N Is Avogadro's Number; and the Factorial Term Is Due to the Indistinguishability of the Gaslike Molecules

Total Page:16

File Type:pdf, Size:1020Kb

N Is Avogadro's Number; and the Factorial Term Is Due to the Indistinguishability of the Gaslike Molecules THE SIGNIFICANT STRUCTURE THEORY APPLIED TO SOME LIQUID ROCKET FUELS BY T. RICHARD SCHMIDT,* MU SHIK JHON,* AND HENRY EYRINGt UNIVERSITY OF VIRGINIA AND UNIVERSITY OF UTAH Communicated March 18, 1968 The significant structure theory of liquids' assumes that some molecules pos- sess solidlike, and some gaslike, degrees of freedom and that the relative contri- butions of each type are given by VS/V and (V - V8)/V, respectively. Here V5 is the molar volume of the solid at the melting point, and V is the molar volume of the liquid. This theory has been successfully applied to a number of liquids2 ranging from liquefied gases, hydrocarbons, molten metals, and fused salts to water and liquid mixtures. Recently, Jhon et al.3 applied the theory with success to simple rocket fuels which have found importance in space programs. In this paper, a further valida- tion of the theory is tested for liquid pentaborane and nitromethane. A parti- tion function is developed and the properties of these compounds are calculated successfully. The Partition Function.-According to the significant liquid structure theory, the partition function of a liquid is written as - (Ifsolid fdeg) &NV/V (fgas) N((V-V.)/V) (N(V V)) ! (1) Here fsolid, fdeg, and fgas are, respectively, the partition functions of the solid, of positional degeneracy, and of the gas; N is Avogadro's number; and the factorial term is due to the indistinguishability of the gaslike molecules. Liquid pentaborane and nitromethane have an abnormally high entropy of fusion (6.48 e.u. for pentaborane4 and 9.48 e.u. for nitromethane5). No solid- state transitions4' 5 have been observed and the increase in volume upon melting is not sufficient to permit the rotation of solidlike molecules in the liquid. Ac- cordingly, the rotational term appears only in the gas partition function, and a six-degree Einstein oscillator term appears in the liquid. The internal rotation of the nitro group in the nitromethane molecule requires discussion. According to Pitzer,6 the potential barrier for rotation of the nitro group is very small (1.1 kcal/mole); consequently, a free rotation of the nitro group is assumed in both the solidlike and gaslike states in the liquid. The earlier models7 of pentaborane were assumed to have a hydrocarbon-like structure of low symmetry. However, several experiments8 with electron dif- fraction, X-ray diffraction, and Raman spectroscopy revealed an unusual pyra- midal structure of C4, symmetry. Considering the above facts, and using standard expressions for f80lid, fgas, and Eyring's expression for fdeg, and applying Stirling's approximation for the fac- torial term in equation (1), the partition functions for pentaborane and nitro- methane take the following forms. 387 Downloaded by guest on September 29, 2021 388 CHEMISTRY: SCHMIDT, JHON, AND EYRING PROC. N. A. S. g E/R T 36 fB Hg = {(1 eaIT e6hi_1 1 - 1/kcT {1 + n(x - 1) exp X [-(aE8/nx(-1 )RT) }N]} . {(27rmkT) / eV 87r2(8r3ABC) /2(kT)h/2 h3 N 4h3 36 N[(V-V.)