tailieunhanh - Ebook Thermodynamics and chemistry: Part 2

(BQ) Part 2 book "Thermodynamics and chemistry" has contents: Mixture, electrolyte solutions, reactions and other chemical processes, equilibrium conditions in multicomponent systems, the phase rule and phase diagrams, galvanic cells. | C HAPTER 9 M IXTURES A homogeneous mixture is a phase containing more than one substance. This chapter discusses composition variables and partial molar quantities of mixtures in which no chemical reaction is occurring. The ideal mixture is defined. Chemical potentials, activity coefficients, and activities of individual substances in both ideal and nonideal mixtures are discussed. Except for the use of fugacities to determine activity coefficients in condensed phases, a discussion of phase equilibria involving mixtures will be postponed to Chap. 13. COMPOSITION VARIABLES A composition variable is an intensive property that indicates the relative amount of a particular species or substance in a phase. Species and substances We sometimes need to make a distinction between a species and a substance. A species is any entity of definite elemental composition and charge and can be described by a chemical formula, such as H2 O, H3 OC , NaCl, or NaC . A substance is a species that can be prepared in a pure state (., N2 and NaCl). Since we cannot prepare a macroscopic amount of a single kind of ion by itself, a charged species such as H3 OC or NaC is not a substance. Chap. 10 will discuss the special features of mixtures containing charged species. Mixtures in general The mole fraction of species i is defined by n def xi D P i j nj or n def yi D P i j nj () (P D1) where ni is the amount of species i and the sum is taken over all species in the mixture. The symbol xi is used for a mixture in general, and yi is used when the mixture is a gas. 222 CHAPTER 9 MIXTURES C OMPOSITION VARIABLES 223 The mass fraction, or weight fraction, of species i is defined by def wi D nM DP i i m j nj Mj () (P D1) where is the mass of species i and m is the total mass. The concentration, or molarity, of species i in a mixture is defined by def ci D ni V The symbol M is often used to stand for units of mol L tion of 0:5 M is 0:5 moles per