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					Chemical Equilibrium
       Collision theory
       Rates of reactions
       Catalysts
       Reversible reactions
       Chemical equilibrium
       Le Chatelier’s Principle
         Concentration
         Temperature
         Volume
         Catalysts
A. Collision Theory
   Reaction rate depends on the collisions
   between reacting particles.
   Successful collisions occur if the
   particles...
     collide with each other
     have the correct orientation
     have enough kinetic energy to break bonds
A. Collision Theory
   Particle Orientation




        Required Orientation




                               Unsuccessful
        Successful Collision    Collisions
     Activation energy: minimum energy required
     for a reaction to occur

         Exothermic                       Endothermic
                      Activation energy




                                 Energy
Energy




           Time                            Time
                           Energy of
                            reaction
A. Collision Theory
   Activation Energy
    depends on reactants
    low Ea = fast rxn rate


                             Ea
   16.2: Rates of Reactions
Chemical kinetics: the study of the rate
 (the speed) of a reaction
Rate of a chemical reaction depends on:

  1. SURFACE AREA
 2. CONCENTRATION of reactants
 3. TEMPERATURE (T) of reactants
 4. Presence/absence of a CATALYST
SURFACE AREA
  Surface Area
   high SA = fast rxn rate
   more opportunities for collisions
   Increase surface area by…
    • using smaller particles
    • dissolving in water
   Effect of Concentration on Rate
Concentration:
 increasing concentration of reactants
 results in more collisions.


 More collisions = increased rate of
 reaction
   Effect of Temperature on Rate
Temperature:
 Increasing T increases particle speed.

 Faster reactants means more collisions
 have the activation energy, which
 increases the rate of the reaction.
Temperature
      Analogy: 2-car collision




           5 mph “fender bender”




         50 mph “high-speed crash”
      Effect of Catalysts on Rate
A catalyst:
  A chemical that influences a reaction, but is not consumed in
  the reaction. (It can be recovered unchanged at the end of
  the reaction.)
  Lowers the activation energy of the reaction.



                                       Activation energy
       Energy




                           Activation energy with catalyst



                Time
Catalysts
   Enzyme Catalysis
    16.1: Reversible Reactions
* Thus far, we have considered only one-way
 reactions: A + B → C + D
Some reactions are reversible:
  They go forward (“to the right”) : A + B → C + D
    and backwards (“to the left”) : A + B ← C + D

 Written with a two-way arrow:
                A+B↔C+D
Examples:
 Boiling & condensing
 Freezing & melting
        Reversible Reactions
o   At chemical equilibrium there is no net
    change in the actual amounts of the
    components of the system.
o   And although the rates of the forward &
    reverse rxns are equal at chemical
    equilibrium, the concentrations of the
    components on both sides of the chem-
    ical eqn are not necessarily the same.
      *In fact they can be dramatically different.
o   Consider a set of escalators as being like
    the double arrows in a dynamic
    equilibrium.
o   The # of people using the up escalator
    must be the same as the # of people
    using the down escalator for the # of
    people on each floor to remain at
    equilibrium
     • However,   the # of people upstairs do not
       have to equal the # of people downstairs
     • Just the transfer between floors must be
       consistent
Examples of irreversible reactions:

 Striking a match / burning paper
 Dropping an egg
 Cooking (destroys proteins)
      16.3: Chemical Equilibrium
For a reversible reaction, when the forward rate equals the
  backward rate, a chemical equilibrium has been
  established.
 Both the forward and backward reactions continue, but
 there is a balance of products “un-reacting” and reactants
 reacting.
             A+B↔C+D

         A   +   B               C    +   D
     Equilibrium Expression
Chemist’s generally express the
position of equilibrium in terms of
numerical values
  These values relate the amounts of
  reactants to products at equilibrium
Consider this hypothetical rxn…
  wA + xB              yC + zD
• Where “w” mols of reactant A and “x”
 mols of reactant B react to give “y” mols
 of product C and “z” mols of product D at
 equil.
         Equilibrium Expression
    We can write a mathematical expression
    to show the ratio of product concs to
    reactant concs called an equilibrium
    expression          y   z
                      [C] [D]
                         w    x
                      [A] [B]
o   The concentration of each substance is raised
    to a power equal to the # of mols of that
    substance in the balanced rxn eqn.
o   The square brackets indicate concentration in
    Molarity (mol/L)
      Equilibrium Expression
                     [C]y [D]z
           Keq=
                        w     x
                     [A] [B]
The resulting ratio of the equilibrium is
called the equilibrium constant or Keq
The Keq is dependent on the temp
  If the temp changes so does the Keq