/V] II (la) i=1 1 ee-hv/kT} , and eE.IR 14 1 (87r3IkT)'12 fCH3NO2 = i(1 -e-/T)6 ¶= 1 - ehvi/kT h { + ( ) p X [-aE8/n(x - l)RT]O} . (27rmkT) 3/ eV 87r2(87r3ABC) 1/2(kT) '/2 )~~~ h3 N h3 (87r31kT)l/2 14 N[(V-V,)/VI h i-lII 1 e-h/T . (lb) Here, x = (V/V,); E8, 0, and V, are the heat of sublimation of the solidlike structure, the Einstein characteristic temperature, and the molar volume of the solidlike structure at the melting point; a is a dimensionless proportionality constant; A, B, and C are the principal moments of inertia of the molecule; I is the moment of inertia of the NO2 group; v is the ith mode of the internal molecular vibration frequency; m is molecular mass; N is Avogadro's number; and n is equal to the number of nearest neighbors around a molecule at the melting point. This is given by n = 12(VS/V.) for a close-packed structure. The other symbols have their usual significance. The parametric values a, E8, 0, and Vs are evaluated in the manner described in the appendix to a recent article (J. Chem. Phys., 44, 1465 (1966)). These values and the molecular constants from reliable reference sources being known, all thermodynamic properties plus the surface tension are calculated using the Burroughs B-5500 computer. The input data used are as follows: For liquid B5H9,4 mol wt = 63.172: A = 1.18344 X 10-38 gm cm2, B = 1.18344 X 10-38 gm cm2, C = 1.169172 X 10-38 gm cm2; the vibrational energies in units of cm-' are: 2600, 2600, 1844, 1413, 1126, 985, 799, 1450, 500, 1870, 1387, 900, 782, 2600, 1500, 1100, 738, 470, 2598(2), 1802(2), 1621(2), 1449(2), 1034(2), 822(2), 700(2), 605(2), 568(2). The numbers in parentheses represent the de- generacies. The parametric values obtained are V, = 90.86 cc, 0 = 40.016, a = 0.002206, and E. = 8896 cal. For liquid CH3NO2,9 mol wt = 61.04; A = 67.2 X 10-40 gm cm2, B = 76 X 10-40 gm cm2, C = 137.9 X 10-40 gm cm2, I = 4.86 X 10-40 gm cm2; the vibra- tional energies in units of cm-' are: 476, 599, 647, 921, 1097, 1153, 1384, 1413, 1449, 1488, 1582, 2965, 3048, 3048. The parametric values obtained are V. = 52.057 cc, 0 = 42.96, a = 0.0000342, and E. = 10,703 cal. Results.-(a) Volume and pressure: The Helmholtz free energy A is given by A = -kT lnf. If A is plotted as a function of V for a constant T and a common Downloaded by guest on September 29, 2021 VOL. 60, 1968 CHEMISTRY: SCHMIDT, JHON, AND FYRING 389 TABLE 1. Calculated and observed molar volumes. Temp. Vcalc. Vobs.* (OK) (cc/mole) (cc/mole) A% B5H, 226.41 (mp) 92.63 92.63 0.00 237.00 93.50 93.87 -0.39 247.00 94.32 94.99 -0.70 261.00 95.53 96.74 -1.25 273.00 96.63 98.09 -1.48 292.3 98.57 100.59 -2.00 313.0 100.91 335.00 (bp) 103.75 CH3NO- 244.73 (mp) 52.18 52.18 0.00 275.11 53.19 297.12 54.02 298.0 54.05 53.88 0.31 323.0 55.11 347.46 56.28 362.44 57.07 374.44 (bp) 57.74 59.6 3.12 * For B5H9 see ref. 10, and for CH3NO2, see refs. 11 and 13. tangent is drawn through the points corresponding to the liquid and vapor phases, then the vapor pressure is given by the slope of the common tangent, and the abscissas of the two points of the common tangent indicate the molar volumes of the liquid and vapor. The results obtained are shown in Table 1 and Figure 1 and are compared with observed values. Good agreement was obtained for both cases. (b) Entropy of the liquid and entropy of vaporization: The entropy is calcu- lated from equation (1) by the relation - (aT ( (kT nf)). (2) - Theamlwi o o Extpurmol for 8Ns AprExpmneracH0,02for 1.0- a. 0.4- FIG. I.