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2/17/2010 Chemical equilibrium Reading: Chapter 15 (omit 15.7) As you read ask yourself … What is meant by chemical equilibrium? How does the equilibrium constant expression depend on the nature of the species (liquids, solids, gases) involved in the equilibrium? How does the magnitude of the equilibrium constant provide information about the relative amounts of reactants and products at equilibrium? How can the direction of a reaction (towards products or toward reactants) be predicted based on the starting amounts? q How can the value of the equilibrium constant be obtained from amounts of reactants and products present in equilibrium? How can the equilibrium concentrations of reactants and products be obtained from the values of the equilibrium constant? Chem 102 1 Chemical equilibrium we have studied physical equilibria con ncentration (mol L-1) chemical reactions can g go forward as well as in the reverse direction concentrations of reactants and products become constant over time 2HI J H2 + I2 H2 + I2 J 2HI HI HI time J H2, I2 Chem 102 H2, I2 time J 2 1 2/17/2010 dynamic equilibrium CO(g) + 2 H2(g) CH3OH(g) Chem 102 3 how can we describe the equilibrium? a specific ratio of concentrations is constant law of mass action: aA+bB dD+eE CO(g) + 2 H2(g) CH3OH(g) based on the balanced chemical equation Chem 102 4 2 2/17/2010 Expt. 1 CH3OH 0.08 CO 0.9 0.8 H2 Expt. 2 0.25 0.75 1.50 Expt. 3 0.6 1.4 1.8 Chem 102 5 K depends on the equilibrium conditions and the nature of the reaction concentrations can be expressed in molarity (moles/L) or in partial pressures, PCO 2 SO2(g) + O2(g) 2 SO3(g) recall: PV = nRT,, so P = (n/V) ( ) RT when substituted into KP expression: Chem 102 6 3 2/17/2010 The equilibrium constant does not have units K is related to rates of reactions and to thermodynamics in thermodynamics the equilibrium constant is defined in terms of activities activity is defined as a ratio of actual concentration or pressure to concentration or pressure of a reference state reference states: 1 M solutions, 1 atm for partial pressures activity = 1 for pure solids and pure liquids aCO = [CO]/co where a is activity and c0 is the reference value 1 M units cancel in ratios we will ignore slight numerical differences between activities and concentrations or pressure and use no units for the equilibrium constant values Chem 102 7 magnitude of equilibrium constants value of KC (or KP ) indicates how far the reaction has proceeded towards products th equilibrium the ilib i constant t t says nothing thi about b t very small very large 10-3 Chem 102 1 10+3 8 4 2/17/2010 K values for related reactions consider the reaction multiples 2 SO2(g) + O2(g) ' 2 SO3(g) KC = [SO3 ]2 [SO2 ]2 [O2 ] If you multiply the coefficients of a chemical equation by a factor Chem 102 9 reverse reactions consider the reaction 2 SO2(g) + O2(g) ' 2 SO3(g) KC = [SO3 ]2 [SO2 ]2 [O2 ] KC’ if yyou reverse the reaction,, Chem 102 10 5 2/17/2010 new reactions Given: 2 NO(g) + Br2(g) ' 2 NOBr(g) 2 NO(g) ' N2(g) + O2(g) What is Kc for KC1=2.0 KC2= 2.1 x 1030 N2(g) + O2(g) + Br2(g) ' 2 NOBr(g) ? if you add reactions to obtain an overall chemical equation, Chem 102 11 Heterogeneous equilibria (section 15.4) CaCO3(s) ' CaO(s) + CO2(g) small amount of CaCO3 large amount of CaO Chem 102 large amount of CaCO3 small amount of CaO 12 6 2/17/2010 heterogeneous equilibria pure solids and pure liquids concentration (mol/L ) of solids and liquids CaCO3(s) ' CaO(s) + CO2(g) Ni(s) + 4 CO(g) ' Ni(CO)4(g) Ag2O(s) + 2 HNO3(aq) ' 2 AgNO3(aq) + H2O(l) solvents are omitted when concentrations are low, because they are effectively a pure liquid Chem 102 13 Calculating equilibrium constants from concentrations Does the data provide equilibrium concentrations? Use initial amounts and mol relationships from balanced chemical equation to calc. equilibrium concentrations put equilibrium concentrations into the equilibrium constant expression Chem 102 14 7 2/17/2010 Example: If the equilibrium concentration of O2 in air is 8 x 10-3M and that of O3 is 9 x 10-32M, what is the equilibrium constant for the conversion of O2(g) to O3(g)? Chem 102 Example: 15 N2O4(g) ' 2 NO2(g) In an experiment 2.00 mol of N2O4(g) are placed in an empty 5.00 L flask and heated to 407 K. A dark red-brown gas appears indicating the formation of NO2. The intensity of the colour indicates that the concentration of NO2 at equilibrium is 0.525 M. What is the equilibrium constant? Chem 102 16 8 2/17/2010 Applications of equilibrium constants Predict the direction of a reaction Use value of Q defined 2 SO3(g) ' 2 SO2(g) + O2(g) Q same way as equilibrium constant expression except the concentrations are not equilibrium values Compare relative magnitude of Q and K Chem 102 17 Example: A mixture of hydrogen, iodine and hydrogen iodide, each at a concentration of 0.0020 M, was introduced into a container heated to 783 K. At this temperature, KP = 46 for the reaction H2(g) + I2(g) ' 2 HI(g) predict whether or not more HI has a tendency p y to form. Chem 102 18 9 2/17/2010 Calculating equilibrium concentrations: write the balanced chemical equation use the equation to make a table of initial concentrations calculate Q, compare to K and predict direction of reaction represent the change in one concentration as x and use the mol relationships to define the changes in all other species in terms of x sum the initial concentration and the change represented by values of x to get expressions for the equilibrium concentrations substitute the equilibrium concentration expressions into the equation for K and solve for x use the value of x to calculate the equilibrium concentrations Chem 102 19 Example In an experiment H2 and CO2 are placed in a flask so that their concentrations are both 0.050 M. The mixture is heated to 420 °C and equilibrium is achieved. What are the concentrations of water and carbon monoxide (the products) at equilibrium? H2 (g) ( ) + CO2(g) ( ) ' H2O (g) ( ) + CO(g) CO( ) Kc = 0.10 0 10 (at ( t 420 °C) Chem 102 20 10 2/17/2010 Example: At 25 °C, the reaction I2(g) + Cl2((g) ' 2 ICl(g) has an equilibrium constant KP = 81.9. Initially a reaction mixture at this temperature contains PI2 = 0.100 atm, PCl2 = 0.100 atm and PICl = 0.100 atm. Calculate the equilibrium partial pressures of I2, Cl2, and ICl. Chem 102 21 Example: What is the equilibrium concentration of the pollutant NO produced in automobile engines at high temperature if the reactants N2(g) and O2(g) are present in their normal ratio in air: 2.24 M N2 and 0.56 M O2? N2 (g) + O2(g) ' 2 NO(g) Chem 102 KC= 1.7 x 10-3 at 2300 K 22 11