Chemical Equilibrium

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Chemical Equilibrium
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Chemical Equilibrium

• The state of a chemical reaction in which its
  forward and reverse reactions occur at equal
  rates so that the concentration of the reactants
  and products does not change with time.

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Law of Mass Action

• aA bB cC ... ltgt pP qQ rR ...
• Equilibrium Constant
• Pp Qq Rr ...
• K ---------------------
• Aa Bb Cc ...

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Meaning of Equilibrium Constant

• Kgtgt1 reaction is product-favored equilibrium
  concentrations of products are greater than
  equilibrium concentrations of reactants.
• Kltlt1 reaction is reactant-favored equilibrium
  concentrations of reactants are greater than
  equilibrium concentrations of products.

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• If K 100 I2 in CCl4 / I2 in water for
  the equilibrium
• I2 in water I2 in CCl4
• What is K for the reverse reaction,
• I2 in CCl4 I2 in H2O?
• 100, 1, 0.01

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Acid-Base Equilibrium in Aqueous Solution

• Acid Dissociation Constant
• HC2H3O2 H2O ltgt H3O C2H3O2-

H3OC2H3O2- K --------------------
-- H2OHC2H3O2
H3OC2H3O2- Ka
KH2O ---------------------- HC2H3O2
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Acid-Base Equilibrium in Aqueous Solution

• Base Dissociation Constant
• NH3 H2O ltgt NH4 OH-

NH4OH- K -----------------
H2ONH3
NH4OH- Kb KH2O
---------------- NH3
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Autoionization of Water

• H2O H2O ltgt H3O OH-

H3OOH- K ----------------- H2O
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Kw K H2O2 H3OOH- 1.0 x 10-14
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Analogy in Semiconductors

• -SiSi- ltgt -SiSi- e-
• -SiSi- ltgt h e-
• K h e-

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e- and h in Semiconductors
Production
Recombination
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Autoionization Equilibria
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e- and h in Semiconductors
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Doping
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Donors and Acceptors in Silicon
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• Which dopant will act as an acceptor for Si?
• B, Ge, As
• As a donor?
• B, Ge, As

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Fermi Level
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Le Chatelier's Principle

• If a stress, such as a change in concentration,
  pressure, temperature, etc., is applied to a
  system at equilibrium, the equilibrium will shift
  in such a way as to lessen the effect of the
  stress.

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Gas Phase Equilibrium

• catalysis
• N2(g) 3 H2(g) ltgt 2 NH3(g) heat
• high pressure and temperature

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The Principle of Le Chatelier

• Changes in Concentration or Partial Pressure
• for N2(g) 3 H2(g) Û 2 NH3(g)
• an increase in N2 and/or H2 concentration or
  pressure, will cause the equilibrium to shift
  towards the production of NH3

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The Principle of Le Chatelier

• Changes in Concentration or Partial Pressure
• for N2(g) 3 H2(g) Û 2 NH3(g)
• likewise, a decrease in NH3 concentration or
  pressure will cause more NH3 to be produced

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The Principle of Le Chatelier

• Changes in Temperature
• for N2(g) 3 H2(g) Û 2 NH3(g) heat
• for an exothermic reaction, an increase in
  temperature will cause the reaction to shift back
  towards reactants

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• The cobalt complexes participating in the
  equilibrium below comprise a humidity sensor.
  From Le Châtelier's principle, when the sensor is
  moist (excess H2O), what color is the cobalt
  complex?
• pink, blue

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• A competition experiment involves O2 and CO
  vying for hemoglobin (Hb) sites, defined by the
  equilibrium
• Hb(O2)4 4 CO Hb(CO)4 4O2
• From Le Châtelier's principle, how is CO
  poisoning reversed?
• decrease O2 pressure, increase O2 pressure,
  remove Hb

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Heterogeneous Equilibrium

• CaCO3(s) heat ltgt CaO(s) CO2(g)

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Gibbs Free Energy and Equilibrium

• DG Reaction
• -------------------------------------
• Negative Spontaneous
• Positive Non-Spontaneous
• Zero Equilibrium
• -------------------------------------

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The Influence of Temperature on Free Energy

• DG, DH, DS
• DG DH - T DS
• DH DS DG
• negative positive negative
• spontaneous at all temperatures

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The Influence of Temperature on Free Energy

• DG, DH, DS
• DG DH - T DS
• DH DS DG
• positive negative positive
• non-spontaneous at all temperatures

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The Influence of Temperature on Free Energy

• DG, DH, DS
• DG DH - T DS
• DH DS DG
• negative negative --------
• spontaneous at low temperatures, nonspontaneous
  at high temperatures

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The Influence of Temperature on Free Energy

• DG, DH, DS
• DG DH - T DS
• DH DS DG
• positive positive --------
• spontaneous at high temperatures, nonspontaneous
  at low temperatures

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Phase Transitions

• H2O(s) -----gt H2O(l) DH gt 0 DS gt 0
• H2O(l) -----gt H2O(g) DH gt 0 DS gt 0
• spontaneous at high temperatures

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Phase Transitions

• H2O(l) -----gt H2O(s) DH lt 0 DS lt 0
• H2O(g) -----gt H2O(l) DH lt 0 DS lt 0
• spontaneous at low temperatures