Lesson 6.1 Energy and Its Units
Suggested Reading
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Zumdahl Chapter 6 Section 1
Essential Question

How is energy described?
Learning Objectives


Differentiate between kinetic and potential energy.
Describe the law of of conservation of energy (the first law of
thermodynamics)
Introduction
Thermodynamics is the study of the relationships between heat and other
forms of energy. Thermochemistry is one area of thermodynamics. It
involves the study of the quantity of heat absorbed or evolved by chemical
reactions. An example of a heat-evolving reaction is the burning of fuel.
There are practical reasons why you would want to know the amount of
heat evolved during the burning of a fuel such as the determination of the
usefulness of the fuel and its associated costs. Indeed, these are
important considerations! There are also theoretical reasons for wanting to
know the quantity of heat involved in a reaction. Heat measurements can
provide data needed to determine whether a particular reaction occurs.
These measurements also enable you to calculate the amount of energy
needed to break a chemical bond, which tells you something about the
strength of that bond.
We have just used the terms energy and heat, assuming you have some
idea of what they mean. But to proceed, you will need precise definitions
of these and other terms. In this lesson, we will define energy and its
different forms and introduce the units for energy.
Energy
We can define energy as the potential or capacity to move matter.
According to this definition, energy is not a material thing but rather a
property of matter. Energy exists in different forms that can be
interconverted. You can see the relationship of a given form of energy to
the motion of matter by following its conversion into different forms.
Consider the conversions of energy in a steam-driven electrical generator.
A fuel is burned to heat water and generate steam, which converts
chemical energy to heat energy. The steam expands against a piston,
which is connected to a drive shaft that turns an electrical coil generating
electricity, which converts part of the heat energy to kinetic energy (energy
of motion), and then to electrical energy. The electrical energy can then be
used to run a motor thereby transforming the electrical energy back to
kinetic energy. Alternatively, you could send the electricity into a light bulb,
converting electrical energy to heat energy and light energy. This example
shows you that energy can exist in different forms, including chemical,
heat, kinetic, light and electrical energy and these different forms of can be
interconverted.
Since this is a chemistry course, we will be primarily concerned with
chemical energy and its transformation during a chemical reaction to heat
energy. In order to understand this we must look at the internal energy of
substances, which is defined in terms of the kinetic and potential energies
of the particles making up the substance.
Kinetic Energy
Kinetic energy is the energy associated with an object by virtue of its
motion. An object of mass m and velocity (speed) ν has kinetic energy Ek
equal to
This formula shows that the kinetic energy of an object depends on both
its mass and its speed, where v is the speed on an individual object or
particle. Thus, a heavy object can move more slowly than a light object
and still have the same kinetic energy. Kinetic energy takes different
forms, including:
Vibrational: The energy due to vibrational motion.
Rotational: The energy due to rotational motion.
Translational: The energy due to motion from one location to another.
These various forms of kinetic energy play an important role in chemistry.
Substances in aqueous and gaseous solutions such as the oceans and air
are in constant motion and generate all of these types of kinetic energy.
Consider the kinetic energy of a person whose mass is 59.0 kg and whose
speed is 28.8 m/s (This is equivalent to a person with a mass of 130 lb
traveling in a car going 60 miles per hour). You substitute the mass and
speed into the equation for kinetic energy giving
The unit of energy comes out of this calculation. The SI unit of energy ,
kg•m2/s2, is given the name joule (J) (pronounced "jewel") after the
English physicist Fames Prescott Joule (1818-1889), who studied the
energy concept. From the equation above we see that a person weighing
130 lb and traveling at 60 miles per hour has a kinetic energy of 2.12 x 104
J, or 21.2 kJ (kilojoules).
The joule is an extremely small unit. To illustrate its size, note that the
watt, which is a measure of the quantity of energy used per unit time,
equals 1 joule per second. An ordinary 100-watt bulb, uses 100 joules of
energy every second. A kilowatt-hour, the unit by which electric energy is
sold, equals 3600 kilowatt-seconds (there are 3,600 seconds in an hour),
or 3.6 million joules. An ordinary household light might use something like
1000 kilowatt-hours (3.6 billion joules) of electricity in a month!
The calorie (cal) is a non-SI unit of energy commonly used by chemists.
The calorie was originally defined as the amount of energy required to
raise the temperature of one gram of water by one degree Celsius.
However, in 1925 the calorie was redefined in terms of the joule as
follows.
1 cal = 4.184 J (exact definition)
Potential Energy
Potential energy is the energy an object has by virtue of its position in a
field of force. For example, water at the top of a dam has potential energy
(in addition to whatever kinetic energy it may poses) because the water is
at a relatively high position in the gravitational force field of the earth. You
can calculate the potential energy of the water using the formula Ep=mgh.
Where Ep is the potential energy of a quantity of water at the top of the
dam, m is the mass of the water, g is the constant acceleration of gravity,
and h is the height of the water measured from some standard level
(usually the surface of the earth). As a quantity of water falls over the dam,
its potential energy decreases from mgh at the top to 0 at the surface of
the earth. The potential energy at the top of the dam is converted to kinetic
energy when the water falls to the earth. As the water falls it moves more
quickly as potential energy decreases and kinetic energy decreases.
Run the simulator below to explore the concepts of potential, kinetic and
thermal (heat) energy.
http://phet.colorado.edu/sims/energy-skate-park/energy-skate-parkbasics_en.jnlp
Consider the total energy of a quantity of water as it moves over the dam.
The water as a whole has kinetic and potential energy. However, the water
is made up of subatomic particles that also have kinetic and potential
energies. The sum of the kinetic and potential energies of the subatomic
particles is referred to as the internal energy, U, of the substance.
Therefore, the total energy, Etotal, of a quantity of water equals the sum of
its kinetic and potential energies as a whole plus its internal energy.
Etotal = Ek + Ep + U
Normally, when you study a substance in the laboratory, the substance is
at rest in a vessel. In this situation, the kinetic and potential energies, as a
whole, are zero. In this case, the total energy of the substance equals it
internal energy, U.
Law of Conservation of Energy
We have discussed situations in which one form of energy can be
converted into another form of energy. For example, when water falls over
the dam, potential energy is converted to kinetic energy. Some of the
kinetic energy of the water may also be converted into random molecular
motion-that is, into internal energy of the water. The total energy, Etotal, of
the water, however, remains constant, Etotal = Ek + Ep + U.
This fact can be stated more generally as the law of conservation of
energy (first law of thermodynamics), which states that energy may be
converted from one form to another, but the total quantity of energy
remains constant.