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

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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