Chapter 14: Chemical Kinetics IIjeloranta/CHEM102/Chem102_Ch14_II.pdf · Chapter 14: Chemical...
Transcript of Chapter 14: Chemical Kinetics IIjeloranta/CHEM102/Chem102_Ch14_II.pdf · Chapter 14: Chemical...
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Chapter 14: Chemical Kinetics II
Chem 102Dr. Eloranta
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Rate Laws
• Experiments would allow you to determine the reaction order and rate constant, but
what if you wanted to know [A] over time?
• Need to “integrate” the rate law (i.e., solve the above differential equation)
If you are familiarwith calculus
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Zero Order Integrated Rate Law
• Plot of [A] vs. time gives a straight line
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First Order Integrated Rate Law
• Plot of ln[A] vs. time gives a straight line
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Second Order Integrated Rate Law
• Plot of 1/[A] vs. time gives a straight line
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Experimental determination of reaction order
Instead of measuring initial rate, could measure [A] as a function of time, then plot. Whichever curve gives a straight line must be the order.
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Half-life, t1/2
Time required for the concentration of a reactant to fall to one-half of its initial value
• Can solve for t1/2
• Example: First order reaction• Take integrated
rate law and plug in 1 and 0.5
(or 2 and 1)
Substitute [A]t in above:
This gives directly:
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Things to consider
• Higher k means faster (shorter) half-life, faster reaction
• Every half-life, concentration decreases by 1/2• After x half-lives, [A] = (1/2)x [A]0
• On your own: Solve for t1/2 for 0 and 2nd order reactions
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Summary of Integrated Rate Laws
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Half-life example
C-14 undergoes first order radioactive decay
with t1/2 = 5730 years
•C-14 produced in atmosphere•Incorporated into plants through photosynthesis
•Incorporated into animals through food
Relative amount of C-14 stays constant in living organisms until they die.
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Half-life example
Suppose a human bone found in a cave has 19.5% of the C-14 found in living organisms. How old is the bone?
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Temperature effects on kinetics
• Hydrogen and oxygen react slowly unless a spark is applied
• then, boom!
• Why?
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Effect of temperature on reaction rate
• Reaction rates are highly dependent on temperature
• Arrhenius Equation (1889) says rate constant depends on temperature Svante Arrhenius
1859-1927Uppsala/Stockholm, Sweden
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Arrhenius Equation
• k = Rate constant• A = Frequency factor
(same units as k)• Ea = Activation energy• R = Gas constant = 8.314
J / (mol K)
• Why? Collision theory...
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Chemical reaction energy diagram
Recall from earlier: • Chemical reactions
involve breaking and forming bonds
• These processes involve changes in (potential) energy Po
tenti
al En
erg
y
Reaction Progress
A + B → C + D
A + B
C + DReactants
Products
(also called reaction coordinate)
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Reaction energy diagramPo
tenti
al Energ
y
Reaction Progress
A + B → C + D
A + B
C + DReactants
Products
Enthalpy (Heat of Reaction): ΔHrxn
Hrxn
• EXOTHERMIC• energy (heat)
released• ΔHrxn is negative• products at
lower energy than reactants
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Reaction energy diagramPo
tenti
al Energ
y
Reaction Progress
A + B → C + D
A + B
C + D
Reactants
Products
• Enthalpy (Heat of Reaction): ΔHrxn
Hrxn
• ENDOTHERMIC• Energy (heat)
absorbed• ΔHrxn is positive• products at
higher energy than reactants
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Collision theory
Think about what happens in a reaction: collision!
O3 + NO → NO2 + O2
OO
O ON
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Collision theory
Think about what happens in a reaction: collision!
O3 + NO → NO2 + O2
OO
OON
Molecules come together to form “transition” (‡) compound:• distorted compared to “favored” configuration• higher energy than both the reactants & products• between reactants and products along the reaction path
‡
[ ](or “activated complex”)
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Collision theory
Think about what happens in a reaction: collision!
O3 + NO → NO2 + O2
OO
OON
Products form:• Moving towards better configuration with a lower
energy
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Reaction coordinatePo
tenti
al Energ
y
Reaction Progress
ΔHrxn
O3 + NO
NO2 + O2
OOO—NO‡
Ea (activation energy)
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Reaction coordinate
ΔHrxnΔHrxn
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Activation energy (Ea)
• Ea is an energy barrier that the reactants must overcome to form the products
• Frequency factor (A): Rate of collisions (collisions / second)• Not all collisions result in a reaction because some do not have
enough energy to overcome Ea
• How could you increase rate constant?• Increase number of collisions per time (A), • Increase the number of collisions with enough energy to
overcome the activation energy (exponential factor),• Decrease activation energy (Ea)
• How do you give the reactants more (kinetic) energy?
