Chapter Equilibrium Practice AP Chemistry Date: Hour:

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1 Chapter Equilibrium Practice AP Chemistry Name: Date: Hour: Review of equilibrium basics ( 13.1: The Equilibrium Condition): state where concentrations of all reactants and products remain constant over time rate of the forward reaction is the same as the rate of the reverse reaction occurs in all closed containers with the human eye, it looks like nothing is happening, but at the molecular level, there is frantic activity Equilibrium happens because the reactants get used up as they change to products, but then the products can collide to produce reactants Concentrations at equilibrium are determined by many factors 13.2 The Equilibrium Constant A way to describe the equilibrium condition is the law of mass action. For a balanced equation aa + bb <===> cc + dd, where the lowercase letters stand for the equation coefficients and the capital letters stand for the chemical formulas, the equilibrium constant K (or K eq ) can be calculated with the formula K = [C]c [D] d products [A] a = [B] b reactants This only works with concentrations (molarities) of the chemicals at equilibrium. Since K is a constant value, if you know the molarities, you can calculate the constant, and if you know the K, you can solve for an unknown molarity. K is just a ratio and has no units! If K > 1, then there will be more products than reactants at equilibrium If K < 1, there will be less products than reactants at equilibrium If K = 1, there will be about the same amounts of reactants and products at equilibrium Do p. 630 #21:

2 Do p. 631 #25: [ ] Do p. 631 #26: [ ] Do p. 631 # 27: [ ] Do p. 631 #28: [ ] 13.3 Equilibrium Expressions Involving Pressures and 13.4 Heterogeneous Equilibria In the equilibrium game we played with the bingo chips, there were two phases: the vapor (gas) and the solid. The vapor/gas is called a dispersed phase because its particles are dispersed (spread out) and its concentration can change. The solid is called the condensed phase because its particles are closed together (condensed), and it has a constant concentration. What are two ways the concentration of a vapor/gas be changed? A solution is also called a dispersed phase. What are two ways the concentration of a solution be changed?

3 In a homogeneous equilibrium, all of the reactants and products are in the same state (all gases or all solutions). In a heterogeneous equilibrium, at least one of the reactants or one of the products is in a different state (usually a pure liquid or pure solid). Because the concentrations (molarities) of pure solids and pure liquids do not change over time, they are not included in equilibrium constant expressions (K). Molarities are often more convenient to use with solutions. With gases, pressures are often more convenient. When using molarities, use the K formula (also called K c, meaning the equilibrium constant using concentrations). When using pressures, use the K p formula (where p stands for pressure). K = K c = [C]c [D] d [A] a [B] b K p = (P C) c (P D ) d (P A ) a (P B ) b Do p. 631 #33: Do p. 631 #34: Do p. 631 #37: [ ]

4 Do p. 631 #38: [4.07] Check for understanding: If K > 1, are there more products or more reactants at equilibrium? Justify your answer Applications of the Equilibrium Constant If we know the equilibrium constant K (or K p ), we can predict the tendency for a reaction to occur whether a given set of concentrations or pressures represents equilibrium what the equilibrium concentrations (or pressures) will be at equilibrium if we are only given the initial (non-equilibrium) concentrations (or pressures) NOTE: At equilibrium, there is ALWAYS some reactant and some product, even if it is just a small amount. The way we check to see if a set of concentrations (or pressures) represent equilibrium is to calculate what is called the reaction quotient (Q). Q has the same formula as K! Q = [C]c [D] d products [A] a = [B] b reactants So what s the difference between Q and K? K s concentrations MUST be at equilibrium. Q s concentrations do NOT have to be at equilibrium. Check for understanding: What does the word quotient in the term reaction quotient mean? There are three possible relationships between Q and K: 1) Q = K This means that the concentrations or pressures used to calculate Q were really at equilibrium. The system is at equilibrium. Concentrations remain the same. 2) Q < K If Q is less than K, that means it has LESS product concentration/pressure than it should have to be at equilibrium. The system is NOT at equilibrium, and the forward reaction will occur at a higher rate until Q = K (equilibrium), so the equilibrium will shift to the right (to make more products). 3) Q > K If Q is greater than K, that means it has MORE product concentration/pressure that it should have to be at equilibrium. The system is NOT at equilibrium, and the reverse reaction will occur at a higher rate until Q = K (equilibrium), so the equilibrium will shift to the left (to make more reactants). NOTE: Q is used to check and see if a system is at equilibrium!

5 Do p. 631 #39: Do pp #41: Do p. 632 #43: [0.080 M]

6 Do p. 632 #45 (HINT: Think BCA table!): [3.4] Do p. 632 #46 (HINT: Think BCA table, but use atm instead of moles): [ ] 13.6 Solving Equilibrium Problems A method called an ICE table (or RICE table) is used to solve equilibrium problems. R stands for reaction (the balanced equation) I stands for initial concentrations C stands for change in concentrations E stands for equilibrium concentrations (They are a LOT like BCA tables, but these are for reactions that do not go to completion.) Use this RICE table to follow along with and complete the example worked on pp : Reaction H 2 (g) + F 2 (g) <==> HF (g) Initial Change Equilibrium