Scanning the leaves of some plants at the base of the rosebush, you notice a few green bugs whose backs give the impression of a shield. A small stick lies temptingly close to your foot. You can’t resist and so you grab the stick and position it for optimal poking, but just as the pointy end gets within a few inches of the insect, which now senses that its own pleasant afternoon is about to be ruined, it raises its hind quarters in the direction of the stick—that is to say, in your direction—exposing its sophisticated defense weaponry.
Our world is filled with aromas and odors, some pleasant and some not so much. What is it that we are smelling when we “smell something”? Well, every odor that we perceive is the result of an interaction between the chemical receptors of our olfactory systems and some chemical compound. A stink bug “stinks” because it produces a highly concentrated solution of volatile compounds that we (and anything else that would dare to disturb it) perceive as malodorous, noxious, and irritating. It’s a pretty ingenious defense system: Make yourself so disgusting, so distasteful, that everyone will leave you alone, or at least think twice before bothering or eating you. Interestingly enough, the primary compounds in the stink bug’s stink bomb are hydrogen cyanide (a highly toxic compound that inhibits cytochrome c oxidase, thereby blocking aerobic respiration) and benzaldehyde.
Benzaldehyde is a compound composed of seven carbon atoms, six hydrogen atoms, and one oxygen atom. One mole of benzaldehyde has a mass of 106.12 grams. It can react with other atoms or compounds to form new compounds (pure substances composed of two or more elements in a fixed proportion). They can be broken down by chemical means to produce their constituent elements or other compounds. They are characterized according to the same systems of traits: physical properties and chemical reactivities.
This chapter focuses on compounds and their reactions. It reviews the various ways in which compounds are represented: Empirical and molecular formulas and percent composition will be defined and explained. There is a brief overview of the major classes of chemical reactions, which we will examine more closely in subsequent chapters, and finally, there is a recap of the steps involved in balancing chemical equations with a particular focus on identifying limiting reagents and calculating reaction yields.
Molecules and Moles
A molecule is a combination of two or more atoms held together by covalent bonds. It is the smallest unit of a compound that displays the identifying properties of that compound. Molecules can be composed of two atoms of the same element (e.g., N2 and O2) or may be composed of two or more atoms of different elements, as in CO2, SOCl2, and C6H5CHO (benzaldehyde). Because reactions usually involve a very large number of molecules, far too many to count individually, we usually measure amounts of compounds in terms of moles or grams, using molecular weight to interconvert between these units.
