The Laws of Thermodynamics

Thermodynamics is the empirical science that describes the state of macroscopic systems without reference to their microscopic structure. The laws of thermodynamics are based on experimental observations. The physical systems described by thermodynamics are considered to be composed of a very large number of microscopic particles (atoms or molecules). In the context of thermodynamics, a macroscopic system is described in terms of the external conditions that are imposed on it, determined by scalar or vector fields, and the values of the corresponding variables that specify the state of the system for given external conditions. The usual fields and corresponding thermodynamic variables are 

This can be put into a quantitative expression as follows the sum of the change in the internal energy A of a system and the work done by the system A IT is equal to the heat 

Heat is not a concept that can be related to the microscopic structure of the system, and therefore Q cannot be considered a state function, and dg is not an exact differential. From the first law, heat is equal to the increase in the internal energy of the system when its temperature goes up without the system having done any work. 

There can be no thermodynamic process whose sole effect is to transform heat entirely to work 

A direct consequence of the second law is that, for a cyclic process which brings the system back to its initial state, 

The Laws provide the framework of the necessary equations for the solution of thermodynamic problems the Data, the information required to arrive at numerical answers through these equations. 

The Zeroth Law When bodies A and B are in thermal equilibrium with body C, they are in thermal equilibrium with each other. It is used to establish temperature as a state property. 

The First Law Energy is conserved. The basis for energy balances, it relates the state functions internal energy and enthalpy, to the path dependent ones work and heat. 

The Second Law Any process whose only effect is the transfer of heat from one temperature level to a higher one, is impossible. It introduces 

The Third Law The absolute entropy of a perfect crystal at zero absolute temperature, is zero. It is used to determine absolute values of entropy, that are essential in calculations involving chemical reactions. NoterTTie development of thermodynamics based on these laws follows the historical approach. For other approaches, see the Preface. 

There are three major laws of thermod5mamics. As already mentioned, the first law is the conservation of energy. When invoking this law, it is essential that all energy sources be considered. For any isolated system, the following statement of the first law always holds. 

If heat is transferred to the gas q 0) and work is done on the gas (w 0), the internal energy increases such that AU = q + w. This is an alternate expression of the first law. 

Is entropy a conserved quantity Consider n moles of ideal gas trapped in a chamber of some volume V. Connected to this chamber by a valve is another chamber of the same volume, but it is evacuated. The gas chambers are an isolated system. If the valve were opened, thereby connecting the two chambers, we would expect gas particles to move into the empty chamber. Since the available volume has doubled, then by Equation 1.7 the change in entropy would be nRln(2) 0. Though energy would be conserved in this process, tirat is, AU = 0, entropy would increase. And if we added another chamber and allowed the gas to expand, entropy would continue to increase. It would not be conserved, and the fact that it would increase for this type of process is significant. 

The flow of gas from occupying a volume V to occupying a volume 2V in the two chamber setup corresponds to our everyday experience. We do not walk into a room and from time to time find all the air is in one half of the room while the other half is evacuated rather, the air has a uniform density throughout the room. It occupies the entire volume, not part of it, because that maximizes the number of configurations or arrangements of gas particles. In the hypothetical example, the expansion of the gas from occupying one chamber to occupying two is a spontaneous process. Once the valve is opened, it happens without external influence. As well, if is an irreversible process. 

Therefore, the tendency for a spontaneous change is connected with the tendency of a system to maximize entropy. 

We will start with the laws of thermodynamics, whose confusing chronology was beautifully described by Atkins and is summarized in another box below. 

Scientific American Library W. H. Freeman, New York, 19S4 

Three of these laws usually do not give students much trouble. The first law, which was actually formulated after the second law, is the conservation of energy, which makes sense to those people who believe you don t get something for nothing. The zeroth and third law deal with temperature, which most students think they understand, but sometimes don t The big problem is usually the second law. 

The purpose of this review is to remind you of a few key concepts. As such, it will not be rigorous, missing out essential caveats and details (isothermal this, adiabatic that) and so on-. At the end of it, however, we hope you will have grasped or reminded yourself of the answers to the following questions  

What is the molecular machinery underpinning these laws  

In an open (size and composition not fixed) chemical system, the quantities p, V, Ni (i = 1,.N) constitute a convenient complete set of independent variables for specifying the thermodynamic state of the system. The pressure p is an intensive property, while the volume V and the mole number iV (t = 1. JV) are extensive variables. Here the numbers of moles of the same species present in different phases are distinguished by different values of the subscript i, so that N is the sum of the total number of species present in each phase. 

The zeroth law of thermodynamics states that there exists an additional intensive variable, temperature T = T(j ,V, AT,), which has the same value for all systems in equilibrium with each other. 

The first law of thermodynamics is conservation of energy, which states that there exists an extensive function U = U p, V, N,), called the internal energy, having the property that for a closed system (one that does not exchange material with its surroundings) the heat added to the system in an infinitesimal process is 

In the classical formulation, the second law of thermodynamics states that there exist an absolute scale for the temperature T and an extensive function S(p, V, Ni), called the entropy, such that for an infinitesimal process in a closed system 

The particular result of statistical mechanics that is sometimes referred to as the third law of thermodynamics appears to be of little use in combustion. 