-Vapor pressure o2- versus temperature. The experimental data (0, A) are obtained from refer- QO- ences 12 and 14, respec- 225 250 275 300 325 350 375 tively. T(OK) Downloaded by guest on September 29, 2021 390 CHEMISTRY: SCHMIDT, JHON, AND EYRING PROC. N. A. S. TABLE 2. Calculated values of the entropy of the liquid and entropy of vaporization. B5H9 CH3NO2 Temp. SL AS, Temp. SL AS, (OK) (e.u.) (exu.) (OK) (e-u.) (e.u.) 226.41 (mp) 35.76 36.11 244.73 (mp) 37.17 40.68 237.00 37.00 34.14 275.11 41.57 35.10 247.00 38.15 32.46 297.12 41.65 31.76 261.00 39.74 30.34 298.0 43.61 31.63 273.00 41.11 28.70 323.0 43.47 28.37 292.3 43.30 26.28 347.46 45.47 25.60 313.0 45.67 24.17 362.44 46.58 24.06 335.0 (bp) 48.19 22.13 374.44 (bp) 47.46 22.91 TABLE 3. Calculated values of heat cappacity, compressibility, and coefficient of thermal expansion. Cv Cp Temp. a X 103 j6 X 10' (cal/gm (cal/gm (OK) (deg-) (atm -1) mole deg) mole deg) -B5H9 226.41 (mp) 0.9454 6.293 19.85 27.07 237.00 0.8694 6.277 20.72 27.18 247.00 0.8841 6.834 21.55 28.00 261.00 0.9325 7.898 22.72 29.37 273.00 0.9844 9.001 23.73 30.61 292.3 1.0799 11.154 25.34 32.64 313.0 1.1972 14.089 27.05 34.83 335.0 (bp) 1.3407 18.151 28.82 37.15 -CEsNO-- 244.73 (mp) 6.0967 1.990 16.02 21.80 275.11 6.7107 2.610 16.78 22.90 297.12 7.336 3.223 17.34 23.83 298.0 7.363 3.250 17.37 23.87 323.0 8.151 4.121 18.01 24.96 347.46 9.012 5.194 18.62 26.03 362.44 9.592 5.989 19.00 26.69 374.44 (bp) 10.090 6.717 19.29 27.22 The entropy of vaporization AS, is equal to S, - St, where SO and St are the en- tropies of the vapor and liquid, respectively.
Recommended publications
  • Guide for the Use of the International System of Units (SI)
    Guide for the Use of the International System of Units (SI) m kg s cd SI mol K A NIST Special Publication 811 2008 Edition Ambler Thompson and Barry N. Taylor NIST Special Publication 811 2008 Edition Guide for the Use of the International System of Units (SI) Ambler Thompson Technology Services and Barry N. Taylor Physics Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 (Supersedes NIST Special Publication 811, 1995 Edition, April 1995) March 2008 U.S. Department of Commerce Carlos M. Gutierrez, Secretary National Institute of Standards and Technology James M. Turner, Acting Director National Institute of Standards and Technology Special Publication 811, 2008 Edition (Supersedes NIST Special Publication 811, April 1995 Edition) Natl. Inst. Stand. Technol. Spec. Publ. 811, 2008 Ed., 85 pages (March 2008; 2nd printing November 2008) CODEN: NSPUE3 Note on 2nd printing: This 2nd printing dated November 2008 of NIST SP811 corrects a number of minor typographical errors present in the 1st printing dated March 2008. Guide for the Use of the International System of Units (SI) Preface The International System of Units, universally abbreviated SI (from the French Le Système International d’Unités), is the modern metric system of measurement. Long the dominant measurement system used in science, the SI is becoming the dominant measurement system used in international commerce. The Omnibus Trade and Competitiveness Act of August 1988 [Public Law (PL) 100-418] changed the name of the National Bureau of Standards (NBS) to the National Institute of Standards and Technology (NIST) and gave to NIST the added task of helping U.S.
