Biology (Holt)

Energy in Chemical Reactions
In chemical reactions, energy is absorbed or released when chemi-
cal bonds are broken and new ones are formed. The graphs shown
in Figure 13 compare a chemical reaction that releases energy with
a chemical reaction that absorbs energy. The freezing and melting of
water are good examples of how energy is released or absorbed dur-
ing chemical reactions. When water freezes, the process that leads
to the formation of ice crystals causes heat energy to be released.
When you fill an ice-cube tray with water and place it in the freezer
to make ice, heat is released from the water as the water freezes.
When you remove ice cubes from the freezer, the ice begins to melt.
When ice melts, it absorbs heat from the environment. When you
hold a piece of ice, your hand gets cold and heat is transferred from
your hand to the ice as the ice begins to melt.
Metabolism (muh TAB uh lihz uhm)is the term used to describe
all of the chemical reactions that occur within an organism. Your
cells get most of the energy needed for metabolism from the food you
eat. As food is digested, chemical reactions convert the chemical
energy in food molecules to forms of energy that can be used by cells.

Activation Energy
The heat from a flame transfers enough energy to ignite the logs in a
campfire. The spark from a spark plug causes the gasoline in an auto-
mobile engine to ignite. In both cases, energy is needed to start a
chemical reaction. The energy needed to start a chemical reaction is
called .To better understand activation energy,
think of rolling a boulder down a hill. To get the boulder rolling down-
hill, you must first push it. Activation energy is simply a chemical
“push” that starts a chemical reaction. Even in a chemical reaction
that releases energy, activation energy must be supplied before the
reaction can occur.

activation energy

SECTION 4Energy and Chemical Reactions 39

Reactants

Products

Energy

Energy released

Reaction progress

Reactants

Products

Energy

absorbed

Energy

Reaction progress

Energy-Releasing Reaction Energy-Absorbing Reaction

Chemical reactions absorb or release energy.

Figure 13 Energy and chemical reactions

http://www.scilinks.org

Topic: Chemical Reactions

Keyword: HX4040