CHEMISTRY
Matter and Change
Chapter 16: Reaction Rates
CHAPTER
16
Table Of Contents
Section 16.1
A Model for Reaction Rates
Section 16.2
Factors Affecting Reaction Rates
Section 16.3
Reaction Rate Laws
Section 16.4
Instantaneous Reaction Rates and
Reaction Mechanisms
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SECTION
16.1
A Model for Reaction Rates
• Calculate average rates of chemical reactions from
experimental data.
• Relate rates of chemical reactions to collisions
between reacting particles.
energy: the ability to do work or produce heat; it
exists in two basic forms: potential energy and kinetic
energy
SECTION
16.1
A Model for Reaction Rates
reaction rate
collision theory
activated complex
activation energy
Collision theory is the key to understanding
why some reactions are faster than others.
SECTION
16.1
A Model for Reaction Rates
Expressing Reaction Rates
• The reaction rate of a chemical reaction is stated
as the change in concentration of a reactant or
product per unit of time.
SECTION
16.1
A Model for Reaction Rates
Expressing Reaction Rates (cont.)
• Reaction rates are determined experimentally
(and always expressed as a positive value).
• Average rate of reaction: the change in
concentration of a reactant or product that
occurs during a specific time interval.
SECTION
16.1
A Model for Reaction Rates
Collision Theory
• Collision theory states that atoms, ions, and
molecules must collide in order to react.
• Correct orientation and
• Sufficient energy
•
(Lego example)
SECTION
16.1
A Model for Reaction Rates
Collision Theory (cont.)
SECTION
16.1
A Model for Reaction Rates
Collision Theory (cont.)
• An activated complex is a temporary, unstable
arrangement of atoms in which old bonds are
breaking and new bonds are forming.
SECTION
16.1
A Model for Reaction Rates
Collision Theory (cont.)
• The minimum amount of energy that reacting
particles must have to form the activated
complex and lead to a reaction is called the
activation energy.
• High activation energy means that few
collisions have the required energy and the
reaction rate is slow.
• What does an exothermic reaction look
like on a plot of energy vs. time?
• Draw a plot in your notes.
SECTION
16.1
A Model for Reaction Rates
Collision Theory (cont.)
SECTION
16.1
A Model for Reaction Rates
Collision Theory (cont.)
SECTION
16.1
A Model for Reaction Rates
Spontaneity and Reaction Rate
• Are more spontaneous reactions faster than
less spontaneous reactions?
• ΔG indicates only the natural tendency for a
reaction to proceed—it does not affect the rate
of a chemical reaction.
SECTION
16.1
Section Check
Which of the following is NOT a
requirement for a reaction to occur,
according to the collision theory?
A. Reacting substances must collide.
B. Reacting substances must be in an
exothermic reaction.
C. Reacting substances must collide in the
correct orientation.
D. Reacting substances must collide with
sufficient energy to form an activated complex.
SECTION
16.1
Section Check
A temporary, unstable arrangement of
atoms in which old bonds are breaking
and new bonds are forming is called ____.
A. reaction complex
B. reaction substrate
C. activated complex
D. activated molecule
SECTION
16.2
Factors Affecting Reaction Rates
• Identify factors that
affect the rates of
chemical reactions.
• Explain the role of a
catalyst.
concentration: a
quantitative measure of
the amount of solute in a
given amount of solvent
or solution
catalyst
Factors such as reactivity,
concentration, temperature, inhibitor
surface area, and catalysts heterogeneous catalyst
affect the rate of a chemical
homogeneous catalyst
reaction.
SECTION
16.2
Factors Affecting Reaction Rates
The Nature of Reactants
• Some substances react more readily than
others. (Alkali metals, halogens)
• The phase of the reactants matters. Some
substances react faster in aqueous
solutions than if combined as solids.
SECTION
16.2
Factors Affecting Reaction Rates
Concentration
• Chemists change reaction rates by changing
concentrations of reactants.
• When concentrations are increased, more
molecules are available to collide per unit
volume, and therefore collisions occur more
frequently.
SECTION
16.2
Factors Affecting Reaction Rates
Surface Area
• Greater surface area allows particles to collide
with many more particles per unit of time.
• For the same mass, many small particles have
more surface area than one large particle.
• Reaction rate increases with increasing surface
area.
• Steel wool v. solid steel example
SECTION
16.2
Factors Affecting Reaction Rates
Temperature
• Increasing temperature generally increases
reaction rate.
• Increasing temperature increases the kinetic
energy of the particles.
• Reacting particles collide more frequently at
higher temperatures.
SECTION
16.2
Factors Affecting Reaction Rates
Temperature (cont.)
• High-energy collisions are more frequent at
a higher temperature.
• As temperature increases, reaction rate
increases.
SECTION
16.2
Factors Affecting Reaction Rates
Temperature (cont.)
SECTION
16.2
Factors Affecting Reaction Rates
Catalysts and Inhibitors
• A catalyst is a substance that increases
the rate of a chemical reaction without
being consumed in the reaction. Note: a
catalyst does not increase the amount of
product .
