Review 2

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Review 2

• Chapter 14
• Chemical Equilibrium

2

• Equilibrium Condition
• Study equilibrium tells us more about whether a
  reaction will occur or not.
• Closed system
• Example At 500K, the reaction PCl3(g)
  Cl2(g) ? PCl5(g) in a closed system
• If start with 1M of reactants, at equilibrium,
  PCl3(g) Cl2(g) 0.52M, PCl5(g) 0.48M
  this gives Kc 1.8
• If start with 1M of product, at equilibrium,
  PCl3(g) Cl2(g) 0.52M, PCl5(g) 0.48M
  this gives Kc 1.8
• If start with 2M of reactants, at equilibrium,
  PCl3(g) Cl2(g) 0.82M, PCl5(g) 1.18M
  this gives Kc 1.8
• If add 1M of Cl2, PCl3(g) 0.30, Cl2(g)
  1.3M, PCl5(g) 0.70M this gives Kc 1.8
• Depend on concentration, energy and organization

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K Law of mass action Expression (Heterogeneous
vs. Homogeneous reactions) Kp, Kc Calculate K
of reversed reaction, ½ of a reaction or doubled
Example 2SO2(g) O2(g) ? 2SO3(g) 4SO2(g)
2O2(g) ? 4SO3(g) SO2(g) ½ O2(g) ?
SO3(g) 2SO3(g) ? O2(g) 2SO2(g)
    Example 2CO(g) O2(g) ? 2CO2(g) K
2.75 x 1020 _at_1000K CO2(g) ? CO(g) ½ O2(g)
K 6.03 x 10-11 _at_1000K
Summarize
If the coef in the reaction is Then K is
Doubled Squared
halved Square root
Reversed in sign Inverted
Multiplied by a constant n Raised to the nth power
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Kp Kc (RT)?n ?n change in gas moles. It is
always product GAS moles reactant GAS moles T
temp in Kelvin , R is gas conatant Reaction
Quotient Q Use the initial concentrations (or
partial pressure pressures) of the reactants and
products to predict the direction of the
reaction. Q?K (shift left), Q?K (shift right),
Q K (at equilibrium). Calculations Example
SO2(g) ½ O2(g) ? SO3(g) K 1.84 at
1000K CO2(g) ? CO(g) ½ O2(g) K 6.03 x
10-11 Find K for the reaction SO2(g)
CO2(g) ? CO(g) SO3(g) K 1.11 x 10-10
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Le Chateliers Principle If a system in
equilibrium is altered in any way, the system
adjust itself by shifting the reaction to
minimize the effect of the change (Use Q to
predict the shift of the reaction) 1) Change of
the concentration or partial pressure of one
reactant or product. An example SO2(g)
NO2(g) ? NO(g) SO3(g) K 1.96 _at_ 1200K
Compare Q and K for shift of reaction.
Pressures of inert materials or non-reactive
materials do not influence Keq. 2) Change of the
volume of the container. In general ? V
shift to the side has more moles. If equal
moles of product and reactant then ?V has no
influence. 3) Change the pressure of the
container. (Change the partial pressure of an
inert gas or a non-reactive material has nothing
to do with the K) ? in Pressure favors side with
low gas moles 4) Change of temperature of the
environment (depend on exothermic or endothermic
reaction, Keq changes with temperature). Inc in
temp favors direction of endothermic 5) Addition
of a Catalyst, no effect on K, Equilibrium is
achieved faster 6) Adding Inert gas No effect
on K
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• Review
• Problems with Equilibrium concentrations given
  and find Kc or Kp
• Problems with initial concentrations and K, find
  eqm concentrations
• Need ICE chart in this one!!
• For ice chart problems, can you use
  Approximations??
• K100 ltlt reactant0 YES!!
• X 100/ reactant0 lt 5 YES!!
• Ice chart has to be in MOLARITY or PARTIAL
  pressure, no other units such as moles etc
  allowed