The first law of thermodynamics is conservation of energy, which states that there exists an extensive function U = U(p, V,Ni called the 

In this and the previous section, we have focused on the role energy plays in chemical reactions. This is an area of science known as thermodynamics, which stems from Greek words meaning movement of heat. The concepts we addressed, such as exothermic and endothermic reactions and entropy, fit neatly in the laws of thermodynamics, which are paraphrased as follows  

Energy is conserved. It may be converted from one form to another, say from potential to kinetic energy, but the total amount of energy in the universe is constant. The energy that an exothermic reaction releases always goes somewhere in the environment, usually in the form of thermal energy (heat). 

Any process that happens by itself results in the net dispersal of energy. The degree of energy dispersal is measured by entropy, which continuously increases because the universe is full of processes that continue to occur all on their own. 

Students often state the laws of thermodynamics this way. You cant win because you cant get any more energy out of a system than you put into it. You can t break even because no matter what you do, some of your energy will be lost as ambient heat. Lastly, you cant get out of the game because you depend on entropy-increasing processes, such as solar nuclear fusion or cellular respiration, to remain alive. 

Some of the sun s dispersed energy is used to drive endothermic reactions that allow for the functioning of living organisms. 

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Thermodynamics. Many definitions and formulations exist for the laws of thermodynamics, a detailed treatment of which may be found in standard engineering texts (2). Definitions that apply best to air conditioning are as follows ... 

Thermodynamics is the branch of science that embodies the principles of energy transformation in macroscopic systems. The general restrictions which experience has shown to apply to all such transformations are known as the laws of thermodynamics. These laws are primitive they cannot be derived from anything more basic. 

Mass-Transfer Principles Dilute Systems When material is transferred from one phase to another across an interface that separates the two, the resistance to mass transfer in each phase causes a concentration gradient in each, as shown in Fig. 5-26 for a gas-hquid interface. The concentrations of the diffusing material in the two phases immediately adjacent to the interface generally are unequal, even if expressed in the same units, but usually are assumed to be related to each other by the laws of thermodynamic equihbrium. Thus, it is assumed that the thermodynamic equilibrium is reached at the gas-liquid interface almost immediately when a gas and a hquid are brought into contact. 

To make the flaw grow, say by 1 mm, we have to tear the rubber to create 1 mm of new crack surface, and this consumes energy the tear energy of the rubber per unit area X the area of surface torn. If the work done by the gas pressure inside the balloon, plus the release of elastic energy from the membrane itself, is less than this energy the tearing simply cannot take place - it would infringe the laws of thermodynamics. 

This leads to what is called the Clausius form of the second law of thermodynamics. No processes are possible whose only result is the removal of energy from one reservoir and its absorption by another reservoir at a higher temperature. On the other hand, if energy flows from the hot reservoir to the cold reservoir with no other changes in the universe, then the same arguments can be used to show that the entropy increases, nr remains constant for reversible processes. Therefore, such energy flows, which arc vciy familiar, are in agreement with the laws of thermodynamics. 

Two of the laws of thermodynamics and two of the laws of motion will cover almost all needs. In the simplest terms, it can be said that ... 

This notation by Kroger-Vink is very intuitive. However, the laws of thermodynamic equilibrium may not be applied to these symbols because the elements are not independent of each other as required by thermodynamics. For example the formation of the interstitial metal ion re-... 

The Thermodynamic or Kelvin Temperature Scale Description of the Kelvin temperature scale must wait for the laws of thermodynamics. We will see that the Kelvin temperature is linearly related to the absolute or ideal gas temperature, even though the basic premises leading to the scales are very different, so that... 

Calculations involving U or Um must wait until the laws of thermodynamics are formulated. 

To integrate equations (1.50) and (1.51), we must know how Z changes with p, V, and T. The laws of thermodynamics that we will discuss next in Chapter 2 will allow us to derive the necessary relationships we will apply the resulting equations in Chapter 3. 

What about points to the right of 2 Can they be reached Consider an adiabatic path from point 1 to point 2a that is also located on the isothermal Qj. The cycle of interest is 1 — 2a —> 2 — 1. Again, two of the three steps are adiabatic. In this case, however, heat is evolved during the 2a —> 2 step from the conversion of work into heat. The complete conversion of work into heat is a well-known phenomenon and is not forbidden by the laws of thermodynamics. Thus, there are states to the right of 2 on the isotherm O2 that are accessible from 1 via an adiabatic path. 

For interesting discussions of the laws of thermodynamics, see J. R. Goates and J. B. Ott, Chemical Thermodynamics An Introduction, Harcourt Brace. Jovanovich, Inc., New York, 1971 R. Battino and S. E. Wood, Thermodynamics An Introduction, Academic Press, New York, 1968 P. A. Rock. Chemical Thermodynamics, University Science Books, Mill Valley, California, 1983 H. A. Bent. The Second Law An Introduction to Classical and Statistical Thermodynamics, Oxford University Press, New York, 1965. 

