Proton Wars (and Woos)

All Rights Reserved, Copyright 1999 by Craig Rusbult
[update in November 2010: The text-and-drawing are unchanged from 1999;
the only changes are these "updates in 2010" and the bottom-of-page links.]

        This web-page is a rough outline of a paper I'll submit to The Journal of Chemical Education in the near future. [update: I did not submit it to JCE in 1999.]  The introduction to my paper will cite two papers from J Chem Ed:  one is by Stephen J. Hawkes (JCE, 1992, pp 542-543), about the mechanism for acid-base reactions;  another (maybe by Miller in JCE, 1992; I'll check this later) discusses the use of anthropomorphic analogies to help students form vivid mental images of chemical phenomena.
        Hawkes explains why common methods of teaching can "create a false concept of the action of an acid, as if it somehow expelled its proton by means of some internal force," and why a more accurate description is that "a base must tear the H+ from the attractive forces holding it to an acid, breaking its bonding by superior force."
        I agree, and for two decades I've been using the conceptual analogies of proton warring and proton wooing for the purpose of helping students to construct vivid mental images of atomic-level events involving protons and electrons within molecules, and to connect these images with symbolic representations of the atomic-level events in chemical equations and electron-dot pictures.
        comment:  In the manuscript I'll submit to JCE, the introduction (and the rest of the paper) will be developed in more detail.  In this draft I'll just outline the basic themes.

        As in other chemical reactions, an essential driving force for acid-base reactions is the attraction between positive and negative charges.  As shown in the center picture below, during a collision a proton (H) is simultaneously between two electron pairs (:), one on Cl- and another on H2O.  Because each electron pair has negative charge, each attracts (and is attracted to) the positively charged proton, and each would like to be near this proton (in the form of a covalent bond involving the proton) after the collision.

        Two possible outcomes are shown.  When the electron-pair on Cl- wins the battle, we get the species on the left side of the equilibrium equation, HCl + H2O.  But when H2O wins the war, we get the right-side species, Cl- + H3O+.  The left and right sides of the equation correspond to victories by Cl and H2O, respectively.
        The diagram above can help students to understand the fundamentals of reactions, and to interconvert between different descriptions (in chemical equations, electron-dot pictures, verbal descriptions, and mental representations) of an acid-base reaction.

        Two Perspectives: Warring and Wooing
        We can think of the collision-interactions in two ways.
        Proton Wars:  Every collision is analogous to a war between bases, and the stronger base takes the proton.
        Proton Woos:  Every collision is analogous to a competitive courtship.  During the collision both electrons make themselves available to the proton, and the proton chooses to depart with the electron-pair that is more attractive, when all factors are considered.
        [update in 2010:  Both perspectives, warring and wooing, are technically equivalent but are mentally different (leading to different mental models), and both can be useful ways to think about the +- interactions (attractions and repulsions) that are key concepts in modern chemistry.  Thinking about the collision as a WAR adopts the perspective of a proton being the passive "reward" won by the stronger competitor, while with a WOO the proton is the active "chooser".]

        In either perspective, an acid is passive.  As a strongly reacting acid, HCl does not "want" to donate a proton, but its conjugate base, Cl-, is not strong enough (or attractive enough) to hold onto a proton during most collisions with H2O.
        By contrast with a misleading equation (HCl --> H+ + Cl-) that implies action by the acid, the representations above -- the Bronsted-Lowry equation (HCl + H2O <--> H3O+ + Cl-) and the electron-dot pictures -- place the focus where it should be, on the bases and (more fundamentally) the base's electrons as these interact with a proton.

        comments:  The basic ideas are outlined above.  In the paper I'll also discuss symbolism, simplification, and three extensions.

        SYMBOLISM:  The meaning of key symbols (H and H+) varies with context.  In a chemical formula (and thus a chemical equation) or in a sentence, H is a neutral H-atom (a proton with an electron);  but in an electron-dot picture, H is a positively charged proton.  In a formula, equation, or sentence, a proton is H+;  but a proton is H in the electron-dot picture of a multi-atom molecule.