    [Show full text]
  • Ideal Gasses Is Known As the Ideal Gas Law
    ESCI 341 – Atmospheric Thermodynamics Lesson 4 –Ideal Gases References: An Introduction to Atmospheric Thermodynamics, Tsonis Introduction to Theoretical Meteorology, Hess Physical Chemistry (4th edition), Levine Thermodynamics and an Introduction to Thermostatistics, Callen IDEAL GASES An ideal gas is a gas with the following properties: There are no intermolecular forces, except during collisions. All collisions are elastic. The individual gas molecules have no volume (they behave like point masses). The equation of state for ideal gasses is known as the ideal gas law. The ideal gas law was discovered empirically, but can also be derived theoretically. The form we are most familiar with, pV nRT . Ideal Gas Law (1) R has a value of 8.3145 J-mol1-K1, and n is the number of moles (not molecules). A true ideal gas would be monatomic, meaning each molecule is comprised of a single atom. Real gasses in the atmosphere, such as O2 and N2, are diatomic, and some gasses such as CO2 and O3 are triatomic. Real atmospheric gasses have rotational and vibrational kinetic energy, in addition to translational kinetic energy. Even though the gasses that make up the atmosphere aren’t monatomic, they still closely obey the ideal gas law at the pressures and temperatures encountered in the atmosphere, so we can still use the ideal gas law. FORM OF IDEAL GAS LAW MOST USED BY METEOROLOGISTS In meteorology we use a modified form of the ideal gas law. We first divide (1) by volume to get n p RT . V we then multiply the RHS top and bottom by the molecular weight of the gas, M, to get Mn R p T .
    [Show full text]
  • Neighbor List Collision-Driven Molecular Dynamics Simulation for Nonspherical Hard Particles
    Neighbor List Collision-Driven Molecular Dynamics Simulation for Nonspherical Hard Particles. I. Algorithmic Details Aleksandar Donev,1, 2 Salvatore Torquato,1, 2, 3, ∗ and Frank H. Stillinger3 1Program in Applied and Computational Mathematics, Princeton University, Princeton NJ 08544 2Materials Institute, Princeton University, Princeton NJ 08544 3Department of Chemistry, Princeton University, Princeton NJ 08544 Abstract In this first part of a series of two papers, we present in considerable detail a collision-driven molecular dynamics algorithm for a system of nonspherical particles, within a parallelepiped sim- ulation domain, under both periodic or hard-wall boundary conditions. The algorithm extends previous event-driven molecular dynamics algorithms for spheres, and is most efficient when ap- plied to systems of particles with relatively small aspect ratios and with small variations in size. We present a novel partial-update near-neighbor list (NNL) algorithm that is superior to previ- ous algorithms at high densities, without compromising the correctness of the algorithm. This efficiency of the algorithm is further increased for systems of very aspherical particles by using bounding sphere complexes (BSC). These techniques will be useful in any particle-based simula- tion, including Monte Carlo and time-driven molecular dynamics. Additionally, we allow for a nonvanishing rate of deformation of the boundary, which can be used to model macroscopic strain and also alleviate boundary effects for small systems. In the second part of this series of papers we specialize the algorithm to systems of ellipses and ellipsoids and present performance results for our implementation, demonstrating the practical utility of the algorithm. ∗ Electronic address: [email protected] 1 I.
    [Show full text]
  • Equipartition of Energy
    Equipartition of Energy The number of degrees of freedom can be defined as the minimum number of independent coordinates, which can specify the configuration of the system completely. (A degree of freedom of a system is a formal description of a parameter that contributes to the state of a physical system.) The position of a rigid body in space is defined by three components of translation and three components of rotation, which means that it has six degrees of freedom. The degree of freedom of a system can be viewed as the minimum number of coordinates required to specify a configuration. Applying this definition, we have: • For a single particle in a plane two coordinates define its location so it has two degrees of freedom; • A single particle in space requires three coordinates so it has three degrees of freedom; • Two particles in space have a combined six degrees of freedom; • If two particles in space are constrained to maintain a constant distance from each other, such as in the case of a diatomic molecule, then the six coordinates must satisfy a single constraint equation defined by the distance formula. This reduces the degree of freedom of the system to five, because the distance formula can be used to solve for the remaining coordinate once the other five are specified. The equipartition theorem relates the temperature of a system with its average energies. The original idea of equipartition was that, in thermal equilibrium, energy is shared equally among all of its various forms; for example, the average kinetic energy per degree of freedom in the translational motion of a molecule should equal that of its rotational motions.
