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Chapter 15
  • Chemical Equilibrium
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Equilibrium Problem
  • Consider the following reaction:
  • CO(g)  +  H2O(g)        CO2(g)  + H2(g)
  • Kp =  0.0611 at 1000 K
  • A reaction mixture initially contains partial pressures of CO and H2O equal to 1.77 atm.  The mixture is allowed to equilibrate at 1000 K.  What are the equilibrium partial pressures of CO2, H2, CO, and H2O?
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Equilibrium Problem
  • For the following reaction, Kc = 255 at 1000 K:
  • CO(g)  +  Cl2(g)         COCl2(g)
  • A reaction mixture initially contains a CO concentration of 0.1500 M and a Cl2 concentration of 0.175 M.  The mixture is then allowed to equilibrate at 1000 K.  What are the equilibrium concentrations of CO, Cl2, and COCl2?
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Equilibrium Problem
  • Kp = 0.11 at 25°C for the decomposition of ammonium hydrogen sulfide:
  • NH4HS(s)         NH3(g)  +  H2S(g)
  • A sample of NH4HS is placed in an evacuated flask and allowed to equilibrate at 25°C.  What are the equilibrium partial pressures of NH3 and H2S?
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Equilibrium Problem: small values of K
  • Kc = 2.99 × 10-7 at 227°C for the reaction:
  • SO2Cl2(g)          SO2(g)  +  Cl2(g)
  • If a reaction mixture initially containing 0.175 M SO2Cl2 is allowed to equilibrate at 227°C, what is the equilibrium partial pressure of Cl2?
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Reaction Quotient
  • The reaction quotient, Q, is defined in a similar fashion as the equilibrium constant.
  • For a reaction:
  • aA + bB ¾® cC + dD
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Reaction Quotient
  • The pressures or concentrations in the reaction quotient are at any point in the reaction.
  • Pure solids, liquids, and solvents do not appear in reaction quotients.
  • Usually, the initial pressures or concentrations are used to calculate the value of the reaction quotient.
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Reaction Quotient
  • The value of the reaction quotient is compared to the value of K to determine which direction the reaction must go to reach equilibrium.
    • If Q < K, reaction goes to equilibrium by moving in the forward direction, making products.
    • If Q > K, reaction goes to equilibrium by moving in the reverse direction, making reactants.
    • If Q = K, reaction is already at equilibrium.
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Reaction Quotient
  • At a particular temperature, Kc = 2.7 for the reaction:
  • CO(g)  +  H2O(g)        CO2(g) + H2(g)
  • A reaction mixture contains the following concentrations:  [CO] = 0.065 M; [H2O] = 0.28 M; [CO2] = 0.31 M; [H2] = 0.22 M.  In which direction must the reaction proceed to reach equilibrium?  What are the equilibrium concentrations?
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LeChâtelier’s Principle
  • If a reaction at equilibrium is disturbed by a change in:  the concentration or pressure of one of the species in the reaction; pressure; or pressure, the reaction will respond in such a way to partially counteract the change.
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Adding or removing a reactant or product
  • N2(g)  +  3 H2(g)          2 NH3(g)
  • Suppose the reaction above is at equilibrium.  Therefore, Q = K.
  • Add some hydrogen gas to the mixture.  Initially, the partial pressure of H2 increases and Q < K.
  • Since Q < K, reaction moves towards products, increasing the partial pressure of NH3, decreasing the partial pressure of N2, and partially counteracting the increase in the partial pressure of H2.
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Adding or removing a reactant or a product
  • N2(g)  +  3 H2(g)          2 NH3(g)
  • Reaction is initially in equilibrium, Q = K.
  • Remove some nitrogen gas.  Initially, the partial pressure of N2 decreases.  This will increase the value of Q.
  • Q > K.  The reaction moves in the reverse direction towards reactants.  This will increase the partial pressure of H2, decrease the partial pressure of NH3, and partially replace the N2 that was removed.
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Pressure or Volume Changes
  • Pressure and/or volume changes affect gaseous species mostly.  Liquids and solids are generally unaffected by pressure changes.
  • For a gas:
    • Pressure is proportional to number of moles.
    • Volume is inversely proportional to pressure.  (Volume increase has the same effect as a pressure decrease.)
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Pressure changes
  • N2(g)  +  3 H2(g)          2 NH3(g)
  • An increase in pressure favors the side of the reaction with the fewer number of moles of gas.
  • A decrease in pressure favors the side of the reaction with the greater number of moles of gas.
  • For reactions with equal number of moles of gas on both sides, or for reactions with no gaseous reactants or products, pressure has no effect.
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Adding or removing reactants or products, pressure changes
  • So far, the changes discussed so far will change the value of Q for the reaction.  The value of K does not change.
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Temperature changes
  • In order to know how temperature affects an equilibrium mixture, you have to be given the DH for the reaction.
  • N2(g)  +  3 H2(g)          2 NH3(g)     DH = –91.8 kJ
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Changes that have no effect on equilibrium mixtures
  • Addition of a species that does not appear in the equilibrium expression.
    • Solid, liquid, or solvent
    • A species that does not appear in the reaction at all.
  • Catalysts have no effect on the composition at equilibrium!!
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LeChâtelier’s Principle
  • Consider the following reaction at equilibrium for which DH  = 16.3 kJ:
  • NO(g)  +  Br2(g)         NOBr2(g)