        SIMPLIFICATION:  I've treated acid-base reactivity as a purely covalent process, ignoring the effects of solvation (and complex species such as H4O9+) and entropy.  { How significant are solvation effects and (especially) entropy considerations?  To what extent is it true that the "stronger" and "more attractive" electron pairs will go away with the proton, especially when the solvent is a competitor?  If we consider only covalent electrical forces, the reaction "HCl (aq) --> H+ + Cl-" is ludicrous, but so is the dissolving of NaCl, which does occur.  { I'll visit the library tonight to get tables of delta-H and delta-S, but so far I haven't tried to estimate these contributions. }   In the JCE paper I'll probably keep it simple by briefly mentioning these factors as outlined in the following paragraph, plus a clarifying endnote, and citing papers with a more technical treatment. }  /   Also, references to simple "electron pairs" ignores the complex spatial distributions and dynamics of electrons.
        An educationally useful approach, especially at the level of general chemistry, is analogous to a strategy used in physics --- thinking in terms of a simplified system plus correction factors.  For example, in physics we can first calculate the motion of a hockey puck simplistically by ignoring friction and air resistance;  then we add correction factors for the effects of friction and air, and notice the difference this makes in our predictions, compare both sets of predictions (using the simplified and modified theories) with observations, and learn.  /   For an acid-base reaction, a covalent proton war is a simplified starting point that can be modified by considering other factors, especially solvation and entropy.  A corrective modification might occur by considering the ways in which solvation affects the characteristics of each electron pair involved in the proton war, when we are thinking about "the factors affecting the strength of a base" as discussed below.

        THREE EXTENSIONS follow:  1) factors affecting basicity/acidity, 2) equilibrium, and  3) an analogous analogy (from proton wars to electron wars).

        An interesting follow-up question for students is:  What makes one electron-pair a stronger warrior, or a more appealing suitor, compared with another electron pair?  Suitable Aesop's Examples, each chosen to illustrate a specific concept, include:
        Comparing the basicity of NH2- and NH3.  If all other factors are equal, in a "conjugate series" the more negative a molecule containing an electron pair, the more negative this electron pair will be, and the stronger it will be as a warrior and wooer, so NH2- (when it grabs a proton to become NH3) is a stronger base than NH3 (when it becomes NH4+).
        The acidity of CH3CH2OH vs CH3COOH, of alcohol vs carboxylic acid:  Comparing the conjugate bases for each, in CH3CH2O- the O has a full negative charge, but in CH3COO- there is resonance so each O has only a half negative charge, thereby making the electron pairs on these Os much weaker as a warrior or wooer, so acetate is a weaker base.  { And its conjugate, acetic acid, is a stronger acid, compared with the corresponding alcohol. }   /   After developing the concept of resonance and its effects on the electron pairs, we can ask students, "If you were a positive proton, which would you prefer being close to, a half-negative charge or a whole-negative charge?"
        CH3COOH vs CF3COOH:  Because F is highly electronegative, compared with H, the Fs will pull electrons away from the COOH end of the molecule, thereby making these O's less negative and less strong as a warrior/wooer, so CF3COO- is a weaker base because it isn't as strong at grabbing protons, and CF3COOH is a more strongly reacting acid because it isn't as strong when it tries to hang onto its protons.
        Other examples (such as HF vs HCl, alkenes vs alkynes,...) can also be used, depending on students' experience.
        REALITY CHECKS:  For each example we can tell students that our chemical intuition, based on a logical analysis of positive-negative attractions, is supported by experimental observations.  { And if our chemical intuitions ever fail, this can be especially interesting! }

        DYNAMIC EQUILIBRIUM:  The type of collision shown above (plus spatial variations) is happening in every part of a solution, over and over, many billions of times each second.  In another paper (unpublished), which outlines an intuitive approach to the quantitative aspects of acid-base equilibria, I ask students to do a thought experiment by imagining a "magic camera" that lets them "see a snapshot" for a typical outcome (defined by what we observe in physical experiments) of these billions of battles, captured at an instant of time.   /   Then we can gather the results from many physical experiments with different acids and bases, and summarize our observations in a table that shows the relative strengths of acids and of their conjugate bases.

        The same type of anthropomorphic analogy that is used for acid-base reactions can help students visualize oxidation-reduction reactions as a battle for electrons (in an electron war) and as a competitive courtship (in an electron woo).   /   And a similar qualitative treatment of empirical data, using tables to show the oxidizers and reducers (or the acids and bases) that are strong and weak, can be done with either electron wars or proton wars.

P.S.  {originally this was in the "simplification" paragraph, but I don't think it's important enough to keep there}  A long time ago, I made Ka calculations of the straightforward type justifiably criticized by Hawkes, and [agreeing with Hawkes] concluded that if Ka is calculated in the same way for both acids -- as " [A-] [H3O+] / [HA] " -- the Ka of HNO3 is smaller than the Ka of H3O+.  Is HNO3 a "strong acid" (i.e., with the majority of molecules losing a proton) due to the high concentration of solvent competitors?

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