• An inhibitor is a substance that slows or
prevents a reaction.(Refrigeration,
preservatives)
SECTION
16.2
Factors Affecting Reaction Rates
Catalysts and Inhibitors (cont.)
• Catalysts lower the activation energy.
• Low activation energy means that more of the
collisions between particles will have sufficient
energy to overcome the activation energy barrier
and bring about a reaction.
SECTION
16.2
Factors Affecting Reaction Rates
Catalysts and Inhibitors (cont.)
• A heterogeneous catalyst exists in a
physical state different than that of the
reaction it catalyzes.
• A homogeneous catalyst exists in the same
physical state as the reaction it catalyzes.
SECTION
16.2
Section Check
Which of the following generally does not
increase the rate of a chemical reaction?
A. increasing concentration
B. adding a catalyst
C. adding an inhibitor
D. increasing temperature
SECTION
16.2
Section Check
High-energy particle collisions are more
frequent:
A. when an inhibitor is present
B. when temperature is decreased
C. when activation energy is higher
D. when temperature is increased
SECTION
16.3
Reaction Rate Laws
• Express the relationship
between reaction rate and
concentration.
• Determine reaction orders
using the method of initial
rates.
reactant: the starting
substance in a chemical
reaction
rate law
specific rate constant
reaction order
method of initial rates
The reaction rate law is an experimentally determined
mathematical relationship that relates the speed of a
reaction to the concentrations of the reactants.
SECTION
16.3
Reaction Rate Laws
Writing Reaction Rate Laws
• A rate law expresses the relationship
between the rate of a chemical reaction
and the concentration of the reactants.
SECTION
16.3
Reaction Rate Laws
Writing Reaction Rate Laws (cont.)
• The symbol k is the specific rate
constant, a numerical value that relates
the reaction rate and the concentrations of
reactants at a given temperature.
• The specific rate constant is unique for every
reaction.
SECTION
16.3
Reaction Rate Laws
Writing Reaction Rate Laws (cont.)
• The reaction order for a reactant defines how
the rate is affected by the concentration of that
reactant.
• Rate = k[H2O2]
• The reaction is first order,
so the rate changes in the
same proportion the
concentration of H2O2
changes.
SECTION
16.3
Reaction Rate Laws
Writing Reaction Rate Laws (cont.)
• 2NO(g) + 2H2(g)
→ N2(g) + 2H2(g)
–Rate = k[NO]2[H2]
–If H2 is doubled, the rate doubles.
–If NO is doubled, the rate quadruples because 22 = 4.
–First-order H2, second-order NO, third-order overall
SECTION
16.3
Reaction Rate Laws
Determining Reaction Order
• The method of initial rates determines
reaction order by comparing the initial rates
of a reaction carried out with varying
reactant concentrations.
• Initial rate measures how fast the reaction
proceeds at the moment when reactants are
mixed.
SECTION
16.3
Reaction Rate Laws
Determining Reaction Order (cont.)
SECTION
16.3
Reaction Rate Laws
Determining Reaction Order (cont.)
• Doubling [A] doubles the reaction rate, so
[A] is first order.
• Doubling [B] quadruples the reaction rate, so
[B] is second order.
• Rate = k[A][B]2
SECTION
16.3
Section Check
What is the overall reaction order of the
following reaction?
Rate = k[A]2[B]2
A. 1st
B. 2nd
C. 3rd
D. 4th
SECTION
16.3
Section Check
In the following reaction, what is the overall
reaction order if doubling [A] results in
quadrupling the reaction rate and doubling [B]
results in a reaction rate eight times faster?
Rate = k[A]m[B]n
A. 12
B. 5
C. 6
D. 10
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
• Calculate instantaneous rates of chemical reactions.
• Understand that many chemical reactions occur in
steps.
• Relate the instantaneous rate of a complex reaction
to its reaction mechanism.
decomposition reaction: a chemical reaction that occurs
when a single compound breaks down into two or more
elements or new compounds
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
instantaneous rate
complex reaction
reaction mechanism
intermediate
rate-determining step
The slowest step in a sequence of steps
determines the rate of the overall chemical
reaction.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Instantaneous Reaction Rates
• This figure shows the concentration of H2O2 over
time during the decomposition reaction
2H2O2(aq) → 2H2O(l) + O2(g).
• The instantaneous
rate is the slope of the
straight line tangent to
the curve at the
specific time.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Instantaneous Reaction Rates (cont.)
• Instantaneous rate can be calculated if the
concentrations are known, the temperature is
known, and the experimentally determined rate
law and specific rate constant at that temperature
are known.
• 2N2O5(g) → 4NO2(g) + O2(g)
• Rate = k[N2O5]
• k = 1.0 × 10–5 s–1 and [N2O5] = 0.350 mol/L
• Rate = (1.0 × 10–5 s–1)(0.350 mol/L) = 3.5 × 10–6
mol/(L•s)
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Reaction Mechanisms
• Most chemical reactions consist of sequences
of two or more simpler reactions.
• Each step is called an elementary step.
• A complex reaction contains two or more
elementary steps.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Reaction Mechanisms (cont.)
