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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 Chemical Kinetics The study of rates of chemical reactions and the ... – PowerPoint PPT presentation

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Title: Chemical Kinetics and Rates of Reaction


1
Chemical Kinetics and Rates of Reaction
  • McGraw-Hill Chemistry Text
  • Read Pg 462-467 and 481-482

2
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).

3
  • 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

4
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

5
  • 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

6
  • The reactants are being used up as the reaction
    takes place.
  • Whats happening to the concentration of products
    over time?
  • It is increasing.

7
  • 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)
8
Factors that affect the Rate
  • Concentration (and pressure)
  • Temperature
  • Amount of surface area
  • Catalysts
  • Nature of reactants

9
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.

10
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

11
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.

12
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.

13
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?

14
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

15
  • 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

16
  • 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

17
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

18
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.

19
  • 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.

20
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)

21
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
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