1. Gas Equilibria The Jungle Begins...

2. What is Equilibrium?  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

3. Using the K eq constant Let's look at the logical consequences of the assumption that the reaction between ClNO2 and NO eventually reaches equilibrium ClNO 2 (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][ClN O]

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] [ClNO2][NO] Or in general terms Keq = [C] c [D] d [A] a [B] b [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. N 2 O 4 --> 2NO 2 at 100 C Colorleless reddish brown Then 2NO 2 -->N 2 O 4 begins to happen until the rates become the same and we have N 2 O 4 2NO 2 What is the equilibrium constant expression for this equation? Keq = (PNO2) (PN2O4) 2 (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 1.Reciprocal Rule N 2 O 4 --> 2NO 2 K original = [NO 2 ] 2 [N 2 O 4 ] For the reverse reaction 2NO 2 --> N 2 O 4 K reverse = 1/K orig

9. Rules of K continued Coefficient Rule N 2 O 4 --> 2NO 2 K original = [NO 2 ] 2 [N 2 O 4 ] If we change the coefficients to 2 and 4 (doubling the molar ratio) K new = K orig 2

10. Rules of K, concluded Multiple Equilibria K1 2NO 2 --> N 2 O 4 K2 N 2 O 4 --> N 2 + O 2 K new 2NO 2 --> N 2 + O 2 K new = K 1 K 2 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 1.Write the equation 2.ICE 3.Find K 4.Plug and Chug (check units) 5.Does this answer make sense?