Part I: Applications of Equilibrium Constants and Le Chateliers Principle

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Part I Applications of Equilibrium Constants
and Le Chateliers Principle

• Thursday, July 12th
• CHM 102

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Applications of Equilibrium Constants

• So far we have
• Calculated equilibrium constants.
• Covered what magnitudes of the Kcs mean.
• Calculated equilibrium concentrations.
• Calculated a new equilibrium constant when a
  chemical equation is flipped or multiplied.
• Now were going to predict directions of
  reactions that arent at equilibrium!

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Q, the reaction quotient

• The reaction quotient, Q, is calculated the same
  way as one would an equilibrium constant
  (products over reactants, etc.).
• The big difference is, the concentrations used to
  calculate Q are not taken at equilibrium!
• The value of Q when compared to K can help us
  predict where a reaction will go (products or
  reactants) at any time during the reaction.

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Q, the reaction quotient

• There are three general rules for comparing Q and
  K
• When Q K, the reaction is already at
  equilibrium.
• When QgtK, concentration of products is too large,
  and the reaction is going to shift to make more
  reactants to bring the system to equilibrium.
• When QltK, too many reactants, so the reaction
  will shift to products to bring the system to
  equilibrium.

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Practice Q, the reaction quotient

• For the reaction
• predict the direction the reaction will
  proceed to reach equilibrium. The Kc for the
  reaction is 50.5 at 448oC, and were starting
  with a 1.0 L container holding 0.02 mol H2, 0.01
  mol I2, and 0.07 mol HI.

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Le Chateliers Principle

• LCs principle helps predict the effects of
  temperature, concentration, and pressure on
  equilibrium.
• It states If a system at equilibrium is
  distrubed by a change in temperature, pressure,
  or concentration of one of its components, the
  system will shift its equilibrium position so as
  to counteract the effect of the disturbance.

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Le Chateliers Principle

• LCs principle helps predict the effects of
  temperature, concentration, and pressure on
  equilibrium.
• It states If a system at equilibrium is
  distrubed by a change in temperature, pressure,
  or concentration of one of its components, the
  system will shift its equilibrium position so as
  to counteract the effect of the disturbance.

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Monkey on a tightrope
DEMO Student with a balance
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LCs Principle Concentration

• Consider a the reaction A B ? C at equilibrium.
  Once were at equilibrium we add more C to the
  mixture. What happens? (hint think of our
  monkey on the tightrope)
• The system is going to attempt to counter what we
  did to it! It will consume some C and work to
  produce more A and B to get back to equilibrium.
• In short, if we add more reactant or product to a
  chemical system already at equilibrium, the
  system will act to use up some of what we put in,
  and will shift to the other side.

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Practice LCs Principle

• Now were looking at the Haber process again.
  Lets say the system is at equilibrium, and we
  remove some ammonia from the system. What
  direction will the reaction shift to achieve
  equilibrium?

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LCs Principle Pressure

• This portion of LCs principle applies to systems
  containing gases.
• Again lets look at the Haber process
• How many moles of gas are on each side?
• If we up the pressure on the gas, with
  temperature and concentration constant, what will
  the system do?
• If we cut the volume of the container the system
  is in by a half, what will the system do?

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LCs Principle Temperature

• LCs principle can take thermodynamics into
  account here.
• If we know whether a process is endothermic (DH gt
  0) or exothermic (DH lt 0), we can predict the
  effects of raising or lowering temperature on the
  equilibrium of a system.
• Temperature, unlike concentrations and pressure,
  actually changes the equilibrium constant.

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LCs Principle Temperature

• Weve got the reaction A B ? C. The DHrxn is
  -18.6 kJ/mol.
• This is an exothermic reaction, so the process
  gives off heat. So lets think of heat as a
  product of the reaction A B ? C heat.
• What will happen if we raise the temperature?
• What will happen if we lower the temperature?

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Practice LCs Principle

• Youve got the reaction B C ? 3D. The DH for
  the reaction is 8.4 kJ/mol. If the reaction is
  at equilibrium at 273 K and we raise the
  temperature to 350 K, which direction will the
  process shift (products or reactants)? What will
  happen to K?

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LCs Principle and Temperature

• This sums up the effects of temperature on
  equilibrium as predicted by LCs principle
• Endothermic reactions Increasing T makes an
  increase in K, and a shift to products.
• Exothermic reactions Increasing T makes a
  decrease in K, and a shift to reactants.

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Exam Review 2

• Thursday, July 12th
• CHM 102