| In the discussion of energy, the fundamental concept | | | | renewable energy source, for example, solar energy, |
| is that of work which is defined as motion against an | | | | is one that is virtually inexhaustible on the human time |
| opposing force. Energy is the capacity to do work. | | | | scale. A nonrenewable energy source, for example, |
| An object traveling at high speed and impacting on | | | | natural gas, is one that can be either completely |
| another object can do more work—can drive the | | | | consumed (during a lifetime or during several |
| object farther against an opposing force—than | | | | lifetimes) or depleted to such an extent that it is no |
| the same object moving slowly. This contribution to | | | | longer economical for humankind to obtain it. About |
| energy, the energy ascribed to motion, is called | | | | 80 percent of commercial energy is obtained from |
| kinetic energy. The kinetic energy of an object of | | | | three kinds of fuel: oil, coal, and natural gas. When |
| mass m traveling at a speed υ is ½ mυ2. An | | | | these fuels burn in air they release energy. They are |
| object may also have energy by virtue of its position. | | | | called fossil fuels because they are believed to have |
| An object high above the surface of Earth has more | | | | formed from the remains of plants and animals |
| energy (can do more work) than one at its surface. | | | | subject to heat and pressure for millions of years. |
| This contribution to the total energy, the energy due | | | | Natural gas is a mixture of methane (CH4), 60 to 90 |
| to position, is called potential energy. The relation | | | | percent, and smaller amounts of other gaseous |
| between the object's position and potential energy | | | | hydrocarbons, including ethane (C2H6), propane |
| depends on the nature of the force field it | | | | (C3H8), and butane (C4H10). It is valued because it |
| experiences. The potential energy of a body of mass | | | | burns hotter and produces less air pollution than other |
| m at a height h above the surface of Earth is mgh, | | | | fossil fuels. Complete combustion of a hydrocarbon |
| where g is the acceleration of free fall at the location. | | | | substance produces carbon dioxide and water. It has |
| More important for chemistry is the potential energy | | | | been estimated that in 2001, 2.39 trillion cubic meters |
| of one charge near another charge. The Coulomb | | | | of natural gas were consumed worldwide, with |
| potential energy of a charge q 1 at a distance r from | | | | estimated remaining reserves of 150 trillion cubic |
| a charge q2 is given by q1 q2 /4πϵ0 r, where ϵ0 | | | | meters. |
| is a fundamental constant called the vacuum | | | | Oil (also referred to as petroleum) is a complex liquid |
| permittivity. Energy is also stored in the | | | | mixture of organic substances, principally of |
| electromagnetic field in the form of photons. The | | | | hydrocarbons containing five to sixteen carbon |
| energy of a photon of radiation of frequency υ is | | | | atoms. Most crude oil, once removed from a well, is |
| hv, where h is Planck's constant. Energy is conserved; | | | | sent by pipeline to a refinery, where it is distilled to |
| that is, the sum of the kinetic and potential energies | | | | separate it into gasoline, heating oil, diesel oil, and |
| of a single body remains constant provided it is free | | | | asphalt. The use of catalysts during the refining |
| of external influences (forces). Thus, a falling weight | | | | process increases the yield of gasoline. |
| accelerates: The fall implies a reduction of potential | | | | Coal is the most plentiful fossil fuel, comprising 80 |
| energy and the acceleration implies an increase in | | | | percent of the fuel reserves of the United States |
| kinetic energy; the sum, though, is constant. | | | | and 90 percent of those of the world. It is a |
| A generalization (which can be interpreted as an | | | | complex mixture of organic compounds and is |
| implication) of the conservation of energy is the first | | | | anywhere from 30 to 95 percent carbon by mass. It |
| law of thermodynamics, which focuses on a property | | | | also contains sulfur compounds. When coal is burned, |
| of a many-body system called the internal energy. | | | | the sulfur is converted to sulfur dioxide, a |
| The internal energy can be interpreted as the sum of | | | | troublesome air pollutant. The description of coal as |