    [Show full text]
  • Atkins' Physical Chemistry
    Statistical thermodynamics 2: 17 applications In this chapter we apply the concepts of statistical thermodynamics to the calculation of Fundamental relations chemically significant quantities. First, we establish the relations between thermodynamic 17.1 functions and partition functions. Next, we show that the molecular partition function can be The thermodynamic functions factorized into contributions from each mode of motion and establish the formulas for the 17.2 The molecular partition partition functions for translational, rotational, and vibrational modes of motion and the con- function tribution of electronic excitation. These contributions can be calculated from spectroscopic data. Finally, we turn to specific applications, which include the mean energies of modes of Using statistical motion, the heat capacities of substances, and residual entropies. In the final section, we thermodynamics see how to calculate the equilibrium constant of a reaction and through that calculation 17.3 Mean energies understand some of the molecular features that determine the magnitudes of equilibrium constants and their variation with temperature. 17.4 Heat capacities 17.5 Equations of state 17.6 Molecular interactions in A partition function is the bridge between thermodynamics, spectroscopy, and liquids quantum mechanics. Once it is known, a partition function can be used to calculate thermodynamic functions, heat capacities, entropies, and equilibrium constants. It 17.7 Residual entropies also sheds light on the significance of these properties. 17.8 Equilibrium constants Checklist of key ideas Fundamental relations Further reading Discussion questions In this section we see how to obtain any thermodynamic function once we know the Exercises partition function. Then we see how to calculate the molecular partition function, and Problems through that the thermodynamic functions, from spectroscopic data.
    [Show full text]
  • Kinetics and Atmospheric Chemistry
    12/9/2017 Kinetics and Atmospheric Chemistry Edward Dunlea, Jose-Luis Jimenez Atmospheric Chemistry CHEM-5151/ATOC-5151 Required reading: Finlayson-Pitts and Pitts Chapter 5 Recommended reading: Jacob, Chapter 9 Other reading: Seinfeld and Pandis 3.5 General Outline of Next 3 Lectures • Intro = General introduction – Quick review of thermodynamics • Finlayson-Pitts & Pitts, Chapter 5 A. Fundamental Principles of Gas-Phase Kinetics B. Laboratory Techniques for Determining Absolute Rate Constants for Gas-Phase Reactions C. Laboratory Techniques for Determining Relative Rate Constants for Gas-Phase Reactions D. Reactions in Solution E. Laboratory Techniques for Studying Heterogeneous Reactions F. Compilations of Kinetic Data for Atmospheric Reactions 1 12/9/2017 Kinetics and Atmospheric Chemistry • What we’re doing here… – Photochemistry already covered – We will cover gas phase kinetics and heterogeneous reactions – Introductions to a few techniques used for measuring kinetic parameters • What kind of information do we hope to get out of “atmospheric kinetics”? – Predictive ability over species emitted into atmosphere • Which reactions will actually proceed to products? • Which products will they form? • How long will emitted species remain before they react? Competition with photolysis, wash out, etc. • Pare down list of thousands of possible reactions to the ones that really matter – Aiming towards idea practical predictive abilities • Use look up tables to decide if reaction is likely to proceed and determine an “atmospheric lifetime”
    [Show full text]
  • Chemical Examples for the Fit Equations
    Chemical Examples for the Fit Equations The following are just a few of the applications for nonlinear curve fitting. A nice introduction to biological applications of nonlinear curve fitting by Dr. Harvey Motulsky is available on the Web 1. a exp(-bx) f = a e-bx Example: Chemical kinetics first order decay of a reactant. A → P gives [A] = [A] o e-kt a(1-exp(-bx)) f = a (1 - e-bx ) 1.2 1 0.8 0.6 0.4 a(1-exp(-bx)) 0.2 0 0 2 4x 6 8 10 Example: Chemical kinetics first order increase of a product. A → P gives [P] = [P] o(1 - e-kt ) a(1-exp(-b(x-c))) f = a (1 - e-b(x-c) ) Example: This form is the same function as above with an x axis offset. For first order kinetics a delay in the time of the start of the reaction of t o would give the rate law [P] = [P] o(1 - e-k(t-to) ) where t o = c in the fitting function. a(1-exp(-bx)) + c f = a (1 - e-bx ) + c Example: This form introduces a constant offset for first order type growth. For a first order chemical reaction [P] = [P] o(1 - e-kt ) + c where the offset, c, is due to miscalibration or an instrumental artifact. This form can be used interchangeably with f = A e-bx + C as given in the next function with A = -a and C = a+c. a exp(-bx) + c f = a e-bx + c 1 0.8 0.6 0.4 a exp(-bx) + c a exp(-bx) 0.2 0 0 2 4x 6 8 10 Example: Chemical kinetics first order decay of a reactant towards equilibrium.
