Absolute Entropy and Entropy Change
Absolute entropy is a measure of the amount of disorder or randomness in a system at 0 Kelvin. Entropy change refers to the change in the level of disorder or randomness in a system as a result of a chemical reaction or physical process. It is a key concept in thermodynamics and is related to the dispersal of energy in a system.
6 Key excerpts on "Absolute Entropy and Entropy Change"
eBook - ePub- Gavin Whittaker, Andy Mount, Matthew Heal(Authors)
- 2000(Publication Date)
- Taylor & Francis(Publisher)
...Under these conditions, the total number of possible ways of arranging the material equals one (w =1), and so the entropy (defined as k B ln w) is equal to zero. Although absolute zero cannot be reached and perfect crystalline solids cannot be made, it is still possible to apply the third law. In practice, the absolute entropy of materials drops to infinitesimally small values at low temperature, and for most purposes equals zero at the low temperatures which can be routinely achieved in the laboratory. Because it is possible to measure entropy changes from a reference point using heat capacity measurements, entropy (unlike the enthalpy and internal energy) has a measurable absolute value for any system. B5 ENTROPY AND CHANGE Key Notes Spontaneous process A spontaneous process has a natural tendency to occur without the need for input of work into the system. Examples are the expansion of a gas into a vacuum, a ball rolling down a hill or flow of heat from a hot body to a cold one. Non-spontaneous process A non-spontaneous process does not have a natural tendency to occur. For a non-spontaneous process to be brought about, energy in the form of work must be put into a system. Examples include the compression of a gas into a smaller volume, the raising of a weight against gravity, or the flow of heat from a cold body to a hotter one in a refrigeration system. Second law of thermodynamics The second law of thermodynamics states that the entropy of an isolated system increases for irreversible processes and remains constant in the course of reversible processes...
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
...The modern statement of the second law of thermodynamics, commonly known as the law of increased entropy, is; • The entropy of any isolated system always increases. An isolated system is a system that cannot exchange energy (heat or work) or mass with its surroundings. Consider entropy in nature as the tendency for the system to become more random. For example, if you take a pack of playing cards that are neatly stacked in order and throw them up into the air, the cards will not tend to fall neatly back into the stack of 52 cards, but will fall into a disordered pile of cards all over the room. For the case of chemical reactions or phase changes, entropy is considered to be made up of all of the possible positions of atoms and molecules that make up the chemical system under specific conditions. An example of this would be the phase changes of water. When one mole of water in the solid state changes to water in the liquid state and then to water in the gaseous state, the entropy of the system is increased at the molecular level. The hydrogen bonding, which acts to give order to the system, is increasingly disrupted at each phase change and the molecules are allowed to move more freely between more possible positions, so the entropy of the system increases. Entropy is directly related to the heat added to the system and, as such, is related to the extent of molecular movement. Entropy is also related to the temperature of a system as long as it is not undergoing a phase change as shown in Fig. 8.7. As the temperature of the solid increases, heat is transferred to the solid, the movement of the atoms or molecules that make up the solid increases, the molecules become more spread out, disorder increases, and so entropy increases. When the melting point of the solid is reached, there is a rapid increase in entropy with no change in temperature until all the solid is converted to the liquid...
eBook - ePubBiomolecular Thermodynamics
From Theory to Application
- Douglas Barrick(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...This applies to any type of isolated system, no matter how complex, including the entire universe. These concepts give rise to another statement of the second law: The entropy of an isolated system (the universe) increases during any spontaneous process. Here, “spontaneous” is synonymous with irreversible. Since all processes that occur in isolation (i.e., without an external driving force) must be spontaneous, this means that all natural processes increase entropy. Although reversible processes do not increase entropy, they do not decrease it. Rather, the second law states that process that decrease entropy do not occur in isolation. ‡ Entropy As a Thermodynamic Potential A key feature of the inequality developed in the previous section is that it makes the entropy into a thermodynamic potential. By potential, we mean that when an isolated system is away from equilibrium, entropy identifies the direction of spontaneous change and provides a driving force for change. Moreover, as a potential, entropy locates the position of equilibrium. Identifying the position of equilibrium is extremely important in analyzing the thermodynamics of chemical reactions. Though readers may be unfamiliar with the concept of a thermodynamic potential, everyone is familiar with potentials associated with simple mechanical systems with just a few degrees of freedom. For a ball on a hill, gravity provides a potential, determining which direction the ball will move (downhill), and where it will come to rest (an equilibrium position, at the bottom of the hill). Both pieces of information can be quantified by taking derivatives, and for the equilibrium position, using the derivative to find a minimum. Potential energy is also important on the molecular scale for determining favorable bonding patterns, and these interactions clearly influence bulk equilibrium properties of a system of molecules...
