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ChemicalEquilibrium

ChemicalEquilibrium - Chemical Equilibrium A Study of...

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1 Chemical Equilibrium A Study of Balance?
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Some Interesting Observations Let’s make a solution by mixing 0.03 M FeCl 3 and 0.10 M KSCN in water… Now let’s play a little! 2
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3 Welcome to a Looooong Discussion Rate = k[HI] 2 Rate = k[H 2 ][I 2 ]
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4 Establishing Equilibrium
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5 2 HI H 2 + I 2 For the forward reaction: Rate f = k f [HI] 2 The rate changes through the kinetic region because the ______________ is changing. This continues until _____________________ For the reverse reaction: Rate r = k r [H 2 ][I 2 ] Same things are true as for forward reaction.
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6 Rate Ratios? In the kinetic region we construct a ratio; ] ][ [ ] [ 2 2 2 I H k HI k rate rate r f r f = OR Which rearranges to: = Q [ H 2 ][ I 2 ] [ HI ] 2 = k f k r i rate r rate f
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7 Continuing… What happens when the reaction reaches equilibrium? 1 = r f rate rate So that [ H 2 ][ I 2 ] [ HI ] 2 = k f k r = K
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For the gas-phase reaction A B the forward reaction rate constant is 3.0 × 10 −4 s −1 and the reverse reaction rate constant is 1.5 × 10 −2 s −1 . What is the value of the equilibrium constant, K eq ? A. 0.02 B. 50 C. 0.0004 D. 2500
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9 Please Notice Q applies in the kinetic region of the reaction While K applies in the equilibrium region But they are both the same ratio And Q will always become K
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10 Chemical Equilibrium When a system is not at equilibrium, it will ALWAYS proceed toward equilibrium. The equilibrium constant value will be the same no matter what the starting conditions. e.g., phase changes e.g., reactions
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11 Equilibrium Constant e.g., 2 NO 2 (g) = N 2 O 4 (g) K expression? describes conditions at equilibrium K is a function of  T measured, tabulated
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12 Writing the Q or K Expression We built this first example with a simple reaction What about a reaction like 2A + B = 3C + 2D ? This could be written as A + A + B = C + C + C + D + D so Q or K would be = {[C][C][C][D][D]}/ {[A][A][B]} or K = [C] 3 [D] 2 /[A] 2 [B]
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13 Examples Write the K expression for the reaction 2H 2 + O 2 = 2H 2 O Now try C 2 H 4 + HCl = C 2 H 5 Cl Next, Hg(NO 3 ) 2 + 2KI = HgI 2 + 2KNO 3 and finally, 2MnO 4 - (aq) + 10Br - (aq) + 16H + (aq) = 2Mn 2+ (aq) + 5Br 2 (aq) + 8H 2 O(l)
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14 Variant Equilibrium Constant When all compounds are in solution... the K expression we’ve used is fine and is referred to as K c where c means concentration. Note that if all compounds are in solution, they are in the same phase. What if all compounds are in the gas phase?
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15 Variant for the Gas Phase We can still construct the K expression However, M = (n/V), one of the terms from the Gas Law Hence, for each component we can substitute (n/V) = P/RT in the K expression Simplifying this will leave a K expression in terms of pressure, not concentration This K is K p , and it differs from K c by some constant factor of RT
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16 Thus… For the reaction N 2 O 4 (g) 2 NO 2 (g) M (N2O4) = P (N2O4) /(RT) and M (NO2) = P (NO2) /(RT) So that K = [NO 2 ] 2 /[N 2 O 4 ] = K c = (P (NO2) .(RT)
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