provocative opinion Arrhenius Confuses Students Stephen J. Hawkes
Oregon State University, Cowallis, OR 97331 The traditional presentation of the Arrhenius acid-base concept before the Brmsted-Lowry confuses and misleads students. The Bronsted-Lowry approach should be presented first, because it is simpler; it involves only the transfer of a proton; and it does not lead to the misconceptions and complications discussed below. The Arrhenius d e f ~ t i o n scould serve later as an historical footnote but not as a viable concept. If a base is defined by its ability to produce OH- as suggested by Arrhenius, it requires a convoluted argument to show that NH3 and HC1 react in an acid-base reaction in the absence of water. It can be done, invoking the fact that NH&1 is produced either in the absence or presence of water and is the "salt" produced from NHdaq) and HCl(aq). The relevance of the OH- still remains as a stumbling block to understanding, even then. This problem leads to other misconceptions. I have seen statements in textbooks that acetic acid and ammonia, for example, react in aqueous solution because they produce Ht and OH- which then interact. The student is thus led away from the simple understanding that the acid and the base interact directly.
542
Journal of Chemical Education
The meaning of acidity also is confused by the Arrhenius defmitions. Water at 100 OC has pK, = 12.3. When asked whether it was, therefore, more acidic than at 25 'C when pK, = 14.0, some of my students argued that the increase in [Hi] made no difference: it was offset by the increase in [OH-]! This is clearly an artifact of the Arrhenius concept, because they gave theopposite response when asked about a decrease in oK. of acetic acid. the basicitv of the acetate ion notwithstanding. The need for an OH group led Arrhenius to propose NH40H as the formula for hydrated ammonia, giving us the "ammonium hydroxide" label on reagent bottles. This misconception is apparently not eliminated even now, for it has appeared on the pages of this Jourml(1) as late as 1991. A student struggling to discard the h h e n i u s concept cannot absorb the concept ofconjugate acids and bases and has difficulty in believing that negative ions are bases unless they are hydroxide ions. The Arrhenius approach requires that they first appreciate that negative ions react with water producing OH- without providing any reason to expect such reaction, while the Brmsted-Lowry concept
.-
makes it obvious that any negative ion must be basic simply because of its negative charge. I t is inherent in human nature that we accept what we are told first and relinquish or change it with difficulty. Like most students of my generation, I learned the Arrhenius concept in my first year of college. (My high school teacher held pre-Arrhenius concepts.) I learned the Brgnsted-Lowry concept as a college senior. Consequently, I found the latter difficult, and it took decades to realize that it is the simpler of the two concepts. We should not confuse another generation of students in this way.
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An acid is a suhstance from which a proton can be removed. A base is a substance that can remove a proton fmm an acid.
I rewmmend these definitions to teachers and publishers and especially to students.
Protons Are Not "Donated The reaction HCl + H '
mugger. It creates a false concept of the action of an acid, as if it somehow expelled its proton by means of some internal force. A base must tear the H+ from the powerful attractive forces holdine it to an acid. breakine its bonding by superior force. My chemistry teacher, William Gerrard, nearly half a century ago provided me the above argument and the simple definitions:
+ CI-
requires an input of 1.4 x lo6 Jlmol (2). When that much energy is required, it makes no more sense to speak of HC1 "donating" a proton than of "donating" your purse to a
Literature Cited 1. Tutt1e.T. R.J. C k m . Educ 1631.68,533. 2. C a l d a t e d horn data In Chase, M. W.; Dsvies, C. A ; Downey, J. R.: Frurip, D. J.; McDonsld, R. A ; Syverud, A. N. JANAF Thermhemiml lbblea, 3rd ed.. Amen. can Chemical Society: Washington. DC, and American Instihlte of Physics: New
Volume 69 Number 7 July 1992
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