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- 1. AP Chemistry - Core Concept Cheat Sheet 20: Equilibrium Key Chemistry Terms Using ICE Charts • Reversible Reaction: Reaction that can go in both directions. • Equilibrium: When the rate of the forward and reverse of a reversible process are equal. • Dynamic equilibrium: The number of reactants and products do not change, but the reaction continues to occur in both directions. • Equilibrium constant expression: Equation showing the ratio of the concentration of products to reactants with the balanced equation coefficients as powers. • Equilibrium constant (K): The value found when equilibrium concentrations are plugged into the equilibrium constant expression. • Homogeneous equilibrium: When all species are the same state of matter. • Heterogeneous equilibrium: When there are at least 2 different states of matter present. • Reaction Quotient (Q): When concentrations at any time are plugged into the equilibrium constant expression. Used to determine if a system is at equilibrium. • Solubility Product (Ksp): Equilibrium constant for a dissolution reaction. • Dissolution reaction: The process of a solid dissolving and forming ions. • Saturated solution: A solution that is at equilibrium. • Solubility: The amount of a solid that will completely dissolve to form a saturated solution. • Le Chatelier’s Principle: A system at equilibrium will readjust to reach equilibrium again when disturbed. • Exothermic reaction: System gives off energy to the surroundings. Energy can be thought of as a product. • Endothermic reaction: System gains energy from the surroundings. Energy can be thought of as a reactant. ICE charts a technique for organizing information in an equilibrium problem • Make a table with the reactants and products across the top. • Place “ICE” down the left hand side, for Initial, Change and Equilibrium. • Fill in any given information from the problem. • Use the balanced equation’s stoichiometric ratio to determine the “change” row. Establishing Equilibrium Equilibrium is not established instantly. The forward reaction must produce products, which can then reform reactants. As the forward rate slows and the reverse rate increases, equilibrium will be established. Equilibrium Constants Writing equilibrium constant expressions • Write the concentration of the products over the concentration of the reactants. • Do not include pure solids or pure liquids—only gases and solutions. • Use the coefficients of the balanced equations as powers for each species. Finding equilibrium constant: • Plug in equilibrium concentrations into the equilibrium constant expression. Solving problems with ICE Charts: • If an equilibrium concentration is known, you can determine the “change” and find the other equilibrium concentrations to plug in and solve for K. • If you don’t know any equilibrium concentrations, write expressions for them and plug in the expressions into your “K” equation to solve for them. o If the K is very tiny (10-5 or smaller), you may approximate that the change is insignificant compared to the original value (if the original value is > 0) e.g.: 0.25 M – x ≈ 0.25 M Solubility Product Writing solubility product expressions • Same as writing K expressions. • With a dissolution reaction, the reactant is always a pure solid and is therefore never included. Reaction Quotient can be used in solubility as well: • If Q = K, it’s at equilibrium (saturated solution). • If Q > K, there are too many ions present, some will re-form solid (precipitate) and leave a saturated solution present. • If Q < K, the solution is unsaturated—more solid could dissolve. Determining solubility: • Set the initial concentration of the solid to “x” and the equilibrium concentration to “0”. Solve with an ICE chart to find “x” and that is the solubility. Le Chatelier’s Principle The system will try to un-do what you did. Change made Add reactant Remove reactant Add a product Remove a product Decrease volume Increase volume Reaction Quotient Reaction will shift towards Products Reactants Reactants Products Side with least gas particles Side with most gas particles • For endothermic reactions, think of energy (temperature) as a reactant and follow guide above. Writing Reaction Quotient Expressions: • For exothermic reactions, think of energy (temperature) as • Same as for “K” expressions a product and follow guide above. Finding reaction quotient: • Plug in concentrations at any time. Changes that do not affect equilibrium: Determining if a system is at equilibrium: • Adding/removing a pure solid or liquid. • If Q = K, it’s at equilibrium • Adding/removing a non-reactive gas. • If Q > K, A reaction will proceed to the left (remove extra • Changing the volume of a reaction with equal number of gas products and form more reactants) to reach equilibrium. particles on each side. • If Q < K, A reaction will proceed to the right (remove extra • Adding a catalyst. reactants & form more products) to reach equilibrium. How to Use This Cheat Sheet: These are the keys related this topic. Try to read through it carefully twice then rewrite it out on a blank sheet of paper. Review it again before the exams. RapidLearningCenter.com ©Rapid Learning Inc. 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