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31,32, & 33- Equilibria of Aqueous solutions and phase diagrams.docx

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McGill University
CHEM 204
Christopher Barrett

CHEM 204 03/29/201304/03/2013 Lecture 31,32, & 33 Martin M. PHGY Tutor Equilibrium in Aqueous Solution and Phase Diagrams Recall: We use Gibbs energy to predict equilibria of chemical reactions. A reaction mixture will keep adjusting until Gibbs is minimized. A negative G is always spontaneous, and a positive G is never spontaneous. Δ R° = Σ n ΔG° (products) — Σ n ΔG° (reactants) By calculating the total ΔfG for a system of reactants and products at all possible combinations, we get this prediction curve. 1. Spontaneous Reactions Note that the equilibrium is never 100%. The lowest Gibbs energy occurs slightly to the left, so there is always a tendency to mix with some of the reactant 2. Non spontaneous reactions: The reaction shouldn't go forward, but it does a little bit because of the entropy of mixing makes the lowest Gibbs energy slightly to right. *If you remove the products as they are made, you can drive the reaction forward. Our body does this for non-spontaneous reactions. 3. Perfect equilibrium: The minimum of the Total Gibbs lies in the middle between the pure reactants and pure products. This is the case for many biochemical equilibria. At equilibrium, the reaction mixture composition is dominated by the entropy of mixing CHEM 204 03/29/201304/03/2013 Lecture 31,32, & 33 Martin M. PHGY Tutor The direction of a reaction is simply the way it will travel to find the minimum Gibbs. Equilibrium= when composition Δ G R 0 Left of minimum: forward reaction spontaneous Right of minimum: reverse reaction spontaneous Q: How do molar Gibbs vary with temperature? A: Gibbs is proportional to entropy at a given T. Therefore, at any given temperature, the 3 phases will have a definite order of Gibbs.  Higher temperatures: gases have the lowest Gibbs  Colder temperatures: solids have the lowest Gibbs The "Stable phase" of a product can then be thought of as ongoing equilibrium reaction between solid↔liquid↔gas. Note: This graph looks different at every pressure, since if we increase the pressure we change the entropy of the gas phase of the material. Crossing of the lines means no phase is preferred. A phase transition occurs here. We can use thi
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