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Gas Equilibria

Gas Equilibria. The Jungle Begins. Reaction is reversible Both products and reactants are present Forward and reverse reactions take place at the same rate BUT the partial pressures must be specified. For the reaction 2A = B. What is Equilibrium?. Using the K eq constant.

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Gas Equilibria

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  1. Gas Equilibria The Jungle Begins. . .

  2. Reaction is reversible Both products and reactants are present Forward and reverse reactions take place at the same rate BUT the partial pressures must be specified For the reaction 2A = B What is Equilibrium?

  3. Using the Keq constant Let's look at the logical consequences of the assumption that the reaction between ClNO2 and NO eventually reaches equilibrium ClNO2(g)+NO(g) --> NO2(g)+ClNO(g) • At equilibrium: rate forward=rate reverse • Substituting the rate laws for the forward and reverse reactions into this equality gives the following result. • : kf(ClNO2)(NO)=kr(NO2)(ClNO) • But this equation is only valid when the system is at equilibrium, as follows. • At equilibrium: kf[ClNO2][NO]=kr[NO2][ClNO]

  4. We should replace the (ClNO2), (NO), (NO2), and (ClNO) terms with symbols that indicate that the reaction is at equilibrium. • By convention, we use square brackets for this purpose. The equation describing the balance between the forward and reverse reactions when the system is at equilibrium should therefore be written • kf[ClNO2][NO]=kr[NO2][ClNO] • Since kf and kr are constants, the ratio of kf divided by kr must also be a constant. This ratio is the equilibrium constant for the reaction, Kc. The ratio of the concentrations of the reactants and products is known as the equilibrium constant expression.

  5. K expression Keq = [ClNO][NO2] [ClNO2][NO] Or in general terms Keq = [C]c[D]d [A]a[B]b For aA + bB --> cC + dD

  6. Or, the expression could be written for gases, in which case we will use pressures rather than moles/liter or molarity. N2O4 --> 2NO2 at 100 C Colorleless reddish brown Then 2NO2 -->N2O4 begins to happen until the rates become the same and we have N2O4 <--> 2NO2 What is the equilibrium constant expression for this equation? Keq = (PNO2) (PN2O4)2

  7. Gaseous Equilibrium Expression • So we can also write the general equilibrium constant as • K = (PC)c(PD)d (PA)a(PB)b Note that the expression will change depending on the form of the equation.

  8. Rules of K • Reciprocal Rule N2O4 --> 2NO2 Koriginal = [NO2]2 [N2O4] For the reverse reaction 2NO2 -->N2O4 Kreverse = 1/Korig

  9. Rules of K continued Coefficient Rule N2O4 --> 2NO2 Koriginal = [NO2]2 [N2O4] If we change the coefficients to 2 and 4 (doubling the molar ratio) Knew = Korig2

  10. Rules of K, concluded Multiple Equilibria K1 2NO2 --> N2O4 K2 N2O4 -->N2 +O2 Knew 2NO2 -->N2 +O2 Knew = K1K2 Want to know why? Read the text for a great explanation!

  11. ICE (ice, ice baby) I initial concentration C change in concentration E final or ending concentration Remember this--you will use it over and over and over and over. . . When? With experimental Data!

  12. Protocol for Gas Equilibrium Problems • Write the equation • ICE • Find K • Plug and Chug (check units) • Does this answer make sense?

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