Chemical Kinetics

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Chemical Kinetics
The area of chemistry that concerns reaction
rates and reaction mechanisms.
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Reaction Rate
The change in concentration of a reactant or
product per unit of time
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2NO2(g) ? 2NO(g) O2(g)
Reaction Rates
1. Can measure disappearance of reactants
2. Can measure appearance of products
3. Are proportional stoichiometrically
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2NO2(g) ? 2NO(g) O2(g)
Reaction Rates
4. Are equal to the slope tangent to that
point
5. Change as the reaction proceeds, if
the rate is dependent upon concentration
?NO2
?t
5
Rate Laws
Differential rate laws express (reveal) the
relationship between the concentration of
reactants and the rate of the reaction.
The differential rate law is usually just called
the rate law.
Integrated rate laws express (reveal) the
relationship between concentration of reactants
and time
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Writing a (differential) Rate Law
Problem - Write the rate law, determine the value
of the rate constant, k, and the overall order
for the following reaction
2 NO(g) Cl2(g) ? 2 NOCl(g)
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Writing a Rate Law
Part 1 Determine the values for the exponents
in the rate law
R kNOxCl2y
In experiment 1 and 2, Cl2 is constant while
NO doubles.
The rate quadruples, so
the reaction is second order with respect to NO
? R kNO2Cl2y
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Writing a Rate Law
Part 1 Determine the values for the exponents
in the rate law
R kNO2Cl2y
In experiment 2 and 4, NO is constant while
Cl2 doubles.
The rate doubles, so the
reaction is first order with respect to Cl2
? R kNO2Cl2
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Writing a Rate Law
Part 2 Determine the value for k, the rate
constant, by using any set of experimental data
R kNO2Cl2
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Writing a Rate Law
Part 3 Determine the overall order for the
reaction.
R kNO2Cl2
2

1
3
? The reaction is 3rd order
Overall order is the sum of the exponents, or
orders, of the reactants
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Collision Model
Key Idea Molecules must collide to react.
However, only a small fraction of collisions
produces a reaction. Why?
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Collision Model
Collisions must have sufficient energy to produce
the reaction (must equal or exceed the activation
energy).
1.
Colliding particles must be correctly oriented to
one another in order to produce a reaction.
2.
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Factors Affecting Rate
Increasing temperature always increases the rate
of a reaction.

• Particles collide more frequently

• Particles collide more energetically

Increasing surface area increases the rate of a
reaction
Increasing Concentration USUALLY increases the
rate of a reaction

Presence of Catalysts, which lower the activation
energy by providing alternate pathways
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Endothermic Reactions
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Exothermic Reactions
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Catalysis

• Catalyst A substance that speeds up a reaction
  without being consumed
• Enzyme A large molecule (usually a protein)
  that catalyzes biological reactions.
• Homogeneous catalyst Present in the same phase
  as the reacting molecules.
• Heterogeneous catalyst Present in a different
  phase than the reacting molecules.

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Lowering of Activation Energy by a Catalyst
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Catalysts Increase the Number of Effective
Collisions
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Reaction Mechanism
The reaction mechanism is the series of
elementary steps by which a chemical reaction
occurs.

• The sum of the elementary steps must give the
  overall balanced equation for the reaction
• The mechanism must agree with the
  experimentally determined rate law

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Rate-Determining Step
In a multi-step reaction, the slowest step is the
rate-determining step. It therefore determines
the rate of the reaction.
The experimental rate law must agree with the
rate-determining step
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Identifying the Rate-Determining Step
For the reaction 2H2(g) 2NO(g) ? N2(g)
2H2O(g) The experimental rate law is R
kNO2H2
Which step in the reaction mechanism is the
rate-determining (slowest) step?
Step 1 H2(g) 2NO(g) ? N2O(g) H2O(g)
Step 2 N2O(g) H2(g) ? N2(g) H2O(g)
Step 1 agrees with the experimental rate law
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Identifying Intermediates
For the reaction 2H2(g) 2NO(g) ? N2(g)
2H2O(g)
Which species in the reaction mechanism are
intermediates (do not show up in the final,
balanced equation?)
Step 1 H2(g) 2NO(g) ? N2O(g) H2O(g)
Step 2 N2O(g) H2(g) ? N2(g) H2O(g)
2H2(g) 2NO(g) ? N2(g) 2H2O(g)
? N2O(g) is an intermediate
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Consider the hypothetical reaction shown
below. Overall reaction 2A C2 ? A2C
C Reaction mechanism Elementary step 1 A C2 ?
AC C slow Elementary step 2 AC A ?
A2C fast
What is the rate Law?
Rate k A1C21
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• Consider the mechanism
• Step 1 ClO-(aq) H2O(l) lt-gt HOCl(aq) OH-(aq)
  K, fast
• Step 2 I-(aq) HOCl(aq) ? HOI(aq) Cl-(aq)
  k1, slow
• Step 3 HOI(aq) OH-(aq) ? OI-(aq) H2O(l)
  k2, fast
• Overall
• ClO- I- ? OI- Cl-
• What is the rate law?
• You might think it is rate kI- HOCl

