Ebook Physical chemistry (6/E): Part 2
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Ebook Physical chemistry (6/E): Part 2
CHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2l Conductivity156Electrical Conductivity of Electrolyte Solutions157SummarySo far, we have discussed only equilibrium properties of systems. Processes in systems in equilibrium are reversible and are comparatively easy to treat. This chapter and the next deal with nonequilibrium processes, which are Ebook Physical chemistry (6/E): Part 2 irreversible and hard to treat. The rate of a reversible process is infinitesimal. Irreversible processes occur at nonzero rates.The study of rate prEbook Physical chemistry (6/E): Part 2
ocesses is called kinetics or dynamics. Kinetics is one of the four branches of physical chemistry (Fig. 1.1). A system may be out of equilibrium becaCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2es are transport processes, and the branch of kinetics that studies the rates and mechanisms of transport processes IS physical kinetics. Even though neither matter nor energy IS being transported through space, a system may be out of equilibrium because certain chemical species in the system are re Ebook Physical chemistry (6/E): Part 2acting to produce other species. The branch of kinetics that studies the rates and mechanisms of chemical reactions is chemicalkinetics or reaction kiEbook Physical chemistry (6/E): Part 2
netics. Physical kinetics is discussed in Chapter 15 and chemical kinetics in Chapter 16.There are several kinds of transport processes. If temperaturCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2on is studied in Sec. 15.2. If unbalanced forces exist in the system, it is not in mechanical equilibrium and parts of the system move. The flow of fluids is the subject offluid dynamics (or fluid mechanics). Some aspects of fluid dynamics are treated in Sec. 15.3 on viscosity. If differences in con Ebook Physical chemistry (6/E): Part 2centrations of substances exist between different regions of a solution, the system IS not in material equilibrium and matter flows until the concentrEbook Physical chemistry (6/E): Part 2
ations and the chemical potentials have been equalized. This flow differs from the bulk flow that arises from pressure differences and is called diffuCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2hrough the system, producing an electric current. Electrical conduction is studied in Secs. 15.5 and 15.6.We shall see that the laws describing thermal conduction, fluid flow, diffusion, and electrical conduction all have the same form, namely, that the rate of transport is proportional to the spati Ebook Physical chemistry (6/E): Part 2al derivative (gradient) of some property.Transport properties are important in determining how fast pollutants spread in the environment (see chap. 4Ebook Physical chemistry (6/E): Part 2
of D. G. Crosby, Environmental Toxicology and Chemistry. Oxford. 1998). Biological examples of transport phenomena include the flow of blood, the difCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2s migration of charged species in an electric field are used to separate biomolecules and played a key role Hl sequencing the human genome (Sec. 15.6).15.2THERMAL CONDUCTIVITYFigure 15.1 shows a substance in contact with two heat reservoirs at different temperatures. A steady state will eventually b Ebook Physical chemistry (6/E): Part 2e reached hl which there IS a uniform temperature gradient dTidx in the substance, and the temperature between the reservoirs varies linearly with V fEbook Physical chemistry (6/E): Part 2
rom T. at the left end to 7> at the right end. (The gradient of a quantity is its rate of change with respect to a spatial coordinate.) The rate of heCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2óđ. the substance’s cross-sectional area in a plane perpendicular to the X axis. Experiment shows that dq'df is also proportional to the temperature gradient dTidx. Thus$=ế <151>dt dxwhere the proportionality constant k is the substance’s thermal conductivity and dq is the heat energy7 that in time Ebook Physical chemistry (6/E): Part 2dt crosses a plane with area 3Í and perpendicular to theV axis. The minus sign occurs because dTdx is positive but dqdt is negative (the heat flows toEbook Physical chemistry (6/E): Part 2
the left in the figure). Equation (15.1) is Fourier’s law of heat conduction.This law also holds when the temperature gradient in the substance is noCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2he laws of heat conduction, apparently suffered from a thyroid disorder and wore an overcoat in summer.)k is an intensive property whose value depends on T. p. and composition. From (15.1), the SI units of k are j K 1 m 1 s-1 = w K-1 nr1, where 1 wan (W) equals 1 J/s. Values of £ for some substances Ebook Physical chemistry (6/E): Part 2 at 25°c and 1 atm are shown in Fig. 15.2. Metals are good conductors of heat because of the electrical-conduction electrons, which move relatively frEbook Physical chemistry (6/E): Part 2
eely through the metal. Most nonmetals are poor conductors of heat. Gases are very poor conductors because of the low density of molecules. Diamond haCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2e a higher thermal conductivity than diamond, but conflicting results from various calculations and from experiments leave this question in doubt, k for an individual single-wall carbon nanotube (like many other intensive properties of nanomaterials) depends on the tube length, which complicates thi Ebook Physical chemistry (6/E): Part 2ngs: see J. R. Lukes and H. Zhong. J Hear Transfer. 129, 705(2007).]Although the system in Fig. 15.1 is not in thermodynamic equilibrium, we assume thEbook Physical chemistry (6/E): Part 2
at any tiny portion of the system can be assigned values of thermodynamic variables such as T. u. s. and p. and that all the usual thermodynamic relatCHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5Electrical Ebook Physical chemistry (6/E): Part 2holds well m most (but not all) systems of interest.*,'() K-' cm-’s '1A10 - ■ Qdumond. Type I)Cut»)I - - _ Feioltr' ■ ’ NaClU)- - fytex tlSSfti)HjCXDI0-J- cci,(0 Ebook Physical chemistry (6/E): Part 2CHAPTER15 Transport ProcessesCHAPTER OUTLINE151 Kinetics152 Thermal Conductivity15.1KINETICS153 Viscosity15.4Diffusion and Sedimentation15.5ElectricalGọi ngay
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