| all the kinetic and potential energies of all the particles | | | | being of high quality is based on its having a low |
| comprising the system. The first law of | | | | sulfur content and high carbon content. Lignite coal |
| thermodynamics states that the internal energy of an | | | | (brown coal) has low carbon content and produces |
| isolated system is constant. The first law is closely | | | | the least energy upon combustion (about 15 kJ/g). |
| related to the conservation of energy, but it | | | | Bituminous coal (soft coal) has higher carbon content |
| acknowledges the possibility of the transfer of | | | | and produces more energy. It is the most |
| energy as heat, which is outside the reach of | | | | extensively used coal. Anthracite coal (hard coal) has |
| mechanics itself. The special theory of relativity | | | | the highest carbon and heat content (about 30 kJ/g), |
| states that the mass of a body is a measure of its | | | | but supplies of it are limited in most places. In 2001, |
| energy: E = mc2, where c is the speed of light. That | | | | 4.41 billion metric tons of coal was consumed |
| is, energy and mass are equivalent and | | | | worldwide, with estimated reserves of 985 billion |
| interconvertible. Changes in mass are measurable only | | | | metric tons. (A metric ton is 1,000 kilograms [2,679 |
| when changes in energy are considerable, which in | | | | pounds].) |
| practice commonly means for nuclear processes. | | | | The combustion of fossil fuels produces carbon |
| In chemistry we are often concerned with the | | | | dioxide gas, a heat-trapping gas. For the past 250 |
| transfer of energy from one location (e.g., a reaction | | | | years (since the beginning of the Industrial |
| vessel) to another (the surroundings of that vessel). | | | | Revolution), the increased use of fossil fuels has |
| One mode of transfer is by doing work. For example, | | | | caused the atmospheric concentration of carbon |
| work is performed when gases evolved in a reaction | | | | dioxide to increase by a factor of about 25 percent. |
| push back a movable wall (e.g., a piston) against an | | | | It is now generally believed that this increase has |
| opposing force, such as that due to the external | | | | produced higher global temperatures—a |
| atmosphere or a weight to which the piston is | | | | phenomenon called the greenhouse effect. |
| attached. Another mode of transfer is as heat. Heat | | | | Commercial nuclear power is generated by nuclear |
| is the transfer of energy that occurs as a result of a | | | | fission reactions. When slow-moving neutrons strike |
| temperature difference between a system and its | | | | nuclei of uranium-235 or plutonium-239, these nuclei |
| surroundings when the two are separated by a | | | | are split, releasing energy. The energy is used to heat |
| diathermic wall (a wall that allows the passage of | | | | water and drive a turbine, in turn producing electrical |
| energy as heat). A metal wall is diathermic; a | | | | energy. Currently nuclear power supplies more than |
| thermally insulated wall is not diathermic. Finally, | | | | 16 percent of the world's total electricity. |
| energy may leave a system as electromagnetic | | | | A typical nuclear reactor utilizes uranium oxide, whose |
| radiation, for example as in | | | | uranium content is approximately 3 percent |
| chemiluminescence—the emission of radiation from | | | | uranium-235, and 97 percent uranium-238, by mass. |
| matter in energetically excited molecular states | | | | During the fission reaction, the uranium-235 is |
| produced in the course of a chemical reaction, and as | | | | consumed and fission products form. As the amount |
| a result of spectroscopic transitions. We may | | | | of uranium-235 decreases and the amounts of fission |
| concentrate on the first two modes of transfer, | | | | products increase, the fission process becomes less |
| work and heat. | | | | efficient. At some point, the spent nuclear fuel is |
| At a molecular level, work is the transfer of energy | | | | removed and stored. Some of the radioactive fission |
| that makes use of or drives the orderly motion of | | | | products, because of their radioactivity and long |
| molecules in the surroundings. The uniform motion of | | | | half-lives, must be stored securely for thousands of |
| the atoms in a piston driven back by expanding gas | | | | years. Thus, nuclear waste management poses a |
| is an example of orderly molecular motion. In | | | | tremendous challenge. |
