1. What is this?

2. Reaction Rates: How fast reactions occur
Kinetics
Reaction Rates:
How fast reactions occur

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

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

5. 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?

6. 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?

7. 2 kinds of rate laws Both determined by experiment
Differential Rate Law
How rate depends on [ ]
Integrated Rate Law
How rate depends on time

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

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

10. 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

11. 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

12. 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 8

13. Rate = k[NH4+] [NO2-] [NH4] [NO2-] Rate 0.1 2 0.2 4 8 [NH4] [NO2-]
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-]

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

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

21. Half-life: First order

22. Half-Life
First order
Second order
Zero Order