1. Kinetics

2. This is important!!!
determine rate laws & units from experimental data
calculate rates & concentrations of reactants or products under given conditions
match mechanisms to rate laws
The next few chapters comprises over 50% of the test!!!

3. Rates how quickly a reaction proceeds to the right (products)
units are D amount of reactants per D time (most commonly M/s)
as [reactants] & [products] change, so does the rate
rates are expressed as positive quantities, so negative sign is used on the slope to make it positive

4. Factors that affect rates
rates increase with more collisions or higher energy collisions, so…
[reactants] – higher [reactant] produce a faster rxn by increasing the frequency of collisions
temp – higher temp means higher energy collisions, collisions more often, and a higher percentage of successful collisions, thus higher rate
state of reactants – liquids & gases react faster because there are more particles available to collide. solids are limited to reacting particles on the surface.
catalyst – presence of catalyst increases rate by changing the mechanism of the rxn to one that has a higher success rate

5. [reactants] & rate
rate law is a math relationship that shows how rate depends on [reactants]
where: a A + b B  products,
the rate law is: Rate = k[A]m[B]n
k = rate constant (determined experimentally)
[A] = molar concentration of A
[B] = molar concentration of B
m & n = small, whole number that relate to # of m’cules that collide and determine the rxn order

6. m & n are NOT the coefficients from the balanced rxn
can ONLY be found experimentally
if Rate = k[A]1[B]2, the rxn is said to be first order with respect to A, second order with respect to B, and third order overall

7. Example
Experiment
[A] (M)
[B] (M)
Rate = -D[A]/Dt (M/s) 1
0.10
0.04 2
0.20
0.08 3
0.32
Determine the rate order with respect to each of the reactants and the rate constant, k.

8. Stoichiometry Example
Consider the reaction between gaseous hydrogen and gaseous nitrogen to produce ammonia gas:
H2 + N2  NH3
At a particular time during the reaction, H2 disappears are a rate of 4.0 M/s.
What is the rate of disappearance of N2?
What is the rate of appearance of NH3?
If NH3 appears at a rate of 3.2 M/s, how fast does H2 disappear?

9. D [] with time
1st order rxn’s rate depends on the [] of a single reactant raised to the 1st power.
to recognize 1st order rxns, graph ln[A] vs. time and you will get a straight line whose slope = -k
2nd order rxn’s rate depends on the [] of a single reactant raised to the 2nd power
to recognize 2nd order rxns, graph 1/[A] vs. time and you will get a straight line whose slope = k

10. Your new favorite table…
Order or reaction
1st
2nd
differential rate law
Rate = k[A] = -D[A]/Dt
Rate = k[A]2 = -D[A]/Dt
integrated rate law
ln[A]t = -kt + ln[A]0
1/[A]t = kt + 1/[A]0
Half-life
t1/2 = 0.693/k
t1/2 = 1/k[A]0
Straight line
ln[A] vs. time
1/[A] vs. time
slope -k k

11. Temp & rate generally rate increases with temp
higher temp = higher energy = more collisions = more successful collisions = higher speed of m’cules
[] changes will not change k, but T will
know the vocab:
activation energy
transition state
catalyst

12. know how to label Ea, Ereac, Eprod, DH, & transition state
know how to recognize endothermic vs. exothermic rxns

13. Reaction Mechanisms step-by-step process of a chem rxn
elementary step = individual step in mechanism
molecularity = # of m’cules participating in an elementary step
unimolecular = 1 m’cule reacting, rate is 1st order
bimolecular = 2 m’cules reacting, rate is 2nd order
termolecular (rare) = 3 m’cules reacting, rate is 3rd order

14. more mechanisms
rxn mechanism is written as a list of steps that add up to the overall rxn:
2A + 2B  A2B2
possible mechanism:
1. A + A  A2 Rate = k[A]2
2. A2 + B  A2B Rate = k[A2][B]
3. A2B + B  A2B2 Rate = k[A2][B2]

15. 1. A + A  A2 Rate = k[A]2 2. A2 + B  A2B Rate = k[A2][B] 3
1. A + A  A2 Rate = k[A]2 2. A2 + B  A2B Rate = k[A2][B] 3. A2B + B  A2B2 Rate = k[A2][B2] intermediate = chemical species that is formed in one elementary step and used in another (A2 & A2B) catalyst = chemical species that is used in one elem step and produced in another (not here) rate determining step = slowest step & the one that determines the rate law for the entire reaction

16. Catalysts
catalyst = used to increase the rate of the rxn, but is not used up in the rxn itself, removes the slowest step in the rxn, lowers the Ea.