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How can I controlthe rate of my
Reactions?
Chemical Chemical KineticsKineticsChapter 15Chapter 15
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!
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 rate = change in Reaction rate = change in concentration of a reactant concentration of a reactant or product with time.or product with time.
Three “types” of rates Three “types” of rates initial rateinitial rateaverage rateaverage rateinstantaneous rateinstantaneous rate
Reaction Rates Reaction Rates Section 15.1Section 15.1
What is similar?
•ratio of two things
•TIME always Bottom !!!!!
Rate = [ x ] t
Change in amount
Change in time
CALCULATING RATE
RATE CALCULATIONS
1. John takes 10 weeks to earn $150. However, Mary earns $150 in 30 days. Calculate the rate at which they both earn money. Which has the larger rate?
Mary John Amount Amount
Time Time Rate ($/day) Rate ($/day)
Larger =
$150 $150
30 days 70 days
$5/day $2/day
Mary
REACTION RATES ! !
RR = [reactants ] t
RR = [products ] t
Blue dye is oxidized Blue dye is oxidized with bleach. with bleach.
Its concentration Its concentration decreases with time.decreases with time.
The rate — the change The rate — the change in dye conc with time in dye conc with time — can be determined — can be determined
from the plot.from the plot.
Blue dye is oxidized Blue dye is oxidized with bleach. with bleach.
Its concentration Its concentration decreases with time.decreases with time.
The rate — the change The rate — the change in dye conc with time in dye conc with time — can be determined — can be determined
from the plot.from the plot.
Dy
e C
on
ce
ntr
atio
nD
ye
Co
nc
en
trat
ion
REACTION RATES
RR = [P ] = - [R ] t t
P =products R = reactants
Relative Rates
Reactant
2A 4B + C
- [A ] = [B ] = [C ] 2 t 4 t t
Rate Expressions
HOW CAN WE CHANGE REACTION RATES ?
• Some reactions need to be fast: airbags.
• Some reactions are slowed; time released pills
Examples:
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 ReactantsExamples
•Packaging materials•Food•Building materials•Clothing •Fireworks
Nature of ReactantsThe materials used
•Activation Energy; Unique to each substance
•Orientation of reactants; depends on reactants
lower concentration of reactants More concentrated
Concentration (M)amount present
Concentration (M)amount present
Examples
•Hydrogen peroxide•Food: vacuum packed•Breathing•Firemen
0.3 M HCl0.3 M HCl
6.0 M HCl6.0 M HCl
ConcentrationsConcentrations
Concentration (M)amount present
•Increasing the number of reactants, the chance of successful Collisions increase.
Physical state Physical state of reactantof reactant
oror
Surface areaSurface area
Surface AreaMore it can be spread out more area
Paper (demo)
MORE
kindle
groundLESS Whole bean
Logs
SURFACE AREAamount 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
TEMPERATUREExamples
•Glowstick•Cook / freeze•Summer / winter•Ice pack / hot packs•Medication
Temperature Temperature
Cold Hot
D9 C30
Temperature
•Temperature is related to Kinetic Energy
Lower T has less E
•T: related to motionLower T = less motion
Collide less
Collision Theory
Collisions
Collisions
EnergyCollisions
Energy
Orientation
NO
NO
YES
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
Inhibitors
Ea