• A reaction mechanism is the complete
sequence of elementary steps that makes
up a complex reaction.
• An intermediate is a substance produced in
one of the elementary steps and consumed in
a subsequent elementary step.
• Intermediates do not appear in the net
chemical equation.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Reaction Mechanisms (cont.)
Chlorine-catalyzed decomposition of ozone
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Reaction Mechanisms (cont.)
• Every complex reaction has one
elementary step that is slower than the
others.
• The slowest elementary step in a complex
reaction is called the rate-determining step.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Reaction Mechanisms (cont.)
SECTION
16.4
Section Check
What is a reaction with two or more
elementary steps called?
A. compound reaction
B. complex reaction
C. multi-step reaction
D. combined reaction
SECTION
16.4
Section Check
What is the slowest step in a complex
reaction called?
A. elementary step
B. reducing step
C. rate-determining step
D. intermediate step
CHAPTER
Reaction Rates
16
Resources
Chemistry Online
Study Guide
Chapter Assessment
Standardized Test Practice
SECTION
A Model for Reaction Rates
16.1
Study Guide
Key Concepts
• The rate of a chemical reaction is expressed as the rate
at which a reactant is consumed or the rate at which a
product is formed.
• Reaction rates are generally calculated and expressed in
moles per liter per second (mol/(L ● s)).
• In order to react, the particles in a chemical reaction must
collide.
• The rate of a chemical reaction is unrelated to the
spontaneity of the reaction.
SECTION
16.2
Factors Affecting Reaction Rates
Study Guide
Key Concepts
• Key factors that influence the rate of chemical reactions
include reactivity, concentration, surface area,
temperature, and catalysts.
• Raising the temperature of a reaction generally
increases the rate of the reaction by increasing the
collision frequency and the number of collisions that
form an activated complex.
• Catalysts increase the rates of chemical reactions by
lowering activation energies.
SECTION
Reaction Rate Laws
16.3
Study Guide
Key Concepts
• The mathematical relationship between the rate of a
chemical reaction at a given temperature and the
concentrations of reactants is called the rate law.
rate = k[A]
rate = k[A]m[B]n
• The rate law for a chemical reaction is determined
experimentally using the method of initial rates.
SECTION
16.4 Instantaneous Reaction Rates and Reaction Mechanisms
Study Guide
Key Concepts
• The reaction mechanism of a chemical reaction must
be determined experimentally.
• For a complex reaction, the rate-determining step limits
the instantaneous rate of the overall reaction.
CHAPTER
16
Reaction Rates
Chapter Assessment
The energy required to initiate a reaction
is called ____.
A. initiation energy
B. activation energy
C. complex energy
D. catalyst energy
CHAPTER
16
Reaction Rates
Chapter Assessment
In general, which of the following does not
cause a reaction rate to increase?
A. increasing surface area
B. increasing temperature
C. increasing volume
D. adding a catalyst
CHAPTER
16
Reaction Rates
Chapter Assessment
What is the overall reaction order of the
following reaction?
Rate = k[A][B]2[C]
A. 1st order
B. 2nd order
C. 3rd order
D. 4th order
CHAPTER
16
Reaction Rates
Chapter Assessment
A substance produced by an elementary
step in a complex reaction that is
consumed later and does not show up in
the net reaction is called a(n) ____.
A. activated complex
B. catalyst
C. enzyme
D. intermediate
CHAPTER
16
Reaction Rates
Chapter Assessment
Increasing the temperature of a reaction
increases the rate of reaction by:
A. increasing the collision frequency
B. increasing the number of
high-energy collisions
C. both a and b
D. none of the above
CHAPTER
16
Reaction Rates
Standardized Test Practice
Which of the following is an acceptable
unit for expressing a rate?
A. mol/L ● s
B. L/s
C. M
D. mL/h
CHAPTER
16
Reaction Rates
Standardized Test Practice
How many moles are in 4.03 × 102 g of
calcium phosphate (Ca3(PO4)2)?
A. 0.721 moles
B. 1.30 moles
C. 1.54 moles
D. 3.18 moles
CHAPTER
16
Reaction Rates
Standardized Test Practice
Doubling the concentration of one
reactant in a reaction causes the reaction
rate to double. What is the order of that
reactant?
A. 1st
B. 2nd
C. unable to determine
D. none of the above
CHAPTER
16
Reaction Rates
Standardized Test Practice
The rate law for the reaction A + B + C →
Product is rate = k[A]2[B][C]. If [A] = 0.350M,
[B] = .500M, [C] = .125M, and k = 6.50 × 10–5
L3/(mol3 ● s), what is the instantaneous rate
of reaction?
A. 2.84 × 10–6 mol/L ● s
B. 4.98 × 10–7 mol/L ● s
C. 5.84 × 10–6 mol/L ● s
D. 2.84 × 10–7 mol/L ● s
CHAPTER
16
Reaction Rates
Standardized Test Practice
H2O2 breaks down to form hydrogen and
oxygen gas in what type of reaction?
A. synthesis
B. double replacement
C. decomposition
D. single replacement
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