all physical and chemical changes have energy considerations associated with
them. To understand how and why these changes happen, an understanding of
energy is required.
Energy is defined as the capacity to do work. Work is done whenever a
force is applied over a distance. Therefore, anything that can force matter to
move, to change speed, or to change direction has energy. The following example
will help you understand this definition of energy. When you charge a battery with
electricity, you are storing energy in the form of chemical energy. The charged
battery has a capacity to do work. If you use the battery to operate a toy car, the
stored energy is transformed into mechanical energy that exerts a force on the
mechanism that turns the wheels and makes the car move. This process continues
until the charge or stored energy is completely used. In its uncharged condition,
the battery no longer has the capacity to do work.
Work itself is measured in joules, and so is energy. In some problems,
however, energy may be expressed in kilocalories. The relationship between
these two units is that 4.18 × 10^3 joules (J) equals 1 kilocalorie (kcal).
Forms of Energy
Energy may appear in a variety of forms. Most commonly, energy in reactions is
evolved as heat. Some other forms of energy are light, sound, mechanical
energy, electrical energy, and chemical energy. Energy can be converted from
one form to another, as when the heat from burning fuel is used to vaporize water
to steam. The energy of the steam is used to turn the wheels of a turbine to
produce mechanical energy. The turbine turns the generator armature to produce
electricity, which is then available in homes for use as light or heat, or in the
operation of many modern appliances.
Two general classifications of energy are potential energy and kinetic
energy. Potential energy is stored energy due to overcoming forces in nature.
Kinetic energy is energy of motion. The difference can be illustrated by a boulder
sitting on the side of a mountain. It has a high potential energy due to its position
above the valley floor. If it falls, however, its potential energy is converted to
kinetic energy. This illustration is very similar to the situation of electrons
cascading to lower energy levels in the atomic model described in Chapter 2.