Thursday, November 11, 2010

Rust And Entropy

I've tried to tackle plenty of news reports and documents that purportedly are based on physics, but really are nothing more than either errors, or complete bastardization of things the writer didn't understand. This is another example, but done by Dan Styer and published in Nov. issue of AJP (D. Styer Am. J. Phys. v.78, p.1077 (2010)). In it, he tackles the claim from several sources that rust occurs due to entropy, i.e. the tendency towards an increase in entropy. He cited several sources for having such a "theme".

This particular quote comes from an online source: Cutler J. Cleveland and Robert Kaufmann, “Fundamental principles of energy,” in Encyclopedia of Earth, last updated 29 August 2008 accessed 12 May 2010. But the spirit of this quote can be found in many places., “Entropy tends to make our eyes grow weaker as we age… entropy causes iron and other metals to rust” appears in Louis M. Savary, Teilhard de Chardin: The Divine Milieu Explained (Paulist Press, Mahwah, NJ, 2007), p. 163., “Entropy imposes itself in the form of limitation and in the natural tendency toward confusion and chaos, toward decay and destruction. Entropy is why ice cream melts and people die. It is why cars rust and forests burn. Preponderantly, the second law describes effects that we don't want to see” appears in Gilbert L. Wedekind, Spiritual Entropy (Xulon, Fairfax, VA, 2003), p. 68.

Of course, anyone who has studied chemistry in college can pick up what's wrong with such a claim. As Styer pointed out, rust occurs when two types of elements interact, i.e. iron and oxygen, via the reaction

Now, one can assume that Fe is in a solid form, while oxygen is in a gaseous form. In general, it is clear that a gas will have a higher entropy than a solid. So what is going on here is that we have an initial condition of a solid iron and a gaseous oxygen, and ending up with a solid rust (iron oxide). This final condition should actually have a lower entropy than solid iron plus oxygen gas. So the entropy of this system should have decreased, not increased as claimed in the cited references.

Of course, the whole entry of the universe does not decrease since the process involved a change in entropy of the surrounding that is higher. But as Styer indicated, the iron+oxygen system itself has a decrease in entropy. Proclaiming that rust occurs because of entropy (i.e. Thermo's 2nd Law) is faulty.


1 comment:

Douglas Natelson said...

A better explanation would involve the Gibbs free energy, but I don't think the statement about entropy is 100% misleading. The reason the reaction runs predominantly one direction (ending in the establishment of equilibrium in terms of energy and particle number between the reactants and products) is precisely the tendency of the whole system (reactants, products, + energy reservoir that is the surroundings) to find a macrostate corresponding to the greatest number of microstates, subject to the constraints that this take place at some temperature (of the environment) and pressure (of the gas).