Chemical
Kinetics
Chapter 15
How can I control
the rate of my
Reactions?
Kinetics
•Study of
speed or
rate of
reactions.
We can use thermodynamics to tell if a reaction is
product or reactant favored.
 But this gives us no info on HOW FAST reaction
goes from reactants to products.
 KINETICS — the study of REACTION RATES
and their relation to the way the reaction
proceeds, i.e., its MECHANISM.


The reaction mechanism is our goal!
Reaction Rates
Section 15.1
 Reaction
rate = change in
concentration of a reactant
or product with time.

Three “types” of rates
 initial rate
 average rate
 instantaneous rate
What is similar?
• ratio of two things
• TIME always Bottom !!!!!
Rate = D [ x ]
Dt
Change in amount
Change in time
CALCULATING
RATE
RATE CALCULATIONS
1. John takes 10 weeks to earn $150.
However, Mary earns $150 in 30 days.
Calculate the rate at which they both earn
money. Which has the larger rate?
Mary
John
Amount $150
Amount $150
70 days
Time 30 days
Time
Rate ($/day) $5/day
Rate ($/day) $2/day
Larger = Mary
REACTION RATES ! !
RR = D [reactants ]
Dt
RR = D [products ]
Dt
Dye Concentration
Blue dye is oxidized
with bleach.
Its concentration
decreases with time.
The rate — the change in
dye conc with time —
can be determined
from the plot.
REACTION RATES
-
RR = D [P ] = D [R ]
Dt
P =products
Dt
R = reactants
Relative Rates
Reactant
2A g
-
4B + C
D [A ] = D [B ] = D [C ]
2D t
4D t
Dt
Rate Expressions
HOW CAN WE CHANGE
REACTION RATES ?
• Some reactions need to be fast:
airbags.
• Some reactions are slowed;
released pills
Examples:
time
Collision Theory
D9 C20
(theory about molecules colliding)
•Rate of reactions
depend molecules
colliding in such a
way that old
bonds break and
new bonds form.
COLLISION THEORY
• C = collisions
• E = energy
• O = orientation
Factors Affecting RXN Rates
• Nature of Reactants
• Temperature
• Concentration
• Surface Area/
Physical state
• Catalysts
Nature of Reactants
• What you use
Nature of Reactants
Examples
•Packaging materials
•Food
•Building materials
•Clothing
•Fireworks
Nature of Reactants
The materials used
•Activation Energy;
Unique to each
substance
•Orientation of reactants;
depends on reactants
Concentration (M)
amount present
lower concentration of reactants
More concentrated
Concentration (M)
amount present
Examples
•Hydrogen peroxide
•Food: vacuum packed
•Breathing
•Firemen
Concentrations
0.3 M HCl
6.0 M HCl
Concentration (M)
amount present
•Increasing the
number of reactants,
the chance of
successful Collisions
increase.
Physical state
of reactant
or
Surface area
Surface Area
More it can be spread out a more area
a
LESS
Whole bean
ground
MORE
kindle
a
Logs
Paper (demo)
SURFACE AREA
amount in contact
Examples
•Fire: solid / liquid fuels
•Food: chew or grinding
•Brewing: tea or coffee
•Grain elevator
Surface Area
• Number of particles that
are exposed.
• More surface area = more
reactions
• More contact = more
Collisions.
Temperature
• Increased temperature causes
increased motion.
• Increases the KINETIC energy
TEMPERATURE
Examples
•Glowstick
•Cook / freeze
•Summer / winter
•Ice pack / hot packs
•Medication
Temperature
Cold
Hot
D9 C30
Temperature
• Temperature is related to
Kinetic Energy
Lower T has less E
• T: related to motion
Lower T = less motion
Collide less
Collision Theory
NO
Collisions
Collisions
NO
YES
Energy
Collisions
Energy
Orientation
Catalyst / Inhibitors
• A substance NOT
permanently changed
during the reaction.
• Changes energy needed
to start reaction
Activation Energy
•Energy needed to
start reaction
Energy Diagram
Catalyzed Reactions
d9 c23
Hydrogen peroxide Rxt.
C34
Catalyst
•Works by changing the
activation energy
required
•The lower the Ea the
greater the rate of the
reaction.
Inhibitors
d9 c38
•Works by changing the
activation energy
required
•The greater the Ea the
lower the rate of the
reaction.
Energy Diagram
Ea
Inhibitors