today reaction coordinates activation energy catalysis
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Principles of Chemistry II © Vanden Bout
Today
Reaction Coordinates
Activation Energy
Catalysis
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We have a balloon with H2 and O2
why is not reacting?
2H2(g) + O2(g) 2H2O(g)
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A. this reaction is not spontaneous at room temperature
B. the reaction is very slow at room temperature
C. the reaction is very slow at these concentrations
D. B & C
E. all of the above
We have a balloon with H2 and O2
why is not reacting?
2H2(g) + O2(g) 2H2O(g)
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When the reaction is very very slow
the problem is typically that the rate constant is very small
What affects the rate constant?
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Arrhenius Picture
Reaction Coordinate
Energy
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A. all have the same energy
B. have a distribution of energies
C. have one of several fixed energies
At a given temperature the molecules in a sample
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How many molecules have enough energy to get over the barrier?
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Arrhenius Law
The rate constant k is a function of temperature
k = A e-Ea/RT
Arrhenius Law
pre-factor barrier
temperature
The higher the temperature the more molecules that have enough energy to make it over the barrier
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What is A?
This is the rate at infinite temperature(not all interactions between the molecules
even with sufficient energy will lead to products)
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Very important in organic chemistry“steric effect”
“steric hindrance”“steric protection”
putting a big unreactive part of the molecule “in the way”
to slow (or stop) the reaction
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Let's make a new Equation
k = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatures
lnk1 = ln A -Ea/RT1
lnk2 = lnA -Ea/RT2
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Let's make a new Equation
k = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatures
lnk1 = ln A -Ea/RT1
lnk2 = lnA -Ea/RT2
subtract to get a new equation that doesn't have A
ln(k2/k1) = -EaR
[ ]1T2 T1
1-
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A. -1.2 x 104 K
B. 1 x 105 J mol-1
C. 1.2 x 104 J mol-1
D. 1 x 105 K
E. -1 x 102 kJ mol-1
The activation energy for this reaction is?
-EaR
[ ]1T2 T1
1-ln(k2/k1) =
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A. -1.2 x 104 K
B. 1 x 105 J mol-1
C. 1.2 x 104 J mol-1
D. 1 x 105 K
E. -1 x 102 kJ mol-1
The activation energy for this reaction is?
Slope = -Ea/REa = -R x slope
Ea = -8.314 J K-1 mol-1 x (-1.2 x 104 K) = 1 x 105 J mol-1
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Why are reactions faster at higher temperatures?
More molecules have sufficient energy to getover the barrier. BIG EFFECT
More molecules have collisions (but this is a very small effect) that is
ignored in Arrhenius view
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Transition State Theory
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Transition State Theory
Two Step Mechanism (Two Barriers)
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What is the rate limiting step?We need to write reaction coordinate vs Free Energy
Reaction Coordinate
deltaG
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How else can affect k?
Change the barrier (mechanism)
Reaction Coordinate
Energy
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Catalyst
Lower the barrier for the reaction (by changing the mechanism)
Is not consumed during the course of the reaction(it can be used over and over again)
However, it might under go chemistry during the reaction,but the original form is regenerated by reaction.
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Decomposition of Hydrogen Peroxide
H2O2(l) O2(g) + H2O(l)
This reaction is very slow at room temperature(thus you can get a bottle of H2O2 at the store)
demo
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What happens when I add the catalyst
H2O2 + MnO2 + 2H+ Mn2+ + 2H2O + O2
Mn2+ + 2H2O2 Mn(OH)2 + 2H+
Mn(OH)2 + H2O2 MnO2 + 2H2O
Note: During the reaction the catalyst changes. But at the end it is back to the same compound!
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How do many catalysts work?
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Catalyzes three chemical reactions2NOx xO2 + N2
2CO + O2 2CO2
2CxHy + (2x+y/2)O2 2xCO2 + yH2O
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Haber Process (Fritz Haber Nobel 1918)Formation of Ammonia
originally osmium and uranium
Now iron (keep out the O2)
Ertl Nobel Prize 2008
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EnzymesBiological Catalysts
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Substrate + Enzyme = Complex = Product + Enzyme
S + E = [SE] = P + E
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Enzyme Name = Function
Glucose Oxidase Oxidizes Glucose
Aromatic Amine Dehydrogenaseremoves Hydrogen from an aromatic amine
Hydrolase Hydrolyze reactions
Isomerase Isomerize molecules
Transferase Transfers functional groups
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Freshman Research Initiative ProjectNanomaterials
Dendrimer encapsulated nanoparticle
small metal particlecan be made of a variety of materials (Au,Ag, Pd, Pt, Cu, Pt/Cu, Pd/Cu,....)
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How good is the catalyst? Measure the kinetics
OH
NO O
-
OH
NH2
Measure the concentration as a function of time.
Kinetics are first order in reactantplot ln[concentration] vs time slope = -k
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Kinetics Wenly Ruan, Alex Guevaraal 2007