chap 14

• Dynamic equilibrium

  A reaction system attains dynamic equilibrium when there is no further change in concentrations
• Equilibrium


  When the forward and reverse reactions are proceeding at the same rate
• Equilibrium is signified with double arrows or the equal sign
• The equilibrium mixture is independent of whether we start on the "reactant side" or the "product side"
• Mass action expression


  A simple relationship among the concentrations of the reactants and products for any chemical system at equilibrium
• Reaction quotient (Q)


  The numerical value of the mass action expression
• Equilibrium constant (Kc)


  The value of the reaction quotient at equilibrium

  
• For chemical equilibrium to exist, the reaction quotient Q must be equal to the equilibrium constant Kc

  
• Determining mass action expression


  Exponents are the same as the stoichiometric coefficients
• Reversing the direction of equilibrium


  The new equilibrium constant is the reciprocal of the original
• Multiplying the coefficients by a factor


  The equilibrium constant is raised to a power equal to that factor
• Adding chemical equilibria


  Their equilibrium constants are multiplied
• The gas law can be used to write the equilibrium constant in terms of partial pressures (Kp)
• Magnitude of K


  Indicates the position of equilibrium: K>>1 (large K) means a large amount of product and very little reactant, K~1 means approximately equal amounts of reactant and product, K<<1 means mostly reactant and very little product
• Relationship between Kp and Kc


  Kp = Kc(RT)^(Δn), where Δn is the change in moles of gas
• Homogeneous reactions


  All reactants and products are in the same phase
• Heterogeneous reactions


  Involve more than one phase
• For heterogeneous equilibria, the equilibrium law is written without concentration terms for pure solids or pure liquids

  
• Ratio of amount of substance to volume of substance


  Constant for a pure liquid or solid
• Concentration of a substance in a solid


  Constant, doubling the number of moles doubles the volume but the ratio of moles to volume remains the same
• Equilibrium law for a heterogeneous reaction


  Written without concentration terms for pure solids or pure liquids, solids and liquids are replaced with "1" in the mass action expression
• If the Kp for the reaction 2NO2(g) N2O4(g) is 0.480 at 25°C, the reaction favors reactant
• Le Châtelier's principle


  A system at equilibrium (Q=K) when upset by a disturbance that causes (Q≠K) will shift to offset the stress
• Shift to the right

  The forward reaction is dominant (Q<K)
• Shift to the left

  The reverse direction is dominant (Q>K)
• Factors affecting quantities present at equilibrium


  • Concentration affects the value of Q and helps predict the reaction progress
  • Pressure and volume of gases both affect the value of Q and predict the reaction progress
• Endothermic reactions


  Increase K with increasing temperature
• Exothermic reactions


  Decrease K with increasing temperature
• Approach to using Le Châtelier's principle


  1. Write the mass action expression for the reaction

  2. Examine the relationship between the affected concentration and Q (direct or indirect)

  3. Compare Q to K
• If NH3(aq) is added or Cu2+ is added to the reaction Cu(H2O)6

  2+

  (aq) + 4NH3(aq) [Cu(NH3)4

  2+

  (aq)] + 6H2O(l), the reaction shifts right
• If pressure is increased in the reaction N2(g) + 3H2(g) 2NH3(g), the reaction shifts right
• If PO4

  1. is removed from the reaction H3PO4(aq) + 3OH-

  (aq) 3H2O(l) + PO4(aq)

  1. , the reaction shifts right
• Adding HCl(aq) or removing AgCl will shift the reaction Ag+

  (aq) + Cl-

  (aq) AgCl(s) to the right
• van't Hoff rule


  Write mass action expression with "heat term" as if it were a concentration and predict shifts as before
• If the reaction H3PO4(aq) + 3OH-

  (aq) 3H2O(l) + PO4(aq)

  1. is exothermic and the system is cooled, the reaction shifts to the right
• If the reaction 2 SO3(g) 2 SO2(g) + O2(g) is endothermic and the temperature is increased, the equilibrium constant Kc increases
• Catalysts have no effect on equilibrium
• The reaction between aqueous cobalt ion and chloride ion [Co(H2O)6]2+

  (pink, aq) + 4Cl-

  (aq) [Co(Cl)4]2-

  (blue,aq) + 6H2O(l) is endothermic
• Equilibrium calculations


  1. Calculating equilibrium constants from known equilibrium concentrations or partial pressures
  2. Calculating one or more equilibrium concentrations or partial pressures using the known value of Kc or KP
• Using an ICE (Initial, Change, Equilibrium) table


  To solve equilibrium problems when given initial conditions and an equilibrium constant