    [Show full text]
  • Section 2 Introduction to Statistical Mechanics
    Section 2 Introduction to Statistical Mechanics 2.1 Introducing entropy 2.1.1 Boltzmann’s formula A very important thermodynamic concept is that of entropy S. Entropy is a function of state, like the internal energy. It measures the relative degree of order (as opposed to disorder) of the system when in this state. An understanding of the meaning of entropy thus requires some appreciation of the way systems can be described microscopically. This connection between thermodynamics and statistical mechanics is enshrined in the formula due to Boltzmann and Planck: S = k ln Ω where Ω is the number of microstates accessible to the system (the meaning of that phrase to be explained). We will first try to explain what a microstate is and how we count them. This leads to a statement of the second law of thermodynamics, which as we shall see has to do with maximising the entropy of an isolated system. As a thermodynamic function of state, entropy is easy to understand. Entropy changes of a system are intimately connected with heat flow into it. In an infinitesimal reversible process dQ = TdS; the heat flowing into the system is the product of the increment in entropy and the temperature. Thus while heat is not a function of state entropy is. 2.2 The spin 1/2 paramagnet as a model system Here we introduce the concepts of microstate, distribution, average distribution and the relationship to entropy using a model system. This treatment is intended to complement that of Guenault (chapter 1) which is very clear and which you must read.
    [Show full text]
  • 4 Pressure and Viscosity
    4 Pressure and Viscosity Reading: Ryden, chapter 2; Shu, chapter 4 4.1 Specific heats and the adiabatic index First law of thermodynamics (energy conservation): d = −P dV + dq =) dq = d + P dV; (28) − V ≡ ρ 1 = specific volume [ cm3 g−1] dq ≡ T ds = heat change per unit mass [ erg g−1] − s ≡ specific entropy [ erg g−1 K 1]: The specific heat at constant volume, @q −1 −1 cV ≡ [ erg g K ] (29) @T V is the amount of heat that must be added to raise temperature of 1 g of gas by 1 K. At constant volume, dq = d, and if depends only on temperature (not density), (V; T ) = (T ), then @q @ d cV ≡ = = : @T V @T V dT implying dq = cV dT + P dV: 1 If a gas has temperature T , then each degree of freedom that can be excited has energy 2 kT . (This is the equipartition theorem of classical statistical mechanics.) The pressure 1 ρ P = ρhjw~j2i = kT 3 m since 1 1 3kT h mw2i = kT =) hjw~j2i = : 2 i 2 m Therefore kT k P V = =) P dV = dT: m m Using dq = cV dT + P dV , the specific heat at constant pressure is @q dV k cP ≡ = cV + P = cV + : @T P dT m Changing the temperature at constant pressure requires more heat than at constant volume because some of the energy goes into P dV work. 19 For reasons that will soon become evident, the quantity γ ≡ cP =cV is called the adiabatic index. A monatomic gas has 3 degrees of freedom (translation), so 3 kT 3 k 5 k 5 = =) c = =) c = =) γ = : 2 m V 2 m P 2 m 3 A diatomic gas has 2 additional degrees of freedom (rotation), so cV = 5k=2m, γ = 7=5.
