Chapter 19, part II Notes

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Chapter 19, part II Notes

• Equilibrium Reactions
• LeChateliers Principle
• Equilibrium Constants

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Reactions so far

• Up to now, we have assumed that reactions go one
  way and stop when all reactants have become
  products.
• This is true sometimes, but not always. There
  are also

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Reversible Reactions

• In a reversible reaction, the reaction occurs
  simultaneously in both directions.
• Products are being formed and are also being used
  to form reactants.

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• 2SO2 O2 D 2SO3

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Equilibrium

• So how much product is formed? It depends on the
  reaction.
• Eventually, the reaction will reach chemical
  equilibrium, where forward and reverse reactions
  take place at the same rate.

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Equilibrium

• This does NOT mean that the amounts of product
  and reactant are the same.
• The equilibrium position is the relative percent
  of products and reactants in a system at
  equilibrium.

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LeChateliers Principle

• Henri LeChatelier studied shifts in the
  equilibrium of reactions and came up with his
  principle. He said
• If a stress is applied to a system in dynamic
  equilibrium, the system changes to relieve the
  stress.

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LeChateliers Principle

• Lets look at how several factors will shift the
  equilibrium point.

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Concentration

• Increasing the concentration of a product or
  reactant in equilibrium will shift the
  equilibrium point away from that substance.
• Decreasing the concentration would have the
  opposite effect.

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H2CO3 D H2O CO2

• Which way will equilibrium shift if adding
• CO2
• H2CO3
• H2O
• Which way will equilibrium shift if removing
• CO2
• H2CO3

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Temperature

• Adding heat to a reaction at equilibrium will
  shift the equilibrium point towards the side that
  absorbs heat.
• Removing heat will shift the equilibrium point
  towards the side that produces heat.

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2SO2 O2 D 2SO3 heat

• Which way will equilibrium shift if heat is
  added?
• Which way will it shift if heat is removed?

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Pressure

• Adding pressure to a system at equilibrium will
  shift the equilibrium point towards the side that
  has the least moles of gas.
• Decreasing pressure shifts equilibrium towards
  the side with the most moles of gas.

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N2 3H2 D 2NH3

• Assuming all the products and reactants are
  gases, which way does equilibrium shift when
  pressure is added?
• Which way does it shift when pressure is relieved?

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Equilibrium Constants

• Generally, the equilibrium point is expressed as
  a numerical value, called the equilibrium
  constant, for any reversible reaction at a given
  temperature.

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Given the equation of a general reaction to be
coefficients

• aA bB D cC dD

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The equilibrium constant would be
CcDd AaBb

• Keq

The brackets around the substances means it is
their concentration (molarity).
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Equilibrium constant

• If the equilibrium constant (Keq) is bigger than
  one, it means the products are favored at
  equilibrium.
• A Keq less than one means the formation of
  reactants is favored at equilibrium.

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N2O4 D 2NO2

• In the above reaction, at the equilibrium point,
  dinitrogen tetroxide has a concentration of
  0.0045M and nitrogen dioxide has a concentration
  of 0.030M. What is the equilibrium constant for
  the reaction?

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N2 3H2 D 2NH3

• Calculate Keq for the reaction if at equilibrium
  in 1.0L there is 0.15mol H2, 0.25mol N2 and
  0.10mol NH3.

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2HI D H2 I2

• Keq for the above reaction is 0.020 if the
  concentration of hydrogen and iodine are both
  0.50M, what is the concentration of HI?