Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances that "go in" to a chemical reaction are called the reactants (by convention, these are usually listed on the left side of a chemical equation), and the substances found that "come out" of the reaction are known as the products (by convention, these are usually found on the right side of a chemical equation). An arrow linking the reactants and products is typically drawn between them to indicate the direction of the chemical reaction. By convention, for one-way reactions (a.k.a. unidirectional), reactants are listed on the left and products on the right of the single-headed arrow. However, you should be able to identify reactants and products of unidirectional reactions that are written in any orientation (e.g. right-to-left; top-to-bottom, diagonal right-to-left, around a circular arrow, etc.) by using the arrow to orient yourself.
2H2O2 (hydrogen peroxide) → 2H2O (water) + O2 (oxygen)
Note: possible discussion
Practice: Identify the reactants and products of the reaction involving hydrogen peroxide above.
Note: possible discussion
When we write H2O2 to represent the molecule of hydrogen peroxide, it is a model representing an actual molecule. What information about the molecule is immediately communicated by this molecular formula? That is, what do you know about the molecule just by looking at the term H2O2? What information is not explicitly communicated about this molecule by looking only at the formula?
While all chemical reactions can technically proceed in both directions some reactions tend to favor one direction over the other. Depending on the degree to which a reaction spontaneously proceed in either both or one direction a different name can be given to characterize the reactions reversibility. Some chemical reactions, such as the one shown above, proceed mostly in one direction with the "reverse" direction happening on such long time scales or with such low probability that, for practical purposes, we ignore the "reverse" reaction. These unidirectional reactions are also called irreversible reactions and are depicted with a single-headed (unidirectional) arrow. By contrast, reversible reactions are those that can readily proceed in either direction. Reversible reactions are usually depicted by a chemical equation with a double-headed arrow pointing toward both the reactants and the products. In practice, you will find a continuum of chemical reactions; some proceed mostly in one direction and nearly never reverse, while others change direction easily depending on various factors like the relative concentrations of reactants and products. These terms are just ways of describing reactions with different equilibrium points.
Use of vocabulary
You may have realized that the terms "reactants" and "products" are relative to the direction of the reaction. If you have a reaction that is reversible, though, the products of running the reaction in one direction become the reactants of the reverse. You can label the same compound with two different terms. That can be a bit confusing. So, what is one to do in such cases? The answer is that if you want to use the terms "reactants" and "products", you must be clear about the direction of reaction that you are referring to - even for when discussing reversible reactions. The choice of terms, "reactants" or "products" that you use will communicate to others the directionality of the reaction that you are considering.
Let's look at an example of a reversible reaction in biology and discuss an important extension of these core ideas that arises in a biological system. In human blood, excess hydrogen ions (H+) bind to bicarbonate ions (HCO3-), forming an equilibrium state with carbonic acid (H2CO3). This reaction is readily reversible. If carbonic acid were added to this system, some of it would be converted to bicarbonate and hydrogen ions as the chemical system sought out equilibrium.
\[HCO_3^−+ H^+\rightleftharpoons H_2CO_3\]
The example above examines and "idealized" chemical systems as it might occur in a test-tube. In biological systems, however, equilibrium for a single reaction is rarely reached as it might be in the test-tube. In biological systems, reactions do not occur in isolation. Rather, the concentrations of the reactants and/or products are constantly changing, often with a product of one reaction being a reactant for another reaction. These linked reactions form what are known as biochemical pathways. The immediate example below illustrates this point. While the reaction between the bicarbonate/proton and carbonic acid is highly reversible, it turns out that, physiologically, this reaction is usually "pulled" toward the formation of carbonic acid. Why? As shown below, carbonic acid becomes a reactant for another biochemical reaction—the conversion of carbonic acid to CO2 and H2O. This conversion reduces the concentration of H2CO3, thus pulling the reaction between bicarbonate and H+ to the right. Moreover, a third, unidirectional reaction, the removal of CO2 and H2O from the system, also pulls the reaction further to the right. These kinds of reactions are important contributors to maintaining the H+ homeostasis of our blood.
\[ HCO_3^- + H^+ \rightleftharpoons H_2CO_3 \rightleftharpoons CO_2 + H_20 \rightarrow waste\]
The reaction involving the synthesis of carbonic acid is actually linked to its breakdown into \(CO_2\) and \(H_2O\). These products are then removed from the system/body when they are exhaled. Together, the breakdown of carbonic acid and the act of exhaling the products pull the first reaction to the right.