14 - Chemical Equilibrium

14 - Chemical Equilibrium - Chapter 14 Chemical Equilibrium...

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Chapter 14: Chemical Equilibrium 14.1Characteristics of Chemical Equilibrium chemical equilibrium – finite concentrations of reactants and products, and these concentrations remain constant equilibrium reaction always results in smaller amounts of products than theoretical yield predicts concentrations of reactants and products at equilibrium provide quantitative way of determining how successful a reaction has been product-favored reaction – products predominate over reactants reactant-favored reaction – equilibrium mixture consists mostly of reactants and little products Equilibrium is Dynamic o macroscopic scale – reaction has “stopped” o nanoscale – forward and reverse reactions occur but at the same rate so there is no net change – dynamic equilibrium Equilibrium is Independent of Direction of Approach o for a specific reaction at a specific temperature, the equilibrium state will be the same no matter what the direction of approach to equilibrium o whether you start with reactants or products, the same equilibrium state is achieved as long as same number of atoms of each type, the volume of container, and temperature remain the same Catalysts Do Not Affect Equilibrium Concentration o if a catalyst is present, the same equilibrium state will be achieved, but more quickly 14.2The Equilibrium Constant equilibrium constant – K c – quotient of equilibrium concentrations of product and reactant substances that has a constant value for a given reaction at a given temperature equilibrium constants can be used to answer: o 1) when equilibrium has been achieved, do products predominate over reactants? o 2) given initial concentrations of reactants and products, in which direction will the reaction go to achieve equilibrium? o 3) what concentrations of reactants and products are present in equilibrium? equilibrium constants are more valuable for fast reactions because you can determine composition soon after reactants are mixed; if not at equilibrium, must use kinetics to predict composition of reaction mixture when concentrations are large enough, value of K c expressed in terms of concentrations does not remain constant, even at same temperature because of noncovalent interactions among molecules (especially ions), that cause them to behave different as their concentrations become larger; true equilibrium constants must be expressed in terms of corrected concentrations called activities Writing Equilibrium Constant Expressions o aA + bB cC + dD becomes K c = [C] c [D] d / [A] a [B] b o concentrations of products over reactants o concentration is raised to power of its stoichiometric coefficient in balanced equation o only concentrations that appear in equilibrium constant expression are gases and solutes in dilute
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14 - Chemical Equilibrium - Chapter 14 Chemical Equilibrium...

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