University:
Winston-Salem Forsyth County Schools
Course:
AP Chemistry
Academic year:

2014
Views:

337

Pages:

1
Author:

Alonzo Riddle

16 • Chemical Equilibria 1. aA +bB + . . . rR +sS + . . . r s [R] [S] ⋅ ⋅ ⋅ Kc = [A]a [B]b ⋅ ⋅ ⋅ and for gases: (P ) r (P ) s Kp = R a S b (PA ) (PB ) 2. K > 1 products favored K < 1 reactants favored 3. Excluded: solids; pure liquids; water (in aqueous solutions) because their [ ]’s do not change. 10. If out of equilibrium: Calculate the reaction quotient (Q) similar to the way an equilibrium constant would be found. If: Q < K forward reaction occurs to reach equilibrium Q > K reverse reaction occurs to reach equilibrium 11. Problem solving: • Set up problems using the “magic box” (or ICE box) C = “change” or ∆. Example: A B+C A B C initial 5.0 M 0M 0M ∆ 4. Convert from Kc to Kp Kp = Kc(RT)∆n where ∆n = moles of gaseous product – moles of gaseous reactant. equilibrium 5. Typical question: Given Kc and the starting concentrations of reactants, find concentrations of products at equilibrium. • “∆” row only follows the stoichiometry of the equation. Example: Kc for acetic acid = 1.8 x 10-5. What is the equilibrium concentration of [H+] in a 0.100 M solution of the acid? 6. Equilibrium constant for a reverse reaction = 1 the value of the forward K reaction. B+C Example: A -6 K = 3.0 x 10 if [A] = 5.0M initially; find [C] at equilibrium. • 7. Equilibrium constant for a doubled reaction = K2. 8. When using Hess’s Law: Koverall = K1 x K2 9. 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. Learn when to make an approximation (needed for multiple choice questions!) 5% rule usually works when value of K is 103 smaller than value of known concentrations. If greater than 5% use the quadratic equation: (not usual on the AP exam) ax2 + bx + c = 0 x= • − b ± b 2 − 4ac 2a Another easy to solve situation is the perfect squares situation. Example: H2 + I2 2HI K = 3.5 x 102 Calculate [HI] when [H2] = [I2] = 0.10 M