       NOTE: this is only for gases!!!
    Equilibrium Constant
Equilibrium constants provide valuable
chemical information
They show whether products or
reactants are favored in a rxn
 always written as a ratio of products over
 reactants
 a value of Keq > 1 means that products
 are favored
 Keq < 1 than reactants are favored
     Sample Problem 1
    Dinitrogen tetroxide (N2O4), a
 colorless gas, and nitrogen dioxide
     (NO2), a brown gas, exist in
equilibrium with each other according
         to the following eqn:
        N2O4(g)     2NO2(g)

    A liter of gas mixture at 10°C at
equilibrium contains 0.0045mol N2O4 &
     0.030 mol NO2. Write the Keq
 expression and calculate Keq for the
                 reaction.
        Analyze: list what we know
    Known:
      [N2O4] = .0045mol/L
      [NO2] = .030mol/L
    Unknown:
      Keq expression = ?
      Keq = ?


o   At equilibrium, there is no net change in
    the amount of N2O4 or NO2 at any given
    instant
      Calculate: solve for unknowns
      The only product of the rxn is NO2, which
      has a coefficient of 2 in the balanced eqn
      The only reactant N2O4 has a coefficient of
      1 in the balanced eqn
    The equilibrium expression is:

            [NO2]2                 [.030M]2
Keq=                     Keq=
            [N2O4]  1
                                   [.0045M]1
o   Keq is equal to: Keq= 0.20
o   Keq < 1, therefore rxn doesn’t favor
    products
            Classwork:
•   A mixture at equilibrium at 827°C contains
    0.552 M CO2, 0.552 M H2, 0.448 M CO, and
    0.448 M H2O.
      CO2(g)+ H2(g)<==> CO(g) + H2O(g)
    a. Write the equilibrium expression for
       the above rxn.
    b. Calculate Keq at this temp?
    c. More CO2 is added to the system,
       which direction will the reaction
       shift?
    d. Are the reactants or products
       favored in this reaction?
* Le Chatelier’s Principle is about
 reducing stress – a stress applied to a
 chemical equilibrium

Relax! Reduce stress brought
 on by chemical equilibrium
with me, Henri Le Chatelier!




                               (1850 – 1936)
     16.4: Le Chatelier’s Principle
Le Chatelier’s Principle:
  When a stress is applied to a system (i.e.
  reactants and products) at equilibrium, the
  system responds to relieve the stress.
  The system shifts in the direction of the
  reaction that is favored by the stress.
  A stress is a change in:
      Concentration
      Temperature
      Volume
      16.5: Stress: Change Concentration
Ex:    Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O
       (pink)                (blue)

Stress          Result
Add Cl1-        Forward rxn favored
                Shifts forward to reduce extra Cl1-
                More CoCl42- will form
Add H2O         Backward rxn favored
                Shifts backward to reduce extra H2O
                More Co(H2O)62+ will form
      16.7: Stress: Change Temperature
Ex: heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O
       (pink)                   (blue)
This reaction is endothermic. For Le Chatelier’s
   principle, consider “heat” as a chemical.
Stress Result
Increase T  Forward rxn favored; shifts forward to
            reduce extra heat
              More CoCl42- will form
Decrease T    Backward rxn favored; shifts backward
              to replace “lost” heat
              More Co(H2O)62+ will form
       16.6: Stress: Change Volume
Ex:     1 N2 (g) + 3 H2(g) ↔ 2 NH3(g)
(1 + 3 = 4 moles of gas) ↔ (2 moles of gas)

Stress       Result
Decrease V Forward rxn favored; shifts forward to side
           with fewer moles of gas (reduces # of
           molecules packed into this smaller volume)

Increase V   Backward rxn favored; shifts backward to
             side with more moles of gas (to fill the
             larger volume with more molecules)
      16.7: Catalysts & Equilibrium

                   MnO2
Ex:    2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g)
  Since a catalyst increases the forward and
  backward rates equally, it will not shift the
  equilibrium.

				
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posted:7/25/2013
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