Iron oxide chemical formula

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