lect5 - GG325 GEOCHEMISTRY week 2 lecture 5 Lecture 5...

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1 GG325 L5, F2009 Lecture 5 Chemical Thermodynamics Pease read chapter Ch3,Ch4 (McSween et al.) -or- Ch2,Ch3 (White) & most detailed -or- Ch3 (Brownlow) & most general ±M i x t u r e s ± Chemical Potential ± Non ideality in gasses and aqueous mixtures GG325 L5, F2009 Thermodynamics of Chemical Mixtures Partial Molar Quantities and Chemical Potential Many thermodynamic properties like G depend on composition, which can be expressed in a multi-component system with a partial molar value . The partial molar value expresses how that property (volume, pressure, enthalpy, entropy) depends on changes in amount of one component For example, we can define the partial molar volume of component a in phase p as: V a p = dV/dn a (at constant T, P) such that V p = Σ V a /n a i GG325 -- GEOCHEMISTRY week 2 : lecture 5
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2 GG325 L5, F2009 Thermodynamics of Chemical Mixtures Partial Molar Volume is useful for understanding the properties of mixtures. Let&s say we wanted to know how much space is occupied by 1 mole of calcite (CaCO 3 ) versus 1 mole of dolomite (CaMg[CO 3 ] 2 ) in an outcrop of limestone. O we might want to know how volume in a magma chamber might change if 1 mole of pure forsterite Mg 2 SiO 4 was turned into 1 mole of forsterite mixed with 20% fayalite (Mg 0.8 Fe 0.2 ) 2 SiO 4 -- often abbreviated as Fo 80 . The partial molar volumes of the components in these systems would give us the answer. GG325 L5, F2009 Thermodynamics of Chemical Mixtures Chemical Potential is Partial Molar Gibbs Free Energy . It is another useful property for understanding mixtures. Chemical Potential, written as & , is change in Gibbs free energy relative to change in a particular component. It is a measure of Free energy change with compositional change. For all components ±i², the finite change version of & is: Σ & i n i = G at constant T, P for one component ±a² and at n other = constant, this becomes & a n a = G or & a = G/ n a GG325 -- GEOCHEMISTRY week 2 : lecture 5
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3 GG325 L5, F2009 Thermodynamics of Chemical Mixtures Chemical Potential is useful for: 1. describing open systems , in which the number moles of at least one component is not fixed due to exchange with the surroundings. 2. studying mixtures and solutions: & provides a means for determining the G of a solution relative to the pure components it is made from. In a closed system at equilibrium, Σ & i n i = 0 Thus & i applies even when the system lacks "pure" phases (e.g., minerals with solid solution or substitution, gas phase mixtures, liquid mixtures, solutes in water, etc. .).
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This note was uploaded on 09/27/2010 for the course GG GG325 taught by Professor Ronjacobs during the Winter '10 term at Hawaii.

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lect5 - GG325 GEOCHEMISTRY week 2 lecture 5 Lecture 5...

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