3.19 : Energy Transfer in Chemical Reactions

Chemical reactions require sufficient energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ones formed. In general, kinetic energy is the form of energy powering any type of matter in motion. Imagine a person building a brick wall. The energy it takes to lift and place one brick on top of another is the kinetic energy—the energy matter possesses because of its motion. Once the wall is in place, it stores potential energy. Potential energy is the energy of position or the energy matter possesses because of the positioning or structure of its components. If the brick wall collapses, the stored potential energy is released as kinetic energy as the bricks fall.

Chemical energy is the form of potential energy in which energy is stored in chemical bonds. When those bonds are formed, chemical energy is invested, and chemical energy is released when they break. Notice that chemical energy, like all energy, is neither created nor destroyed; instead, it is converted from one form to another.

Chemical reactions that release more energy than they absorb are characterized as exergonic. The catabolism of the foods is an example of an exergonic reaction. Chemical energy stored in the food is absorbed as fuel for our body, but some energy is released, such as heat. In contrast, chemical reactions that absorb more energy than they release are endergonic. These reactions require energy input, and the resulting molecule stores the chemical energy in the original components and the energy that fueled the reaction. Because energy is neither created nor destroyed, the energy needed for endergonic reactions comes from exergonic reactions in many cases.

In addition to chemical energy, mechanical, radiant, and electrical energy are essential in human functioning.

• Mechanical energy, which is stored in physical systems such as machines, engines, or the human body, directly powers the movement of matter. When you lift a brick into place on a wall, your muscles provide the mechanical energy that moves the brick.
• Radiant energy is energy emitted and transmitted as waves rather than matter. These waves vary from long radio waves and microwaves to short gamma waves emitted from decaying atomic nuclei. The full spectrum of radiant energy is referred to as the electromagnetic spectrum. The human body uses the ultraviolet energy from the sunlight to convert a compound in skin cells to vitamin D. The human eye evolved into seeing the wavelengths that comprise the colors of the rainbow, from red to violet, so that range in the spectrum is called "visible light."
• Electrical energy, supplied by electrolytes in cells and body fluids, contributes to the voltage changes that help transmit impulses in nerve and muscle cells.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 2.3:Chemical Reactions.