Notes

We experience reactions on a daily basis. From the food we cook
and how our body uses that food to the cars we drive, if it weren’t for
reactions, these phenomena would not exist. We know reactions
follow rules based on the elements or compounds involved. But is
that all that reactions boil down to? Let’s take a closer look at how
reactions really do tie all sciences together.
Systems and Surroundings
In science, we need to distinguish between the system and the
surroundings. The system​ involves the components directly related
to an interaction. Systems can be open or closed. This is determined
by what interaction, if any, the system has with the surroundings. The
surroundings ​refer to the environment in which the system interacts.
In the case of a reaction, the system generally involves the reactants
and products of the reaction. The surroundings would be the
container and atmosphere around the reaction.
This is important to keep in mind because reactions occurring in our
everyday lives are generally not isolated in a closed system. There is
almost always an interaction with the surrounding environment. Think
about a car using gasoline. The reaction occurs in the engine,
allowing the car to move. The engine is in contact with the metal
pieces and air surrounding it. What do you notice when you stand
next to a car that has been running for some time? The exterior of the
car is hot. The actual hood was never involved in the reaction of the
gasoline, but you can tell that something has happened based on the
temperature of the hood.

Energy
The reason the hood of a car gets hot after it has been running for some time is all due to energy.
The definition of energy is “the ability to do work.” Work is determined by the change in position,
speed, state, or form of matter. Therefore, when we combine the two definitions, we can say that
energy is the capacity to change matter. When we think of the amount of energy something has, we
can think of it as its capacity to cause things to happen. The standard symbol for energy is E, and its
standard unit is the joule​ (J).

1

At the macroscopic scale, energy appears in many ways, such as motion, light, sound, electrical and
magnetic fields, and thermal energy (heat). Energy at the macroscopic scale can be accounted for as
a combination of kinetic energy and potential energy. Kinetic energy ​is associated with the motion of
an object’s particles, while potential energy ​is associated with the relative position of an object’s
particles. Energy, whether kinetic or potential, can be defined as “the ability to do work.” Since work is
determined by force and distance, energy is able to apply a force, causing a change in location.
Energy ​at the microscopic scale can be modeled either
as motions of particles or as stored in force fields (electric,
magnetic, gravitational) that mediate interactions between
particles. Figure 1 shows magnetic fields using iron filings. This
concept includes electromagnetic radiation, a phenomenon in
which energy stored in fields moves across space (light, radio
waves) with no supporting matter medium.

In all cases of energy, we need to distinguish between the system and the surroundings. The system
involves the components directly related to the energy being viewed. The surroundings ​refer to the
environment in which the system interacts. In figure 1, the system is just the magnet. The
surroundings would include the iron filings as well as the air surrounding the magnet.
Macroscopic Energy
Macroscopic​ energy​ descriptions include an object’s motion (movement), or kinetic energy;
gravitational potential energy; and sound, light, and thermal energy (an object’s heat capacity).
Macroscopic energy passes through a boundary layer of the system and reacts to the environment or
another system. This larger view of energy includes the total mechanical energy of the system
associated with the macroscopic position and motion of the system as a whole.
Microscopic Energy
Microscopic​ energy​ descriptions include the internal energy of
particles; the motion of particles or force fields (electric, magnetic,
gravitational); and the relative position between particles in an
object and Earth’s gravitational field.
Scientists view microscopic energy as energy at a molecular level
within the system; they consider it the system’s internal energy. The
energy of molecules is kinetic and potential, just as any other system. However, the kinetic energy is
present in the random motion of the molecular particles, as seen in figure 2.