1. Chemical Kinetics Collision Theory: How reactions takes place
Reaction Rates:
How fast reactions occur
Reaction Mechanisms
Resource:

2. Why are kinetics important?
In order to control processes.
speed up useful reactions that occur too slowly
slow down reactions that are harmful
Example:
Catalysts are used in our cars to rapidly convert toxic substances into safer substances
Refrigerators are used to slow the process of spoiling in food

3. Collision Theory How do reactions occur at the molecular level?
Molecules collide with each other
Form activated complex
collisions
correct and incorrect collisions

5. The area under the curve is a measure of the total number of particles present.

7. Svante Arrhenius
Did some fancy math to figure out that number of collisions alone don’t account for reaction rates
He found that reactants also require:
Activation energy (Ea - energy to break bonds)
Right orientation
transition state

8. Not all collisions leads to a reaction For effective collisions proper orientation of the molecules must be possible

10. What affects reaction rate?
Temperature
concentration and temperature
Increased number of collisions
More molecules have enough activation energy
Remember Maxwell-Boltzmann distribution
Increased temperature, distribution flattens out
More molecules have Ea

11. What affects reaction rate?
Higher concentration
Number of collisions increased
concentration
Increased surface area

12. What affects reaction rate?
Catalysts
Def’n: substance that speeds up a rxn w/o being used up itself
Number of collisions with Ea increase
Ea lowers
Catalysts hold molecules in right orientation
Homogeneous catalyst (same phase of matter)
Demo: Catalysis by Co2+
Heterogeneous catalyst (different phase)
catalyst

14. What is this?

15. How do we measure rxn rates?
Rates must be measured by experiment
Indicators that a reaction is happening
Color change
Gas formation
Precipitate formation
Heat and light
Many ways to measure the rate
Volume / time
Concentration / time
Mass / time
Pressure / time

16. How do we measure rxn rate?
A  B
How fast product appears
How fast reactant disappears

17. Forward vs Reverse Rxn Some rxns are reversible
After a sufficient amount of product is made, the products begin to collide and form the reactants
We will deal only w/ rxns for which reverse rxn is insignificant
2 N2O5(aq)  4 NO2(aq) + O2 (g)
Why is reverse rxn not important here?

18. Rate Law
Math equation that tells how reaction rate depends on concentration of reactants and products
Rates = k[A]n
K = rate constant / proportionality constant
n = order of reaction
Tells how reaction depends on concentration
Does rate double when concentration doubles?
Does rate quadruple when concentration doubles?

19. 2 kinds of rate laws Both determined by experiment
Differential Rate Law
How rate depends on [ ]
Integrated Rate Law
How rate depends on time

20. Differential Rate Law 2 methods Graphical analysis
Method of initial rates

21. Graphical Analysis Graph [ ] vs. time Take slope at various pts
Evaluate rate for various concentrations

22. Graphical Analysis When concentration is halved… [N2O5] (M) Rate (M/s)
Rate is halved
Order = 1
Rate = k[N2O5]1
[N2O5]
(M)
Rate
(M/s)
1.0 2
0.5
0.25

23. Graphical Analysis When concentration is doubled… [NO2] (M) Rate (M/s)
Rate is quadrupled
Order = 2
Rate = k[N2O5]2
[NO2]
(M)
Rate
(M/s)
1.0 2
2.0 8
4.0 32

24. Method of Initial Rates
Initial rate calculated right after rxn begins for various initial concentrations
NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l)
Rate = k [NH4+]n[NO2-]m
[NH4+]
[NO2-]
Rate (M/s)
0.1 2
0.2 4 6

25. [NH4] [NO2-] Rate 0.1 2 0.2 4 6 [NH4] [NO2-] Rate 0.1 2 0.2 4 8
When [NO2] doubles, rate doubles,
First order with respect to (wrt) NO2
n = 1
When [NO2] doubles, rate doubles,
First order with respect to (wrt) NO2
m = 1
Rate = k[NH4+] [NO2-]

26. Calculate k, using any of the trials, you should get the same value
Try this one:
[NH4+]
[NO2-]
Rate (M/s)
0.1 2
0.2 8
Rate = k [NO2-]2
Calculate k, using any of the trials, you should get the same value

27. Integrated Rate Law Tells how rate changes with time
Laws are different depending on order
Overall reaction order is sum of exponents
Rate = k  zero order
Rate = k[A]  first order
Rate = k[A]2  second order
Rate= k[A][B]  second order

28. First order integrated rate law
Rearrange and use some calculus to get:
This is y = mx + b form
A plot of ln[A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you know the concentration at any time

29. Second order integrated rate law
Rearrange and use some calculus to get:
This is y = mx + b form
A plot of 1/[A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you can now the concentration at any time

30. Zero order integrated rate law
Rearrange and use some calculus to get:
This is y = mx + b form
A plot of [A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you can now the concentration at any time

31. Graphs give order of rxn
Use graphs to determine order
If [A] vs time = zero order
If ln [A] vs time = first order
If 1/ [A] vs time = second order

32. Half-life Def’n: time it takes for concentration to halve
Depends on order of rxn
At t1/2 [A]=[A]0/2

34. Half-Life
First order
Second order
Zero Order

35. Reaction Mechanism Reactions occur by a series of steps =
Example:
Overall reaction: NO2 + CO  NO + CO2
occurs by following steps
Step 1:
Step 2:

36. Intermediates Two molecules of NO2 collide
Oxygen is transferred, making NO3, the intermediate
Intermediates are temporarily formed during a reaction
They are neither a reactant nor a product &
Get used up in reaction

38. Rules for Reaction Mechanisms
Sum of elementary steps = overall balanced rxn
Mechanism must agree with experimental rate law

39. Elementary Step
Steps in reaction from which a rate law for step can be directly written
2 molecules of NO2 need to collide, therefore…
Rate = k [NO2]2

40. Molecularity Rate law written based on molecularity
Number of things that have to collide
Unimolecular – rxn depends on 1 molecule
Bimolecular – rxn depends on 2 molecules
Termolecular – rxn depends on 3 molecules
Very rare!

41. Give molecularity and rate law:
Unimolecular (first order)
rate=k[A]
Bimolecular (second order)
rate=k[A][B]

42. Rate Determining Step
The slowest step in mechanism determines overall rate
Rate cannot be faster than slowest step
Demo: Filling bottle with funnel
Overall rate law can be written from molecularity of slowest step

43. How are mechanisms determined?
Rate law is determined using experiment (method of initial rates, etc.)
Chemist uses intuition to come up w/ various mechanisms
Narrows down choices using rules for mechanisms
No mechanism is ever absolutely proven