Chemical Equilibrium

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Chemical Equilibrium

• Chapter 17

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The Equilibrium State

• Chemists are interested in these reversible
  reactions. One example is the following

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• Graphs of reactant and product concentrations
  change with time as shown below.

(a) Only 0.04 M N2O4 present initially (b) Only
0.08 M NO2 present initially
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• The equilibrium expression compares reactant and
  product concentrations.

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• Write the Kp and Kc expressions for 2
  N2O5(g) ? 4 NO2(g) O2(g)
• The equilibrium concentrations for the reaction
  between CO and Cl2 to form carbonyl chloride
  (phosgene gas) CO(g) Cl2(g) ? COCl2(g) at 74C
  are CO 1.2 x 102 M, Cl2 0.054 M, and
  COCl2 0.14 M. Calculate Kc and Kp.

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• Methane (CH4) reacts with hydrogen sulfide to
  yield H2 and carbon disulfide, a solvent used in
  manufacturing. What is the value of Kp at 1000 K
  if the partial pressures in an equilibrium
  mixture at 1000 K are 0.20 atm of CH4, 0.25 atm
  of H2S, 0.52 atm of CS2, and 0.10 atm of H2?

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• Heterogeneous Equilibrium When reacting species
  are in different phases, solid and liquid phases
  are excluded from the expression because their
  concentrations do not change.
• For CaCO3(s) ? CaO(s) CO2(g)Kc CO2
  because CaCO3 and CaO are solids.

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• Write the equilibrium equation for each of the
  following reactions
• (a) CO2(g) C(s) ? 2 CO(g)
• (b) Hg(l) Hg2(aq) ? Hg22(aq)
• (c) 2 Fe(s) 3 H2O(g) ? Fe2O3(s) 3 H2(g)
• (d) 2 H2O(l) ? 2 H2(g) O2(g)

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• We can make the following generalizations
  concerning the composition of equilibrium
  mixtures
• If Kc gt 103, products predominate over reactants.
  If Kc is very large, the reaction is said to
  proceed to completion.
• If Kc is in the range 103 to 103, appreciable
  concentrations of both reactants and products are
  present.
• If Kc lt 103, reactants predominate over
  products. If Kc is very small, the reaction
  proceeds hardly at all.

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• The reaction quotient (Qc) is obtained by
  substituting initial concentrations into the
  equilibrium constant. Predicts reaction
  direction.Qc gt Kc System proceeds to form
  reactants.Qc Kc System is at equilibrium.Qc lt
  Kc System proceeds to form products.

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• Predicting the direction of a reaction.

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• The equilibrium constant (Kc) for the formation
  of nitrosyl chloride, from nitric oxide and
  chlorine gas
• 2 NO(g) Cl2(g) ? 2 NOCl(g) is 6.5 x 104
  at 35C. In an experiment, 2.0 x 102 moles of
  NO, 8.3 x 103 moles of Cl2, and 6.8 moles of
  NOCl are mixed in a 2.0-L flask. In which
  direction will the system proceed to reach
  equilibrium?

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The equilibrium constant Kc for the reaction of
H2 with I2 is 57.0 at 700K. If 1.00 mol of H2 is
allowed to react with 1.00 mol of I2 in a 10.0 L
reaction vessel at 700 K, what are the
concentrations of H2, I2 and HI at equilibrium?
What is the composition of the equilibrium
mixture in moles?
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Using Equilibrium Constants

• Knowing K allows us to calculate equilibrium
  concentrations from initial concentrations.
• We use the Initial Change Equilibrium method.
• Use Kc24 to determine equilibrium concentrations.

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• A mixture of 0.500 mol H2 and 0.500 mol I2 was
  placed in a 1.00-L stainless steel flask at
  700C. The equilibrium constant Kc for the
  reaction H2(g) I2(g) ? 2 HI(g) is 57 at this
  temperature. Calculate the equilibrium
  concentrations.
• If the starting concentration of HI was 0.040 M,
  calculate the new equilibrium concentrations.
• If the initial concentrations are H2 0.100 M
  and I2  0.200 M, calculate the equilibrium
  concentrations.

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Steps to follow in calculating equilibrium
concentrations from initial concentrations
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Le Châteliers Principle

• Le Châteliers principle If an external stress
  is applied to a system at equilibrium, the
  system adjusts in such a way that the stress
  is partially offset.

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• Concentration Changes
• - The concentration stress of an added reactant
  or product is relieved by reaction in the
  direction that consumes the added substance.
• The concentration stress of a removed reactant or
  product is relieved by reaction in the direction
  that replenishes the removed substance.

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• Haber process for synthesis of ammonia.
• N2(g) 3 H2(g) ? 2 NH3(g) Kc 0.291 at
  700 K
• Given an equilibrium mixture of 0.50 M N2, 3.00 M
  H2, and 1.98 M NH3 at 700 K, what happens when
  the concentration of N2 is increased to 1.50 M?
• Le Châteliers principle tells us the reaction
  will relieve the stress by converting the N2 to
  NH3.

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• The reaction of iron(III) oxide with carbon
  monoxide occurs in a blast furnace when iron ore
  is reduced to iron metal
• Fe2O3(s) 3 CO(g) ?2 Fe(l) 3 CO2(g)
• Use Le Châteliers principle to predict the
  direction of reaction when an equilibrium mixture
  is disturbed by
• (a) Adding Fe2O3 (b) Removing CO2 (c) Removing CO

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• Volume and Pressure Changes Only reactions
  containing gases are affected by changes in
  volume and pressure.
• Increasing pressure Decreasing volume
• PV nRT tells us that increasing pressure or
  decreasing volume increases concentration.

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• N2(g) 3 H2(g) ? 2 NH3(g) Kc 0.291, 700 K

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• Consider the reaction N2O4(g) ? 2 NO2(g), taking
  place in a cylinder with a volume 1 unit.

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• The Volume is then halved, which is equivalent to
  doubling the pressure.
• Since Q gt K, the product is too high and the
  reaction progresses in the reverse direction.

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• Does the number of moles of reaction products
  increase, decrease, or remain the same when each
  of the following equilibria is subjected to a
  decrease in pressure by increasing the volume.
• PCl5(g) ? PCl3(g) Cl2(g)
• CaO(s) CO2(g) ?CaCO3(s)
• 3 Fe(s) 4 H2O(g) ? Fe3O4(s) 4 H2(g)

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• The following picture represents the equilibrium
  mixture for the gas-phase reaction A2 ? 2A.
• Draw a picture that shows how the concentrations
  change when the pressure is increased by
  decreasing the volume.

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• Temperature Changes Changes in temperature can
  change the equilibrium constant.
• Endothermic processes are favored when
  temperature increases.
• Exothermic processes are favored when
  temperature decreases.

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• Consider the reaction N2(g) 3 H2(g) ? 2
  NH3(g) which is exothermic by 92.2 kJ.

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• In the first step of the Ostwald process for
  synthesis of nitric acid, ammonia is oxidized to
  nitric oxide by the reaction
• 4 NH3(g) 5 O2(g) ? 4 NO(g) 6 H2O(g) ?H
  905.6 kJ
• How does the equilibrium amount vary with an
  increase in temperature?

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• The following pictures represent the composition
  of the equilibrium mixture at 400 K and 500 K for
  the reaction A(g) B(g) ? AB(g).
• Is the reaction endothermic or exothermic?

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• Catalysis No effect.