    [Show full text]
  • Electrons and Holes in Semiconductors
    Hu_ch01v4.fm Page 1 Thursday, February 12, 2009 10:14 AM 1 Electrons and Holes in Semiconductors CHAPTER OBJECTIVES This chapter provides the basic concepts and terminology for understanding semiconductors. Of particular importance are the concepts of energy band, the two kinds of electrical charge carriers called electrons and holes, and how the carrier concentrations can be controlled with the addition of dopants. Another group of valuable facts and tools is the Fermi distribution function and the concept of the Fermi level. The electron and hole concentrations are closely linked to the Fermi level. The materials introduced in this chapter will be used repeatedly as each new device topic is introduced in the subsequent chapters. When studying this chapter, please pay attention to (1) concepts, (2) terminology, (3) typical values for Si, and (4) all boxed equations such as Eq. (1.7.1). he title and many of the ideas of this chapter come from a pioneering book, Electrons and Holes in Semiconductors by William Shockley [1], published Tin 1950, two years after the invention of the transistor. In 1956, Shockley shared the Nobel Prize in physics for the invention of the transistor with Brattain and Bardeen (Fig. 1–1). The materials to be presented in this and the next chapter have been found over the years to be useful and necessary for gaining a deep understanding of a large variety of semiconductor devices. Mastery of the terms, concepts, and models presented here will prepare you for understanding not only the many semiconductor devices that are in existence today but also many more that will be invented in the future.
    [Show full text]
  • Chapter 7. Statistical Mechanics
    Chapter 7. Statistical Mechanics When one is faced with a condensed-phase system, usually containing many molecules, that is at or near thermal equilibrium, it is not necessary or even wise to try to describe it in terms of quantum wave functions or even classical trajectories of all of the constituent molecules. Instead, the powerful tools of statistical mechanics allow one to focus on quantities that describe the most important features of the many-molecule system. In this Chapter, you will learn about these tools and see some important examples of their application. I. Collections of Molecules at or Near Equilibrium As noted in Chapter 5, the approach one takes in studying a system composed of a very large number of molecules at or near thermal equilibrium can be quite different from how one studies systems containing a few isolated molecules. In principle, it is possible to conceive of computing the quantum energy levels and wave functions of a collection of many molecules, but doing so becomes impractical once the number of atoms in the system reaches a few thousand or if the molecules have significant intermolecular interactions. Also, as noted in Chapter 5, following the time evolution of such a large number of molecules can be “confusing” if one focuses on the short-time behavior of any single molecule (e.g., one sees “jerky” changes in its energy, momentum, and angular momentum). By examining, instead, the long-time average behavior of each molecule or, alternatively, the average properties of a significantly large number of PAGE 1 molecules, one is often better able to understand, interpret, and simulate such condensed- media systems.
    [Show full text]
  • Johnson Noise and Shot Noise: the Determination of the Boltzmann Constant, Absolute Zero Temperature and the Charge of the Electron
    Johnson Noise and Shot Noise: The Determination of the Boltzmann Constant, Absolute Zero Temperature and the Charge of the Electron MIT Department of Physics Some kinds of electrical noise that interfere with a measurement may be avoided by proper attention to experimental design such as provisions for adequate shielding or multiple grounds. Johnson noise [1] and shot noise, however, are intrinsic phenomena of all electrical circuits, and their magnitudes are related to important physical constants. The purpose of this experiment is to measure these two fundamental electrical noises. From the measurements, values of the Boltzmann constant, k, and the charge of the electron, e will be derived. PREPARATORY QUESTIONS To illustrate the dilemma faced by physicists in 1900, consider the highly successful kinetic theory of gases 1. De¯ne the following terms: Johnson noise, shot based on the atomic hypothesis and the principles of noise, RMS voltage, thermal equilibrium, temper- statistical mechanics from which one can derive the ature, Kelvin and centigrade temperature scales, equipartition theorem. The theory showed that the well- entropy, dB. measured gas constant Rg in the equation of state of a 2. What is the Nyquist theory ?? of Johnson noise? mole of a gas at low density, 3. What is the mean square of the fluctuating compo- nent of the current in a photodiode when its average P V = RgT (1) current is Iav ? is related to the number of degrees of freedom of the The goals of the present experiment are: system, 3N , by the equation 1. To measure the properties of Johnson noise in a k(3N) R = = kN (2) variety of conductors and over a substantial range g 3 of temperature and to compare the results with the Nyquist theory; where N is the number of molecules in one mole (Avo- gadro's number), and k is the Boltzmann constant de- 2.
    [Show full text]