The Elephant in the Water
At some point during my personal education in chemistry, I abandoned the use of “H+” to represent “what forms when an acid is placed in water” and switched over to writing “H3O+.” The way I see it, the moment came when my desire to be right and rigorous finally surpassed my urge to be efficient—taking the time to write the extra symbols was suddenly worth the trouble, once I realized that it was apparently a matter of correctness. What one rarely considers as an undergrad is the idea that “H+” wouldn’t have survived to the present day if it didn’t have a kernel of truth to it. What beleaguered chemists hiding out in dark, dusty laboratories are still fighting for the proton? What evidence could possibly bolster these defenders of the proton? Read on!
There is an interesting pedagogical dimension to this whole discussion. Oftentimes when students are learning a new concept or how to solve a new type of problem, functional shortcuts become apparent. These tricks minimize the mental effort associated with problem solving while working with high enough frequency to be tolerable—any trick that works more than 90% of the time is a winner! The catch, of course, is that shortcuts leave out important conceptual details and leave the student’s learning at a disadvantage. As a result, teachers tend to be highly opposed to them while students lap them up. Complicating the situation further, the effectiveness of a particular trick depends on the thoroughness of the teacher and the complexity of assigned problems.
I can’t speak for the broader community, but there was a time when I branded the use of “H+” one of these counterproductive tricks, and I think it’s a fairly common sentiment. For a very wide range of problems, simply writing “H+” works. The nugget of knowledge that the proton is not bare in acidic solutions is very rarely essential to the solution of a problem. Perhaps that fact annoys a lot of teachers—students can get by ignoring it, even though the claim has broader bogus implications (e.g., acids just fall apart in the gas phase, other bare cations can exist in aqueous solution, etc.). The feeling of annoyance encourages the idea in professors that the perpetuation of “H+” is a student-driven conspiracy!
I think there’s more to the story. As I mentioned before, “H+” probably wouldn’t have survived to the present day if there wasn’t some truth to it. It wouldn’t have made it if teachers over the years didn’t smile at a student scribbling “H+” and admit there might be something to it other than the drive for spending as little time as possible on one’s chemistry homework. But what could it be?
If we accept that bare protons in aqueous solutions are bogus—which they are—the tendency is to move to the next simplest description, which involves the proton “coordinated” (covalently bound, really) to one water molecule. Enter the hydronium ion, H3O+. Hydronium has a wealth of pedagogical advantages. First and foremost, it reminds us of the importance of water in the chemistry of acids. Acids don’t just fall apart in the gaseous phase, and basic water is essential for acid dissociation. In this sense, the use of hydronium ion is admirable.
In another sense, however, the use of hydronium ion is…misleading. If we want to fit the proton in with other solvated cations (and that is what it is after all, right?), using the formula H3O+ makes no sense. Like other solvated cations, the proton is surrounded by more than one water molecule in aqueous solution. Writing H3O+ in this context makes about as much sense as writing FeH2O3+ to represent aqueous iron(III). On the contrary, evidence suggests that aqueous protons may be surrounded by as many as six water molecules coordinated directly or indirectly. Might the formula [H(H2O)6]+ be a better choice?
Even the defenders of the proton would admit that [H(H2O)6]+ is a handful for the chemist interested in acid-base chemistry to write. It’s overkill. Instead, they support a much more subtle representation: H+(aq)! The (aq) phase designator is something of a double-edged sword here. On the one hand, it obscures the issue of how many waters are actually involved with the proton (but let’s face it, most of us don’t really care…) and masks the importance of water molecules in aqueous acid-base processes (how many students don’t know that “aq” stands for “aqueous,” or what “aqueous” really means?). On the other hand, it allows us to mentally file the aqueous proton away with other aqueous cations: Fe3+(aq), Li+(aq), Mg2+(aq), etc. The phase designator is something of a cop out in these cases, too, but it conveys the key idea that waters are bound to the cation in a coordinate covalent fashion (which is true for the proton too, by the way!).
So perhaps we ought to re-brand the defenders of the proton as the “defenders of the aqueous phase designator.” My sense is it would be very difficult to drag most general chemistry instructors kicking and screaming away from the use of H3O+, but it’s a noble goal. Consider me a convert…!