Rate Laws and Order of
Reaction
Read 6.3 p 372-378
Q 1-6 p 377
Chemical
Kinetics
Concentration and Rate
Each reaction has its own equation that
gives its rate as a function of reactant
concentrations.
this is called its Rate Law
• A rate law is an equation expressing the
rate of a reaction in terms of the [molar]
of the species involved in the reaction.
Chemical
Kinetics
aA + bB  cC + dD
• The rate is expressible as
rate = k[A]m[B]n
• The powers m and n are kinetic orders,
• A and B are the chemical substances
• k is the rate constant, different values for
each rxn
Chemical
Kinetics
Order of a Reaction
• The “order” of a chemical reaction is the
number of chemical [] terms upon which the
rate depends
• Rate = k[A]
- first order rate;
 the rate only depends on one [] term
• Rate = k[A]2
- second order rate
• Rate = k[A][B] - second order rate
Chemical
Kinetics
Concentration and Rate
Compare Experiments 1 and 2:
when [NH4+] doubles, the initial rate doubles.
Chemical
Kinetics
Concentration and Rate
Likewise, compare Experiments 5 and 6:
when [NO2-] doubles, the initial rate doubles.
Chemical
Kinetics
Concentration and Rate
This equation is called the rate law,
and k is the rate constant.
Chemical
Kinetics
Rate Laws
• Exponents tell the order of the reaction with
respect to each reactant.
• This reaction is
First-order to [NH4+]
First-order to [NO2−]
• The overall reaction order can be found by
adding the exponents on the reactants in the
rate law.
• This reaction is second-order overall.
Chemical
Kinetics
Recall………………………..
• A rate law shows the relationship between the reaction
rate and the concentrations of reactants.
 For gas-phase reactants use Partial Pressure instead of [A].
• k is a constant that has a specific value for each reaction.
• The value of k is determined experimentally.
“Constant” is relative herek is unique for each rxn
k changes with T
Chemical
Kinetics
Temperature and Rate
• Generally, as temperature
increases, so does the
reaction rate.
• This is because k is
temperature dependent.
Chemical
Kinetics
Determining Rxn Order from
Experimental Data – Ex.1
• Four experiments were conducted to discover
how the initial rate of consumption of BrO3ions in the rxn varies as the concentrations of
the reactants are changed.
• Use the experimental data in the table below
to determine the order of the reaction with
respect to each reactant and the overall
order.
• Write the rate law for the rxn and determing
the value of k.
Chemical
Kinetics
Data – Ex. 1
Initial [ ]
mol L-1
Initial Rate
Experiment
BrO3-
Br-
H+
mol L-1 s-1
1
0.10
0.10
0.10
1.2x10-3
2
0.20
0.10
0.10
2.4x10-3
3
0.10
0.30
0.10
3.5x10-3
4
0.20
0.10
0.15
5.4x10-3
Chemical
Kinetics
Homework
• Pg. 377 # 1-6
• Lab Exercise 6.4.1 The Sulfur Clock
“determine the order of the reaction”
p.404-405
Chemical
Kinetics
Review of Chemical Kinetics
First order
Second order
Second order
Linear
Exponential
Exponential
-dependent on
one molecule
-dependent on two
molecules
-dependent on two
molecules
Rate
Laws
Graph of
rate vs
concn
Chemical
Kinetics
First-Order Processes
Consider the process in
which methyl isonitrile is
converted to acetonitrile.
CH3NC
CH3CN
How do we know this
is a first order rxn?
Chemical
Kinetics
Reaction Mechanisms
Read 6.4 p 383-390
The sequence of events that describes
the actual process by which reactants
become products is called the reaction
mechanism.
Chemical
Kinetics
Reaction Mechanisms
• Reactions may occur all at once or
through several discrete steps.
• Each of these processes is known as an
elementary reaction or elementary
process.
Chemical
Kinetics
Reaction Mechanisms
•
The molecularity of a process tells how many
molecules are involved in the process.
•
The rate law for an elementary step is written
directly from that step.
Chemical
Kinetics
Multistep Mechanisms
• In a multistep process, one of the steps will
be slower than all others.
• The overall reaction cannot occur faster than
this slowest, rate-determining step.
Chemical
Kinetics
Slow Initial Step
NO2 (g) + CO (g)  NO (g) + CO2 (g)
• The rate law for this reaction is found
experimentally to be
Rate = k [NO2]2
• CO is necessary for this reaction to occur, but the
rate of the reaction does not depend on its
concentration.
• This suggests the reaction occurs in two steps.
Chemical
Kinetics
Slow Initial Step
• A proposed mechanism for this reaction is
Step 1: NO2 + NO2  NO3 + NO (slow)
Step 2: NO3 + CO  NO2 + CO2 (fast)
• The NO3 intermediate is consumed in the second step.
• As CO is not involved in the slow, rate-determining step, it does
not appear in the rate law.
Chemical
Kinetics
Fast Initial Step
• The rate law for this reaction is found
(experimentally) to be
• Because termolecular (= trimolecular)
processes are rare, this rate law
suggests a two-step mechanism.
Chemical
Kinetics
Fast Initial Step
• A proposed mechanism is
{Step 1 is an equilibriumit includes the forward and reverse reactions.}
Chemical
Kinetics
Fast Initial Step
• [NO] [Br2] = [NOBr2]
Chemical
Kinetics
Questions
• Q 2 p 390
• Q 1-3 p 391
Chemical
Kinetics
Unit Review
• Chapter 5: Self Quiz Q 1-18 p 355 Q
2,4,6,9,14,16
• Chapter 6: Self Quiz 1-18 p 407, Q
4,7,910,13-16 p 408-409
Chemical
Kinetics