Ionization, of strong electrolytes

The double arrow represents the condition of equilibrium that exists between the non-ionized and the ionized species of the electrolyte. Since ionization of strong electrolytes is practically complete there will not be much gain in studying this type of reaction from the point of view of equilibrium. Equilibria involving weak electrolytes, where there is only partial ionization, are of considerable importance. 

THE IONIZATION OF STRONG ELECTROLYTES By William D. Harkins Kent Chemical Laboratory, University op Chicago Commimicated by J. Stieglitz, April 14, 1920... 

Chemists use a double arrow to represent the ionization of weak electrolytes and a single arrow to represent the ionization of strong electrolytes. Because HQ is a strong electrolyte, we write the equation for the ionization of HCl as follows ... 

Ionization of strong electrolytes. Molecular states of nitric acid and perchloric 285 acid... 

The law is in fact of very wide application it holds for nonionic as well as ionic reactions. The degree of ionization of weakly ionized substances can be calculated with high precision according to the law. But the behavior of strong electrolytes does not conform as closely to this law, and the law is of value only in a qualitative fashion to predict the extent of the ionization of these substances. In this connection we may recall Rule 4 for writing ionized equations, which directed to treat all strong electrolytes as if they were completely ionized. 

Strong electrolytes are substances that are completely ionized when they are dissolved in water, as represented in Fig. 4.4(a). We will consider several classes of strong electrolytes (1) soluble salts, (2) strong acids, and (3) strong bases. 

For equivalent concentrations, solutions of strong electrolytes contain many more ions than do solutions of weak electrolytes. As a result, solutions of strong electrolytes are better conductors of electricity. Consider two solutions, 1M HCl and 1M HC2H3O2. Hydrochloric acid is almost 100% ionized acetic acid is about 1% ionized. (See Figure 15.3.) Thus, HCl is a strong acid and HC2H3O2 is a weak acid. Hydrochloric acid has about 100 times as many hydronium ions in solution as acetic acid, making the HCl solution much more acidic. 

His three early Leipzig papers (5-7) represent a synthesis of concepts that he was well qualified to make. Working in Ostwald s laboratory, he must have absorbed some of the mass of electrochemical information which appeared a few years later in Ostwald s two-volume work on the history and theory of electrochemistry (H). He was thoroughly familiar with the second-law thermodynamics of Thomson and Clausius, and with the more recent pronouncements of van t Hoff and Helmholtz. Nernst was also imbued with the atomism of Dalton and Boltzmann, in v hich respect he differed from Ostwald and Helmholtz, and he had accepted Arrhenius s recently published (12,13) hypothesis of the complete dissociation of strong electrolyses in solution. However, his conductance work in Kohlrausch s laboratory had given him a lively appreciation of the effects of incomplete ionization of weak electrolytes. 

Not only the conductance but also the colligative properties of strong electrolytes show deviations from the values to be expected on the basis of complete ionization. The freezing-point depression of, for example, a NaCl solution is less than we would expect for 2 moles of ions per mole of NaCl the van t Hoff factor approaches 2 only in very dilute solutions (Table 12.2, page 217). These diminutions in the colliga-... 

In contrast to the situation with strong electrolytes, the degree of ionization of weak electrolytes is in good agreement with the law of chemical equilibrium. For the reaction... 

The previous example involved a reaction of strong electrolytes, which appear to ionize or dissociate completely in aqueous solution. There are only six common strong acids (acids that appear to ionize completely). In addition to HCl, the others are HCIO4, HI, HBr, HNO3, and H2SO4. Other acids are weak acids, which means that they ionize only partially in aqueous solution. NaOH and other metal hydroxides are strong bases. There are many weak bases, including ammonia (NH3). 

A weak electrolyte is a compound that dissolves in water mostly as molecules. Only a few of the dissolved solute molecules undergo ionization, producing a small number of ions in solution. Thus solutions of weak electrolytes do not conduct electrical current as well as solutions of strong electrolytes. When the electrodes are placed in a solution of a weak electrolyte, the glow of the light bulb is very dim. In an aqueous solution of the weak electrolyte HF, a few HF molecules ionize to produce and F ions. As more and F ions form, some recombine to give HF molecules. These forward and reverse reactions of molecules to ions and back again are indicated by two arrows between reactant and products that point in opposite directions ... 

