As stated previously, a chemical change occurs by breaking the bonds within a substance, rearranging the atoms that had previously been connected, and then generating new bonds, in order to produce a new chemical. The corresponding chemical reaction, which is the combination of these molecular-level processes, is symbolically represented in a chemical equation.
To date, chemists have discovered 118 unique elements. However, as discussed in Chapter 3, most atoms are not naturally stable, as they do not possess fully-satisfied valence shells. Instead, these elements must bond with one another, and millions of ionic compounds and covalent molecules have been produced, as a result. Once generated, these substances can be exposed to one another, or to additional types of atoms, in a theoretically-infinite number of combinations. Scientists predict that over 1018, or one quintillion, of these interactions have the potential to initiate viable chemical reactions.
The amount of time that would be required to physically study this number of reactions is prohibitively large. Instead, chemists use currently-available data and observations to find reactivity themes that can, in turn, be applied to make predictions about chemical combinations that have not yet been investigated. The most common patterns that are used to classify chemical reactions will be described in the following sections of this chapter. These themes will be discussed in a consistent manner, so that they can be easily compared to one another. In particular, each pattern will be
- identified using a vocabulary term,
- described verbally,
- represented symbolically in a chemical equation, using the placeholder letters "A," "D," "Q," and "Z," and
- exemplified in a chemically-correct reaction equation.
By first using placeholder letters to represent a reaction theme, the unique characteristic of each pattern can be more easily-identified and, therefore, applied to classify an authentic chemical reaction.