VIEWS: 5 PAGES: 12 POSTED ON: 9/14/2012
Fall 2007 Aqueous Environmental Geochemistry Brooklyn College The City University of New York Natural Systems Open and Close Natural Systems • Natural systems can be large or small • Different phases • In relative sense • Reaction rates vs. Flux rates Gibbs Phase Rule • F=C–P+2 What do C and P mean? • Example: H2O (ice) = H2O (liquid) = H2O (vapor) • F = C’-R – P + 2: What are C’ and R? Give one example. Ehthalpy (H) • Heat content at constant pressure • Heat of formation from elements • ΔHf0 for (most stable form) element is zero • ΔHr0 : Heat exchange ΔHr0 : H>0, endothermic ΔHr0 H<0, exothermic • How T & P change affect the equilibrium of a reaction? Entropy (S) • Degree of randomness or disorder T Cp ST S 0 dT 0 T • Cp: heat capacity. Energy needed to raise the temperature of 1 mole by 1 K • At 0 K, most substances S=0 • S increases as temperature increases Gibbs Free Energy (G) • At equilibrium condition: ΔGr0 = - RT ln Keq R: gas constant, 1.9872 cal/mol K T: 25 ºC, or 298.15 K • Can be transformed into: ΔGr0 = -0.59248 ln Keq = -1.3642 log Keq Direction of Reaction ΔGr = ΔGr0 + RT ln Q • Q: reaction quotient • Sign of ΔGr indicate the direction of reaction Chemical Potential (µ) • Similar to Gibbs Free Energy • µi = µi0 + RT ln(αi) - Chemical potential is for individual component - αi is activity, which is different than concentration! • At equilibrium, is the same for the same component in different phases! Solutions • In solutions (αi) = λi · Ni -λ is the rational activity coefficient - for ideal solutions, λ ≈ 1 for major components (Raoult’s Law); - for ideal solutions, λ ≠ 1 for minor components or dilute solutions (Henry’s Law) - See Fig. 1.3 for differences Solid Solutions (binary mixing) • Only equations to remember: ln λA = α0 NB2 ln λB = α0 NA2 Can be treated as exchange reaction: KAX AmA BNB Kex KEX BmB ANA Keq Dependence on T • Empirical constants • Determined in experiments • Table 1.1 (P39-45) • Equation: F log Keq A BT D log T ET C T 2 GT 1/ 2 T2 • Calculate ΔGr0 , ΔHr0 ΔCpr0 ΔSr0 • ΔHr0 controls the dependence.
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