HOCl is an intermediate though and therefore
cannot be in the rate law as we do not know its
concentration. We therefore need to substitute
for it.
rate K ClO- I- OH- -1
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Collision Model
Key Idea Molecules must collide to react.
However, only a small fraction of collisions
produces a reaction. Why?
26
Collision Model
Collisions must have sufficient energy to produce
the reaction (must equal or exceed the activation
energy).
1.
Colliding particles must be correctly oriented to
one another in order to produce a reaction.
2.
27
Factors Affecting Rate
Increasing temperature always increases the rate
of a reaction.

• Particles collide more frequently

• Particles collide more energetically

Increasing surface area increases the rate of a
reaction
Increasing Concentration USUALLY increases the
rate of a reaction

Presence of Catalysts, which lower the activation
energy by providing alternate pathways
28
Endothermic Reactions
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Exothermic Reactions
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Catalysis

• Catalyst A substance that speeds up a reaction
  without being consumed
• Enzyme A large molecule (usually a protein)
  that catalyzes biological reactions.
• Homogeneous catalyst Present in the same phase
  as the reacting molecules.
• Heterogeneous catalyst Present in a different
  phase than the reacting molecules.

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Lowering of Activation Energy by a Catalyst
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Catalysts Increase the Number of Effective
Collisions
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Not Tested or taught, but could show up on AP Test
Determining Order withConcentration vs. Time data
(the Integrated Rate Law)
Zero Order
First Order
Second Order
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Not Tested or taught, but could show up on AP Test
Problem Find the integrated rate law and the
value for the rate constant, k
A graphing calculator with linear regression
analysis greatly simplifies this process!!
(Click here to download my Rate Laws program for
theTi-83 and Ti-84)
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Not Tested or taught, but could show up on AP Test
Time vs. H2O2
Regression results
y ax b a -2.64 x 10-4 b 0.841 r2
0.8891 r -0.9429
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Not Tested or taught, but could show up on AP Test
Time vs. lnH2O2
Regression results
y ax b a -8.35 x 10-4 b -.005 r2
0.99978 r -0.9999
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Not Tested or taught, but could show up on AP Test
Regression results
y ax b a 0.00460 b -0.847 r2 0.8723
r 0.9340
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Not Tested or taught, but could show up on AP Test
And the winner is Time vs. lnH2O2
1. As a result, the reaction is 1st order
2. The (differential) rate law is
3. The integrated rate law is
4. Butwhat is the rate constant, k ?
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Not Tested or taught, but could show up on AP Test
Method 1 Calculate the slope from the Time vs.
lnH2O2 table.
Now remember
? k -slope
k 8.32 x 10-4s-1
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Not Tested or taught, but could show up on AP Test
Finding the Rate Constant, k
Method 2 Obtain k from the linear regresssion
analysis.
Regression results
y ax b a -8.35 x 10-4 b -.005 r2
0.99978 r -0.9999
Now remember
? k -slope
k 8.35 x 10-4s-1
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Not Tested or taught, but could show up on AP Test
Rate Laws Summary
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Not Tested
Heterogeneous Catalysis
Carbon monoxide and nitrogen monoxide adsorbed on
a platinum surface
Step 1 Adsorption and activation of the
reactants.
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Not Tested
Heterogeneous Catalysis
Carbon monoxide and nitrogen monoxide arranged
prior to reacting
Step 2 Migration of the adsorbed reactants on
the surface.
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Not Tested
Heterogeneous Catalysis
Carbon dioxide and nitrogen form from previous
molecules
Step 3 Reaction of the adsorbed substances.
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Not Tested
Heterogeneous Catalysis
Carbon dioxide and nitrogen gases escape (desorb)
from the platinum surface
Step 4 Escape, or desorption, of the products.
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Not Tested, or taught, but could show up on AP
Test
The Arrhenius Equation

• k rate constant at temperature T
• A frequency factor
• Ea activation energy
• R Gas constant, 8.31451 J/Kmol

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Not Tested, or taught, but could show up on AP
Test
The Arrhenius Equation, Rearranged

• Simplifies solving for Ea
• -Ea / R is the slope when (1/T) is plotted
  against ln(k)
• ln(A) is the y-intercept
• Linear regression analysis of a table of (1/T)
  vs. ln(k) can quickly yield a slope
• Ea -R(slope)