Brief Introduction to the Three Laws of Thermodynamics Kenneth L. Stevenson Purdue Uniuersity a t Fort Wayne Ft. Wayne, Indiana 46805 Taken together, the three laws of thermodynamics, in all their concise simplicity, lead to a vast hody of understanding of energy transformations and equilibria in all kinds of processes from chemical reactions to windmills. Even so, the laws are often difficult to comprehend on a "gut" level because most textbook approaches to them utilize a great deal of mathematics very early in their presentations, which is quite proper for serious students of thermodynamics but which leaves many other people behind. This very short article will attempt to give a fairly non-mathematical feeling for the three laws. The first law is essentially the same as the law of conservation of energy. The energy absorbed or emitted by a system undergoing some kind of process must be accounted for in all the different kinds of energy; e.g., electrical, mechanical, chemical, or heat. Because heat plays an important role in the development of the second law, the different kinds of energy are classified as being either heat, which is energy which flows between bodies a t two different temperatures, or work, which encompasses energy that actually causes some kind of motion in large bodies. The first law then says that
where A E is the change in energy of a system, and q and w are the heat and other kinds of energy added to the system. While the first law is very useful in tabulating energy changes for various processes, it does not allow one to predict whether or not that process can occur. I t simply says that if there is an energy flow, AE, i t must be accounted for as either heat or work. Yet we are all intuitively aware that the direction of energy flow is governed by some natural law. Water spontaneously flows downhill, a coiled spring unwinds when released, and wood hums unassisted when ignited. These processes repmducibly occur this way; one never finds, for example, water spontaneously flowing uphill. The law that prescrihes the direction of energy flow, and hence the familiar continuity of events, is the second law of thermodynamics. One statement of the second law says that heat will not flow spontaneously between two bodies unless they are a t different temperatures. When heat does flow, the hotter hody suffers a reduction in temperature, and the cooler body warms up. If heat is allowed to flow between the bodies indefinitely it eventually must cease because the two bodies will come to the same temperature. Although the total energy residing in the two bodies taken together 330 / Journal of Chemical Education
A thorough treatment of thermodynamics requires mathematics. But sometimes the math seems to obscure the basic ideas. At the request of a high schcal teacher, Ken Stevenson has written the following article presenting the hasic ideas behind the three laws of thermodynamics, uncluttered hy equations. We hope you will find it useful. If you would like to see more articles of this type in High Schwl Forum, send your suggestions to Dr. J . Dudley Herron, Department of Chemistry, Purdue University, West Lafayette, IN 47907.
has not changed, something has happened to the bodies, namely, they have lost the ability to exchange heat with each other. Another way of saying this is that their heat has become degraded or useless. This "degradedness" of the two bodies with respect to each other is a kind of quantity just as energy is a quantity, and it has been given the name, entropy. The second law says, then, that if a spontaneous process is to occur within a system there must be an increase in degradedness or entropy of the entire universe, which consists of the system and its surroundings. This statement goes somewhat beyond our first simple statement in that i t does not mention heat. In fact, there need not he an exchange of heat between the system and the surroundings in order that a process is to he spbntaneous. All that has to happen is that the total entropy change of system plus surroundings must he positive. Thus, total entropy change for a process is taken as the criterion for spontaneity. If the second law is valid everywhere in the universe, then a rather interesting philosophical point arises. For, as spontaneous processes occur, and entropy of the universe increases, there results a degradation of the ability of the universe to cause further energy transformations and events. Eventually, the entropy should reach a maximum such that no further events can occur. At this point the universe is a completely helpless, homogeneous soup a t a uniform temperature, the victim of the "Heat Death". On a more familiar level, the second law is responsible for the loss in efficiency in energy conversion processes. A heat engine is a device that derives work from the flow of heat from a high temperature reservoir such as a firebox to a low temperature sink such as a cooling tower or exhaust system, as in a steam engine or internal combustion engine. One is faced with the paradox that (1) heat must flow from a high temperature reservoir to a low temperature sink if the process is to occur at all, but (2) some of the heat must he converted to work before it gets to the low temperature sink if the engine is to he of any use. Some heat must be spent in the cold sink in order to cause the heat to flow, and since the heat flows, some of it can be transformed into work. This means that any heat engine is forced to be less than 10090efficient in converting heat into work. In fact, Camot showed that the fraction of
heat which could be converted to work is equal to the temperature difference between the heat reservoir and cold sink divided by the absolute temperature of the heat reservoir. Any energy converter suffers from this same kind of difficulty. For example, a water wheel cannot convert all of the potential energy of the water into rotary energy in the wheel because in order for the wheel to turn, water must flow over the wheel and away from it, hut this carries away some of the original energy of the water in the form of kinetic energy. Every time an energy conversion occurs, there is an expenditure of energy lost to the surroundings. This loss is the tax that is being paid to the second law in order that the spontaneous process may occur. The third law is the one which allows the evaluation of the absolute entropy of any material, and leads to the interpretation of entropy as the degree of randomness or a state of probability of a system. It says that at absolute zero temperature the absolute entropies of all perfectly crystalline materials are identical and arbitrarily set at 0. The interpretation of entropy as disorder or randomness follows from the recognition that a t absolute zero a perfect crystal is perfectly ordered, i.e., if one were to draw a
three-dimensional picture of such a crystal be would have to produce only one unequivocal picture of it. At temperatures higher than absolute zero, the atoms in the crystal vibrate in excited states, imperfections occur, and eventually the lattice breaks up. The crystal becomes more and more disordered and the number of pictures of how the crystal might look becomes larger and larger. The bridge between classical thermodynamics and statistics is made by postulating that the entropy of any system is determined by the number of ways of arranging the particles in that system without changing the state. In other words, entropy is directly related to probability. Thus, natural processes always occur such that the universe evolves from a state of lower to a state of higher probability. SuQgestionstor Further Reading Mahan. B. H., "Elementary Chemical Themadynsmies", W. A. Benjamin, Nerv York,
1969.
Lewis, G.N., andRandall, M., reviaion by Pifzer, K . S.. a n d B m r . L . . "Thermcdynam-
i c i ' , McCraw-Hill, NewYork. 1961. Bent. H.A.,*The Second Law", Oxford University Proos, New Ymk. 1965. Moo-, W. J.. "Physical b e m i s t w " . 4th Ed.. Prcnthe-Hall, Englewoad Cliffs, NJ., 1912. Zemsruky, M. W., J. CHEM. EDUC., 51.511974). S c i e n r i f k ~ m e r i c o224, ~ (Sept., 1971) Speeielisueon"Eneqyand Powr.'
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