11CHAPTER 17aChemical EquilibriumChemical Equilibrium2Basic Concepts Reversible reactions do not go to completion.z They can occur in either directionz Symbolically, this is represented as: The reactions proceed until the composition of the reaction mixture reaches a state of minimum free energy.() () () ()←→ggggD d + C cB b + A a3Basic Concepts Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate.z It is a stage in a chemical reaction when there is no further tendency for the concentrations or the partial pressures of a reaction mixture composition to change.4Basic Concepts Chemical equilibria are dynamic equilibria.z Molecules are continually reacting, even though the overall composition of the reaction mixture does not change.z The criteria that identify chemical equilibrium are:1. The forward reaction and its reverse reaction are both taking place.2. The forward and reverse reactions are occurring at equal rates – there is no net change.5Nitrogen dioxide shown immediately after expanding2NO2(g) Æ N2O4(g)NO2colorlessThe color of the reaction mixture never become colorless.The reaction has established equilibrium.6Basic Concepts Any chemical reaction will establish equilibrium if it is carried out in a closed container.z If the reaction appears to have gone to completion, the equilibrium lies to the right.z This reaction is product favored.()()forwardreverseleft rightreactants products→←7Basic Concepts An example of a reaction that is product favored is the following. Under the proper conditions, hydrogen gas and oxygen gas react to form water vapor. After equilibrium is established, the amount of reactants remaining are negligible.z This reaction lies far to the right. z The reaction essentially proceeds to completion.22 2() ()22forwardggreverseHOHO→+←8Basic Conceptsz If the reaction appears to occur only to a slight extent, the reaction lies to the left.z This reaction is reactant favored.()()forwardreverseleft rightreactants products→←9Basic Concepts An example of a reaction that is reactant favored is the following. In a closed container at 25.0oC, the decomposition of calcium oxide to calcium and oxygen gas occurs. After equilibrium is established, the amount of products are negligible.z This reaction lies far to the left. z The reaction essentially does not occur.() () 2()22forwardssgreverseCaO Ca O→++←10Characteristics of Chemical Equilibrium An example of chemical equilibrium is the reaction between water vapor and carbon monoxide in a closed container at a high temperature. If equal moles of water vapor and carbon monoxide are placed in the closed container, the initial concentration of reactants begins to decrease. The concentration of the products increases until the reaction appears to stop.2() () 2 2() ()forwardggggreverseHO CO H CO→++←11Figure 6.2: Changes in concentration with time for the reactionThe concentration of the reactants and the products do not change.2() () 2 2() ()forwardggggreverseHO CO H CO→++←12Characteristics of Chemical Equilibrium To determine if a chemical reaction is at equilibrium, change the conditions in which the reaction is occurring.z If the reaction is at equilibrium, the composition of the reaction will change to reestablish equilibrium.13Characteristics of Chemical Equilibrium One example of a dynamic equilibrium can be shown using radioactive 131I as a tracer in a saturated PbI2solution.2*22HO2-2(s) (aq) (aq)[1] Place solid PbI in a saturated PbI solution.PbI Pb 2 I[2] Stir for a few minutes, then filter the solution.Some of the radioactive iodine will go into soluti .on+→←+149.2 Equilibrium and the Law of Mass Action Mathematician Cato Guldberg and chemist Peter Waage (from Norway) proposed the law of mass action. They determined a mathematical relationship that summarizes the composition of a reaction mixture at equilibrium.15Equilibrium and the Law of Mass Action()()32222SOSO OPKPP=Using different initial concentrations of reactant gases, they were able to determine that the quantity of the below equation was the same for every experiment.2 SO2(g) + O2(g) ⇄ 2 SO3(g)16Equilibrium and the Law of Mass ActionThe equilibrium constant was the same.Different initial compositions of reactants were allowed to establish equilibrium.17Equilibrium and the Law of Mass Action The constant “K” is characteristic of the composition of a reaction mixture at equilibrium at a given temperature.z It is called the equilibrium constanttantsreacofpressurespartialproductsofpressurespartialK =18Equilibrium and the Law of Mass Action The Law of mass action states thatz For any reaction between ideal gases, the composition of the reaction mixture can be expressed in terms of an equilibrium constant.z Each partial pressure of the reactants and products is raised to a power equal to the stoichiometric coefficient in the balanced chemical equation for the reaction.19Equilibrium and the Law of Mass Action() () () ()gg g ga A + b B c C + d D→←bBaAdDcCPPPPK =20Equilibrium and the Law of Mass Action The equilibrium constants involving reactions for substances that form ideal solutions in a liquid solvent are shown as concentrations, M.E.g. An aqueous solution of HCl would be shown as [HCl] in the equation.21The Equilibrium Constant One of the fundamental ideas of chemical equilibrium is that equilibrium can be established from either the forward or reverse direction.22The Equilibrium Constant For a simple one-step mechanism reversible reaction such as: The rates of the forward and reverse reactions can be represented as: kfand krare rate constants that have been determined experimentally. The constant gives us information concerning[][][][]ffrrRate A B which represents the forward rate.Rate k C D which represents the reverse rate.k==(g) (g) (g) (g)A B C D→++←23The Equilibrium Constant When system is at equilibrium:Ratef= Rater[][] [][][][][][]frfrSubstitute for the rate relationship to give:kAB kCDwhich rearranges toCDkkAB==24The Equilibrium Constant Because the ratio of two constants is a constant we can define a new constant as follows :[][][][]fcrckK andkCDKAB==25The Equilibrium Constant Similarly, for the general