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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
3 Welcome to a Looooong Discussion Rate = k[HI] 2 Rate = k[H 2 ][I 2 ]

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4 Establishing Equilibrium
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
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
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
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]
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?
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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