Lect w3 152_d2 - arrhenius and catalysts_alg (1)
Transcript of Lect w3 152_d2 - arrhenius and catalysts_alg (1)
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Temperature and Rate
• We have seen that, generally, as temperature increases so does the reaction rate.
• But the powers in the rate law do NOT change
• This means that k is temperature dependent.k
Temperature
We need a microscopic modelto explain this
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Three conditions must be met at the molecular level if a reaction is to occur:
The molecules must collide;
They must be positioned so that the reacting groups are together in a transition state between reactants and products;
The collision must have enough energy to form the transition state and convert it into products.
Collision Rate Model
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Molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.
Right Orientation
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Activation Energy• There is a minimum amount of energy required
for reaction: the activation energy, Ea.
• Just as a ball cannot get over a hill if it does not have enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.
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The higher the temperature, the more molecules have energy to overcome the
activation energy barrier.
Low T
High T
Extra moleculesat high T thatexceed the Ea
Enough Energy
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Examining a reaction
Consider the process in which methyl isonitrile is converted to acetonitrile.
CH3NC CH3CN
This reaction is suspected to be first order…
Design an experiment to determine the order of the reaction.
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Reaction Coordinate Diagrams
Reaction coordinate diagrams help visualize energy changes throughout a process.
ReactionCoordinate
CH3NC CH3CN
rearrangement of
methyl isonitrile.
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k and Catalysis
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Temperature and Rate
• We have seen that, generally, as temperature increases so does the reaction rate.
• But the powers in the rate law do NOT change
• This means that k is temperature dependent.k
Temperature
We need a microscopic modelto explain this
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Reaction Coordinate Diagrams
E Reactants
E Products∆E reaction
E activated complex
Ea
Notice Ea is not related to ∆E
UA GenChemReaction progress
Po
ten
tia
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ne
rgy
SOLUTION
A key reaction in the upper atmosphere is
O3(g) + O(g) 2O2(g)
The Ea(fwd) is 19 kJ, and the Hrxn (∆E) for the reaction is -392 kJ. Draw a reaction energy
diagram for this reaction, postulate a transition state, and calculate Ea(rev).
O3+O
2O2
transition state
Ea= 19kJ
Hrxn = -392kJ
Ea(rev)= (392 + 19)kJ =
411kJ
Your Turn
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The Arrhenius Equation
RTEa
Aek
ln k = ln A - Ea/RT
ln
k2
k1
=Ea
R-
1
T2
1
T1
-
where k is the rate constant at TEa is the activation energyR is the energy gas constant
= 8.3145 J/(mol K)T is the Kelvin temperature
A is the collision frequency factor
Temperature Effects
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Suppose a chemical reaction has an activation energy of 76 kJ/mol.
By what factor is the rate of reaction at 50 oC increased over its rate at 25
oC?k2=rate constant @ 50ºC k1=rate const @ 25ºC
k2/k1 = 10.7 over 10 times faster!
Typical Problems
lnk2
k1
1323.15K
1298.15K
-=-76000 J
8.3145 J/mol K
lnk2
k1
= 2.37
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The decomposition of hydrogen iodide,
Ea = ___________ kJ/mol
2HI(g) H2(g) + I2(g)
has rate constants of 9.51x10-9 L/mol*s at 500. K and 1.10x10-5 L/mol*s at 600. K.
Find Ea.
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SOLUTION
The decomposition of hydrogen iodide,
2HI(g) H2(g) + I2(g)
has rate constants of 9.51x10-9L/mol*s at 500. K and 1.10x10-5 L/mol*s at 600. K. Find Ea.
lnk2
k1=
Ea-R
1
T2
1
T1
-
ln1.10x10-5L/mol*s
9.51x10-9L/mol*s
1
600K
1
500K-Ea = -
(8.314J/mol*K)
Ea = 1.76 x 105 J/mol = 176 kJ/mol
Answer
/
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Series of plots of concentra-tion vs. time Initial
rates Reaction orders
Rate constant (k) and actual
rate law
Integrated rate law (half-life,
t1/2)
Rate constant and reaction
order
Activation energy, Ea
Plots of concentration
vs. time
Find k at varied T
Determine slope of tangent at t0 for
each plot
Compare initial rates when [A]
changes and [B] is held constant and
vice versa
Substitute initial rates, orders, and concentrations
into general rate law: rate = k [A]m[B]n
Use direct, ln or inverse plot to
find order
Rearrange to linear form and
graph
Find k at varied T
Overview
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Catalysts speed up reactions by altering the mechanism to lower the activation energy
barrier, but they are not consumed.
MnOMnO22 catalyzes decomposition of H catalyzes decomposition of H22OO22
2 H2 H22OO22 2 H 2 H22O + OO + O22
Catalysis
Uncatalyzed reactionUncatalyzed reaction
Catalyzed reactionCatalyzed reaction
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Imagine trying to get a bunch of cattle to where you want them to go without any
help…
“Cattle-ists”
Will they get there very
quickly on their own? Will they
take the shortest path do
get there? Is there a way to
help?
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What if there was someone there to herd the cattle and guide them along a more efficient
path?
“Cattle-ists”
The end result is the same – but
the reaction takes a more efficient path.
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Catalytic Converters
13.6
CO + Unburned Hydrocarbons + O2 CO2 + H2Ocatalytic
converter
2NO + 2NO2 2N2 + 3O2
catalyticconverter
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CatalysisCatalysis
3) Enzymes — biological catalysts3) Enzymes — biological catalysts3) Enzymes — biological catalysts3) Enzymes — biological catalysts
Enzymes are specialized organic
substances, composed of polymers of amino acids (proteins), that
act as catalysts to regulate the speed of
the many chemical reactions involved in
the metabolism of living organisms.
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Summary Activity:Fireflies flash at a rate that is temperature dependent.
At 29 ˚C the average firefly flashes at a rate of 3.3 flashes every 10. seconds.
At 23 ˚C the average rate is 2.7 flashes every 10. seconds.
Use the Arrhenius equation to determine the activation energy (kJ/mol) for the flashing process.