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Temperature effects on energy
• In a given sample of molecules, the energy follows Boltzmann distribution
• Higher T: more molecules have enough energy to overcome activation energy
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Arrhenius Law
• Experimentally determine A, Ea, etc.
• Measure k at varying temperatures (must be expressed in Kelvin)
• Plot: ln(k) vs (1/T)
“Arrhenius plot”
• slope = -Ea / R• intercept = ln(A)
y = m x + b
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Arrhenius Law
• Ea = 93.1 kJ / mol• A = 4.36 x 1011 M-1s-1
“Arrhenius plot”
Linear least squares fitusing computer
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Two-point form of Arrhenius Equation
Make measurements at two temperatures only
Note: Not recommended unless the datais verified to be of Arrhenius form.
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Reaction mechanisms
• Most reactions occur in multiple steps rather than in
a single collision event!• Mechanism: The exact molecular pathway that reactants
follow to become products• Elementary reactions: The individual steps in a reaction
mechanism (cannot be broken down further). Takes place in a single collision.
• Add elementary reactions to get the overall reaction (reaction mechanism)
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Example
Overall reaction:
but we know that N2O2 is detected during reaction
Elementary Steps:
N2O2 is an “intermediate”: Species that appears in mechanism, but not overall reaction:• formed in elementary step, then consumed• usually short-lived and often difficult to detect
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Rate laws for elementary reactions
• “Molecularity” = Number of reactants in an elementary step• Rate law follows stoichiometry (molecularity)
For elementary reactions only! (not overall reaction)
Unimolecular: A → Products Rate = k[A]
Bimolecular: A + A → Products Rate = k[A]2
Bimolecular: A + B → Products Rate = k[A][B]
Termolecular: A + A + A → Products Rate = k[A]3
Termolecular: A + A + B → Products Rate = k[A]2[B]
Termolecular: A + B + C → Products Rate = k[A][B][C] }Very
rare
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Example
What is the overall rate?? We will see in a bit...
Rate = k[NO]2
Rate = k[N2O2][O2]
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Kinetics for multi-step reactions
• Usually one step is fast and the other is slow• The slow step will control the overall rate of the reaction
(“rate-limiting” step)
• If first step is slow:
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Kinetics for multi-step reactions
• Usually one step is fast and the other is slow• The slow step will control the overall rate of the reaction
(“rate-limiting” step)
• If last step is slow:• Intermediate will increase in concentration enough to
drive the slow reaction• Steady state approximation
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Example
What is the overall rate?? Rate = k[NO]2
Rate = k[NO]2
Rate = k[N2O2][O2]
Slow
Fast
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Catalysis
• Recall: One way to increase rate constant is to reduce the activation energy
• A catalyst can alter the reaction mechanism such that the activation energy is reduced
Pote
nti
al Energ
y
Reaction Progress
(the double humpIs not important)
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Example: Catalytic destruction of ozone
• Ozone: O3
• Absorbs UV light in the stratosphere and prevents it from reaching the Earth’s surface (skin cancer)
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Example: Catalytic destruction of ozone
Chlorofluorocarbons (CFCs) • Example: CFC-11 = CCl3F• Very unreactive (inert) so they were used as coolants,
propellants, etc.• Stratosphere: ~ 20 km altitude, lots of UV light from sun
C Cl
Cl
Cl
F
hν
C• •Cl
Cl
Cl
F +
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Example: Catalytic destruction of ozone
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + O3 → O2 + O2O3 + hν → O + O2
CCl3F + hν → Cl + CCl3
Cl atom acts as a catalyst: Provides an alternative pathway for reaction to take place.
(cyclic reaction: Cl is re-formed)
O + O2 → O3
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Montreal Protocol
• Banned CFCs in 1987
• Ozone hole will recover in ~100 years
• Good example of legislation solving an environmental problem
http://cdiac.ornl.gov/ftp/oceans/CFC_ATM_Hist/CFC_ATM_Hist_2015/Fig1.png
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Heterogeneous catalysts
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Catalytic converters
(health hazard)
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Catalytic converters
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Despite more cars and more miles driven!
Catalytic converters
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Enzymes
• Biological catalysts which increase reaction rates for biochemical reactions
• Enzyme (usually a protein) has an “active site” where reactants can bind
Example: Sucrase
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E + S ⇌ ES (Fast)ES → E + P (Slow, rate limiting)
(Michaelis-Menten kinetics)
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Tips for this section
• Understand/Identify integrated rate laws:• Equations• Plots, slope, intercept, etc.
• Half-life• Reaction order• Reaction coordinate - identify activation energy,
enthalpy, transition state, etc.• Elementary steps, rates• Catalysis, how do catalysts increase reaction rate?• Lots of calculations!