Kinetics Equilibrium Dr Ron Rusay Spring 2004 Copyright

Dynamic Equilibrium “The Pennies” • Organize into groups of 3 -4. • Dr. R will assign your group a number. • In your group, select one person as: 1) Money Keeper 2) Recorder 3) Transfer Agent (Calculating agent) 4) Auditor (Can be combined with #3) • Send the Recorder to see Dr. R. for your accounting form. Be sure to record other group member names on the form. • After recording all of the names send the Money Keeper to see Dr. R. for your capital stake. • Await instructions for phase I.

Dynamic Equilibrium “The Pennies” Phase I: Phase II: There will be 8 phases. You can begin and go through them at your own pace. Give Dr. R. the accounting form when completed.

Pennies Results

Dynamic Equilibrium “The Pennies” Simulator: http: //mc 2. CChem. Berkeley. EDU/Java/equilibrium/ Class Results http: //ep. llnl. gov/msds/chem 120/equil-graph. html

Dynamic Equilibrium “The Pennies” http: //ep. llnl. gov/msds/chem 120/equil-graph. html

Chemical Equilibrium

Reaction Diagram of O 3 decomposing

Chemical Equilibrium ðMost of the reactions considered until now have had reactants react completely to form products. These reactions “went” only in one direction. [ ] ðSome reactions can react in either direction. They are “reversible”. [ ]When this occurs some amount of reactant(s) will always remain at the end of change. Weak acids and bases follow this type.

Chemical Equilibrium (Definitions) ðA chemical system where the concentrations of reactants and products remain constant over time. ðOn the molecular level, the system is dynamic: The rate of change is the same in both the forward and reverse directions.

Law of Mass Action (The Equilibrium Expression) ð For a reaction: • j. A + k. B l C + m. D ð The law of mass action is represented by the Equilibrium Expression: where K is the Equilibrium Constant. (Units for K will vary. )

Equilibrium Expression ð ð 4 NH 3(g) + 7 O 2(g) 4 NO 2(g) + 6 H 2 O(g) The following Equilibrium Expression can have the amounts uniformly expressed as units of concentration mol/L (M), or as units of pressure (atm).

Equilibrium Expressions If a reaction is re-written where the reactants become products and products-reactants, the new Equilibrium Expression is the reciprocal of the old. Knew = 1/Koriginal When the entire equation for a reaction is multiplied by a molar factor n, Knew = (Koriginal)n

Le Châtelier’s Principle ð. . . If change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.

Le Châtelier’s Principle

Changes on the System ð Concentration: The system will shift concentrations away from the added component. K remains the same.

Changes on the System (continued) The Effect of Catalysts ðA catalyst lowers the activation energy barrier for any reaction…. in both forward and reverse directions! ð A catalyst will decrease the time it takes to reach equilibrium. ð A catalyst does not effect the composition of the equilibrium mixture.