Part I: Intro to Equilibrium

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Part I Intro to Equilibrium

• Wednesday, July 11th
• CHM 102

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What is equilibrium?

• Consider equilibrium as a balanced tightrope
  walker. A successful tightrope walker may lean
  right or may lean left, but will settle at
  equilibrium (hopefully).
• In a scientific sense, equilibrium occurs when
  opposing reactions are occurring at the same rate.

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What is equilibrium?

• Take for example
• While dinitrogen tetroxide is colorless, the
  nitrogen dioxide is brown, so the color can give
  a clue to where equilibrium lies.
• Equilibrium does NOT MEAN THAT A REACTION HAS
  STOPPED AND NOTHING IS PROCEEDING!

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What does it mean?

• So in opposing reactions like we just saw, once
  the system reached equilibrium, there would be
  essentially no color change. The reaction
  forming the colorless gas is going at the same
  rate as the reaction forming the brown gas!
• If we look at the concentrations of products and
  reactants after the system has reached
  equilibrium we can put a number to our
  observations!

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What is this number?

• The number that can help us figure out where an
  equilibrium will lie (products or reactants) is
  called the equilibrium constant.
• The equilibrium constant is expressed using
  concentrations in the following way

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How do we write it?

• Lets take a look at the Haber process.
• Fun History fact In WWI, it was used for the
  production of ammonia by the Germans, which was
  used in explosives. Previously they had imported
  chilean saltpeter (mined from an island of bird
  poop) as a source for nitrates.

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Lets write one again!

• Lets look at the equililbrium of BaI2(s)
  dissolving in water

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One more time!

• Lets look at the acid-base equilibrium of
  hydrofluoric acid losing an H to water!

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What does the Kc tell us?

• If the equilibrium constant is much less than 1,
  the process youre looking at is reactant
  favored.
• If the equilibrium constant is much greater than
  1, the process is product favored!

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What happens to Kc when I flip the equation or
multiply it?

• Lets go back to where we started, knowing at
  25oC the K for the reaction is 25
• If you want the equilibrium constant for the
  reverse equation, its just 1/Kforward, or 1/25
  0.04.
• If you want the equilibrium constant if you
  multiplied the equation by a constant, just raise
  K by the constant you multiplied by, so K is now
  252, or 625!

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Practice Manipulating Kcs

• Lets go back to the Haber process. If the Kc
  value for the Haber process shown here
• at room temperature and pressure is .0005,
  then what is the equilibrium constant for the
  reaction shown here?

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How does k compare to Kc?

• Do not confuse the rate constant (little k) with
  the equilibrium constant (big K), as theyre two
  totally different quantities!
• We just learned about the rate constant, little
  k, and how it ties in with the speed of a
  reaction.
• Big K, the equilibrium constant, tells you where
  a reaction is going to end up, not how fast it
  will get there.
• Moral of the story little k tells you speed,
  big K tells you where youll be.

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Part II Calculating Equilibrium Constants

• Wednesday, July 11th
• CHM 102

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How do we actually find KC?

• There are a few different problem types for you
  to know for finding equilibrium constants.
• Not to scare you, but these WILL follow you
  through the course and to the final if you dont
  spend some time and pick up how to do them!

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Type I Equilibrium Conc. are given

• H2 and N2 are allowed to react via the Haber
  process and come to equilibrium at 25oC and 1
  atm, and at equilibrium, the concentration of N2
  is 0.5 M, the concentration of H2 is 0.6 M, and
  NH3 is 0.0073 M. What is the equilibrium
  constant under these conditions?

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Type I Equilibrium constant is given

• H2 and N2 are allowed to react via the Haber
  process and come to equilibrium at 25oC and 1
  atm, and at equilibrium, the concentration of N2
  is 0.5 M, and the concentration of H2 is 0.6 M.
  What is the equilibrium concentration of NH3 if
  the equilibrium constant under these conditions
  is 0.0005?

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Type II ICEbox problems

• ICE stands for Initial, Change, and Equilibrium
  concentrations of your products and reactants.
  The box means were going to be making a table
  of these concentrations that looks something like
  what you see below.

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Type II ICEbox problems with xs

• You initially have 0.1 M O2(g) in a reaction
  vessel at 25oC. The Kc at this temperature is
  110-5 for the process shown below. What are the
  equilibrium concentrations under these conditions?

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Type III ICEbox problems without xs

• You initially have 0.1 M O2(g) in a reaction
  vessel at 25oC. If 0.987 M of O2 remains at
  equilibrium, what is the equilibrium constant for
  this reaction at 25oC?

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More practice Equilibrium

• A 2.00 L flask is filled with 4.0 mol I2(g) and
  2.0 mol H2 gas at 273 K. If Kc at this
  temperature, is 40.6, what are the equilibrium
  concentrations of H2, I2, and HI?

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More practice Equilibrium

• If equilibrium concentrations of H2, I2, and HI
  are 0.6 M, 0.5 M, and 0.1 M at 650 K, what is the
  equilibrium constant under these conditions?

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More practice Equilibrium

• Initial concentrations of H2, I2, and HI are
• 0.25 M, 0.5 M, and 0.125 M at 780 K. If the
  final concentration of HI is .09M, what is the
  equilibrium constant under these conditions?

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More practice Equilibrium

• Based on the graph below at 273 K, equilibrium
  constant under these conditions?