1. Rates of Reactions

2. Rates
Speed or rate is measured by the amount of change that occurs in a given time interval

3. REACTION RATE
The amount of time it takes to use up the reactants (decrease in reactant concentration) OR
The amount of time it takes to make products (increase in product concentration)

4. I.A. Reaction Rates 2 NO2  2 NO + O2
Reaction Rate-change in concentration of reactants and products in a given time period
Consider the following reaction:
2 NO2  2 NO + O2
time (sec) [NO2] M M M M M
Sketch a graph with t on the x-axis and M on the y-axis.

5. I.A. Reaction Rates 2 NO2  2 NO + O2
How much NO is present at the beginning of the reaction?
time (sec) [NO2] M M M M M
[NO]

6. I.A. Reaction Rates 2 NO2  2 NO + O2
How much NO2 was used up in the first 5 seconds?
How much NO was produced?
time (sec) [NO2] M M M M M
[NO]
0 M
Calculate the moles of NO produced for each value of t (always use the total moles of NO2 used up to that point).

7. Collision Theory
Reaction rate depends on the collisions between reacting particles.
Effective collision Reaction occurs
Elastic collisions  No reaction occurs
Effective collisions occur if the particles...
collide with each other
have the correct orientation
have enough kinetic energy to break bonds

8. Unsuccessful Collisions
Particle Orientation
Required Orientation
Unsuccessful Collisions
Successful Collision

9. I.B. Collision Theory
Elastic Collisions
Effective Collisions

10. Activation Energy (Ea)
Even though the boulder will be more stable at the bottom of the hill, it needs a push to get up and over the bump first.
depends on reactants
low Ea = fast rxn rate

11. Activation Energy (Ea)
Minimum energy required for a reaction to occur
Transition state-complex that is neither reactant nor product
Activated Complex
Activation
Energy
Energy of products
∆H - Difference between energy
of reactants and products
Energy of reactants

12. Draw an energy diagram for an exothermic reaction
Draw an energy diagram for an exothermic reaction. Label the axes, reactants, products, activated complex, activation energy, and change in enthalpy.

13. Factors Affecting Reaction Rate
1. Nature of the Reactants
2. Concentration of Reactants
3. Surface Area of Reactants
4. Temperature
5. Catalysts

14. Factors Affecting Reaction Rate
1. Nature of the Reactants
Certain bonds are easier to break than others
Ca(s) + 2H2O(l)  H2(g) + Ca(OH)2(aq)
2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq)
Na reacts with water faster than Ca because Na is more reactive.

16. 2. Concentration high concentration =
the higher the concentration → the more molecules there are → the more collisions take place creating product

17. 3. Surface Area
the more surface a molecule can touch → the faster the reaction
Increase surface area by…
using smaller particles
dissolving in water

18. Temperature high temp = fast reaction rate
the hotter the molecules → the more they move → the more they collide & change into product

19. Lower temp, more reactant particles have low amts of kinetic energy
As temperature increases, more reactant particles have higher kinetic energy
Only a small # of particles will have enough energy to form an activated complex.

20. Catalysts
5. Catalyst
substance that increases reaction rate without being consumed in the reaction
lowers the activation
energy, more collisions
will have enough energy
to pass over the energy barrier
reaction is faster because a greater fraction of collisions is effective at any given point

21. Catalysts in body
Enzymes
Breakdown sugars, proteins, etc

22. The decomposition of ozone to oxygen is catalyzed by Cl.
Reaction Mechanisms
Many reactions occur in several steps.
2 O3 (g)  3 O2 (g)
The decomposition of ozone to oxygen is catalyzed by Cl.

23. Cl + O3  ClO + O2 Elementary step O3  O + O2 Intermediates
Reaction Mechanisms
Cl + O3  ClO + O2 Elementary step
O3  O + O2 Intermediates
ClO + O  Cl + O2 Add steps
2 O3  3 O2

24. Determine the original equation. Rate-determining step.
Reaction Mechanisms
2 NO  N2O2 Fast
N2O2 + H2  N2O + H2O Slow
N2O + H2  N2 + H2O Fast
Determine the original equation.
Rate-determining step.

25. What is the overall reaction? How many elementary steps?
Reaction Mechanisms
HBr + NO2  HOBr + NO Slow
HBr + HOBr  H2O + Br2 Fast
What is the overall reaction?
How many elementary steps?
What are the intermediates?
What is the rate-determining step?