The Elephant in the Water

Just say no to H+...?

Just say no to H+…?

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?

The true nature of the aqueous proton is just a little more complicated than hydronium ion...

The true nature of the aqueous proton is just a little more complicated than hydronium ion…

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…!

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  1. And when they draw mechanisms, I am moving toward H-OH2+ so they can clearly and explicitly show where the electrons are and where they’re moving to. Same with H-OSO3H. That’s how I always draw it on the board during class. I haven’t yet, but I’m moving toward counting off for arrows from H3O+ that show the O-H bond breaking.


  2. All this is assuming that beginners can be immediately aware of the finest details of the story, and that the first time they touch a steering wheel they’re completely able to win a F1 race. Sorry, they aren’t.

    What is needed, in my opinion, is to completely forget the existence of water when introducing proton-transfer systems, even if we know that water is heavily involved in 99.9% of acid-base systems we’re practically dealing with. And only when the mechanism is clear – yes, azmanam, showing the electrons on the move is really effective – then let water be.
    Because it’s not simple to catch at a glimpse the fact that water is at the same time an acid and a base and a solvent and the strangest substance in the universe and something else.

    Of course, if you’re addicted to an Arrheniusian way of thinking, e.g. in balancing equilibrium equations (that stuff which leads to all those relaxing homework exercises, which mostly are requiring the student just the pH value that the textbook is asking for… never mind if they find quite different values when they’re looking at a pH-meter), then of course H+ is by far easier.

    In the simple and safe world of our ancestors, of course covalent bonds were clearly different things from hydrogen bonds, dative bond were going around with their nice arrows and so on.

    But I feel more than a sensation that most students trained that way are not so able to understand the structure of a H-bonded network, nor can explain the unique features of the substance H2O, and surely have problems even to tell at which dilution sulfuric acid loses its terrible aggressiveness. Don’t ask them why aluminum or iron solutions become jellyfish-like!

    OK, I know a lot of teachers which are still speaking about “salt hydrolysis” and even call ammonia solutions “ammonium hydroxide”. Most of them are nice fellows, good parents, neighbors and citizens: surely better than me. So, were’s the problem with H+?

    Unluckily, as I was born in 1960, I feel uncomfortable with a chemical weltanschaaung which was rotting and misleading (“confusing”, as Hawkes wrote ages ago) even in the Fifties, when it was already straightforward that oxonium structure is just a first approximation. By the way, the radical, definitive difference between H+ and Fe3+, should had been clear even in 1913!

    I’m trying to teach chemistry to people who are supposed to be social and cultural leaders in 2050. If, at that time, they’ll decide to convert to phlogiston and caloric, at least I hope they don’t blame me! ;)


    1. Well said, Sergio. There are folks out there who have staked their entire careers on the strange properties of water!

      What’s most frustrating to me is that changes to the educational status quo often seem to require sweeping reorganizations of the entire curriculum…and it’s difficult to see the difference between “the way we teach is falling short” and “one can’t expect a novice to understand a topic of this complexity.”


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