Studio 9a

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Studio 9a

• 10/30/06

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How do the Precipitation Reactions Differ?

• What assumption did you make about the
  precipitation reactions?

You assumed that all three reactions went to
completion.
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Equilibrium

• The third precipitation reaction was an example
  of a system in equilibrium.
• Pb2(aq) 2I-(aq) ? PbI2(s)
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• What is the first thing that comes to mind when
  you think of the word equilibrium?

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• Equilibrium does NOT mean that ALL things are
  equal.  In chemical terms, it means that the rate
  at which products are formed from reactants and
  the rate at which reactants are formed from
  products are equal.

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What's the rate of reaction?

• For a simple reactions A B, rate kA.

Most reactions slow down as they proceed (go to
completion) and as the concentration/s of the
reactant/s decreases (the rate approaches zero.)
k is the rate constant which varies with
individual reaction (reactant and product)
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Forward and Reverse Rates

• Reactions in equilibrium have both a significant
  forward and reverse rate of reaction. A ? B

Forward reaction  A ? B Reverse reaction  B ? A
When equilibrium is reached, the rates of the
forward and reverse reaction are equal AND are
NOT equal to 0.
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Forward Rate Reverse Rate

• Pb2(aq) 2I-(aq) ? PbI2(s)
• How do you results support the assignment of
  equilibrium to the reaction shown
• Pb2(aq) 2I-(aq) ? PbI2(s)