that there is a definite state of equilibrium between the const.i.tuents of the solution; that is, there is a definite relation between the undissociated acetic acid and its ions, which is characteristic for the prevailing conditions. It is not, however, a.s.sumed that this is a condition of static equilibrium, but rather that there is continual dissociation and a.s.sociation, as represented by the opposing reactions, the apparent condition of rest resulting from the fact that the amount of change in one direction during a given time is exactly equal to that in the opposite direction. A quant.i.tative study of the amount of undissociated acid, and of H^{+} ions and C_{2}H_{3}O_{2}^{-} ions actually to be found in a large number of solutions of acetic acid of varying dilution (a.s.suming them to be in a condition of equilibrium at a common temperature), has shown that there is always a definite relation between these three quant.i.ties which may be expressed thus:

(!Conc'n H^{+} x Conc'n C_{2}H_{3}O_{2}^{-})/Conc'n HC_{2}H_{3}O_{2} = Constant!.

In other words, there is always a definite and constant ratio between the product of the concentrations of the ions and the concentration of the undissociated acid when conditions of equilibrium prevail.

It has been found, further, that a similar statement may be made regarding all reversible reactions, which may be expressed in general terms thus: The rate of chemical change is proportional to the product of the concentrations of the substances taking part in the reaction; or, if conditions of equilibrium are considered in which, as stated, the rate of change in opposite directions is a.s.sumed to be equal, then the product of the concentrations of the substances entering into the reaction stands in a constant ratio to the product of the concentrations of the resulting substances, as given in the expression above for the solutions of acetic acid. This principle is called the !Law of Ma.s.s Action!.

It should be borne in mind that the expression above for acetic acid applies to a wide range of dilutions, provided the temperature remains constant. If the temperature changes the value of the constant changes somewhat, but is again uniform for different dilutions at that temperature. The following data are given for temperatures of about 18C.[1]

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MOLAL

FRACTION

MOLAL CONCENTRA-

MOLAL CONCENTRA-

VALUE OF CONCENTRATION

IONIZED

TION OF H^{+} AND

TION OF UNDIS-

CONSTANT CONSTANT

ACETATE^{-} IONS

SOCIATED ACID

______________

__________

__________________

__________________

__________

1.0

.004