Download Reaction Rate-Catalyst

Chapter 13: Reaction Rate
• Rate = ∆ [A]/ ∆t
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Related to
(1) Concentration: Rate Laws
(2) Temperature
(3) Catalyst
Chapter 13: Reaction Rate: Effect of
Temperature
• CONSIDERATIONS
– As temperature increases, the rate increases for most
reactions
– The rate constant is temperature related.
• Reaction Rate is Proportional to Temperature.
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NOT ALWAYS A LINEAR
RELATIONSHIP
Temperature and Rate
Reaction Rate: Effect of Temperature
The Collision Model
• The collision model: in order for molecules to
react they must collide.
• The greater the number of collisions the faster the
rate.
Reaction Rate: Effect of Temperature
The Collision Model
Reaction Rate: Effect of Temperature
The Collision Model
• The more molecules present, the greater the
probability of collision and the faster the rate.
• The higher the temperature, the more energy
available to the molecules and the faster the rate.
• In order for reaction to occur the reactant
molecules must collide in the correct orientation
and with enough energy to form products.
Reaction Rate: Effect of Temperature
Activation Energy
• Arrhenius: molecules must posses a minimum
amount of energy to react.
• In order to form products, bonds must be broken
in the reactants.
– Bond breakage requires energy.
• Activation energy, Ea, is the minimum energy
required to initiate a chemical reaction.
Temperature and Rate
Activation Energy
Reaction Rate: Effect of Temperature
Activation Energy
• Consider the rearrangement of acetonitrile:
H3C N C
H3C
N
C
H3C C N
– In H3C-NC, the C-NC bond bends until the C-N
bond breaks and the NC portion is perpendicular to
the H3C portion. This structure is called the activated
complex or transition state.
– The energy required for the above twist and break is
the activation energy, Ea.
– Once the C-N bond is broken, the NC portion can
continue to rotate forming a C-CN bond.
Temperature and Rate
Activation Energy
Reaction Rate: Effect of Temperature
The Arrhenius Equation
• Arrhenius discovered most reaction-rate data
obeyed the Arrhenius equation:
k  Ae
 Ea
RT
– k is the rate constant, Ea is the activation energy, R is the gas
constant (8.314 J/K-mol) and T is the temperature in K.
– A is called the frequency factor.
– A is a measure of the probability of a favorable collision.
– Both A and Ea are specific to a given reaction.
Reaction Rate: Effect of Temperature
The Arrhenius Equation
• If we have a lot of data, we can determine Ea and
A graphically by rearranging the Arrhenius
Ea
equation:
ln k  
 ln A
RT
• If we do not have a lot of data, then we can use
k1 Ea  1
1
ln

  
k2
R  T2 T1 
Problem
• A certain first order reaction has a rate constant of
2.75E-2s-1 at 20 ºC. What is the value of “k” at 60 º
C if (a) Ea = 75.5 kJ/mole
Problem: Solution
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• VARIABLES
Ln (k1/k2) =Ea/R (1/T2 – 1/T1)
k1 = 2.75E-2s-1
T1 = 20 º C + 273 K/ º C = 293 K
T2 = 60 º C + 273 K/ º C = 333 K
k2 = x
R = 8.314 Joules/(k) (mole)
Problem: Solution
• Ln (k1/k2) =Ea/R (1/T2 – 1/T1)
• LN (2.75E-2/x) = 75.5 kJ/0.008314 kJ/Kmole (1/333 K –1/293 K)
• Ln (2.75E-2/x) = 75.5kJ/0.008314 kJ/Kmole (-4.10E-4)
• Ln (2.75E-2/x) = -3.723
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take ex or antilog/inverse of both sides
• 2.75E-2/x = 2.416E-2
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x = k2 =
1.1s-1
Reaction Mechanism
• Few reactions go to products in one step.
• Determination of number of steps and
formulas of compounds constitutes a
reaction mechanism.
Reaction Mechanism
STEP ONE:
NO (g) + O3 (g)  NO2 (g) + O2 (g)
STEP TWO:
NO2 (g) + O (g)  NO (g) + O2 (g)
SUMMARY: O3(g) + O(g) 2O2(g)
Chapter 13: Reaction Rate-Catalyst
• A catalyst changes the rate of a chemical reaction.
• There are two types of catalyst:
– homogeneous, and
– heterogeneous.
Reaction Rate: Catalyst
Homogeneous Catalysis
O3(g) + O(g) 2O2(g)
– STEP ONE:
– STEP TWO:
NO (g) + O3 (g)  NO2 (g) + O2 (g)
NO2 (g) + O (g)  NO (g) + O2 (g)
– SUMMARY: O3(g) + O(g) 2O2(g)
• Catalysts usually increase rate by lowering the
activation energy for a reaction.
Reaction Rate: Homogenous Catalyst
• Catalysts usually increase rate by lowering the
activation energy for a reaction.
• Increases the number of effective collisions.
Catalysis
Homogeneous Catalysis
Chapter 15: Reaction Rate &
Heterogeneous Catalyst
• The catalyst is in a different phase than the
reactants and products.
• Typical example: solid catalyst, gaseous reactants
and products (catalytic converters in cars).
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First step is adsorption (the binding of reactant
molecules to the catalyst surface).
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Adsorbed species (atoms or ions) are very
reactive.
• Molecules are adsorbed onto active sites (red
spheres) on the catalyst surface.
Reaction Rate: Heterogeneous Catalyst
Heterogeneous Catalysis
Reaction Rate: Heterogeneous Catalyst
Enzymes
• Enzymes are biological catalysts.
• Enzymes have very specific shapes.
• Substrates undergo reaction at the active site of an
enzyme.
• A specific reactant/substrate fits a specific active
site
Reaction Rate: Heterogeneous Catalyst
Enzymes