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Ionization or Dissociation? Emeric Schultz Department of Chemistry, Bloomsburg University, Bloomsburg, PA 17815 Chemistry is a language. Unfortunately for many students it is a “very” foreign language, one most never master. This “language problem” is exacerbated in several instances in that the way a term is used in chemistry is sometimes not congruent with the normal meaning of that word in English. A case in point is the use of the term dissociation in contexts in which it is not only confusing from the standpoint of poor correlation to common English usage, but is also in conflict with physical reality. The word dissociate is a truncation of dis-associate. The English context is “to take apart that which has associated”. Applying this sense of the word, the logical chemical context would seem to be “to separate into the component units of the associated substance”. The word dissociation works nicely to describe what happens when NaCl, NH4 Cl, other ionic substances, and strong mineral bases, such as NaOH, are added to water. It does not work well for describing what happens to all other acids and bases when these substances are added to water. The component parts of HF are not H + and F{; nor are the components of NH3, NH4+ and OH{ (especially hard to conceptualize, unless of course one starts with NH4OH). Some textbooks, however, appear to recognize this “problem”. The following “definitions” are found in a standard text (1): “Dissociation refers to the process in which a solid ionic compound, such as NaCl, separates into [emphasis added] its ions in solution. Ionization refers to the process in which a molecular compound separates to form [emphasis added] ions in solution.” A review of texts available in my department with post1992 publication dates yielded some very interesting findings on the way authors used the terms ionization and dissociation in referring to the behavior of weak and strong acids and weak bases in aqueous solution. Of the eleven textbooks reviewed, three (2–4) used the term dissociation exclusively. Two texts (5, 6) were interesting hybrids in that the two terms were used in different and inconsistent ways (see below). Six texts (1, 7–11) used ionization fairly consistently. There was a strong, albeit not perfect, correlation between authors who consistently showed acid ionizations as reactions of water with acids to yield hydronium ion (and the accompanying conjugate base) and authors who used the term ionization exclusively. One text (5) refers to reactions in solution as ionizations; one refers to the “ionization equilibrium for HA” as being represented by “Ka…the acid-dissociation constant” (12); one uses the term “percent ionization” with dissociation constants (13); and one has a section heading “Dissociation of water” (14). Another text (6) consistently uses dissociation in describing the reaction of acids in water. This text goes to great pains not to use H+ anywhere in development of the acidity concept, H3 O+ being used exclusively. The three important equilibrium constants in acid–base chemistry are referred to as follows: “the water-dissociation equilibrium constant, Kw” (15); “the acid-dissociation equilibrium constant, Ka” (16); and “a base-ionization equilibrium constant, Kb” (17). In many of the texts there is a different formalism in the way acid reactions are presented in the chapter on “chemical reactions in solution” (H+ used) versus how these reactions are written in the “acid/base chapters” (H3O + used). This mixed use of H + and
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H3 O + varies widely (and in my opinion with great inconsistency) in the texts reviewed. At the heart of acid/base concepts in aqueous solution is the nature of water. The use of the term dissociation in the context of water is one that I find particularly unsettling and unnecessary. We spend a considerable amount of effort treating water as the quintessential polar covalent compound (consider all the important concepts that flow from this) and throw all this effort away by “disassociating water” into its ions! Although I am not a historian of chemistry, I suspect that the term dissociation was once an appropriate description for what was believed to be happening to the species in question in aqueous solution. The Arrhenius definition of acidity is consistent with this supposition. We now know better, and the words we use should reflect that. I believe that a unifying conceptual framework can be obtained by explicitly focusing on the role of water, not only in acid/base reactions, but in all processes in which a substance dissolves or partially dissolves in water. Such an approach also provides for a consistent and physically appropriate usage for the terms dissociation and ionization. This approach starts with the recognition that substances either “interact” or “react” with water. The extent of this interaction or reaction can be either partial or complete (weak or strong). If a substance has interacted with water, its component pieces are in solution; for molecular substances, this is the hydrated molecular substance; for ionic substances these are its component (hydrated) ions. An the ionic substance undergoes “dissociation into its component ions”. (Mineral bases, such as NaOH, are basic as a consequence of being ionic salts of a special type, hydroxides.) If a substance has reacted with water, both water and the substance must change. If the substance is an acid (weak or strong), an ionization reaction occurs with water (to produce the hydronium ion and the conjugate base), whereas if it is a base (ammonia or organic amine), a different ionization reaction occurs with water (to produce the hydroxide ion and the conjugate acid). This “two step” approach, first solvation (interaction), then possible reaction with water (or other species), allows for the incorporation of other reactions in solution. For instance, hydrolysis reactions would involve dissociation of a salt, for example NaF or FeCl3 , into component ions and then the further ionization (partial or complete) occurring by reaction with water (hydrolysis). In the case of FeCl3, of course, the solvation sphere now comes into play. It is important to recognize that what has happened to water is the common strand in all the processes described above that are classified as ionizations. Water is always ionized in the reaction, whereas in some hydrolysis reactions (those of anions acting as bases), the species that reacts with water is “neutralized”. This ionization of water is the key to charge balance in these reactions: charge is either produced from neutral species or transferred from one species to another with water acting as a reactant in both cases. From this viewpoint the neutralization of acidic or basic solutions is therefore deionization. All other common reactions that occur in water, such as solvation and precipitation of salts, complex ion formation, proteins binding to substrates, can be viewed as association/disassociation phenomena.
