Stoichiometry of Iron Oxides
The law of definite proportion in action
Iron rusts. We all know that. But the chemical reactions behind rust are surprisingly complicated! We will return to them at the end of the course when we discuss corrosion (see section 11.6 in the text). Here, we focus simply on the main compounds formed between iron and oxygen. That there is more than one may surprise you. That there are not an infinite number of them (and in fact, there almost is, as well shall see!) is an example of the law of definite proportion, an idea so central to chemistry it is almost taken for granted today.
If iron, Fe, and oxygen, O2, are in contact at high enough temperatures (and with plenty of oxygen around), the grey or reddish compound that forms is called iron(III) oxide with the formula Fe2O3 and the common name (used to identify the natural mineral on Earth and, for that matter, Mars, where its presence is a striking chemical clue to the past history of Mars) hematite. We can write a balanced net reaction for this synthesis very simply:
Note that the number of atoms of each element is the same on each side of the reaction: four iron atoms and six oxygen atoms. This is required by the law of mass conservation and the immutability of elements (in the absence of nuclear reactions, which we will not consider in Chem 5). That the ratio (iron atoms)/(oxygen atoms) is 4/6, or 2/3, and nothing else is an example of the law of definite proportions.
If iron is heated in the presence of less oxygen, it is possible to synthesize another iron oxide, Fe3O4, called magnetite. This is the magnetic "loadstone" known for centuries as a naturally occurring mineral. It is black in color, and its balanced net synthesis reaction is
much like for hematite, but with different combining proportions: 3/4 instead of 2/3, but still a simple ratio of small integers.
Two other points about net reactions are worth mentioning here. First, note that the physical state of each element or compound is indicated in parentheses after the formula: (s) for solid and (g) for gas here. (Other common options include (l) for pure liquid and (aq) for "aqueous solution".) Next, note that we could multiply each of the so-called stoichiometric coefficients by any number and still be OK. (These coefficients are the integers written in front of each compound.) So instead of 3, 2, and 1 in the reaction above, we could multiply by two and write
and still express mass conservation and definite proportion ideas correctly.
But why 2/3 or 3/4? Why not 1/1, or 7/12, or any of a zillion other possibilities? The answer is couched in the detailed atomic structure of Fe and O atoms, a topic for Chem 6; for now, we can state that oxygen atoms in these types of compounds (an non-metal like oxygen combined with a metal like iron) exist in the compound to a very good approximation as discrete ions with the formula O2, a dianion: di- because there is an excess of two negative charges and anion to indicate that the excess charge is negative rather than positive. The law of conservation of charge tells us something about the charge on the iron atoms. Consider Fe2O3 first. It is electrically neutral: no net charge. If the three oxygen anions each carry a charge of 2e, then the total for the three is 6e. This must be balanced by the two iron atoms, which, therefore, must be cations, ions with a net positive charge. This is really as far as we can go with conservation laws alone. It could be that one of the two irons is neutral and the other is +6e, or maybe both are +3e, or maybe even one is 10e and the other is +16e, for all we know now. Other evidence is needed to decide the case, and when that other evidence is invoked (we will see some of it in Chem 5 and more in Chem 6), we find that both iron ions have a +3e charge: Fe3+. This charge is the origin of the (III) part of the name iron(III) oxide.
So what about Fe3O4? Again, the oxygens all together have 4(2e) = 8e net charge. But we cannot give the three irons equal positive charges to balance this: 8/3 isn't an integer! Again, further information is needed, and the answer turns out to be that in magnetite, two irons are iron(III), but one is iron(II), Fe2+. Now we're balanced: 3e + 3e + 2e = +8e.
Finally, we must mention the curious third common mineral containing iron and oxygen known as wüstite. Since oxygen is O2 and since we have found Fe2+ in magnetite, it would seem that a simple FeO compound should exist. In fact, it almost does, but pure FeO cannot be synthesized. Instead, wüstite is an example of a nonstoichiometric compound. It has a variable composition from sample to sample that spans the atomic ratios of roughly 85 to 95 iron atoms per 100 oxygen atoms. We write this as the formula Fe0.85O1.00, for example, with the curious fractional subscripts. In wüstite, almost all irons are Fe2+, but some are Fe3+. This is an example of a so-called mixed valence compound, as is magnetite (i.e., the same element has two distinct charge forms in one compound), but because the Fe3+/Fe2+ ratio is continuously variable (and here is why there is "almost an infinite number of iron oxides"), the compound is nonstoichiometric. Problem set 1 lets you think about these ideas further.
If you would like to learn more about these minerals (hematite is a common mineral used in jewelry, and powdered hematite is red rouge used in polishing applications), here are some links you might enjoy along with a couple of pictures. Note the iron nails and iron filings on the magnetic magnetite!
Photos © 1997 - 2000 Hershel Friedman, www.minerals.net
The latest (January 6, 2004) photo from the Mars rover Spirit, showing the "rust color," due to iron oxides! Some of the rocks are probably hematite.