How can I control the rate of my Reactions? Chemical Kinetics Chapter 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 concentration of a reactant or product with time.  Three “types” of rates  initial rate  average rate  instantaneous rate Reaction Rates Section 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 = $ days 70 days $5/day$2/day Mary

REACTION RATES ! ! RR =  [reactants ]  t RR =  [products ]  t

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. 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. Dye Concentration

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 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

lower concentration of reactantsMore concentrated Concentration (M) amount present

Examples Hydrogen peroxide Food: vacuum packed Breathing Firemen

0.3 M HCl 6.0 M HCl Concentrations

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  more area Paper (demo) MORE kindle ground LESS Whole bean Logs  

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 Collisions Energy Collisions Energy Orientation 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