Nils Walter: Chem 260Coupled reactions can overcome an unfavorable ∆∆∆∆rG@ equilibrium:bBaAdDcCaaaaK××=OKRTGrln−=∆⇒⇒⇒⇒products will dominate in equilibriumbBaAdDcCraaaaKG×>×⇒>⇒<∆10Oexergonic:BUT: If an endergonic reaction (∆∆∆∆rG > 0) is coupled with a strongly exergonic one (∆∆∆∆rG ‘ < 0):∆∆∆∆rG+ ∆∆∆∆rG‘< 0OOO Oenzymes use ATP:bBaAdDcCraaaaKG×<×⇒<⇒>∆10O⇒⇒⇒⇒reactants will dominate in equilibriumendergonic:Nils Walter: Chem 260Sample problem 1:Estimate the composition of a solution in which G6P and F6P are in equilibrium at 25oC, and draw a graph to show how the spontaneity of the reaction varies with composition; ∆∆∆∆rG = + 1.7 kJ mol-1ONils Walter: Chem 260Sample problem 2:In an industrial process N2at 1 bar is mixed with H2at 3 bar and the two gases are allowed to reach equilibrium with the product ammonia in a reactor of constant volume. At the temperature of the reaction, K = 977. What are the equilibrium partial pressures of the three gases?Nils Walter: Chem 260Le Chatelier’s principleWhen a system at equilibrium is subjected to a disturbance, its composition adjusts so as to tend to minimize the effect of the disturbance⇒⇒⇒⇒When a system at equilibrium is heated,a) the equilibrium composition of anexothermic reaction will shift towards reactants,b) while the equilibrium composition of anendothermic reaction will shift towards productsWhy?∆∆∆∆rG = ∆∆∆∆rH - T∆∆∆∆rS O OOand neither reaction enthalpy nor entropy shall vary with T⇒⇒⇒⇒qualitatively:change in ∆∆∆∆rG = -(change in T) x ∆∆∆∆rS
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