Chemical Equilibrium

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Chemical Equilibrium
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Recognizing Rxn Equilibrium

• 1. The system is closed.
• Opposite rxns occur at the same rate.
• Equilibrium is reached by either starting w/
  reactants or products.
• Temp is constant.
• Radioactive tracers can prove th rxn proceeds
  even w/ no net observable changes dynamic
  equilibrium
• Equilibrium does NOT mean conc of PR are equal,
  but th RATES ARE EQUAL!

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Equilibrium Constant, Keq

• A ltgt B
• Once equilibrium is reached, the ratio of partial
  pressures of A and B is constant.
• Law of Mass Action
• Shows the relationship between the concs
  (expressed as partial pressures for gases as
  molarities for solutions) of the R P present at
  equilibrium in any reaction.

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Equilibrium Constant, Keq-cont

• aA bB ltgt cC dD
• Equilibrium-constant Expression
• Keq CcDd
• AaBb
• Temp dependent
• Dimensionless no units !

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Magnitude of Keq

• Keq can be very large or very small
• CO(g) Cl2 (g) ltgt COCl2 (g)
• Keq PCOCl2 1.49 x 108
• PCOPCl2
• For Keq to be so large gtgt 1, the numerator,
  PCOCl2, must be large
• The equilibrium lies to the right (toward the P)
• Keq ltlt1 equilibrium lies to the left (reactants
  dominate)

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The Equilibrium Constant

• The Magnitude of Equilibrium Constants
• If K ltlt 1, then reactants dominate at equilibrium
  and the equilibrium lies to the left.

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Direction of Chemical Equilibrium Keq

• Since equilibrium can be approached fr either the
  P or R, wh way we write the equation is arbitrary
• N2O4 ltgt 2NO2
• Keq (PNO2)2 6.46
• P N2O4
• 2NO2 ltgt N2O4
• Keq P N2O4 0.155
• (PNO2)2
• The Keq expression for a rxn written in one
  direction is the reciprocal of the one for the
  rxn written in the reverse direction.

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Other Ways to Manipulate Eq Keq Values

• When using Hesss Law to obtain a net equation by
  adding equations canceling like terms
• The Keq of a rxn in the reverse direction is the
  inverse of Keq of the forward rxn.
• The Keq of a rxn th has been multiplied by a
  number is the Keq raised to a power equal to th
  number.
• The Keq for a net rxn made of 2 or more steps is
  the product of the Keq for the individual steps.

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Heterogeneous Equilibria

• Homogeneous equilibria -involve subs all in the
  same phase
• Heterogeneous equilibria - subs are in diff
  phases
• Partial pressures of gases are used in Keq
  expression.
• Molar conc of dissolved species are used.
• Pure solids, pure liquids, and solvents are NOT
  included in Keq expression.
• these subs must be present for equilibrium

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Solids do NOT effect Equilbria
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Calculating Keq

• If we know the equil conc of at least one species
  in a rxn, we can use stoichiometry of the rxn to
  determine the conc of the other species at
  equilibrium.

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Calculating Keq-cont

• Tabulate the known initial equilibrium concs of
  all species in the Keq expression.
• For those where initial equil conc are known,
  calc change in conc.
• Use stoich of rxn to calc changes in conc for all
  other species.
• Fr initial change in conc, calc the equil conc.

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Applications of Keq

• Allows us to predict
• 1) the direction in wh the rxn will proceed to
  achieve equilibrium
• 2) calculate the conc of RP when equilibrium
  has been reached

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Applications of Keq-cont.

• Predicting the Direction of Rxn
• Q - rxn quotient - calc by substituting in
  partial press or conc into Keq expression
• Q Keq if sy is at equilibrium
• Q gt Keq - reaction proceeds to the left to
  reach equil
• Qlt Keq - reaction proceeds to the right

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Applications of Keq-cont

• Calculating Equilibrium Constants
• - Again use the known initial equil conc
• - Set up a ICE table
• - Calc unknown equil conc

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Le Chatliers Principle

• If a sy at equilibrium is disturbed by a change
  in temp, press, or the conc of comps, the sy will
  shift the eq position to counteract the effect of
  the disturbance.

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Le Chatliers Principle-cont

• Change in R or P Conc
• Adding a R or P disturbs equil
• Equil will re-establish by consuming the added R
  or P
• Removing R or P
• Equi will re-establish by forming more sub
• N2(g) 2H2(g) ltgt 2NH3(g)
• Add N2 or H2 shifts right
• Add NH3 shifts left
• Remove N2 or H2 shifts left
• Remove NH3 shifts right

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Le Chatliers Principle-cont

• Effects of Volume Press Change
• ONLY effect gas phase!
• A decrease in vol will press
• Eq will shift to decrease the press
• Eq will shift to the side with the least number
  of gas particles
• N2O4(g) ltgt 2NO2(g)
• press will shift to the left

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Le Chatliers Principle-cont

• Effect of Temp Change
• Temp will change the Keq
• Endothermic R heat ltgt P
• the temp will favor the endothermic rxn
• T Keq
• Exothermic R ltgt P heat
• Decreasing the temp will favor the exo rxn
• Increasing T decrease in Keq

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Heat stresses equilibrium
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Le Chatliers Principle-cont

• The Effect of Catalysts
• Lowers the Ea for both the forward reverse rxns
• Increases the rate at wh equil is achieved, it
  will not shift the equilibrium or change the
  equilibrium mixture.

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Haber Process

• N2(g) 2H2(g) ltgt 2NH3(g)
• High conc of H2(g) N2(g) maintained.
• NH3(g) (g) th forms is removed.
• High temp to the forward rxn.
• 300-600C
• Catalyst used to rxn rate (est equilibrium
  faster)
• High press used 200-600 atm

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