is equal to 1 calorie of energy (1 cal = 4.184 J), the unit of energy formerly used in
scientific work. A joule is a small unit, and the kilojoule, kJ, equal to 1000 J is widely
used in describing chemical processes. The “calorie” commonly used to express the
energy value of food (and its potential to produce fat) is actually a kilocalorie, kcal,
equal to 1000 cal.
The science that deals with energy in its various forms and with work is
thermodynamics. There are some important laws of thermodynamics. The first law
of thermodynamics states that energy is neither created nor destroyed. This law is also
known as the law of conservation of energy. As an example of the application of this
law, consider Figure 1. The energy associated with cultivating the land enters the system
as chemical energy in the form of diesel fuel, and the oxygen from the air required for
its combustion. This is a valuable form of concentrated chemical energy that can be
used to propel a tractor or truck, in a turbine attached to a generator for the generation
of electrical energy, or as a fuel to generate heat in an oil-fired furnace. The fuel is
burned in the tractor’s engine, and more than half of its energy is dissipated as heat to
the surroundings. The rest is used to move the tractor and dirt. The energy originally
contained in a concentrated useful form in the diesel fuel is not destroyed, but it is
dissipated in a dilute form, mostly to warm the surroundings very slightly. The energy
that was originally present as a very useful form in the diesel fuel has not been destroyed,
but it has been dispersed in a form that is no longer of practical use.
The first law of thermodynamics must always be kept in mind in the practice of green
chemistry. The best practice of green chemistry and, indeed, of all environmental science,
requires the most efficient use of energy as it goes through a system. The availability
of energy is often the limiting factor in using and recycling materials efficiently. If
enough energy is available, almost anything can be done. For example, many water-
deficient areas of the world, such as the northern coast of Africa, Israel, and Saudi Arabia
are adjacent to limitless supplies of ocean water. If sufficient energy were available,
seawater could simply be distilled to provide an abundant supply. But the consumption
of energy would be prohibitive. It is not only a question of availability of energy, but
rather of energy that can be used in a way that does not do unacceptable environmental
harm. Many countries, including the U.S. have vast deposits of coal. But to rely on
coal for all the energy that various societies perceive that they need in the future would
place huge amounts of carbon dioxide in Earth’s atmosphere almost certainly resulting
in intolerable global warming.
6.2. Radiant Energy from the Sun
The sun is the ultimate source of most of the energy that we use. How much energy
does Earth receive from the sun? If the sun were to abruptly “go out” (not to worry, it
won’t happen for another billion years or so) we would quickly find out, because within
hours Earth would become a frozen rock in space. In fact, the solar flux, which is the
rate at which solar energy is transmitted through space at Earth’s distance from the sun is
1.34 × 10^3 watts/m^2. What this means, as illustrated in Figure 6.2, is that a 1 square meter
area (a square just over 3 feet to the side) with the sun shining perpendicular to it just
136 Green Chemistry, 2nd ed