Equilibrium

Reaction Graphs Initially … A B or Reactants  Products Then A  B Ao ‘Equilibrium is established’ Then A  B Ao Bo Concentrations Time vs Finally … A  B Rates equal Note: the process is dynamic

Physical vs. Chemical Physical - between states of matter H2O(l) H2O(g) Chemical - When the rates of forward and reverse reactions are equal. N2O4(g)  2NO2 (g)

Equilibrium Constant (Kc) N2O4(g)  2NO2 (g) Equilibrium concentrations (in M/L) [NO2] [N2O4] Exp # [NO2]2 [N2O4] [NO2] [N2O4] 0.00465 0.00466 0.00460 0.00463 0.0547 0.0457 0.0475 0.0523 0.0204 0.643 0.448 0.491 0.594 0.0898 1 2 3 4 5 0.0851 0.102 0.0967 0.0880 0.227 Average = 4.63 x 10-3

Equilibrium Constant (Kc) aA + bB  cC + dD Kc = Products reactants = [C]c[D]d [A]a[B]b This works for partial pressures, too.

The meaning of K What does it mean if K is greater than 1? (Hint: How is K derived?) What does it mean if K is less than 1? Ao Bo Concentrations Time

KP bB  cC Kc = [C]c [B]b P KP =

There is a relationship between pressure and concentration. PV=nRT Rearranges to … P= n V ( ) RT Note: n/V = concentration

Relationship between Kc & KP KP = Kc(RT)Δn Δn from the coefficients of the balanced equation T(temperature) in kelvin What’s “R”? 0.08206

Reaction quotient (Qc) The quantity obtained by substituting the initial concentrations into the equilibrium constant expression. N2 + 3H2  2NH3 Example: At the start of a reaction, there are 0.249 mol, N2 0.00321, mol H2 and 0.000642 mol NH3 in a 3.50L reaction vessel at 200ºC. If the equilibrium constant for the reaction is .065, decide whether the system is at equilibrium.

Reaction quotient (Qc) Qc > Kc proceeds from right to left Qc = Kc at equilibrium Qc < Kc proceeds from left to right Products Reactants

Problems Page 644 - #10, 13, 19, 20a, 21 – 24, 27, 29, 31, 32, 35, 37, 39

Part II ICE & Le Chat

Solving Equilibrium Problems 1. Balance the equation. 2. Write the equilibrium expression. 3. List the initial concentrations. 4. Calculate Q and determine the shift to equilibrium.

Solving Equilibrium Problems 5. Define equilibrium concentrations. 6. Substitute equilibrium concentrations into equilibrium expression and solve. 7. Check calculated concentrations by calculating K.

I C E I = Initial C = Change E = Equilibrium

I C E A  B How do we calculate the concentrations of A and B in equilibrium? Kc must be given. Suppose that A is initially 0.850 mol/L and Kc =24.0 I = Initial C = Change E = Equilibrium 0.850 0 -x x (0.850-x) x [B] [A] = [x] [0.850-x] Kc =

I C E I = Initial C = Change E = Equilibrium A  B 0.850 0 -x x 0.850 0 -x x (0.850-x) x = [x] [0.850-x] Kc = 24.0 Now solve

Le Châtelier’s Principle . . . if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.