In Chapter 2 we used the laws of thermodynamics to write equations that relate internal energy and entropy to heat and work. 

Why Do We Need to Know This Material The laws of thermodynamics govern chemistry and life. They explain why reactions take place and let us predict how much heat reactions release and how much work they can do. Thermodynamics plays a role in every part of our lives. For example, the energy released as heat can be used to compare fuels, and the energy resources of food lets us assess its nutritional value. The material in this chapter provides a foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions. 

This important formula, which can be derived more formally from the laws of thermodynamics, applies when any change takes place at constant pressure and temperature. Notice that, for a given enthalpy change of the system (that is, a given output of heat), the entropy of the surroundings increases more if their temperature is low than if it is high (Fig. 7.16). The explanation is the sneeze in the street analogy mentioned in Section 7.2. Because AH is independent of path, Eq. 10 is applicable whether the process occurs reversibly or irreversibly. 

The most common states of a pure substance are solid, liquid, or gas (vapor), state property See state function. state symbol A symbol (abbreviation) denoting the state of a species. Examples s (solid) I (liquid) g (gas) aq (aqueous solution), statistical entropy The entropy calculated from statistical thermodynamics S = k In W. statistical thermodynamics The interpretation of the laws of thermodynamics in terms of the behavior of large numbers of atoms and molecules, steady-state approximation The assumption that the net rate of formation of reaction intermediates is 0. Stefan-Boltzmann law The total intensity of radiation emitted by a heated black body is proportional to the fourth power of the absolute temperature, stereoisomers Isomers in which atoms have the same partners arranged differently in space, stereoregular polymer A polymer in which each unit or pair of repeating units has the same relative orientation, steric factor (P) An empirical factor that takes into account the steric requirement of a reaction, steric requirement A constraint on an elementary reaction in which the successful collision of two molecules depends on their relative orientation. 

The laws of thermodynamics are the cornerstones of any description of a system at equilibrium. The First Law, also known as the Law of Conservation of Energy states that energy cannot be created or destroyed, i.e., the energy of the universe is constant. Thus if the internal... 

Addressing the second question first leads to a critical constraint when thinking about new, more sustainable, technological developments, that is, the universal applicability of the laws of thermodynamics to aU physical, chemical and biological processes. A central and inescapable fact is the inevitability of waste formation. One statement of the second law of thermodynamics says that heat cannot be converted completely into work. Or, in other words, the energy output of work is always less than the energy transformed to accomplish it. A consequence of this is that, even in principle, it is impossible for any real process to proceed without the generation of some sort of waste. 

Examples of the utility and need for new, advanced-performance materials are numerous. For example, in turbine engines today, the need is to be able to increase operating temperatures by 100—150 °C. The laws of thermodynamics allow significant fuel efficiency to be gained as the temperature increases. However, the material, particularly for the turbine blades, must be able to handle these increased temperatures. This material need illustrates the connection between product application and material performance. 

This chapter will describe how we can apply an understanding of thermodynamic behavior to the processes associated with polymers. We will begin with a general description of the field, the laws of thermodynamics, the role of intermolecular forces, and the thermodynamics of polymerization reactions. We will then explore how statistical thermodynamics can be used to describe the molecules that make up polymers. Finally, we will learn the basics of heat transfer phenomena, which will allow us to understand the rate of heat movement during processing. 

The modeler can constrain the initial equilibrium state in many ways, depending on the nature of the problem, but the number of pieces of information required is firmly set by the laws of thermodynamics. In general, the modeler sets the temperature and provides one compositional constraint for each chemical component in the system. Useful constraints include... 

As in the classical expression (25) the quantity ip can be inferred directly as representing the statistical analogue of the Helmholtz free energy. The average behaviour of the canonical ensemble thus obeys the laws of thermodynamics. 

In thermodynamics the state of a system is specified in terms of macroscopic state variables such as volume, V, temperature, T, pressure,/ , and the number of moles of the chemical constituents i, tij. The laws of thermodynamics are founded on the concepts of internal energy (U), and entropy (S), which are functions of the state variables. Thermodynamic variables are categorized as intensive or extensive. Variables that are proportional to the size of the system (e.g. volume and internal energy) are called extensive variables, whereas variables that specify a property that is independent of the size of the system (e.g. temperature and pressure) are called intensive variables. 

Many attempts have been made to deduce thermodynamics from statistical mechanics, and no one can doubt the intimate relationship or even the complete identity of the two sciences. Nevertheless, it has not hitherto been found possible to proceed by a single path unambiguously from simple statistical assumptions to the laws of thermodynamics. This we hope to have accomplished in this paper and the following. 

Our goal in this chapter is to help you learn the laws of thermodynamics, especially the concepts of entropy and free energy. It might be helpful to review Chapter 6 on thermochemistry and the writing of thermochemical equations. The concept of Gibbs free energy (G) will be useful in predicting whether or not a reaction will occur spontaneously. Just like in all the previous chapters, in order to do well you must Practice, Practice, Practice. 

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