Rates of Reactions

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Rates of Reactions
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Rates

• Speed or rate is measured by the amount of change
  that occurs in a given time interval

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REACTION RATE

• The amount of time it takes to use up the
  reactants (decrease in reactant concentration)
• OR
• The amount of time it takes to make products
  (increase in product concentration)

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I.A. Reaction Rates

• Reaction Rate-change in concentration of
  reactants and products in a given time period
• Consider the following reaction
• 2 NO2 ? 2 NO O2

time (sec) NO2 0 4.0 M 5.0 3.1
M 10.0 2.5 M 15.0 2.1 M 20.0 1.8 M Sketch a
graph with t on the x-axis and M on the y-axis.
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I.A. Reaction Rates

• 2 NO2 ? 2 NO O2
• How much NO is present at the beginning of the
  reaction?

time (sec) NO2 0 4.0 M
5.0 3.1 M 10.0 2.5 M 15.0 2.1 M
20.0 1.8 M
NO
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I.A. Reaction Rates

• 2 NO2 ? 2 NO O2
• How much NO2 was used up in the first 5 seconds?
• How much NO was produced?

time (sec) NO2 0 4.0 M
5.0 3.1 M 10.0 2.5 M 15.0 2.1 M
20.0 1.8 M
NO
0 M

• Calculate the moles of NO produced for each value
  of t (always use the total moles of NO2 used up
  to that point).

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Collision Theory

• Reaction rate depends on the collisions between
  reacting particles.
• Effective collision? Reaction occurs
• Elastic collisions ? No reaction occurs
• Effective collisions occur if the particles...
• collide with each other
• have the correct orientation
• have enough kinetic energy to break bonds

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• Particle Orientation

Required Orientation
Unsuccessful Collisions
Successful Collision
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I.B. Collision Theory
Elastic Collisions
Effective Collisions
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Activation Energy (Ea)
Even though the boulder will be more stable at
the bottom of the hill, it needs a push to get up
and over the bump first.

• depends on reactants
• low Ea fast rxn rate

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Activation Energy (Ea)

• Minimum energy required for a reaction to occur
• Transition state-complex that is neither reactant
  nor product

Activated Complex
Energy of products
?H - Difference between energy of reactants and
products
Energy of reactants
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• Draw an energy diagram for an exothermic
  reaction. Label the axes, reactants, products,
  activated complex, activation energy, and change
  in enthalpy.

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Factors Affecting Reaction Rate

• 1. Nature of the Reactants
• 2. Concentration of Reactants
• 3. Surface Area of Reactants
• 4. Temperature
• 5. Catalysts

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Factors Affecting Reaction Rate

• 1. Nature of the Reactants
• Certain bonds are easier to break than others
• Ca(s) 2H2O(l) ? H2(g) Ca(OH)2(aq)
• 2Na(s) 2H2O(l) ? H2(g) 2NaOH(aq)
• Na reacts with water faster than Ca because Na is
  more reactive.

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• 2. Concentration

high concentration the higher the
concentration ? the more molecules there are ?
the more collisions take place creating product
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• 3. Surface Area
• the more surface a molecule can touch ? the
  faster the reaction
• Increase surface area by
• using smaller particles
• dissolving in water

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Temperature

• 4. Temperature
• high temp fast reaction rate
• the hotter the molecules ? the more they move ?
  the more they collide change into product

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• Lower temp, more reactant particles have low amts
  of kinetic energy
• As temperature increases, more reactant particles
  have higher kinetic energy
• Only a small of particles will have enough
  energy to form an activated complex.

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Catalysts

• 5. Catalyst
• substance that increases reaction rate without
  being consumed in the reaction
• lowers the activation
• energy, more collisions
• will have enough energy
• to pass over the energy barrier
• reaction is faster because a greater fraction of
  collisions is effective at any given point

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• Catalysts in body
• Enzymes
• Breakdown sugars, proteins, etc

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Reaction Mechanisms

• Many reactions occur in several steps.
• 2 O3 (g) ? 3 O2 (g)
• The decomposition of ozone to oxygen is catalyzed
  by Cl.

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Reaction Mechanisms

• Cl O3 ? ClO O2 Elementary step
• O3 ? O O2 Intermediates
• ClO O ? Cl O2 Add steps
• 2 O3 ? 3 O2

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Reaction Mechanisms

• 2 NO ? N2O2 Fast
• N2O2 H2 ? N2O H2O Slow
• N2O H2 ? N2 H2O Fast
• Determine the original equation.
• Rate-determining step.

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Reaction Mechanisms

• HBr NO2 ? HOBr NO Slow
• HBr HOBr ? H2O Br2 Fast
• What is the overall reaction?
• How many elementary steps?
• What are the intermediates?
• What is the rate-determining step?