| contrast, heat is the transfer of energy that makes | | | | Scientists hope to someday use controlled nuclear |
| use of or causes disorderly motion in the | | | | fusion to produce energy. Nuclear fusion, which |
| surroundings. When we say that a chemical reaction | | | | involves the coming together of light nuclei to form |
| gives out heat, we mean that energy is leaving the | | | | heavier ones, is the process by which stars generate |
| reaction vessel and stimulating thermal motion | | | | energy. In order for nuclear fusion to occur, the nuclei |
| (random molecular motion) in the surroundings. | | | | must have extremely high temperatures. Research |
| The energy of a chemical system is stored in the | | | | has focused on the fusion of deuterium (hydrogen-2) |
| potential and kinetic energies of the electrons and | | | | nuclei and tritium (hydrogen-3) nuclei, a process that |
| atomic nuclei. This stored energy is sometimes | | | | requires about 50 million degrees Celsius. The principal |
| referred to as chemical energy; however, this is only | | | | renewable energy sources are biomass from crops |
| a shorthand way of referring to the kinetic and | | | | such as trees and corn, hydropower from flowing |
| potential energies of all the particles in an element or | | | | rivers, geothermal power from heat stored in Earth, |
| compound. The internal energy of a system changes | | | | wind energy from the movement of winds, and solar |
| when a chemical reaction occurs because the | | | | energy from the Sun. |
| electrons and nuclei settle into different | | | | Wood is part of an array of plant matter referred to |
| arrangements, as in the change of partnerships of H | | | | as biomass that can be burned to produce energy. |
| and O atoms in the reaction 2H2(g) + O2(g) → | | | | The combustible substances in biomass are primarily |
| 2H2O(g). The energy released in a chemical reaction | | | | carbohydrates (and of these, primarily cellulose). |
| can be transferred to the surroundings (and put to | | | | Cellulose, whose simplest or empirical formula is |
| use) in a variety of ways regardless of the manner in | | | | CH2O, undergoes combustion to form carbon dioxide |
| which the energy accumulated in the first place. Thus, | | | | and water. Wood fuels continue to be used in the |
| energy may escape as heat and be used to raise the | | | | rural areas of developing countries. Hydroelectric |
| temperature of the surroundings, including raising the | | | | power is a well-developed energy source. Today, |
| temperature of water that is then employed in a | | | | hydropower provides about 19 percent of the world's |
| turbine to do work. The energy may also escape as | | | | electricity supply. Because it is a clean, renewable |
| work. The work may be accomplished electrically, as | | | | source of energy, hydropower should continue to |
| when electrons are driven through an external circuit | | | | serve as a vital energy source. |
| and used to drive an electric motor. | | | | There has been a rapid growth in the use of wind |
| Atomic nuclei are also centers of energy storage as | | | | turbines to generate electricity. In 2001 the amount |
| a result of their internal structures. This energy is | | | | of electricity generated in this way worldwide |
| released when the nucleons (protons and electrons) | | | | corresponded to the amount that would have been |
| undergo rearrangement and thereby change the | | | | obtained from burning 15 million barrels of oil. Although |
| strength of their interactions. The changes in energy | | | | this represents only about 0.05 percent of worldwide |
| are so great that they give rise to measurable | | | | energy production in 2001, this fraction will increase. |
| changes of mass. For all chemical processes, the | | | | Solar energy is the most significant and promising |
| changes in mass accompanying acquisition or loss of | | | | renewable energy source. Solar energy is converted |
| energy are totally negligible. | | | | to electricity by solar cells (also known as |
| Modern societies rely on a variety of energy sources | | | | photovoltaic cells). A great deal of solar energy is |
| to heat homes, propel transportation vehicles, and | | | | used currently in what is known as passive heating |
| produce goods for shelter, food, health care, and | | | | (which can be directly experienced as the heat gain in |
| entertainment. Some of these sources are | | | | a greenhouse caused by sunlight). |
| renewable, whereas others are nonrenewable. A | | | | |