Kinetics …or Reaction Rates. Change The ice melted. The Coke went flat. The nail rusted.
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Transcript of Kinetics …or Reaction Rates. Change The ice melted. The Coke went flat. The nail rusted.
Reaction Rate•Consider:
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)Rate = –mol Mg = –mol HCl = t 2t
mol MgCl2 = mol H2 t t
Reaction Rate of a one-way reaction
• Will be determined by looking at the reactants only
• Decreases as time progresses. Why?
• What eventually happens to the amount of reactant?
Reaction Rate of an equilibrium rxn
• Will be determined by looking at the reactants only
• Decreases as time progresses. Why?
• What eventually happens to the rate?
• Does the reaction stop?
Necessities of Reaction•Sufficient energy--Activation
energy (Ea)
Rxn progress
En
erg
y
Ea
2HI
H2 + I2
Activated Complex
ΔH
Factors that affect Reaction Rate
•Concentration•Temperature•Surface Area•Presence of a catalyst•Rate-determining step…or the
slowest step in a rxn mechanism
Rate Law•Relates the rate as a function
of the reactant(s)•Units of rate are always
mol/L-s unless otherwise stated
•Two types: –Differential– Integrated
Differential Rate Law•For the rxn: A B…
differentialRate = k[A]x
k is the rate law constant…units are dependent on the order
x is the order of reactant A
Order
•may not be determined by looking at the coefficients of the reactants•unless the rxn is the rate-determining step
•must be determined experimentally
Order
•If the rate does not change when the reactant is doubled, then the order is zero; and rate is dependent solely on the value of kRate = k[A]0 or Rate = k
Order
•If the rate doubles when the reactant is doubled, then the order is first; and rate is dependent on the concentration of the reactantRate = k[A]1
Order
•If the rate quadruples when the reactant is doubled, then the order is second; and rate is dependent on the square of the concentration of the reactantRate = k[A]2
Overall Order of Reaction
•is the sum of the individual orders
•When Rate = k[A]0, the overall order is zero
•When Rate = k[A]1, the overall order is one
Overall Order of Reaction
•If a differential rate law for the following rxn:A + B Cis Rate = k[A]1[B]2
•then the overall order is three–What must the units of k be in this reaction?
Integrated Rate LawFor the rxn:
2N2O5 4NO2 + O2
The following data were collected:
[N2O5] Time(s)1.00 0
0.88 200
0.78 400
0.69 600
0.61 800
Integrated Rate Law
If we think the reactant is zero order:
Rate = k[N2O5]0
Integrating the rate law gives us:
[N2O5] = -kt + [N2O5]o
Integrated Rate Law
Since the plot gave a curve rather than a line, the order of the N2O5 cannot be zero.
If we try looking at it as if it were first order, then we will need to integrate the rate law.
Integrated Rate Law
If we think the reactant will be first order:
Rate = k[N2O5]1
Integrating the rate law gives us:
ln [N2O5] = -kt + ln[N2O5]o
Integrated Rate Law
If we think the reactant will besecond order:
Rate = k[N2O5]2
Integrating the rate law gives us:
[N2O5]-1 = kt + [N2O5]o-1
First-order Half-Life
• All first-order reactions have half-lives independent of the initial concentration of the reactant.
• All radioactive decays follow first-order kinetics.
First-order Half-Life Problem
• On November 23, 1999 I had Technetium-99 injected into my bloodstream for a bone scan. The half-life of Tc-99 is 6.0 hours.– What percentage of the original
amount of Tc-99 is left in my body today?
Second-order Half-Life• All second-order reactions have
half-lives dependent on the initial concentration of the reactant.
• A second half-life will be longer than a first half-life because the initial concentration changes
• For each successive half-life, [A]o is halved; thus, for each successive half-life, the half-life is doubled
Second-order Half-Life Problem
• The decomposition of NOCl is a second- order reaction where
k = 4.00 x 10-8s-1.– For an initial concentration of
0.50M, what is the half-life?– How much is left after 1 x 108s? – What is the half-life for an initial
concentration of NOCl of 0.25M?
