Chemistry-Unit 3: Energy and Heating/Cooling
Energy is a substance-like quantity that is always involved whenever a system
undergoes change (hotter-colder, faster-slower, higher-lower).
A key to understanding energy is to recognize that energy is always and everywhere
only energy. Energy is stored in a system in several different "accounts" and can be
transferred between system and surroundings in different ways, but it does not come
in different forms. When there is a change in the way the system stores energy or if
energy is transferred between system and surroundings, something about the system
changes, but the energy remains the same.
One difficulty we have in understanding energy is that our everyday use of words can
sometimes muddy the waters. For example, use of the word "heat" can leave the
Impression that it is somehow different from energy. It would be better if we viewed
"heat" as one of the ways that energy is transferred from one object to another. While
It is helpful to say that we "heat" an object (as a shortcut for "transfer energy to"), it is
not useful to say that an object stores "heat". It's fine to describe an object that stores
a lot of thermal energy as "hot", but saying that it stores a lot of "heat" confuses
energy with a way that it is moved from one object to another.
Heating a system increases its thermal energy (E) through the collisions of more
energetic particles with particles of lower energy; as a result, the particles in the
system move more rapidly than before. Use of the -Ing ending helps us view "heating"
as a process of energy transfer through collisions of particles rather than as something
different from energy. The quantity of energy transferred in this way is often referred
to as "heat" (assigned the variable name Q), but it is important to remember that it is
imply energy. Conversely, a system cools when its particles transfer thermal energy
through collisions) to particles in the surroundings. This process lowers the amount of
thermal energy (E) stored by the system.
Temperature is a useful tool because it allows us to assign a numerical value that helps
us describe the thermal energy of a system (or surroundings). It is important to
recognize that temperature and energy are not the same. Changes in temperature
(AT) help us to determine the amount of thermal energy gained or lost by a system, as
we shall discuss at a later time.
We're now ready to discuss the role of energy during phase change. We'll first
examine what happens when a solld melts. As energy is transferred into the system,
the thermal energy (and motion) of the particles Increases. At some temperature, the
particles are vibrating to and fro so rapidly that they can no longer maintain the orderly
arrangement of a solld. They break free of the attractions and begin to move around
more freely-they become "liquid". We use another account to describe the way the
system stores energy when the particles exist as a liquid rather than as a solid; we call
this phase energy, Eph. Particles in the liquid phase store more phase energy than do
particles in the solid phase.
As you recall from the experiment, during the melting of the solid, the temperature
remained more or less constant, despite the fact that energy was being continually
transferred to the system. To explain this, consider the fact that energy is required to
