Chemical Equilibria: The Extent Of Chemical Reactions

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Chemical EquilibriaThe Extent Of Chemical
Reactions

• Textbook Chapter 17 (page 714)
• No reaction goes to 100.
• Many reactions are reversible.
• Reactions incomplete because at equilibriumrate
  of forward reaction rate of reverse reaction
• (not because reaction stops).

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Dynamic Equilibrium

• forward (L to R) and reverse (R to L) reactions
  occur simultaneously.

brown
colourless
Equilibrium concentrations the same, whatever
the starting composition.
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Equilibrium Constant

• At equilibrium, concentrations given by Law of
  Mass Action

• equilibrium constant
• (often omit subscript eq)
• Law of Mass Action first deduced from
  experiment, then proved from laws of
  thermodynamics

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Relation between Keq and Rate Coefficients

• Take system with elementary reactions, eg

For Elementary reactions (unimolecular and
bimolecular)
At equilibrium forward rate reverse rate, so
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Example
2NO(g) O2 (g) ? 2NO2 (g) net reaction
Rate rate of slow step k N2O2O2
So rate k N2O2 O2 k K NO2 O2 hence
rate constant ? NO2 O2 consistent with
observed rate law
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Kc Equilibrium Constant with respect to
Concentration

• units M (mol L-1)
• Units of Kc depend on values of a, b, c d

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Kp Equilibrium Constant with respect to Partial
Pressure

• Reaction involving gases

• pA partial pressure of A at equilibrium...

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Example

• Cl2 is placed in a vessel and heated to 1400 K.

• When equilibrium reached we find
• pCl2 1 atm, pCl 2.97 x 102 atm.
• What is the value of Kp at 1400 K?

(units!)
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Relationship Between Kp Kc

• pV nRT

• so Kc Kp(RT)-1 for this particular reaction
• In general

Dn (ngas products ngas reactants)
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Example

• Having found Kp at 1400 K, find Kc at 1400 K

consistent units!
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Magnitude Of Equilibrium Constants

• A particular Kc has meaning only in relation to
  particular equation.
• If the reaction has same number of moles on each
  side (ie Kc is dimensionless)
• if Kc gt 1 mostly products at equilibrium
• if Kc lt 1 mostly reactants at equilibrium
• if Kc 1 equal number of moles reactant
  product

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Using Equilibrium Constants

• Determined by experiment can be predicted from
  theory for certain systems
• Independent of initial concentrations
• Independent of the presence of inhibitors or
  catalysts
• Dependent on temperature (T must be given for
  each Kc value)