1. Collision Theory and Reaction Rates

2. Collision Theory
Chemical reactions only occur when atoms, ions, or molecules collide.
The species must make contact for a reaction to occur.
Collisions must have proper orientation and sufficient energy to be successful.

3. Orientation matters
The image to the right shows the only orientation that will lead to a reaction
If the two molecules don’t collide at the right orientation they will rebound

4. Orientation matters

5. Energy matters
The activation energy needs to be enough to create the activated complex
If it’s too much it might drive the reaction the opposite way
If it’s too little the activated complex won’t form
Activated complex formed

6. Ex: Respiration Photosynthesis

7. Other connections
The graph you saw for the activation energy helps you to identify if a reaction is exothermic or endothermic
The activation energy is connected to the overall energy of the reaction
It takes energy to break bonds (Activation energy) and when bonds are reformed some energy is released.

8. Quiz check! True or false:
As long as there is enough energy the reaction will go forward
A poor orientation can be overcome with more energy
When a product is being formed and is in a high energy intermediate step we call that
Homogenous reaction
Heterogeneous reaction
Activated complex
Rebounded actuality

9. Reaction Rates
The rate of a chemical reaction is affected by many factors, including:
Concentration
Pressure
Temperature
Surface area
Catalyst

10. Concentration of reactants
as concentration of reactants increases the rate of reaction generally increases
Remember - Particles (molecules & atoms/ions) have to collide in order for a reaction to occur.
Ex:
If you're driving a car, which freeway would you have a HIGHER chance of getting into a car accident?

11. Pressure
If the pressure of a gas is increased by decreasing the volume, the same amount of material is now in a smaller area; therefore increasing the concentration in a given area. The reaction will occur faster
Volume 1 : More space (Fewer collisions)
Volume 2: Less space (More collisions)

12. Temperature of the reaction
Increasing temperature increases the speed of the particles increasing the rate of a reaction
Decreasing the temperature decrease the speed of the particles decreasing the rate of a reaction
food spoils more quickly at room temperature than in a refrigerator
bacteria grow faster at RT than at lower temperatures

13. Presence of a catalyst
a substance that increases the rate of a reaction without being consumed in the reaction
Enzymes
biological catalysts
proteins that increase the rate of biochemical reactions
Catalytic converters
Expensive metals (platinum) allow harmful gases to be broken down into H2O, CO2, N2

14. Surface Area
as surface area increases the rate of reaction generally increases
Ex: A chunk of iron rusting releases heat VERY SLOWLY.
Small amount of heat
Iron rusting in a hand warmer releases heat VERY QUICKLY
LARGE AMOUNT OF HEAT

15. Question What is not something that affects reaction rate?:
Orientation
Presence of a catalyst
Temperature
Concentration of reactants

16. Reaction Rates
The speed of an object or event is the change that occurs in a given time interval.
Speed of a car = change in distance
change in time
= Δd Δt
Remember, the term change (Δ) always refers to final value minus initial value.

17. Reaction Rates
Similarly, the rate (or speed) of a reaction can be determined:
Rate = change in concentration (or moles) of product
change in time
Rate = Δ (concentration or moles) Δt

18. Question How is reaction rate measured? a. Speed/Distance
b. Distance/time
c. Mols of product/mols of reactant
d. Concentration/time

19. Comprehension check
What 2 factors of collision theory influence a reaction?
Pick one of the variables that impacts rate of a reaction. Explain how changing that variable impacts the rate of a reaction