Chemistry Reference
In-Depth Information
Kinetic energy, which is often called the energy of motion, is the energy
that an object has due to its mass and velocity (think speed). When the
bowling ball was sitting on the shelf, its velocity was zero, so its kinetic
energy was zero. As it was falling, it was accelerating toward your foot, and
its kinetic energy continued to increase. Where did the kinetic energy come
from? As the bowling ball lost potential energy, because its height above
your foot decreased, it gained kinetic energy as its velocity increased. So,
the energy didn't just “appear.” The gravitational potential energy was sim-
ply converted to kinetic energy.
Just like macroscopic (large) objects, microscopic (tiny) objects such as
atoms can store energy due to their positions. We call the energy that at-
oms store chemical
energy. This chemical energy is stored within atoms as
their potential to form bonds with other atoms, and it is released when new
bonds are made. As in the example with the bowling ball, the energy re-
leased will take on a different form than the stored energy. Most often, the
chemical energy is released in the form of heat.
When wood is burned in a fireplace, energy is released as the atoms
from the wood combine with the atoms of oxygen in the air to make new
compounds, such as carbon dioxide and carbon monoxide. However, be-
fore the wood will burn, you need to put energy into the wood to get it
started. This initial energy, perhaps from burning kindling, is used to break
bonds between atoms in the wood so that they can make new bonds. If the
energy that we put into getting the wood burning was less than the energy
that we got from the wood while it was burning, it would be an inefficient
way to try to heat a room.
Reactions that release more energy than they take in are called exo-
thermic reactions. In this type of reaction, the potential energy of the prod-
ucts is lower than the potential energy of the reactants, with the extra energy
being released, usually in the form of heat. For this reason, exothermic
reactions will heat the area around them. An example of an equation for
an exothermic reaction is shown here:
CH
4(g)
+ 2O
2(g)
CO
2(g)
+ 2H
2
O
(g)
+ 890.4 kJ of energy
As you can see from the equation, energy is released and can be thought
of as a product of the reaction. What the equation doesn't show is that a
certain amount of energy is required to get this reaction started. The energy
required to get a chemical reaction to start is called activation energy. When
you rub a match across a strip of sandpaper, the friction generates the heat
that acts as the activation energy to get the burning reaction started.
