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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
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