3.18 : Types of Chemical Reactions: Exchange and Reversible

An exchange reaction is a chemical reaction in which both synthesis and decomposition occur, chemical bonds are both formed and broken, and chemical energy is absorbed, stored, and released.

A special kind of exchange reaction is the oxidation-reduction reaction, or the redox reaction. These reactions involve the transfer of electrons from one compound to another. The electrons in these reactions commonly come from hydrogen atoms, which consist of an electron and a proton. A molecule gives up a hydrogen atom in the form of a hydrogen ion (H⁺) and an electron, breaking the molecule into smaller parts. The loss of an electron (oxidation) releases a small amount of energy; both the electron and the energy are then passed to another molecule in the process of reduction or the gaining of an electron. These two reactions always happen together, and when an electron is passed between molecules, the donor is oxidized, and the recipient is reduced. Oxidation-reduction reactions often happen in a series so that a molecule that is reduced is subsequently oxidized, passing on not only the electron it just received but also the energy it received.

In theory, any chemical reaction can proceed in either direction under the right conditions. In reversible reactions, reactants turn into products, but when the product's concentration goes beyond a certain threshold (characteristic of the particular reaction), some of these products convert back into reactants. At this point, product and reactant designations reverse. This back and forth continues until a certain relative balance between reactants and products occurs—a state called equilibrium. A chemical equation with a double-headed arrow pointing towards both the reactants and products often denotes these reversible reaction situations.

For example, in human blood, hydrogen ions (H⁺) bind to bicarbonate ions (HCO₃^-), forming an equilibrium state with carbonic acid (H₂CO₃).

However, biological reactions rarely obtain equilibrium because the concentrations of the reactants or products or both are constantly changing, often with one reaction's product a reactant for another. If we added carbonic acid to this system, some of it would convert to bicarbonate and hydrogen ions. When there is excess hydrogen ions in the blood, forming carbonic acid will be the reaction's major direction.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 24.1: Overview of Metabolic Reactions and Openstax, Biology 2e, Section 2.1: Atoms, Isotopes, Ions and Molecules: The Building Blocks.