Chemical Reactions

Chemical Reactions

A chemical reaction occurs when a chemical bond is formed, broken, or rearranged. Chemical reactions are written using symbols, which are known as chemi-cal equations. Any number that is written in a smallerletter that appears below the level of the main text (a subscript) indicates the atoms are joined by chem-ical bonds. A number written as a prefix shows the number of unjoined atoms or molecules. For example, for H₂O, the number “2” indicates that two hydrogen atoms are bonded together with one oxygen atom to form the water molecule. If an equation used the term “2H,” it would mean there were two unjoined hydro-gen atoms. A chemical equation contains the kinds and number of reacting substances (reactants), the chemical composition of the results (products), and, if the equation is balanced, the relative proportion of each reactant and each product. Four types of chemical reactions are important to the study of physiology: synthesis, decomposition, exchange, and reversible.

Synthesis Reactions

Chemical reactions change the bonds between atoms, molecules, and ions to generate new chemical com-binations. Synthesis is a reaction that occurs when two or more reactants (atoms) bond to form a more complex product or structure. The formation of water from hydrogen and oxygen molecules is a synthesis reaction. Synthesis always involves the formation of new chemical bonds, whether the reactants are atoms or molecules. Synthesis requires energy, important for growth and repair of tissues. Synthesis is symbol-ized as: A + B → AB.

Decomposition Reactions

Decomposition is a reaction that occurs when bondswithin a reactant molecule break, forming simpler atoms, molecules, or ions. For example, a typical meal contains molecules of sugars, proteins, and fats that are too large and too complex to be absorbed and used by the body. Decomposition reactions in the digestive tract break these molecules down into smaller frag-ments before absorption begins. Decomposition is symbolized as: AB → A + B.

Exchange Reactions

In an exchange reaction, parts of the reacting mol-ecules are shuffled around to produce new products. For example, an exchange reaction is the reaction of an acid with a base, which forms water and a salt. Exchange reactions are symbolized as: AB + CD → AD + CB.

Reversible Reactions

A reversible reaction is one where the products of the reaction can change back into the reactants they originally were. These reactions can proceed in oppo-site directions, depending on the relative proportions of reactants and products as well as how much energy is available.

So, if A + B → AB, then AB → A + B. Many important biological reactions are freely reversible. Such reactions can be represented as the equation A+B→AB.

1. Describe four kinds of chemical reactions.

2. What are the structural formulas for synthesis reactions and exchange reactions?

3. Explain decomposition reactions.