Module 2 Notes copy.docx - I Chemical Equilibrium A Concept...

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I. Chemical Equilibrium A. Concept of Equilibrium 1. Most chemical equations are reversible processes (products of rx. can form reactants) 2. Equilibrium – [both species] are constant because the two processes are occurring at the same rate 3. Equilibrium is in a dynamic state (both forward and reverse rx. occur, but [prod] and [reac] does not change over time) 4. Equilibrium can be established starting with only reac, only prod, or a mixture B. Equilibrium Constant 1. Reaction Quotient ( Q c ) – fraction we construct using a balanced chemical equation 2. Changes as a reaction takes place For aA + bB cC + dD , Q c = [ C ] c [ D ] d [ A ] a [ B ] b 3. Equilibrium constant ( K c ) – equal to Q c a. Overall, its’ just (products/reactions) b. Called the equilibrium expression and known as the law of mass action c. Doesn’t change as a reaction takes place K c = [ C ] c [ D ] d [ A ] a [ B ] b = Q C 4. Three possible outcomes for A + B C with 1 mole of A and 1 mole of B a. Reaction goes to completion, mostly containing C (product) High equilibrium constant (>100 OR >1*10 2 ) Said to “lie to the right” or “favor products” b. Reaction doesn’t really occur, mostly containing A and B (reactants) Small equilibrium constant (<.01 OR <1*10 -2 ) Said to “lie to the left” or “favor reactants” c. Reaction occurs but doesn’t finish, containing relatively equal amounts of A, B, and C (products and reactants) In between small and large (.01<x<100 OR 1*10 -2 <x<1*10 2 ) 5. Heterogeneous Equilibria a. Means going from one phase to another b. Only include gaseous and aqueous species in equilibrium expressions Solids and pure liquids are not included because their concentrations are constant 6. Calculating Equilibrium Constants a. RICE Tables Reaction Initial Concentration ( M ) Change in Concentration ( M ) Equilibrium Concentration ( M ) i.e. Reaction cis -stilbene trans -stilbene Initial Concentration ( M ) 0.850 0
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Change in Concentration ( M ) x + x Equilibrium Concentration ( M ) 0.850– x x 24.0 = [ trans – stilbene ] [ cis– stilbene ] = x .850 x → x = 0.816 M C. Le Châtelier’s Principle 1. When we change the conditions of a system at equilibrium, the system shifts in the direction that minimizes the effect of the change 2. Change conditions by adding/removing products and reactants, change in system volume and pressure, or a change in temperature 3. Adding or Removing Products and Species a. When Q > K , the rx. shifts left When Q < K , the rx. shifts right When Q = K , the rx. is in equilibrium
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