lec13_whc - Last Time Equilibrium from initial conditions...

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Last Time • Equilibrium from initial conditions – Molar proportions – Equilibrium tables – using “x” • Changing reaction conditions – LeChatelier’s principle
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Le Chatelier’s Principle • Equilibrium adjusts to release stress • Explains changes in reactions due to – n – number of moles used – P – pressure of components – T – temperature of reaction • Changes in n, P, and T effect Q and K
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Addition of Reagents • Changes Q, and returns to equilibrium, K aA bB K c = • Increase A, more B forms, K c is constant • Increase B, more A forms, K c is constant [B] b [A] a
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Reaction Compression • Reaction responds by decreasing pressure • Increase pressure – favors fewer particles • Decrease pressure – favors more particles 2SO 2 (g) + O 2 (g) 2SO 3 (g) 2HBr(g) H 2 (g) + Br 2 (g)
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Reaction Temperature • Depends on reaction enthalpy, H H < 0, exothermic reaction – Increase T, favors reactants – Decrease T, favors products H > 0, endothermic reaction – Increase T, favors products – Decrease T, favors reactants
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aA bB Initial conc [A] [B] Change in conc. –ax +bx Equilibrium conc [A] – ax [B]+ bx Note: If V constant (V initial = V equil ) : • in solution, • gas phase reaction in container of fixed volume, • or gas phase reaction with n gas =0 Æ can just work in concentrations: Use K c to solve for x in mol/L . e.g. See text example 13 … Work with moles or concentrations?
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Equilibrium Tables with K p aA(g) bB(g) Initial P P Ai P Bi Initial moles. P Ai × V i /RT P Bi × V i /RT Change in moles. –ax +bx Equilibrium moles. P Ai × V i /RT – ax P Bi ×
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lec13_whc - Last Time Equilibrium from initial conditions...

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