aA +bB + … ↔ rR +sS +.. K c = [R] r [S] s [A] a [B] b K > 1 products favored K < 1 reactants favored In heterogeneous equilibria only use gases & aqueous! Omitted solids; pure liquids; water (in aqueous solutions) because their [ ]’s do not change. 2 and for gases: Kp = (P R ) r (P S ) s (P A ) a (P B ) b

2 X (g) + Y (g)   3 Z (g) The reaction mixture represented above is at equilibrium at 298 K, and the molar concentrations are [X] = 2.0 M, [Y] = 0.5 M, and [Z] = 4.0 M. What is the value of the equilibrium constant for the reaction at 298 K? (A) 0.50 (D) 16 (B) 2.0(E) 32 (C) 4.0 3

The diagram represents a mixture of NO 2(g) and N 2 O 4(g) in a 1.0 L container at a given temperature. The two gases are in equilibrium according to the equation 2 NO 2(g)   2 N 2 O 4(g). Which of the following must be true about the value of the equilibrium constant for the reaction at this temperature? (A)K = 0(E) There is not (B)0 < K < 1enough information (C) K = 1to determine the (D) K > 1relative value of K. 4 = NO 2(g) = N 2 O 4(g)

K p = K c (RT) Δ n Based upon Δ n; when reactant gas moles are equal to product gas moles K p = K c 5 K p using pressures of gases; DO NOT use brackets, must use (P) notation for equilibrium expression.

Equilibrium constant for a reverse reaction = 1/K the value of the forward reaction Equilibrium constant for a doubled reaction = K 2 When using Hess’s Law: K overall = K 1 x K 2 6

Le Châtelier’s Principle: effect of changes in concentration, pressure, & temperature. Equilibrium always “shifts” away from what you add. “Stress” means too much or too little: chemical, heat, or room. 7

H 2(g) + I 2(g)   2 HI (g) Δ H > 0 Which of the following changes to the equilibrium systems represented above will increase the quantity of HI (g) in the equilibrium mixture? I.Adding H 2(g) II.Increasing the temperature III.Decreasing the pressure (A)I only(C) I and II only (B)III only(D) II and III only (E) I,II, and III 8

If out of equilibrium: Calculate the reaction quotient (Q) similar to the way an equilibrium constant would be found. If: Q < Kforward reaction occurs to reach equilibrium Q > Kreverse reaction occurs to reach equilibrium 9

Set up problems using the ICE box Ex: H 2(g) + I 2(g)   2 HI (g) M Initial Δ equilibrium “  ” row only follows the stoichiometry of the equation.

K c general types K a acid equilibrium constants, weak acids to moderate acids; strong acids K a >>>1 K b base equilibrium constants, weak bases to moderate; strong bases K b >>>1 K w ion product constant for water; basis for pH 25˚C K sp solubility product constant, point of saturation for “insoluble” precipitation reactions K p using pressures of gases; DO NOT use brackets, must use (P) notation for equilibrium expression. 11

K sp KaKa KpKp KbKb B Looks like K p for this year?

Topic: Equilibrium 2008 #1 13

B #1 K b problem

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