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Entropy and Free Energy Thermodynamics: the science of energy transfer – Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored
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What is thermodynamics Thermodynamics tells us NOTHING about the rate of reaction. The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.)
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Entropy Entropy, S: Measure of dispersal or disorder. Can be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no disorder and S = 0. If temperature change is very small, can calculate entropy change, S = q/T (heat absorbed / T at which change occurs) Sum of S can give total entropy at any desired temperature.
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Entropy Examples (positive S) Boiling water Melting ice Preparing solutions CaCO 3 (s) CaO (s) + CO 2 (g)
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Entropy Examples (negative S) Molecules of gas collecting Liquid converting to solid at room temp 2 CO (g) + O 2 (g) 2 CO 2 (g) Ag + (aq) + Cl - (aq) AgCl (s)
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Entropy Generalizations S gas > S liquid > S solid Entropies of more complex molecules are larger than those of simpler molecules (S propane > S ethane >S methane ) Entropies of ionic solids are higher when attraction between ions are weaker. Entropy usually increases when a pure liquid or solid dissolves in a solvent. Entropy increases when a dissolved gas escapes from a solution
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Laws of Thermodynamics First law: Total energy of the universe is a constant. Second law: Total entropy of the universe is always increasing. Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.
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Calculating S o system S o system = S o (products) - S o (reactants) Can also relate surroundings to the system! S o surroundings = q surroundings / T = - H system / T
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Calculating S o universe S o universe = S o surroundings + S o system S o universe = - H system / T + S o system Can use 2 nd law to predict whether a reaction is product-favored or reactant-favored! The higher the temperature, the less important the enthalpy term is!
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Gibbs Free Energy G system = - T S universe = H system - T S system G o system = H o system - T S o system G o rxn = H o rxn - T S o rxn
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Gibbs Free Energy G o system or G o rxn If negative, then product- favored. If positive, then reactant-favored. G o reaction = G f o (products) - G f o (reactants)
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Gibbs Free Energy G is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction! Know the meaning of enthalpy-driven vs. entropy-driven reactions. Gs are additive!
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Thermodynamics and K If not at standard conditions, G = G o + RT ln Q (Equilibrium is characterized by the inability to do work.) At equilibrium, Q = K and G = O Therefore, substituting into previous equation gives 0 = G o + RT ln K and G o = - RT ln K(can use Kp or Kc)
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First law: Total energy of the universe is a constant. Second law: Total entropy of the universe is always increasing. Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0. Entropy : time’s arrow Absolutely MUST learn table in Chapter highlights! Thermodynamics and Time
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Thermodynamics and K Understand relationship between G o, K, and product-favored reactions! G o 1product-favored G o =0 K=1 G o >0 K<1reactant-favored
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