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Information • Textbooks • Media • Resources Extending this approach to the quantitative arena, two domains of equilibrium constants emerge: (i) dissociation constants such as Ksp, Kf (complex ion), Km (Michaelis constant for enzymes); and (ii) ionization constants such as Kw, Ka, Kb. Ionization constants are instantly recognizable as having either H3 O + or OH{, whereas dissociation constants do not contain these species. My own special plea is that we banish H+ from the our discussion of acids and bases. This can be done. One of the texts reviewed for this paper does not even use H+ in redox reactions (12)! A clearer distinction needs to be made between the processes of dissociation and ionization and the attendant equilibrium constants associated with these processes, both in texts and in the classroom. My own belief is that the “inconsistency” in texts by and large is inadvertent and reflects the fact that the authors, like the rest of us, understand what these similar terms mean in the context used and use them interchangeably. However, we certainly cannot assume that the same applies for students who are learning this for the first time. I think that we need to continue to look for better ways to communicate the discipline we love, by using more learner-friendly vocabulary (where possible), as well as by being more consistent in concept development. Specifically, I believe that the term ionization is a more consistent, physically correct and English-congruent alternative to the use of dissociation for all acid/base phenomena in aqueous solution (including the autoionization of water), and should therefore be universally adopted in this context.
Literature Cited 1. Whitten, K. W.; Gailey, D. G.; Davis, R. E. General Chemistry with Qualitative Analysis, 4th ed.; Saunders: Fort Worth, 1992; p 127. 2. Zumdahl, S. S. Chemistry, 3rd ed.; D. C. Heath: Lexington, MA, 1993. 3. Silberberg, M. Chemistry, The Molecular Nature of Matter and Change; Mosby: St Louis, 1996. 4. Kask, U.; Rawn, J. D. General Chemistry; Wm. C. Brown: Dubuque, 1993. 5. Brown, T. L.; LeMay H. E.; Bursten, B. E. Chemistry, The Central Science, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1994. 6. Bodner, G. M.; Pardue, H. L. Chemistry, An Experimental Science, 2nd ed.; Wiley: New York, 1995. 7. Umland, J. B. General Chemistry; West: Minneapolis/St. Paul, 1993. 8. Atkins, P. W.; Beran, J. A. General Chemistry, 2nd ed.; Scientific American: New York, 1992. 9. Hill, J. W.; Petrucci, R. H. General Chemistry; Prentice Hall: Upper Saddle River, NJ, 1996. 10. Gillespie, R. J.; Eaton, D. R.; Humphreys, D. A.; Robinson, E. A. Atoms, Molecules, and Reactions: An Introduction to Chemistry; Prentice-Hall: Englewood Cliffs, NJ, 1994. 11. Kotz, J. C.; Treichel, P. Chemistry & Chemical Reactivity, 3rd ed.; Saunders: Fort Worth, 1996. 12. Brown, T. L.; LeMay H. E.; Bursten, B. E. Chemistry, The Central Science, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1994; p 580. 13. Brown, T. L.; LeMay H. E.; Bursten, B. E. Chemistry, The Central Science, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1994; p 582. 14. Brown, T. L.; LeMay H. E.; Bursten, B. E. Chemistry, The Central Science, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1994; p 568. 15. Bodner, G. M.; Pardue, H. L. Chemistry, An Experimental Science, 2nd ed.; Wiley: New York, 1995; p 602. 16. Bodner, G. M.; Pardue, H. L. Chemistry, An Experimental Science, 2nd ed.; Wiley: New York, 1995; p 606. 17. Bodner, G. M.; Pardue, H. L. Chemistry, An Experimental Science, 2nd ed.; Wiley: New York, 1995; p 619.
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