Chap15-Part2

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Unformatted text preview: EXAM‐3: April 15th Friday 2011 (not April 11th)  Syllabus: Chapters 15, 16 and 17    Exam ‐2 Grades Online    KEY will be posted by tonight    Exam‐1 Grades curved   1  N 2O4 ( g) ⇔ 2 NO2 ( g) N 2O4 → 2 NO2 Rate = k f [ N 2O4 ] 2 NO2 → N 2O4 Rate = k r [ NO2 ]2 [ NO2 ]2 k f ⇒ = = equilibrium cons tan t [ N 2O4 ] kr € 2  •  No maAer the starBng composiBon of reactants and  products, the same raBo of concentraBons is achieved at  equilibrium.  •  For a general reacBon   the equilibrium constant expression is   where Kc is the equilibrium constant.  3 Law of Mass AcHon  The law of mass acBon is universal, applicable under any circumstance. However, for  reacBons that are complete, the result may not be very useful. Here is the mass acBon  law by using a general chemical reacBon equaBon in which reactants A and B react to  give product D and E.  where    a, b, d, e = small integers       [A], [B], [D], [E] = equilibrium concentraBons         in units of M     Kc = “the equilibrium constant”  (a)  The relaBonship between the equilibrium concentraBons and the equilibrium   constant depends only on the stoichiometry of the reacBon and not the mechanism.  (b) The equilibrium constant does depend on the temperature.  4  Law of Mass AcHon  Cato Maximilian Guldberg (1836‐1902) & Peter Waage (1833‐1900)  5  N2O4(g)  ↔ 2 NO2(g)  6  Consider the reacBon A + B ↔ C + D.  Assume that both the   forward and reverse reacBons are elementary steps and that the   value of the equilibrium constant, Kc,  is very large.  (a) Which species predominate at equilibrium, reactants or products?  (b) Which reacBon has the larger rate constant, the forward  or the reverse?  7  Consider the reacBon A + B ↔ C + D.  Assume that both the   forward and reverse reacBons are elementary steps and that the   value of the equilibrium constant, Kc,  is very large.  (a) Which species predominate at equilibrium, reactants or products?  products:  (b) Which reacBon has the larger rate constant, the forward  or the reverse?  8  Consider the reacBon A + B ↔ C + D.  Assume that both the   forward and reverse reacBons are elementary steps and that the   value of the equilibrium constant, Kc,  is very large.  (a) Which species predominate at equilibrium, reactants or products?  products:  (b) Which reacBon has the larger rate constant, the forward  or the reverse?  forward:  9  10  Law of Mass AcHon:  Gas Phase  where    a, b, d, e = small integers     PA, PB, PC, PE = equilibrium parBal pressures in units of atm     Kp = “the equilibrium constant”  11  RelaHonship Between Kp and Kc  aA + bB ⇔ dD + eE d e PP K p = D a Eb PA PB PA = [ D]d [ E ]e & Kc = [ A]a [ B]b n A RT n = ( A ) RT = [ A]RT etc . V V ([ D]RT ) d ([ E ]RT ) e [ D]d [ E ]e ( RT ) d ( RT ) e ⇒ Kp = = a b ([ A]RT ) ([ B]RT ) [ A]a [ B]b ( RT ) a ( RT ) b ⇒ K p = K c ( RT ) d + e− a− b L − atm ( R = 0.08206 ) mol − K K p = K c ( RT ) Δn 12  assume T = 500 K       **units**  13  The diagram shows an equilibrium   mixture in soluBon for the reacBon   A + X ↔ AX.    Is Kc greater than or less than 1?  14  The diagram shows an equilibrium   mixture in soluBon for the reacBon   A + X ↔ AX.    Is Kc greater than or less than 1?  15  Consider the reacBon of carbon monoxide  gas with chlorine gas at 100oC to form  phosgene gas:  CO(g) + Cl2(g) ↔ COCl2(g)  Because the numerator is much larger than  the denominator, [COCl2] >> [CO][Cl2].  If K >> 1  “the equilibrium lies to the right”  products predominate  If K << 1  “the equilibrium lies to the lej”  reactants predominate  16  (a) Write an expression for Kc for the following reacBon:  (b) Does the following reacBon lie to the right, favoring product  formaBon, or to the lej, favoring reactant formaBon?  17  (a) Write an expression for Kc for the following reacBon:  (b) Does the following reacBon lie to the right, favoring product  formaBon, or to the lej, favoring reactant formaBon?  18  (a) Write an expression for Kc for the following reacBon:  (b) Does the following reacBon lie to the right, favoring product  formaBon, or to the lej, favoring reactant formaBon?  to the lej, favoring reactant formaBon  19  (1)  The equilibrium constant of a reacBon in the reverse direcBon   is the inverse of the equilibrium constant in the forward direcBon.  20  (2) The equilibrium constant of a reacBon that has been mulBplied  by a number is the equilibrium constant raised to a power equal  to that number.  21  (3) The equilibrium constant for a net reacBon made up of two or  more steps is the product of the equilibrium constants for the single  steps.  22  Consider the equilibrium   Calculate the equilibrium constant Kc for this reacBon, given that  23  Consider the equilibrium   Calculate the equilibrium constant Kc for this reacBon, given that  24  Methanol can be produced by the following reacBon:   CO(g)  + 2 H2(g)  ↔  CH3OH(g)  An equilibrium mixture in a 2.00‐L container is found to contain 0.0406 mol  CH3OH, 0.170 mol CO, and 0.302 mol H2. Calculate Kc.  25  Methanol can be produced by the following reacBon:   CO(g)  + 2 H2(g)  ↔  CH3OH(g)  An equilibrium mixture in a 2.00‐L container is found to contain 0.0406 mol  CH3OH, 0.170 mol CO, and 0.302 mol H2. Calculate Kc.  26  Homogeneous equilibria involve reactants and products in the same phase.  Examples:  N2O4(g)  ↔ 2 NO2(g)      CO(g) + Cl2(g) ↔ COCl2(g)      CO(g)  + 2 H2(g)  ↔  CH3OH(g)  Heterogeneous equilibria involve at least one reactant or product in a diﬀerent phase.  Examples:  PbCl2(s)  ↔  Pb2+(aq)  +  2 Cl‐(aq)     H2O(l)  +  CO32‐ (aq)  ↔  OH‐(aq)  +  HCO3‐(aq)     CO2(g)  +  H2(g)  ↔  CO(g)  +  H2O(l)     SnO2(s)  +  2 CO(g)  ↔  Sn(s)  + 2 CO2(g)  27  ...
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## This note was uploaded on 09/13/2011 for the course CHEM 102 taught by Professor Todd during the Spring '08 term at UNC.

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