Zero-order Half-Life• Most often occur when a catalyst is
needed for the reaction to proceed.
• The catalyst determines how much reactant will be used.
• Since the rate is constant the half-life may be determined using either the rate law constant or the rate itself.
Reaction Mechanism
•Most reactions do not occur in a single step
•Rather, they happen in a series of steps called elementary steps
•The sum of the elementary steps gives the overall reaction.
Reaction Mechanism• Intermediates are substances that
are formed in one elementary step and consumed in a subsequent elementary step. They are rarely part of the rate law.
•Catalysts are substances added to a step that are also produced in a subsequent step. They are rarely part of the rate law.
Reaction Mechanism•Rate-determining step—the
slowest step. If it is the first or only step, then the rate law may be written from its molecularity.–The differential rate law may be written from the reactants in the rate-determining step.
Reaction Mechanism•Consider the following elementary
steps: Step 1: OCl- + H20 HOCl + OH- fast
Step 2: HOCl + I- HOI + Cl- slow
Step 3: HOI + OH- H20 + OI-
fast• What is the overall balanced equation?• What is the differential rate law?• What substance(s) is an intermediate?
A catalyst?
Reaction Mechanism• What is the overall balanced
equation?
• What is the differential rate law?
• What substance(s) is an intermediate? A catalyst?
OCl- + I- Cl- + OI-
Rate = k[I-]
HOCl, HOI, OH-; H2O
Reaction Mechanism•A catalyst is a substance that
increases the rate of reaction.– It does so by lowering the Ea required for the reaction to occur.
Collision Theory• Molecules must move toward each
other (through random motion).• Molecules must hit with the proper
orientation.• Molecules must hit with sufficient
energy.• Molecules will separate after
reaction occurs.
Collision Theory
• Plot 1/T vs. lnk to determine Ea or A.
• If you have two sets of data…
ln k2 = Ea 1 1_ k1 R T1 T2
Activation Energy Problem
Given the following kinetics data for the reaction: NO(g) + O3(g) NO2(g) + 02(g)
Determine the activation energy.
T (K) k (L/mol-s)195 1.08 x 109
230 2.95 x 109
260 5.42 x 109
298 12.0 x 109
369 35.5 x 109
Reaction Mechanism
• A series of elementary steps must satisfy two requirements for the reaction:– The sum of the elementary steps
must give the overall balanced equation for the reaction.
– The mechanism must agree with the observed rate law.
Catalyst– provides a surface whereby the
reacting molecules might position themselves more favorably for collision
– lowers Ea
– Homogeneous (i.e. enzymes) are in the same phase as the reacting molecules
– Heterogeneous (i.e. Pt or Pd pieces in the catalytic converter of a car) are in a different phase and promote adsorption
Enzymes•are proteins in living
organisms that catalyze biological reactions.–Salivary amylase–Lactase
Inhibitors•are elements or compounds
used to decrease the rate of a reaction.–Tetraethyl lead
•Oops! Along came catalytic converters…
–Methyl t-butyl ether (MTBE) and ethanol
Collision Theory• Concentration• Temperature• k is a measure of the fraction of
collisions with sufficient energy to produce a reaction– k = Ae –Ea/RT (Arrhenius equation)
•A—frequency factor (motion/orientation)
•Ea—activation energy (J/mol)
Reaction Mechanism Problem #1
• The balanced equation for the reaction of nitric oxide with hydrogen is2NO + 2H2 2H20 + N2
The experimentally determined rate law is: rate = k[NO]2[H2]
The following mechanism has been proposed:
Reaction Mechanism Problem #1 (cont.)
NO + H2 N + H2O (slow)
N + NO N2O (fast)
N2O + H2 N2 + H2O (fast)
Is this mechanism consistent with theobserved rate law?
k1
k2
k3
Reaction Mechanism Problem #2
NO + H2 N + H2O (fast, with equal rates)
N + NO N2O (slow)
N2O + H2 N2 + H2O (fast)
Is this mechanism consistent with theobserved rate law?
k1
k2
k3
k-1