eBook - ePub- Dov M. Gabbay, Paul Thagard, John Woods, Dov M. Gabbay, Paul Thagard, John Woods(Authors)
- 2011(Publication Date)
- North Holland(Publisher)
...Entropy in Chemistry Robert J. Deltete 1. Introduction Contemporary textbooks in physical chemistry and chemical thermodynamics regularly refer to the importance of the concept of entropy in describing the course of chemical reactions and the conditions for chemical equilibrium (e.g., [ Winn, 1995, p. 63]). This was not always the case. In fact, for the most part, it was quite the opposite for a long time, enough so that two recent authors could subtitle a paper “the tortuous entry of entropy into chemistry” [ Kragh and Weininger, 1996 ]. In this essay, I begin in Section II with a brief description of the entry of entropy into physics through the work of Rudolf Clausius. I then sketch, in Sections III and IV, the productive use to which the concept was put in the work of Josiah Willard Gibbs and Max Planck, before turning in Section V to the reasons that most chemists did not follow Gibbs and Planck. Section VI offers some speculations on how resistance to entropy on the part of chemists was gradually overcome. 2. Clausius on Entropy The essential step leading to the concept of entropy was taken by Clausius in 1850, when he argued that two laws are needed to reconcile Carnot's principle about the motive power of heat with the law of energy transformation and conservation. Efforts to understand the second of the two laws finally led him in 1865 to his most concise and ultimately most fruitful analytical formulation. In effect, two basic quantities, internal energy and entropy, are defined by the two laws of thermodynamics. The internal energy U is that function of the state of the system whose differential is given by the equation expressing the first law, (1) where đ Q and đ W are, respectively, the heat added to the system and the external work done on the system in an infinitesimal process. 1 For a simple fluid, the work is given by the equation 1 I have altered Clausius' notation, and also Gibbs's in what follows, to conform to contemporary usage...
eBook - ePub- Wolfgang Hofkirchner, Wolfgang Hofkirchner(Authors)
- 2013(Publication Date)
- Routledge(Publisher)
...The difference is called entropy production, σ. d S – q/T = σ. (3) σ never can be negative. Positivity of σ expresses the unidirectionality of spontaneous changes, σ is the “time arrow”. Entropy production is a measure of changes. When nothing happens σ is zero. σ > 0 is a sign that something happened. The great success of the entropy approach is classical thermodynamics. A theory describing systems in equilibrium or undergoing reversible processes and is particularly applicable to isolated systems, or to systems with isolation (walls). In isolated systems the equilibrium state is characterized by entropy maximum. When S = S 0 there is no place for further changes, the system is “dead”. Non-equilibrium thermodynamics describes the processes, and provides tools to calculate non-equilibrium entropy changes. There were attempts to derive thermodynamics on information basis. In a wonderful paper “Information and thermodynamics” Rothstein [52] described the connection. He argued that entropy is the missing information. Rothstein said, that from an informational viewpoint quantity of heat is energy transferred in a manner which has eluded mechanical description, about which information is lacking in terms of mechanical categories, and entropy can be interpreted as a measure of missing information relative to some standard state. Rothstein used this relation to give thermodynamics an informational formulation. “The basic laws of thermodynamics can be stated as: a. The conservation of energy b. The existence of modes of energy transfer incapable of mechanical description c. The third law is true by definition, for a perfectly ordered state at absolute zero there is no missing information.” 2.3 Statistical Entropy In 1872 Boltzmann defined entropy in terms of possible microstates S = k log W (4) where W is the so called thermodynamic probability, or the number of microstate referring to the same macrostate...
eBook - ePub- Warren C. Strahle, William A. Sirignano, William A. Sirignano(Authors)
- 2020(Publication Date)
- Routledge(Publisher)
...2 CHEMICAL THERMODYNAMICS 2.1 INTRODUCTION Thermodynamics is an empirical science dealing with the properties of substances and their energetics. It applies to systems of a single phase and multiple phases. For purposes here it also applies to systems of varying chemical composition. The subject of chemical thermodynamics applies to fixed mass and flowing systems, just as in the case of fixed composition systems. The subject simply becomes a bit more complex when chemical change is involved, than in the case of fixed composition systems. Since combustion involves composition changes, however, this complexity must be accepted as necessary. Thermodynamics enables us to calculate the energetics of system changes in composition. As such it enables us to determine, for example, the temperature and pressure changes when a system undergoes a chemical transformation. It will be seen that thermodynamics can also be used to tell us what the composition change will be when a system undergoes a reaction. It is not used, however, to determine rates of chemical transition. That is the subject of a following chapter on chemical kinetics. The subject of thermodynamics is only concerned with beginning and end thermodynamic states for a system, with no concern for the process path between them. Nevertheless, it is an essential science in combustion. 2.2 PROPERTIES OF SUBSTANCES We shall deal with solids, liquids and gases. For solids and liquids we will use the actual measured thermodynamic properties of the substances. So, for example, any thermodynamic equation of state such as a relation between pressure, temperature and volume for liquids and solids will be the actual measured relation. For gases, however, but with good approximation, we will always assume them to be thermally perfect gases. The term “thermally perfect” means that the gas obeys the equation of state (2.1) where V is a fixed system volume, ρ is pressure and T is absolute thermodynamic temperature...
