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Chemistry LibreTexts

6.2: The Nature and Types of Energy

  • Page ID
    174936
  • Learning Objectives

    • Define energy, distinguish types of energy, and describe the nature of energy changes that accompany chemical and physical changes
    • Distinguish the related properties of heat, thermal energy, and temperature
    • Define and distinguish specific heat and heat capacity, and describe the physical implications of both
    • Perform calculations involving heat, specific heat, and temperature change

    Energy is one of the most fundamental and universal concepts of physical science, but one that is remarkably difficult to define in way that is meaningful to most people. This perhaps reflects the fact that energy is not a “thing” that exists by itself, but is rather an attribute of matter (and also of electromagnetic radiation) that can manifest itself in different ways. It can be observed and measured only indirectly through its effects on matter that acquires, loses, or possesses it. Energy can be defined as the capacity to supply heat or do work. One type of work (\(w\)) is the process of causing matter to move against an opposing force. For example, we do work when we inflate a bicycle tire—we move matter (the air in the pump) against the opposing force of the air already in the tire.

    History of Energy - "Living Force" vs. "Dead force"

    The concept that we call energy was very slow to develop; it took more than a hundred years just to get people to agree on the definitions of many of the terms we use to describe energy and the interconversion between its various forms. But even now, most people have some difficulty in explaining what it is; somehow, the definition we all learned in elementary science ("the capacity to do work") seems less than adequate to convey its meaning.

    Although the term "energy" was not used in science prior to 1802, it had long been suggested that certain properties related to the motions of objects exhibit an endurance which is incorporated into the modern concept of "conservation of energy". René Descartes (1596-1650) stated it explicitly: In the 17th Century, the great mathematician Gottfried Leibniz (1646-1716) suggested the distinction between vis viva ("life force") and vis mortua ("dead force"), which later became known as kinetic energy (1829) and potential energy (1853).

    SI Unit of Energy - The joule

    The basic unit of energy is the joule. One joule is the amount of work done when a force of 1 newton acts over a distance of 1 m; thus 1 J = 1 N-m. The newton is the amount of force required to accelerate a 1-kg mass by 1 m/sec2, so the basic dimensions of the joule are kg m2 s–2.

    \[1\, J = 1\, kg\, m^2 s^{–2}:\]

    Types of Energy

    Energy can be characterized through its observed properties. All the types can be broadly divided into two types principal types: potential and kinetic energy. However, there are additional flavors of both that are commonly discussed in the study of chemistry.

    Kinetic Energy

    Whatever energy may be, there are basically two kinds. Kinetic energy is associated with the motion of an object, and its direct consequences are part of everyone's daily experience; the faster the ball you catch in your hand, and the heavier it is, the more you feel it. Quantitatively, a body with a mass \(m\) and moving at a velocity \(v\) possesses the kinetic energy

    \[KE= \dfrac{1}{2} m v^2 \label{KE}\]

    Example \(\PageIndex{1}\)

    What is the kinetic energy of a 4.25 g bullet shot from a rifle a velocity of 965 m s–1.

    Solution

    This is a direct application of the definition of kinetic energy (Equation \ref{KE}):

    \[\begin{align*} KE &= \dfrac{1}{2} (0.00425\, kg) (965\, m \,s^{–1})^2 \\[4pt] &= 1.980\, J \end{align*}\]

    Potential Energy

    Potential energy is energy a body has by virtue of its location, composition, or condition. However, there is more: the body must be subject to a "restoring force" of some kind that tends to move it to a location of lower potential energy. Think of an arrow that is subjected to the force from a stretched bowstring; the more tightly the arrow is pulled back against the string, the more potential energy it has. More generally, the restoring force comes from what we call a force field— a gravitational, electrostatic, or magnetic field. We observe the consequences of gravitational potential energy all the time, such as when we walk, but seldom give it any thought.

    If an object of mass \(m\) is raised off the floor to a height \(h\), its potential energy increases by \(mgh\), where \(g\) is a proportionality constant known as the acceleration of gravity; its value at the earth's surface is 9.8 m s–2.

    \[PE = mpg \label{PE}\]

    Example \(\PageIndex{2}\)

    Find the change in gravitational potential energy of a 2.6 kg textbook that falls from the 66-cm height of a table top onto the floor.

    Solution

    This is a direct application of Equation \ref{PE}

    \[\begin{align*} PE &= m g h \\[4pt] &= (2.6\, kg)(9.8 \,m s^{–2})(0.66 m) \\[4pt] &= 16.8\, kg\, m^2 s^{–2} \\[4pt] &= 16.8\, J \end{align*}\]

    Similarly, the potential energy of a particle having an electric charge \(q\) depends on its location in an electrostatic field. This would be an electric potential energy. and has a difference functional form that gravitational energy (Equation \ref{PE}).

    Chemical Energy

    Electrostatic potential energy plays a major role in chemistry; the potential energies of electrons in the force field created by atomic nuclei lie at the heart of the chemical behavior of atoms and molecules. "Chemical energy" usually refers to the energy that is stored in the chemical bonds of molecules. These bonds form when electrons are able to respond to the force fields created by two or more atomic nuclei, so they can be regarded as manifestations of electrostatic potential energy. In an exothermic chemical reaction, the electrons and nuclei within the reactants undergo rearrangement into products possessing lower energies, and the difference is released to the environment in the form of heat.

