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Chemical Reactions and Molecules

Chemical Bonds: The Quest for Stability

All elements are most stable when their outermost shell is filled with electrons according to the octet rule. This is because it is energetically favorable for atoms to be in that configuration and it makes them stable. However, not all elements have enough electrons to fill their outermost shells. For this reason, atoms form chemical bonds with other atoms. As a result, they obtain the electrons they need to attain a stable electron configuration.

When two or more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water molecule, H2O, consists of two hydrogen atoms and one oxygen atom; these bond together to form water, as illustrated in the image below. Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer shells.


Two or more atoms may bond with each other to form a molecule. When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule forms. Image Attribution: OpenStax Biology

Chemical Reactions

Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances in the beginning of a chemical reaction are the reactants. They are usually found on the left side of a chemical equation. Conversely, the substances at the end of the reaction are the products. They are usually found on the right side of a chemical equation.

An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction; this direction is not always a “one-way street.” For the creation of the water molecule shown above, the chemical equation would be:

\(\mathrm{2H} + \text{O} \longrightarrow \mathrm{H_2O}\)

An example of a simple chemical reaction is the breaking down of hydrogen peroxide molecules, each of which consists of two hydrogen atoms bonded to two oxygen atoms (H2O2). The reactant hydrogen peroxide is broken down into water, containing one oxygen atom bound to two hydrogen atoms (H2O), and oxygen, which consists of two bonded oxygen atoms (O2).

Balanced Equations

In the equation below, the reaction includes two hydrogen peroxide molecules and two water molecules. This is an example of a balanced chemical equation, wherein the number of atoms of each element is the same on each side of the equation. According to the law of conservation of matter, the number of atoms before and after a chemical reaction should be equal, such that no atoms are, under normal circumstances, created or destroyed.

\(\mathrm{2H_2O_2} \text{ (hydrogen peroxide)} \longrightarrow \mathrm{2H_2O} \text{ (water)} + \mathrm{O_2} \text{ (oxygen)}\)

Compounds vs Mononuclear Molecules

All of the reactants and products of this reaction are molecules. That is, each atom remains bonded to at least one other atom. However, in this reaction only hydrogen peroxide and water are representatives of compounds. They contain atoms of more than one type of element. Molecular oxygen, on the other hand, as shown in the image below, consists of two doubly bonded oxygen atoms. Thus, it is not classified as a compound but as a mononuclear molecule.


The oxygen atoms in an O2 molecule are joined by a double bond. Image Attribution: OpenStax Biology

Reversible reactions and equilibrium

Some chemical reactions, such as the one shown above, can proceed in one direction until the reactants are all used up. The equations that describe these reactions contain a unidirectional arrow and are irreversible.

Reversible reactions are those that can go in either direction. In reversible reactions, reactants are turned into products, but when the concentration of product goes beyond a certain threshold (characteristic of the particular reaction), some of these products will be converted back into reactants. At this point, the designations of products and reactants reverse.

This back and forth continues until a certain relative balance between reactants and products occurs—a state called equilibrium. We often denote these situations of reversible reactions by a chemical equation with a double headed arrow pointing towards both the reactants and products.

\(\mathrm{HCO_3^-} + \mathrm{H^+} ↔ \mathrm{H_2CO_3}\)

In biological reactions, however, equilibrium is rarely obtained because the concentrations of the reactants or products or both are constantly changing, often with a product of one reaction being a reactant for another. To return to the example of excess hydrogen ions in the blood, the formation of carbonic acid will be the major direction of the reaction.

However, the carbonic acid can also leave the body as carbon dioxide gas (via exhalation) instead of being converted back to bicarbonate ion. Consequently, this drives the reaction to the right by the chemical law known as law of mass action. These reactions are important for maintaining the homeostasis of our blood.

\(\mathrm{HCO_3^-} + \mathrm{H^+} ↔ \mathrm{H_2CO_3} ↔ \mathrm{CO_2} + \mathrm{H_2O}\)

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