Reactants and Products

FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry; Biochemistry

ABSTRACT

The concept of reactants and products in chemical reactions is described, and their importance in all related fields is elaborated. Reactants and products are an obligate pair in that one cannot occur without the other. They are also relative terms defined by the direction in which a chemical reaction proceeds.

The Relationship between Reactants and Products

The concept of "up" is inseparable from the concept of "down." In the same way, the concept of reactants is inseparable from that of products. When atoms or molecules are brought together under the proper conditions, the energy relationships that govern how electrons behave in chemical bonds allow new bonds to form and existing bonds to change in specific ways, and a chemical reaction takes place. Bonds may break apart, form anew, or simply rearrange. When this happens, the chemical identities of the reactants change. They cease to be the elements or compounds they were before the interaction and become new compounds as a result of the various changes they have gone through. In other words, they transform from reactants, or starting materials, into products.

A chemical reaction can be indicated by the following general equation:

A + B → C + D

This particular representation indicates that two compounds, A and B, undergo a reaction to produce two products, C and D. It should be obvious that many other representations, indicating different numbers and types of materials, are possible. By convention, the starting materials—the reactants—are presented on the left, the products on the right. Since the arrow always points from the reactants to the products, however, it is equally valid to write an equation from right to left. It is also true that virtually all reactions that proceed in one direction can be made to proceed in the opposite direction as well, causing the products of a reaction to undergo the "reverse reaction" and re-form the original reactants. That is, the equation

A + B → C + D

indicating that A and B react to produce C and D can usually also be written as

C + D → A + B

indicating that C and D react to produce A and B. What were originally the reactants in the first reaction are the products in the second. The designation of which compounds or elements are reactants and which are products is thus relative, depending on which reaction is desired.

Many reaction systems enter a condition called equilibrium. In such cases, the forward reaction, in which the reactants form the products, occurs at the same time as the reverse reaction, in which the products re-form the reactants. At equilibrium, these two opposing reaction processes take place at exactly the same rate, meaning that A and B are forming C and D at the same time and to the same extent that C and D are re-forming A and B. This condition is indicated in the reaction equation by replacing the one-way arrow with a double half arrow that points in both directions:

A + B ⇌ C + D

Writing Equations with Reactants and Products

In order to provide the precise, usable information that a reaction equation should convey, such as the chemical identities of the reactants and products, a chemical formula that uniquely and unequivocally identifies each reactant and product is required. The chemical formula also uses numerical subscripts and coefficients to identify all the atoms or molecules that are present and how many of each. For example, the equation

2Al + Fe₂O₃ → 2Fe + Al₂O₃

clearly states that two atoms of aluminum react with one molecule of iron oxide to produce two atoms of iron and one molecule of aluminum oxide. It also states that each molecule of iron oxide is composed of two atoms of iron and three of oxygen, while each molecule of aluminum oxide is composed of two atoms of aluminum and three of oxygen.

For most organic compounds, the reaction equation is written using a pictorial representation of the three-dimensional molecular structure of each reactant and product rather than just their chemical formulas.

PRINCIPAL TERMS

• chemical formula: the combination of symbols and numerical coefficients that specifies the number and identity of the different atoms involved in a chemical reaction or molecular structure.
• chemical reaction: a process in which the molecules of two or more chemical species interact with each other in a way that causes the electrons in the bonds between atoms to be rearranged, resulting in changes to the chemical identities of the materials.
• compound: a chemically unique material whose molecules consist of several atoms of two or more different elements.
• element: a form of matter consisting only of atoms of the same atomic number.
• equilibrium: the state that exists when the forward activity of a process is exactly equal to the reverse activity of that process.

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