lect4 - GG325 - GEOCHEMISTRY week 2: lecture 4 Lecture 4...

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1 GG325 L4, F2009 Lecture 4 Thermodynamic considerations Pease read chapter Ch3, Ch4 (McSween et al.) -or- Ch2, Ch3 (White) & most detailed -or- Ch3 (Brownlow) & most general ± Chemical Thermodynamics & Into and The Phase Rule ±E n t h a l p y ± Entropy ± The Gibbs Function ± G and K GG325 L4, F2009 Chemical Thermodynamics Thermodynamics is the study of the energetics of physical and chemical transformations of matter. The energetics of a " system " are described with physical (e.g., temperature = T, pressure = P) and chemical (e.g., composition = X 1 , X 2 , X 3 ,…) quantities. A system can contain more than one chemical component (e.g., H 2 O) and more than one physiochemical phase (ice, water vapor, liquid water). GG325 -- GEOCHEMISTRY week 2 : lecture 4
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2 GG325 L4, F2009 Definitions & system - a collection of matter in an identifiable place and condition. A system can contain more than one chemical component and more than one physiochemical phase . &A component is a chemical entity (e.g., H 2 O) that can be used to describe compositional variation in a system. Normally, we choose the minimum number of components (C) to describe the system: C = n ± r , where n is the number of chemical species and r is the number of reactions that can occur between these species. A component need not physically exist in a system (e.g., plagioclase can be described as a solid- solution mixture of albite and anorthite components, but neither exists in a plagioclase grain of intermediate composition, like labradorite or andesine). phase is a physical form of that chemical (ice, water vapor, liquid water, e.g., quartz, chert, cristobalite). GG325 L4, F2009 The Phase Rule: This provides a basis for thermodynamically describing a system by telling us the number of system variables (P, T, composition X 1 , X 2 , X 3 , etc.) we must define for a unique set of conditions to apply F = C - P +2 (Sometimes written as f = c - φ +2) F = degrees of freedom (we want this to be a small number) C = components (chemical constituents) P = phases (i.e., minerals) Some Examples: a). A univariant system. .. (see figure at right) H2O (l) H2O (v) (=steam) P=2 (aqueous and vapor) C=1 (water) F=1 (T or P) i.e., if we know T, then we can determine P, or vice-versa GG325 -- GEOCHEMISTRY week 2 : lecture 4
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3 GG325 L4, F2009 Some More Examples: b). A simple system. .. dissolved silica SiO 2 (s) + H 2 O (l) H 4 SiO 4 (aq) P=2 (aqueous and solid) C=2 (SiO 2 and water) F=2 (e.g., T & P) c). Another simple system. ..”salt” water NaCl (s) + H 2 O (l) Na+ (aq) + Cl- (aq) P=2 (aqueous and solid) C=3 (water, sodium ions, chloride ions) F=3 (e.g., T & P and either Na+ or Cl-) d) The forsterite-fayalite-melt system. .. Mg 2 SiO 4 + Fe2 2 SiO 4 form the solid solution mineral olivine, so they count as one phase. P = 2 (melt + olivine) C = 2 (forsterite + fayalite) F = 2 (e.g., T & P) GG325 L4, F2009 In a complex natural system, such as sea water , or basaltic melt and crystals , there are significantly more components than this (for instance, other minerals with additional chemical elements, gasses such as water and carbon dioxide, etc. .) Many trace elements are present in such fleeting quantities that
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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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lect4 - GG325 - GEOCHEMISTRY week 2: lecture 4 Lecture 4...

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