When elemental iron corrodes it combines with oxygen in the air to ultimately form red brown iron(III) oxide called rust. (a) If a shiny iron nail with an initial mass of \(23.2 \mathrm{g}\) is weighed after being coated in a layer of rust, would you expect the mass to have increased, decreased, or remained the same? Explain. (b) If the mass of the iron nail increases to \(24.1 \mathrm{g}\), what mass of oxygen combined with the iron?

When we talk about a chemical reaction, we're referring to the process where substances, known as reactants, transform into different substances called products. In the context of the corrosion of iron, the reactants are iron (Fe) and oxygen (O2), and the product is iron(III) oxide (rust). During this process, new chemical bonds are formed, and old ones are broken.

In our exercise, the shiny iron nail represents the pure reactant, while the rust covering it is the product. Understanding this conversion is crucial because it illustrates how the properties of substances change; in this case, iron changes from a shiny metal to a brittle, reddish-brown compound. A key point to note is that in chemical reactions, mass is conserved. That means the total mass of the reactants must equal the total mass of the products. This conservation is applicable to our rusted iron nail scenario, where the increase in mass is a direct result of oxygen from the air incorporating into the iron to form rust.

Oxidation and reduction are two halves of a type of chemical reaction called a redox reaction. Oxidation involves the loss of electrons, whereas reduction involves the gain of electrons. Let's consider our corroding iron example. The iron is said to oxidize as it loses electrons to oxygen, which is being reduced as it gains those electrons. This electron exchange is a fundamental aspect of the corrosion process.

When we look closely at the iron nail before and after rusting, we can think of the iron atoms on the surface of the nail as having 'given up' some of their electrons to oxygen atoms from the air. As a result, the iron atoms become positively charged iron ions (Fe3+), and oxygen, which picks up the electrons, is reduced, forming oxide ions (O2−). These ions then bond together to form the compound iron(III) oxide (Fe2O3), commonly known as rust.

Stoichiometry is the aspect of chemistry that deals with quantifying the amounts of reactants and products in a chemical reaction. In our textbook's exercise, stoichiometry allows us to calculate the mass of oxygen that has combined with the iron on the corroding nail.

Using stoichiometric calculations, we can determine that the mass increase of the iron nail after rusting is due to the additional mass of the oxygen atoms that have chemically bonded with iron. From our given data: the initial mass of the iron nail was 23.2 g, and after rusting, it increased to 24.1 g. By applying the equation from the solution step, \(\text{Mass of oxygen} = \text{Final mass} - \text{Initial mass of iron}\), we find that the combined mass of oxygen is precisely \(24.1\text{ g} - 23.2\text{ g} = 0.9\text{ g}\).

Grasping the concept of stoichiometry is essential because it not only helps in understanding how the atoms combine in fixed ratios but it also equips one to accurately predict the quantities involved in the reactions. This knowledge is incredibly beneficial in fields ranging from material science to environmental engineering.