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Tuesday, June 7th • Take out your notebook and answer the following: What does “K” represent? 2. How does K help us make predictions about our reactions? 1. K: The equilibrium constant • Each reaction has a specific equilibrium constant (K) that determines how much product and how much reactant exist at equilibrium • Temperature dependent, but we’ll worry about that later! • No units on K! • The equilibrium constant is found by looking at the ratio of the concentrations of products to reactants • A large value of K (much greater than 1) means that at equilibrium, there are substantially more products than reactants • “Product favored” • A little value of K (much less than 1) means that at equilibrium, there are substantially more reactants than products • “Reactant favored” Equilibrium Expressions • To find K, you need an equilibrium expression for your reaction • Equilibrium expression rules: 1. 2. 3. 4. Products divided by reactants Multiply things that are on the same side of the arrow Coefficients become exponents • Make sure your equation is balanced!! Work in terms of concentration or pressure For the following reactions… a. Write the Keq expression b. Given the equilibrium concentrations, state whether each equilibrium is product-favored, reactant-favored, or fairly even ([products] [reactants]) – do this without doing calculations! 1. N2(g) + 3 H2(g) ⇌ 2 NH3(g) At equilibrium: 2. [H2] = 2.00 M [NH3]= 0.01 M [H+] = 0.001 M [F-]= 0.001 M HF(aq) ⇌ H+(aq) + F-(aq) At equilibrium: 3. [N2] = 1.50 M [HF] = 0.55 M Fe3+(aq) + SCN- (aq) ⇌ FeSCN2+(aq) At equilibrium: [Fe3+] = 0.55 M [SCN-] = 0.001 M [FeSCN2+]= 0.001 M For the following reactions… a. Write the Keq expression b. Given the equilibrium concentrations, state whether each equilibrium is product-favored, reactant-favored, or fairly even ([products] [reactants]) – do this without doing calculations! c. Calculate K 1. N2(g) + 3 H2(g) ⇌ 2 NH3(g) At equilibrium: 2. [H2] = 2.00 M [NH3]= 0.01 M [H+] = 0.001 M [F-]= 0.001 M HF(aq) ⇌ H+(aq) + F-(aq) At equilibrium: 3. [N2] = 1.50 M [HF] = 0.55 M Fe3+(aq) + SCN- (aq) ⇌ FeSCN2+(aq) At equilibrium: [Fe3+] = 0.55 M [SCN-] = 0.001 M [FeSCN2+]= 0.001 M Check in •Write a 2-3 sentence summary of what you know about equilibrium expressions •Write one question you still have about equilibrium. • It’s totally ok if it’s something that you think you’re “supposed to know” by now Messing With Equilibrium Iodine gas and hydrogen gas react to form hydrogen iodide in an equilibrium system. 1. Write a balanced chemical reaction for this equation 2. Write an equilibrium expression for this system 3. If, at equilibrium, the concentration of all three substances is 0.05 M, what is the value of K? 4. Imagine a bunch of hydrogen gas is injected into the system. 1. 2. Do you think the system is still at equilibrium? Why or why not? What do you think the system will do in response to this disruption? Le Châtelier’s Principle • Le Châtelier’s Principle: a system at equilibrium will react in response to a disruption. The responses will be in an effort to return to equilibrium • In other words, if you mess with the system, the system will try to fix itself! Examples of disruptions • N2 (g) + 3H2 (g) ⇌ 2NH3 (g) • For each of the following disruptions, predict how the system will react in response • Adding more product • More reactants will be made • Adding more reactant • More products will be made • Taking out some of the product • More products will be made • Taking out some of the reactant • More reactants will be made • Increase the pressure of the system • System wants to decrease pressure, does so by creating fewer moles of gas, so more product will be made • Decrease the pressure of the system • System wants to increase pressure, does so by creating more moles of gas, so more reactant will be made Tonight’s Homework • Le Châtelier’s Principle • Reminder: Heat and Energy test retake deadline is one week from today • Equilibrium mini-test is FRIDAY