Environmental Chemistry

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Environmental Chemistry
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3. Chemical Kinetics

• The study of the rates and mechanisms of chemical
  reaction.
• Rate of reaction - the amount of chemical change
  that takes place in a given interval of time.

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The Rate of Chemical Reaction

• The rate at which reactants are consumed or
  products are produced in a chemical reaction
• Will a reaction occur?
• Collision Theory
• for a reaction to occur
• -reactant particles must collide
• -collision must have a certain minimum
  amount of energy Activation Energy
• -reactants may require a specific orientation

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Potential Energy Diagrams

Activation Energy, Ea
Avg. Energy of Products, PEP
E
?E energy absorbed during the reaction
Avg. Energy of Reactants, PER
Endothermic Reaction
reaction progress
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Rate Law

• For General Reaction
• aA bB ? cC dD
• The rate law generally has a form
• Rate kreactant 1mreactant 2n
• For the reaction above,
• Rate kAaBb
• where k in the rate law is called the rate
  constant

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Order of Reaction

• The sum of all the exponents of the concentration
  terms in the rate equation

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Sample Problem 12

• How long will it take the carbon monoxide (CO)
  concentration in room to decrease by 99 percent
  after the source of carbon monoxide is removed
  and the windows are opened? Assume the first
  order rate constant for removal (due to dilution
  by incoming clean air) is 1.2/hr. No chemical
  reaction occurring.

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Half-Life (t½)

• It is defined as the time required for the
  concentration of a chemical to decrease by
  one-half (for example, C 0.5C0).

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Sample Problem 13

• Subsurface half-lives for benzene, TCE, and
  toluene are listed as 69, 231, and 12 days,
  respectively. What are the first-order rate
  constant for all three chemicals.

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Sample Problem 14

• After a Chernobyl nuclear accident, the
  concentration of 137Cs in milk was proportional
  to the concentration of 137Cs in the grass that
  cows consumed. The concentration in the grass
  was, in turn, proportional to the concentration
  in the soil. Assume that the only reaction by
  which 137Cs was lost by soil was through
  radioactive decay and the half-life for this
  isotope is 30 years. Calculate the concentration
  in milk shortly after the accident was 12,000
  bequerels (Bq) per liter. (Note A bequerel is a
  measure of radioactivity 1 bequerel equals 1
  radioactive disintegration per second.)

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Effect of Temperature on Rate Constants

• Arrhenius equation
• k Ae (Ea/RT)
• Where
• A preexponential factor (same as k)
• Ea activation energy (kcal/mole)
• R - gas constant
• T - temperature (K)

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Sample Problem 15

• The rate constant for carbonaceous biochemical
  oxygen demand (CBOD) at 20oC is 0.1/day. What is
  the rate constant at 30oC? Assume Ea 1.072.

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