CHEM1212: Chapter 15
CHEM1212: Chapter 15 CHEM1212
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Brittany Ariana Borzillo
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This 3 page Class Notes was uploaded by Brittany Ariana Borzillo on Tuesday September 27, 2016. The Class Notes belongs to CHEM1212 at University of Georgia taught by Donald Wayne Suggs in Fall 2015. Since its upload, it has received 13 views. For similar materials see Freshman Chemistry II in Chemistry at University of Georgia.
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Date Created: 09/27/16
Chapter 15: Chemical Equilibrium Principle of Microscopic Reversibility The elementary steps in a reaction mechanism are reversible Reverse process o A process where it is possible to return to the starting conditions along the exact same path without altering the surroundings It is possible to reverse all chemical reactions A reaction will not be represented with a double arrow if a reverse reaction is unlikely or if no noticeable quantity of reactant is produced by the reverse reaction Equilibrium State The state of a chemical system where the rate of the forward reaction is equal to the rate of the reverse reaction o When achieved the concentration of all species in solution is constant as the forward and reverse reactions take place Equilibrium Constants Equilibrium constant expression o Relates the concentrations of reactants and products at equilibrium at a particular temperature to a numerical constant Equilibrium constant o K>>1 Concentration of products is much larger than concentration of reactants at equilibrium K(forward)>>K(reverse Product-favored reaction o K<<1 Concentration of products is much smaller than the concentration of reactants at equilibrium K(forward)<<K(reverse) Reactant-favored reaction o K=1 Significant amounts of both products and reactants are found K(forward)=K(reverse) Writing Equilibrium Constant Expressions aA + bB cC +dD k = [C] [D] [A] [B]b The product concentrations are multiplied in the numerator o Each raised to the power of its stoichiometric coefficient The reactant concentrations are multiplied in the denominator o Each raised to the power of its stoichiometric coefficient Pure solids do not appear in equilibrium constant expressions o When the pure solid is a separate phase the equilibrium concentrations of the species are considered independent of the solid Pure liquids and solvents do not appear in equilibrium constant expressions o Concentrations of liquids/solvents are usually very large compared to reacting species Gas equilibria o Concentrations can be expressed as partial pressure K pK (cT) n T = Temperature in Kelvin R is the Ideal Gas Constant n= moles product – moles reactant Manipulating Equilibrium Constant Expressions multiplying equation by constant K =(K ) n new old reverse reaction direction Knew=1/K old combining reactions Knew=K 1 K 2 Determining an Equilibrium Constant with Experimental Data concentrations are substituted into the equilibrium constant equation unknown concentration o stoichiometric relationships are used to calculate needed concentrations o concentrations are substituted into the equilibrium constant equation Calculating Equilibrium Concentrations the reaction quotient can be used to determine whether a system is at equilibrium the reaction quotient ratio can be written for any reaction aA + bB cC +dD c d Q = [C] [D] [A] [B]b comparing Q and K allows to determine whether a system is at equilibrium o Q<K Not equilibrium More products need to be produced for equilibrium Will proceed forward o Q>K Not equilibrium Products need to be consumed to reach equilibrium Will produce in the reverse o Q=K Equilibrium No further change in concentrations Addition or Removal of a Reactant or Product Le Chatelier’s Principle o If a chemical system at equilibrium is disturbed to the point that it is no longer at equilibrium, the system will respond by reacting forward or reverse so as to counteract the disturbance, resulting in a new equilibrium Change in the Volume of the System Most important to gas equilibria o Pressure increases as volume decreases Change in Temperature Only circumstance that the equilibrium constant changes System responds by shifting to the addition or loss of heat Addition of Heat o Exothermic reactions shift left o Endothermic reactions shift right Loss of Heat o Exothermic reactions shift right o Endothermic reactions shift left Can estimate new equilibrium constant with van’t Hoff equation ln(2 ) = -H (1 – 1) (k1) = R (T2– T 1
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