2Chemical Equilibrium Reversible Reactions: A chemical reaction in which the productscan react to re-form the reactantsChemical Equilibrium:When the rate of the forward reactionequals the rate of the reverse reactionand the concentration of products andreactants remains unchanged2HgO(s) 2Hg(l) + O2(g)Arrows going both directions ( ) indicates equilibrium in a chemical equation
32NO2(g) 2NO(g) + O2(g) Remember this? Why was it so important to measure reaction rate at the start of the reaction(method of initial rates?)
4jA + kB lC + mD Law of Mass Action For the reaction: Where K is the equilibrium constant, and is unitless
5Product Favored Equilibrium Large values for K signify the reaction is “product favored” k > 1When equilibrium is achieved, most reactant has been converted to product
6Reactant Favored Equilibrium Small values for K signify the reaction is “reactant favored” k < 1When equilibrium is achieved, very little reactant has been converted to product
7Writing an Equilibrium Expression Write the equilibrium expression for the reaction:2NO2(g) 2NO(g) + O2(g)K = ???
8Conclusions about Equilibrium Expressions The equilibrium expression for a reaction is the reciprocal for a reaction written in reverse2NO2(g) 2NO(g) + O2(g)2NO(g) + O2(g) 2NO2(g)
9Conclusions about Equilibrium Expressions When the balanced equation for a reaction is multiplied by a factor n, the equilibrium expression for the new reaction is the original expression, raised to the nth power.2NO2(g) 2NO(g) + O2(g)NO2(g) NO(g) + ½O2(g)
10Equilibrium Expressions Involving Pressure For the gas phase reaction:3H2(g) + N2(g) 2NH3(g)K and Kp are NOT interchangeable!
11Heterogeneous Equilibria The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids presentWrite the equilibrium expression for the reaction:PCl5(s) PCl3(l) + Cl2(g)PuresolidPureliquid
12CAN you…..Write an Keq expression?Tell how Keq changes as the stoichiometry changes?Convert Keq in terms of Kc vs Kp?Explain what K means for the rxn?
13jA + kB lC + mD The Reaction Quotient When the reaction is NOT at equilibrium for a given time, the reaction quotient, Q takes the place of K, the equilibrium constant, in the law of mass action.jA + kB lC + mDUseful in determining what will happen underspecial conditions.
14Significance of the Reaction Quotient If Q = K, the system is at equilibriumIf Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achievedIf Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved
15Solving for Equilibrium Concentration Consider this reaction at some temperature:H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0Assume you start with 8 molecules of H2O and 6 molecules of CO. How many molecules of H2O, CO, H2, and CO2 are present at equilibrium?Here, we learn about “RICE” – the most important problem solving technique in the second semester. You will use it for the next 4 chapters!
16Solving for Equilibrium Concentration H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0Step #1: We write the law of mass action for the reaction:
17Solving for Equilibrium Concentration Step #2: We “RICE” the problem, beginning with the R for balanced reactionReactionH2O(g) + CO(g) H2(g) + CO2(g)Initial:Change:Equilibrium:86-x-x+x+x8-x6-xxx
18Solving for Equilibrium Concentration Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for xH2O(g) + CO(g) H2(g) + CO2(g)Equilibrium:8-x6-xx
19Solving for Equilibrium Concentration Step #4: Substitute x into our equilibrium concentrations to find the actual concentrationsH2O(g) + CO(g) H2(g) + CO2(g)Equilibrium:8-x6-xxEquilibrium:8-4=46-4=24
20LeChatelier’s Principle When a system at equilibrium is placedunder stress, the system will undergo achange in such a way as to relieve thatstress.Translated: The system undergoes a temporary shift in order to restore equilibrium.
21LeChatelier Example #1A closed container of ice and water is at equilibrium. Then, the temperature is raised.Ice + Energy WaterThe system temporarily shifts to the _______ to restore equilibrium.right
22LeChatelier Example #2A closed container of N2O4 and NO2 is at equilibrium. NO2 is added to the container.N2O4 (g) + Energy 2 NO2 (g)The system temporarily shifts to the _______ to restore equilibrium.left
23LeChatelier Example #3A closed container of water and its vapor is at equilibrium. Vapor is removed from the system.water + Energy vaporThe system temporarily shifts to the _______ to restore equilibrium.right
24LeChatelier Example #4A closed container of N2O4 and NO2 is at equilibrium. The pressure is increased.N2O4 (g) + Energy 2 NO2 (g)The system temporarily shifts to the _______ to restore equilibrium, because there are fewer moles of gas on that side of the equation.left