Strong electrolytes are dissociated into ions that are also paired to some extent when tlie charges are high or the dielectric constant of the medium is low. We discuss their properties assuming that the ionized gas or solution is electrically neutral, i.e. 

Fig. 15. Ion movements in the electro dialysis process. Courtesy U.S. Agency for International Development, (a) Many of the substances which make up the total dissolved soHds in brackish water are strong electrolytes. When dissolved in water, they ionize ie, the compounds dissociate into ions which carry an electric charge. Typical of the ions in brackish water are Cl ,, HCO3, , and. These ions tend to attract the dipolar water molecules...

Strong electrolyte A compound that is completely ionized to ions in dilute water solution, 37 Strontium, 543 Strontium chromate, 434 Structural formula A formula showing the arrangement of bonded atoms in a molecule, 34,579-580,586,590,593, 597... 

The acidic character of acids depends on the availability ofhydrogen ions in their solution. An acid X3 is said to be stronger than another acid X2 if, in equimolar solutions, X3 provides more hydrogen ions than does X2. This will be possible provided that the degree of dissociation of X3 is greater than that of X2. Based on the Arrhenius theory of electrolytic dissociation, solutions may be classified in the manner shown in Figure 6.1. If the ionization of an acid is almost complete in water, the acid is said to be a strong acid, but if the... 

MN = M+ + bland a fraction a of the molecules MN may be dissociated into ions M+ and M". With strong electrolytes such as, salts and strong acids in water, the substances are known to be fully ionized, so that a = 1. The Onsager equation in these cases takes the following (original) form ... 

Let the electrolysis of dilute sulfuric acid (so-called electrolysis of water) with a platinum cathode and a platinum anode be considered next. Pure water is a very weak electrolyte and consequently a very poor conductor of electricity. It dissociates very slightly into H+ ions (it may be recalled that in fact, H+ ions does not remain as such but forms hydronium in H30+ by combining with a molecule of water, H+ + H20 H30+) and OFT ions. In the presence of little sulfuric acid (or for that matter any other strong electrolyte) the conductivity, i.e., ionization is greatly increased. The acidified water now contains H+ ions, OFT and SC3 ions. During electrolysis with platinum electrodes, H+ ions are attracted to the cathode, where each ion gains an electron and becomes a hydrogen atom ... 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

We will follow the same procedure as in the naphthalene/benzene example above. You may wish to look over these examples in parallel to see exactly where the difference between an electrolyte and nonelectrolyte manifests itself. We will again begin by calculating the freezing point, ATf. The problem gives us the value of Kf. In solution, the strong electrolyte, sodium sulfate, ionizes as ... 

The EPR spectra of electrolytically produced anion radicals of Q -aminoanthraquin-ones were measured in DME and DMSO. The isotropic hyperfine coupling constants were assigned by comparison with the EPS spectra of dihydroxy-substituted antraquinones and molecular-orbital calculations. Isomerically pure phenylcarbene anion (PhCH ) has been generated in the gas phase by dissociative electron ionization of phenyldiazirine. PhCH has strong base and nucleophilic character. It abstracts an S atom from and OCS, an N atom from N2O, and an H atom from... 

Additives may also be incorporated into the electrolyte solution to enhance selectivity, which expresses the ability of the separation method to distinguish analytes from each other. Selectivity in CZE is based on differences in the electrophoretic mobility of the analytes, which depends on their effective charge-to-hydrodynamic radius ratio. This implies that selectivity is strongly affected by the pH of the electrolyte solution, which may influence sample ionization, and by any variation of physicochemical property of the electrolyte solution that influences the electrophoretic mobility (such as temperature, for example) [144] or interactions of the analytes with the components of the electrolyte solution which may affect their charge and/or hydrodynamic radius. 

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