1. Explaining Reaction Rates
6.3

2. Collision Theory
The rate of a reaction is proportional to the number of collisions per second among the reactant molecules
Anything that can increase the frequency of collision should increase the rate

3. BUT….
for most reactions it is impossible for every one of the collisions between the reactants to actually result in a chemical change
In a gas or a liquid, molecules of the reactants undergo an enormous number of collisions with each other each second.
If each collision were effective, all reactions would be over in an instant
Of all the collisions that occur, only a very small fraction actually results in a chemical change.

4. Orientation:
movie

5. Orientation:
When 2 reactants collide, their atoms must be oriented correctly in order for a reaction to occur

6. Activation Energy

7. Activation Energy
Minimum kinetic energy (KE) that molecules require in order to be able to react

8. Temperature Increasing temperature increases reaction rate
Temperature is a measure of the average kinetic energy of the particles in a substance
Particles with higher kinetic energy will:
Move faster (increasing the collision frequency)
Collide with higher energy (more effective collisions)
Temperature affects both collision frequency and fraction of effective collisions

9. The Maxwell-Boltzmann apparatus
Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms
Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

10. The Maxwell-Boltzmann apparatus
Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms
Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

11. The Maxwell-Boltzmann apparatus
Maxwell and Boltzmann performed an experiment to determine the kinetic energy distribution of atoms
Because all atoms of an element have roughly the same mass, the kinetic energy of identical atoms is determined by velocity (KE= ½mv2)

12. The Maxwell-Boltzmann distribution
The resulting disk looks like this:
Basically, if we plot the intensity of the dots on a graph we get a graph of fraction of atoms/molecules vs. kinetic energy:
Fraction of molecules
Kinetic energy 
Molecules hit disk first
Molecules hit disk last

13. Maxwell-Boltzmann Distribution

14. Temperature and Kinetic Energy

15. Temperature and reaction rate
4/28/2017
Demonstrations: Mg + O2, H2 + O2
By increasing the temperature, a small number of molecules reach Ea. The reaction is exothermic, further increasing temperature and causing more molecules to reach Ea, etc.
Draw the M-B distribution for H2 + O2 before heat was applied. Show how heat affects the diagram.
movie
Shift due to higher temperature
Fraction of molecules
Kinetic energy  Ea

16. Concentration For solutions
Increasing concentration will increase reaction rate
More particles per unit volume
Increases the collision frequency but not the fraction of effective collisions

17. Concentration

18. Pressure (same as concentration for gases)
Increasing pressure will increase reaction rate
More particles per unit volume
Increases the collision frequency but not the fraction of effective collisions

19. Surface Area Increasing surface area increases reaction rate
Increases reaction sites
Affects collision frequency but not fraction of effective collisions

20. Surface Area

21. Surface Area

22. Catalyst
For some reactions, the activation energy is so high that the reaction will either not occur or will occur very slowly
A catalyst is regenerated in the process (i.e. not used up)
A catalyst increases the rate of reaction by lowering the activation energy
Can make the orientation of molecules more favourable, therefore stabilizing the activated complex
A catalyst increases the fraction of effective collisions but not the collision frequency

23. Maxwell-Boltzman Distribution

24. Potential Energy Diagram

25. Catalyst

26. Catalyst
movie

27. Nature of Reactant Atomic structure (periodic trends, activity series)
Ions in solution tend to have a rapid reaction rate
No bonds to be broken
Positive and negative charges attract
In molecular reactions, bonds must first be broken before new bonds can form
If molecules are large, or have strong covalent bonds reaction rate will be slower

28. Nature of Reactant Can increase or decrease rate
Affects the fraction of effective collisions but not the collision frequency