Skip to main content
Biology LibreTexts

Chemical Reactions

  • Page ID
  • Chemical reactions

    Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances that "go into" a chemical reaction are called the reactants (by convention, these are usually listed on the left side of a chemical equation), and the substances that are found to "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, 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 one-way reactions that are written in any orientation (e.g., right-to-left, top-to-bottom, diagonal right-to-left, around a circular arrow, etc.)

    \[\underbrace{2H_2O_2}_{\text{hydrogen peroxide}} → \underbrace{2H_2O}_{\text{water}} + \underbrace{O_2}_{\text{oxygen}}\]

    Note: practice


    Note: possible discussion

    When we write \(H_2O_2\) to represent the molecule 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 \(H_2O_2\)?



    Note: use of vocabulary



    \[HCO_3^−+ H^+ \rightleftharpoons H_2CO_3 \label{2}\]

    The examples above examine "idealized" chemical systems as they might occur in a test tube. In biological systems, however, equilibrium for a single reaction is rarely obtained as it might be in the lab. In biological systems, reactions do not occur in isolation. 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 above illustrates this and another caveat. While the reaction between the bicarbonate/proton and carbonic acid is highly reversible, it turns out that physiologically this reaction is usually "pulled" towards the formation of carbonic acid. Why? As shown below, carbonic acid becomes a reactant for another biochemical reaction: its conversion 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 reaction, the removal of CO2 and H2 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.