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Ch 17 Reaction Rates

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					Reaction Rates

       Chapter 17
Expressing reaction
rates
                          ∆ quantity
     Average rate =   - ________________
                          ∆ time



     This equation defines the average rate at
      which reactants produce products.
Chemists look at changes in molar
concentration (mol/L or M) of reactants or
products per unit time.

Average rate is expressed as mol/L· s

Reaction rates are determined
experimentally by measuring concentrations
in a reaction.

Rates CANNOT be calculated from balanced
equations.
Average reaction rates

• ALWAYS POSITIVE
• Negative sign (-) in front of
  equation is used to calculate
  rate of consumption of
  reactants.
The Collision Theory

• Atoms, ions, and molecules
  must collide in order to react.
• Explains why reactions occur
  and how rates of chemical
  reactions can be modified.
The Collision Theory

1. Reacting substances (atoms, ions,
   or molecules) must collide.
2. Reacting substances must collide
   with the correct orientation.
3. Reacting substances must collide
   with sufficient energy to form the
   activated complex.
2H2 + O2 → 2H2O

• According to the collision
  theory, H2 and O2 must collide in
  order to produce water (H2O).
CO + NO2 → CO2 + NO

• In this reaction, CO and NO2
  must also collide to produce
  CO2 and NO.
• However, only a small fraction
  of collisions produce reactions.
Orientation

• In the previous example, few
  collisions result in reactions
  because the C in CO must
  collide with the O in NO2 for a
  temporary bond to form.
• Any other contact results in
  rebound (or bouncing off) of
  reactants.
Activated Complex

• When orientation is correct at
  collision, and intermediate
  substance (activated complex)
  is formed.
• Often acts as a transition state
  (or first step) leading to the
  products.
Activation energy

• Collisions between reactants
  must also have enough
  activation energy in order to
  produce the activated complex.
• Correct orientation (at collision)
  will not produce the activated
  complex if there is insufficient
  energy.
Exothermic reaction
Endothermic reaction
Influence of Spontaneity

• Spontaneity only indicates the
  natural tendency for a reaction
  to occur.
• Spontaneity will NOT affect the
  reaction rate.
17.2 Factors affecting
reaction rates
1. Nature of reactants
  Reactive nature of the reactants
  Some react more readily than others
Factors cont

2. Concentration
     Higher concentrations of
  reacting particles increases the
  rate of reaction.
     Higher concentrations
  increases number of particles
  present and increases number
  of collisions.
Factors cont

3. Surface Area
     Increasing surface area
  increases reaction rates.
     Increased surface area gives
  more room for collisions in the
  same amount of time.
Factors cont

4. Temperature
     Increasing temperature
   increases the reaction rates.
     Higher temperatures raises
   kinetic energy of reacting
   particles and raises frequency
   of collisions and collision
   energy.
Factors cont

5. Catalysts
     Increase reaction rates
   without being consumed in the
   reaction (metabolic reactions).
     Catalysts (enzymes) lower
   the activation energy needed
   for a reaction to occur which
   increases the reaction rate.
Enzyme catalyzed
reaction
Factors cont

6. Inhibitors
     Catalysts that slow or stop
   reactions.
     Food industry: preservatives
   to prevent spoilage or weed
   herbacides that stop weed
   growth.
Catalysts

Heterogenous: exist in a physical
 state different than that of the
 reaction it catalyzes.
Homogenous: exist in the same
 physical state as the reaction it
 catalyzes.
17.3 Reaction Rate
Laws
• Rate law - equation that
  expresses the mathematical
  relationship between the rate of
  reaction and the concentration
  of reactants
• A → B Rate = k [A]
• k=specific rate constant
  [A]=concentration of A
Specific rate constant
k
• unique for every reaction
• Concentration per time; usually in
  mol/s
• Reaction rate is directly proportional
  to molar concentration
• k does NOT change w/concentration
  but will change with temperature
Instantaneous reaction
rates
• Rate of decomposition at a
  specific time
• Use rate law: Rate=k [A]
• Constant at specific
  temperature and concentration
  of reactant will be given
Reaction Mechanisms

• Most chemical reactions consist of a
  series of simple reactions
  (elementary reactions)
• A complex reaction consists of 2 or
  more elementary reactions
• Reaction mechanism is made of the
  complete sequence of elementary
  reactions that make the complex
  reaction
Intermediates

• A substance produced in one
  elementary step that is
  consumed in the next
  elementary step
Rate-determining step

• Reactions cannot occur any
  faster than the slowest
  elementary step.
• The slowest elementary step is
  the rate-determining step since
  it controls the rate of the
  reaction.

				
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