1. Reaction Rates Standard 8: Matter SC.912.P.10.5 SC.912.P.10.7
Standard 10: Energy
SC.912.P.12.12

2. Have you ever wondered why some chemical reactions, like the burning of a match, take place at a fast rate, while others, like the spoiling of milk, take place slowly?
The rate of a chemical reaction is the speed at which
the reactants are converted to products.
The rate of reaction depends on the particular reactants
and bonds involved.

3. THE COLLISION THEORY!!! In order for reactions to occur,
atoms/molecules must COLLIDE!
Once they collide, they MAY interact…
What do you mean, they MAY interact?????
They must collide:
the right way around AND
with enough energy for bonds to break.

4. What does all of this look like???
Only ONE collision will result in a chemical reaction between these two molecules!
Collision 1
Collision 2
Collision 3
Collision 4
Of the collisions shown in the diagram,
only collision 1 may possibly lead on to a reaction.

5. ACTIVATION ENERGY (Ea)
Okay, what we know….
Bonds have to BREAK and then REFORM in order for a reaction to occur.
It takes ENERGY for these bonds to break in the first place!
This energy required to initially break the bonds is
appropriately called the
ACTIVATION ENERGY (Ea)
Activation energy animation

6. MAKING BONDS RELEASES ENERGY THEREFORE IS EXOTHERMIC.
BREAKING BONDS REQUIRES ENERGY
THEREFORE IS ENDOTHERMIC.
Energy of products<Activation Energy<Energy of reactants
Energy of reactants<Activation Energy<Energy of products

7. What is a CHEMICAL REACTION RATE?
The speed at which reactants are converted to products.
What is ACTIVATION ENERGY?
The energy initially required to break apart the bonds of a molecule.
What is the difference between an
EXOTHERMIC and ENDOTHERMIC reaction?
Exothermic makes bonds by releasing energy.
Endothermic breaks bonds by requiring the use of energy.

8. Factors which affect reaction rates…
The SURFACE AREA of solid reactants
With more exposed surface, there are more atoms/molecules
that can react with something else
This can be done by breaking something up into smaller pieces.
Zinc powder provides a contact area
THOUSANDS of times greater than that!
Zinc metal has contact area of 6 cm3
Because of this, if ignited, burns explosively
If ignited, does nothing

9. Factors which affect reaction rates continued…
2. The CONCENTRATION of the reactants
Low concentration
High concentration
The higher the concentration of a solution, the greater the
number of particles there are to react with something else…
Something to think about…
Pure oxygen has FIVE TIMES the concentration of oxygen molecules
than air has at the same pressure: so a substance that oxidizes in air
oxidizes more vigorously in pure oxygen.

10. Factors which affect reaction rates continued
3. The TEMPERATURE of the reactants
As the temperature increases, the atoms/molecules
move around faster and have greater kinetic energy!
Therefore, they will collide more frequently,
and the energy of the collisions is much higher!
Remember – to be effective, the energy of the collisions must be
equal to or greater than the activation energy

11. What are the three factors that affect reaction rates?
1. The SURFACE AREA of solid reactants
2. The CONCENTRATION of the reactants
3. The TEMPERATURE of the reactants

12. How else can we make a reaction occur faster???
Through the use of a catalyst.
A catalyst is a substance which speeds up
the rate of reaction without being used up in the reaction.
Catalysts do NOT appear among the final products of reactions they accelerate

13. Some everyday examples of catalysts:
Most of the enzymes in your body are catalysts.
Many of the chemical reactions that are necessary
for life to occur run too slowly without being catalyzed by an enzyme.
Without catalysts we could not exist.
The catalytic converter is a part of all modern car exhaust systems.
This turns many of the pollutants (primarily hydrocarbon (C-H) fragments and
carbon monoxide (CO) in the exhaust into carbon dioxide (CO2) and water (H2O).

14. Summary Chart
The rate of reaction depends on the particular reactants and bonds involved.
Two molecules must collide with enough energy for bonds to break – the energy required to do this is called the activation energy.
If heat/energy is absorbed in the reaction, it is an ENDOTHERMIC reaction (bonds are BROKEN).
If heat/energy is lost in the reaction, it is an EXOTHERMIC reaction (bonds are FORMED).
Three factors that affect reaction rates are:
Surface area – the greater the surface area, the faster the reaction
Concentration – the more particles involved, the faster the reaction
Temperature – the higher the temperature, the faster the reaction
A catalyst may also be used to speed up the rate of reaction. Catalysts are not used up in the reaction, therefore they don’t appear in the reactants or the products.

15. Reaction Rate/Catalyst Quiz
1. Which statement best explains the role of a catalyst in a chemical reaction? A catalyst
a. is added as an additional reactant and is consumed.
b. limits the amount of reactants used during a reaction.
c. changes the kinds of products generated by the reaction.
d. provides an alternative energy pathway, decreasing activation energy.
2. When a catalyst is added to a reaction, what will be the net effect of the catalyst? It will
a. increase the amount of product generated.
b. decrease the amount of product generated.
c. decrease the time the reaction takes.
d. increase the time the reaction takes.

16. Refer to this diagram to answer questions 3, 4, and 5.

17. 3. Which best describes segment L?
a. activation energy
b. reaction heat
c. catalytic reaction
d. kinetic reaction
4. Which section represented on the graph would be changed if a catalyst were added to the reaction?
a. O
b. L
c. N
d. K
5. How can the reaction rate be increased?
a. decrease the size of N
b. decrease the size of L
c. increase the size of O
d. increase the size of M