    Thermal Energy

    Kinetic energy is associated with motion, but in two different ways. For a macroscopic object such as a book or a ball, or a parcel of flowing water, it is simply given by Equation \ref{KE}. But as we mentioned above, when an object is dropped onto the floor, or when an exothermic chemical reaction heats surrounding matter, the kinetic energy gets dispersed into the molecular units in the environment. This "microscopic" form of kinetic energy, unlike that of a speeding bullet, is completely random in the kinds of motions it exhibits and in its direction. We refer to this as "thermalized" kinetic energy, or more commonly simply as thermal energy. We observe the effects of this as a rise in the temperature of the surroundings. The temperature of a body is direct measure of the quantity of thermal energy is contains.

    Once kinetic energy is thermalized, only a portion of it can be converted back into potential energy. The remainder simply gets dispersed and diluted into the environment, and is effectively lost. To summarize, then:

    • Potential energy can be converted entirely into kinetic energy..
    • Potential energy can also be converted, with varying degrees of efficiency,into electrical energy.
    • The kinetic energy of macroscopic objects can be transferred between objects (barring the effects of friction).

    Interconversion of Energies

    Like matter, energy comes in different types. Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed. This observation is expressed in the law of conservation of energy:

    law of conservation of energy

    During a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form.

    When one substance is converted into another, there is always an associated conversion of one form of energy into another. Heat is usually released or absorbed, but sometimes the conversion involves light, electrical energy, or some other form of energy. For example, chemical energy (a type of potential energy) is stored in the molecules that compose gasoline. When gasoline is combusted within the cylinders of a car’s engine, the rapidly expanding gaseous products of this chemical reaction generate mechanical energy (a type of kinetic energy) when they move the cylinders’ pistons. The law of conservation of energy is also one version of the first law of thermodynamics, as you will learn later.

    Transitions between potential and kinetic energy are such an intimate part of our daily lives that we hardly give them a thought. It happens in walking as the body moves up and down. Our bodies utilize the chemical energy in glucose to keep us warm and to move our muscles. In fact, life itself depends on the conversion of chemical energy to other forms. So when you go uphill, your kinetic energy is transformed into potential energy, which gets changed back into kinetic energy as you coast down the other side. And where did the kinetic energy you expended in peddling uphill come from? By conversion of some of the chemical potential energy in your breakfast cereal.

    Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, it has kinetic energy that can be used to do work and produce electricity in a hydroelectric plant (Figure \(\PageIndex{1}\)). A battery has potential energy because the chemicals within it can produce electricity that can do work.

    Two pictures are shown and labeled a and b. Picture a shows a large waterfall with water falling from a high elevation at the top of the falls to a lower elevation. The second picture is a view looking down into the Hoover Dam. Water is shown behind the high wall of the dam on one side and at the base of the dam on the other.

    Figure \(\PageIndex{1}\): (a) Water that is higher in elevation, for example, at the top of Victoria Falls, has a higher potential energy than water at a lower elevation. As the water falls, some of its potential energy is converted into kinetic energy. (b) If the water flows through generators at the bottom of a dam, such as the Hoover Dam shown here, its kinetic energy is converted into electrical energy. (credit a: modification of work by Steve Jurvetson; credit b: modification of work by “curimedia”/Wikimedia commons).

    • When drop a book, its potential energy is transformed into kinetic energy. When it strikes the floor, this transformation is complete. What happens to the energy then? The kinetic energy that at the moment of impact was formerly situated exclusively in the moving book, now becomes shared between the book and the floor, and in the form of randomized thermal motions of the molecular units of which they are made; we can observe this effect as a rise in temperature.
    • Much of the potential energy of falling water can be captured by a water wheel or other device that transforms the kinetic energy of the exit water into kinetic energy. The output of a hydroelectric power is directly proportional to its height above the level of the generator turbines in the valley below. At this point, the kinetic energy of the exit water is transferred to that of the turbine, most of which (up to 90 percent in the largest installations) is then converted into electrical energy.
    • Will the temperature of the water at the bottom of a water fall be greater than that at the top? James Joule himself predicted that it would be. It has been calculated that at Niagra falls, that complete conversion of the potential energy of 1 kg of water at the top into kinetic energy when it hits the plunge pool 58 meters below will result in a temperature increase of about 0.14 C°. (But there are lots of complications. For example, some of the water breaks up into tiny droplets as it falls, and water evaporates from droplets quite rapidly, producing a cooling effect.)
    • Chemical energy can also be converted, at least partially, into electrical energy: this is what happens in a battery. If a highly exothermic reaction also produces gaseous products, the latter may expand so rapidly that the result is an explosion — a net conversion of chemical energy into kinetic energy (including sound).

    What is the zero of Energy and does it matter?

    You might at first think that a book sitting on the table has zero kinetic energy since it is not moving. But if you think about it, the earth itself is moving; it is spinning on its axis, it is orbiting the sun, and the sun itself is moving away from the other stars in the general expansion of the universe. Since these motions are normally of no interest to us, we are free to adopt an arbitrary scale in which the velocity of the book is measured with respect to the table; on this so-called laboratory coordinate system, the kinetic energy of the book can be considered zero.

    We do the same thing with potential energy. If the book is on the table, its potential energy with respect to the surface of the table will be zero. If we adopt this as our zero of potential energy, and then push the book off the table, its potential energy will be negative after it reaches the floor.

    Summary

    Energy is the capacity to do work (applying a force to move matter). Kinetic energy (KE) is the energy of motion; potential energy is energy due to relative position, composition, or condition. When energy is converted from one form into another, energy is neither created nor destroyed (law of conservation of energy or first law of thermodynamics). Matter has thermal energy due to the KE of its molecules and temperature that corresponds to the average KE of its molecules. Heat is energy that is transferred between objects at different temperatures; it flows from a high to a low temperature. Chemical and physical processes can absorb heat (endothermic) or release heat (exothermic). The SI unit of energy, heat, and work is the joule (J).

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