Reaction Rates

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• Reaction Rates
• and
• Reaction Mechanisms
• UNIT XI
• (Ch. 17)

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Reaction Rate

• The speed at which a chemical reaction proceeds.
  Usually expressed as the rate at which a reactant
  disappears or the rate at which a product is
  formed.
• Average reaction rate ?quantity/?time

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Collision Theory

• States that atoms, ions and molecules must
  collide in order for them to react. But only a
  fraction of those collisions result in a reaction
  taking place.

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Reaction Requirements

• For a reaction to take place, the particles
  (e.g., molecules) must react
• in the proper orientation, and
• with sufficient energy to form an Activated
  Complex

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Activation Energy, Ea

• The minimum amount of energy required to form the
  activated complex.

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Reaction Rate

• The speed at which a chemical reaction proceeds.
  Usually expressed as the rate at which a reactant
  disappears or the rate at which a product is
  formed.
• Many reaction rates double for every 10oC
  increase in temperature
• Factors that influence reaction rates
• Nature of the reactants
• Surface area
• Temperature
• Concentration
• Presence of catalysts

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Nature of the Reactants

• Ionic Reactions instantaneous
• Reactions involving bond rearrangement or
  electron transfer occur more slowly
• Neutral molecule reactions slower
• Activated Complex formed by the colliding
  molecules
• Activation Energy - energy required to form the
  activated complex.

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Reversible Reactions

• Reversible Reaction - a reaction that can go
  forward or backward. A reaction is said to go to
  completion if it proceeds until all of one of the
  reactants is completely used up. However, some
  reactions are reversible.
• e.g., H2 I2 2 HI
• H2 I2 2 HI
• or, H2 I2 2 HI (reversible)
• Reversible reactions eventually reach equilibrium
  (kf kb), where kf is the rate of the forward
  reaction, and kb is the rate of the reverse (or
  backward) reaction.

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Surface Area

• In a heterogeneous reaction involving a solid as
  one of the phases, it is important that the
  liquid or gas with which it is reacting can
  contact the surface of the solid. Therefore, the
  surface area is a very important factor in
  determining the overall reaction rate.

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Temperature

• Speed and Kinetic Energy increase as temperature
  increases. Frequency of collisions will increase.
  Thus reaction rates will increase with
  Temperature.
• Increasing the temperature increases the number
  of particles that have the required activation
  energy (see diagram).

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Energy Distribution
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Concentration

• Increasing the concentration increases the
  probability of collisions. Therefore, the
  reaction rate increases.
• H2 I2 2 HI
• rate1 µ I2
• rate1 k1 I2
• rate2 k2 H2
• therefore, the overall reaction rate
• rate k H2I2
• where "k" is the specific rate constant

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Catalysis

• A catalyst increases the rate of a reaction
  without being involved in the reaction (no
  permanent change).
• Serves to reduce the activation energy.
• Heterogeneous Catalyst - two phases
• Homogeneous Catalyst - one phase
• Forms an intermediate compound(s) that reacts
  more readily than the uncatalyzed reactants.
  Activation Energy is reduced.
• Inhibitor(s) - ties up a reactant so it cannot
  react (food, drugs)

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Pressure

• Increasing the pressure on gas reactants reduces
  the volume and increases the concentration.
• Thus, the rate µ pressure
• Homogeneous Reactions - reactants are in the same
  phase
• Heterogeneous Reactions - reaction takes place at
  the interface between two phases.

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Rate Laws

• For the reaction
• 2 H2(g) 2 NO(g) N2(g) 2 H2O(g)
• rate ? H2
• rate ? NO2
• ? rate ? H2 NO2
• or, rate k H2 NO2
• This last expression is call the rate law.

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Reaction Order

• The exponent of the concentration of a substance
  in the rate law is call the rate order.
• Each substance in a reaction will have its own
  rate order.

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Overall Rate Order

• The overall rate order of a chemical reaction is
  the sum of the order for the individual reactants
  in the rate law.
• a A b B products
• The general rate law for such a reaction is
• rate kAmBn
• If the reaction occurs in a single step, then ma
  and nb. That often is not the case. (see the
  reaction above involving H2 and NO to form N2 and
  H2O).

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Determining Rate Order
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Instantaneous Reaction Rates
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Reaction Mechanisms

• A Complex Reaction is one that consists of two or
  more elementary steps. The complete sequence of
  these elementary steps that make up a complex
  reaction is called a reaction mechanism.
• An intermediate is a substance that is produced
  by one elementary step and consumed in a
  subsequent elementary step.

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Rate-determining Step

• A B C
• rate kAB slow
• and C D E
• rate kCD fast
• The overall rate for the production is determined
  by the slower step - in this case the first
  reaction.