Chemical Kinetics and Rates of Reaction

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Chemical Kinetics and Rates of Reaction

• McGraw-Hill Chemistry Text
• Read Pg 462-467 and 481-482

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Chemical Kinetics

• The study of rates of chemical reactions and the
  mechanisms (or steps) by which a chemical
  reaction takes place.
• Reaction rates vary greatly some are very fast
  (burning) and some are very slow (disintegration
  of a plastic bottle in sunlight).

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• Rate of reaction the change in concentration of
  a reactant or product per unit of time.
• Rxn Rate (avg) ? reactant or product
• ? time
• Note square brackets mol/L

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Example problem

• If the concentration of reactant X after 50 s in
  a reaction was 0.0079 mol/L and after 100 s was
  0.0065 mol/L, calculate the average reaction
  rate.
• Rate ?X / ?t
• (0.0065 0.0079 M) / (100-50 s)
• - 0.000028 M/s

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• Concentration of reactant decreases with time, as
  it is being used up
• The rate is fastest when concentration of
  reactants is greatest and slows when
  concentration of reactants are less

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• The reactants are being used up as the reaction
  takes place.
• Whats happening to the concentration of products
  over time?
• It is increasing.

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• In the beginning of the reaction, product is
  formed quickly, but slows over time because less
  reactants means less reactants collisions
  producing the new product.

Concentration of product (mol/L)
Time (s)
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Factors that affect the Rate

• Concentration (and pressure)
• Temperature
• Amount of surface area
• Catalysts
• Nature of reactants

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Concentration

• A higher concentration of reactants leads to more
  effective collisions per unit time, which leads
  to an increasing reaction rate
• We are not increasing the amount being made for a
  given balanced equation with limiting reactants,
  we are only speeding up how quickly those
  products are made.

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Pressure

• affects the rate of reaction, especially when you
  look at gases.
• When you increase the pressure, the molecules
  have less space in which they can move. That
  greater concentration of molecules increases the
  number of collisions.
• When you decrease the pressure, molecules don't
  hit each other as often. The lower pressure
  decreases the rate of reaction

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Temperature

• Temperature is a measure of the kinetic energy of
  a system, so higher temperature implies higher
  average kinetic energy of molecules and more
  collisions per unit time.

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Surface Area

• Reducing the size of particles increases the rate
  of a reaction because it increases the surface
  area available for collisions to take place. This
  increases the number of collisions.

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Catalysts

• A catalyst is a substance that speeds up a
  reaction without being used up itself.
• Some reactions have catalysts that can speed them
  up, but for many reactions there is no catalyst
  that works.
• How do they work?

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1. A catalyst provides a surface on which the
reaction can take place.

• One or more reactants are adsorbed to the surface
  (stick to it)
• Lets look at the example of ethene undergoing
  hydrogenation (breaking double bond and making
  single bonds while adding hydrogen)
• Usually VERY slow

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• Interaction between the catalyst and the reactant
  (maybe weakening bonds, or an actual reaction
  with the surface)
• The reactants may collide on the catalyst or one
  is prepared and then collide together

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• The product is desorbed, breaks away from the
  catalyst once formed
• Nickel and platinum are excellent heterogeneous
  catalysts. They dont adsorbed too much or too
  little.
• Hydrogenation of vegetable oil to make margarine
  uses a nickel catalyst

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2) A catalyst lowers the activation energy

• AE the minimum amount of energy needed for a
  reaction to take place.
• Reactants require less energy to react.
• By providing an alternative mechanism (step in
  the reaction)
• Sometimes catalysts seem to be used up, but they
  will regenerate at the end of the reaction

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Nature of Reactants

• Reactants with large number of chemical bonds
  that have to be broken, or with strong chemical
  bonds that have to be broken, will lead to a
  slower rate.
• The strong bonds will cause a high activation
  energy, leading fewer collisions being
  successful.

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• More complex reactant molecules with more atoms,
  will have more difficulty lining up properly to
  have a successful reaction. This will also lead
  to a lower reaction rate.

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Good rule of thumb

• Ions will react more quickly, because already
  broken up.
• Simple molecules (non-metals covalently bonded)
  will take more time
• Complex molecules and ionic compounds (metal and
  non-metal bonded) will take the most time because
  the strong ionic bonds need lots of energy to be
  broken, and the complex molecules will be hard to
  have proper geometry (line up)

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Practice

• Place the following reactions from fastest to
  slowest, based on the previous slides.
• 2Cu2 2I- ? 2 Cu I2
• 2NaCl ? 2Na Cl2
• Cl2 ? 2Cl

Ans 1 is fastest, then 3, and 2 is slowest