I was reminded of this article by Michelle Francl[1], where she poses the question “What anchor values would most benefit students as they seek to hone their chemical intuition?” She gives as common examples: room temperature is 298.17K (actually 300K, but perhaps her climate is warmer than that of the UK!), the length of a carbon-carbon single bond, the atomic masses of the more common elements.
Well, one of my own personal favourites is anchoring chemical timescales. From 10^{-18} s (that of electron dynamics, and presumably the fastest processes in chemistry) to 10^{+18} (approximately the age of the universe). And (for a unimolecular process) this can be reduced to this equation: Ln(k/T) = 23.76 – ΔG^{‡}/RT I quoted this equation in a recent post, since it gives you the fastest possible chemical reaction if ΔG^{‡ } is set to zero (which of course is not a reaction but a vibration), but which gives you a good estimate of how fast a process will be for any given value of a barrier. It can of course also be solved for e.g. the required barrier to achieve a half-life equivalent to the age of the universe. So, perhaps in increments of orders of 3 magnitudes (of which there are 13 covering the above span) would anyone like to contribute either:
- Their own favourite chemical anchor, or
- Their own favourite example of a chemical timescale bounded by the above limits?
(I did start a list of the latter for our own students, but it is still pretty sparse!)
References
- M. Francl, "Take a number", Nature Chemistry, vol. 5, pp. 725-726, 2013. http://dx.doi.org/10.1038/nchem.1733
Tags: /RT, chemical intuition, chemical timescale, chemical timescales, favourite chemical anchor, Michelle Francl, possible chemical reaction, United Kingdom
I’m not sure this really counts so much as an anchor, but I do find useful as a mental guide sets of photos demonstrating what a mole of each of a collection of materials looks like, e.g. this one: http://fphoto.photoshelter.com/image/I0000Ml_f1mIoHjs
(Though, they’re better if the photographer includes a common reference item for size.)
It probably is warmer where I am than the UK; at least my office is generally closer to 300 K than 298. But I’m trying, too, to get students to work with just one significant figure, hence 300 K, not 298.17…
I like the time idea — here is one version of what I share with students: http://blogs.nature.com/thescepticalchymist/files/2013/08/maps-and-anchors.pdf