Kinetics(Reaction Rate)

How Fast Does the Reaction Go

Rates of reactionKINETICS

ObjectivesTo understand that a chemical reaction

involves collisions between particlesTo be able to describe the four factors

which will affect the rate of a chemical reaction.

To describe the factor that can not be changed and affect the rate of a chemical reaction.

Collision Theory In order to react molecules and atoms must

COLLIDE with each other.They must hit each other with enough energy

and with the correct orientation.The more “frequently” the particles hit, the

faster the reaction will go.A COLLISION THAT RESULTS IN PRODUCT

IS CALLED EFFECTIVE COLLISIONAnything that increase these effective

collisions will make the reaction faster.

How do we make the reaction go faster?

There are four things that we can change to make the reaction go faster.

TemperatureSurface areaConcentrationUsing a catalyst

TemperatureWhen we increase the

temperature we give the particles energy, this makes them move faster, which means they collide with other particles more often.

So the reaction goes faster.

Surface area

If we make the pieces of the reactants smaller we increase the number of particles on the surface which can react.

This makes the reaction faster.

The particles on the surface can react

When cut into smaller pieces the particles on the inside can react

Concentration

If we make one reactant more concentrated (like making a drink of orange squash more concentrated)

There are more particles in the same volume to react

So the reaction goes faster.

There are less red particles in the same volume so there is less chance of a collision

There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Factors that Affect RateCatalysts- substances that speed up a

reaction without being used up.(enzyme).– Speeds up reaction by giving the

reaction a new path.– The new path has a lower activation

energy.– More molecules have this energy.– The reaction goes faster.

Inhibitor- a substance that blocks a catalyst.

Factors that Affect RateSummary

Temperature– Higher temperature faster particles.– More and harder collisions.– Faster Reactions.

Concentration– More concentrated means molecules

closed together.– Collide more often.– Faster reaction.

Particle size– Molecules can only collide at the

surface.– Smaller particles bigger surface area.– Smaller particles faster reaction.– Smallest possible are molecules or

ions.Dissolving speeds up reactions.

– Getting two solids to react with each other is slow.

Using a catalystA catalyst is a chemical which is

added to a reaction.It makes the reaction go faster.The catalyst does not get used up in

the reaction.

Potential Energy Diagrams

Represent the changes in potential energy that occur during a chemical reaction.

The reaction coordinate represents the progress of the reaction.

Remember in terms of energy content

HIGH ENERGY =UNSTABLELOW ENERGY = STABLE

Classification of reactions

Exothermic Rx: releases energy. Heat part of the products.

Endothermic Rx: absorbs energy. Heat part of the reactants.

DefinitionsActivation Energy: the minimum amount

of energy needed to start up a chemical reaction.

Activated Complex: a temporary intermediate product. Highest point of the diagram. Is UNSTABLE.

Catalyzed Reactions

Need less energy to start up the reaction. The presence of a catalyst LOWERS THE ACTIVATION ENERGY!!!

Ene

rgy

Reaction coordinate

Reactants

Products

Ene

rgy

Reaction coordinate

Reactants

Products

Activation Energy - Minimum energy to make the reaction happen

Ene

rgy

Reaction coordinate

Reactants

Products

Activated Complex or Transition State

Ene

rgy

Reaction coordinate

Reactants

Products

Overall energy change (DH)

Thermochemistry

The study of the changes in heat energy that accompany chemical reactions and physical changes.

What is DH?

DH is the enthalpy, or“heat of reaction.” The amount of energy released or absorbed during a chemical reaction.By convention

DHrxn = Hproducts - Hreactants

What is an exothermic reaction?

A reaction that gives off or releases heat. Since...

DHrxn = Hproducts - Hreactants

then DHrxn will be a negative number

Ene

rgy

Reaction coordinate

Reactants

Products

Overall energy change (DH)

What is an endothermic reaction?

A reaction that absorbs heat. Since…

DHrxn = Hproducts – Hreactants

then DH will be a positive number.

Energy

Reaction coordinate

Reactants

Products

Overall energy change (DH)

Heats of Reaction Table IThe new reference table (Table I) shows different heats of reaction for some common reactions.

EXOTHERMIC RXRelease heat.DH for reaction is negativeThe value for DH is the amount of heat

released during the rxWe write the heat it as PRODUCT in the

chemical equationThe greater the magnitude of DH the

more stable the product will be!!!!

Exothermic vs Endothermic

Absorb heatDH for reaction is positiveThe value for DH is the amount of heat

absorbed during the rxWe write the heat it as REACTANT in the

chemical equationThe greater the magnitude of DH the

more UNstable the product will be!!!!

ENDOTHERMIC RX

MC Q-FACTORS1. 22. 13. 24. 25. 46. 47. 38. 19. 310. 2

11. 412. 213. 114. 215. 216. 1

INTERPRETING REACTION COORDINATES handout

1) B2) A3) G4) G5) A6) F7) F8) F9) F

10) B11) E12) I13) D14) J15) C16) H17) H18) A

HEATS OF REACTION MC1) 12) 13) 44) 25) 26) 27) 28) 19) 1

10) 411) 412) 1

PE DIAGRAMS FOR RX 1. EXO2. -303. 204. 105. 606. ENDO7. 258. 209. 3010. SAME

11. H12. E13. B14. G15. I16. F17. A18. C19. D