1. Reaction Rates: 2 NO2  2 NO + O2 change in conc. 1. slope =
change in time
1. slope =
2. consumption of reactants per time
–[NO2] t
concentration (M)
[NO2]
3. production of products per time
+[NO] t t
4. Are equalized to a
stoich. coefficient of 1
2 NO
–[NO2]
2 t =
[NO]
2 t =
[O2] t
1 O2
5. Slow down during rxn
(reactant collisions decrease)
time (t)

2. minimum E required to start reaction
The Collision Model
reactant bonds break, then product bonds form
successful
reactants
Collision
products
Reaction rates depend on collisions
between reactant particles by:
collision frequency
collision energy
proper orientation
activation energy:
minimum E required to start reaction
(Eact )
unsuccessful

3. 4 Factors That Affect Reaction Rates:
Concentration
Temperature
(exposed) Surface Area (particle size)
Catalyst
↑Conc. , ↑collision Frequency
↑Temp , ↑k
↑collision Frequency
↑collision Energy
Eact
(more particles over Eact)
↓size, ↑surface area, ↑collision Frequency
Catalyzed
↑ rate by ↓Eact by…
changing the mechanism
energy
(intermediate)

4. Reaction Mechanisms mechanism: Step 1: NO2 + NO2  NO3 + NO (slow)
Step 2: NO3 + CO  NO2 + CO2 (fast)
NO2 (g) + CO (g)  NO (g) + CO2 (g)
Stoich of mech. and overall equation must match
The overall reaction occurs only as fast as the slowest, rate-determining step. (RDS)
Rate law depends on reactants in the slow step:
rate = k [NO2]2
A reactant NOT in the slow RDS is 0th order
[CO]0
Intermediate (NO3) is produced then consumed
(must be substituted out of rate law) 4

5. Rate Laws rate = k[A]x[B]y[C]z recall…
Rate equations (or rate laws) have the form:
rate = k[A]x[B]y[C]z
rate constant
order with respect to reactants
A, B, & C
…or…
number of each particle involved
in collision that affects the rate
overall order of reaction = x + y +…
Example:
overall order
= ___ order
(4 particles in collision)
rate = k[BrO3–][Br–][H+]2
4th

6. Orders in Rate Laws …only found experimentally (from data).
…do NOT come from the coefficients of reactants of an overall reaction.
…represent the number of reactant particles (coefficients) in the RDS of the mechanism.
…zero order reactants have no effect on rate b/c they do not appear in the RDS of the mechanism (coefficient of 0 in RDS).
…typically 0, 1, 2, but can be any # or fraction

7. Half-life (t1/2): time at which half of initial amount remains.
[A]t = 0.5 [A]0
initial concentration at time, t = 0
given on exam
concentration at time, t
= t1/2
0.693 k
Half-life (t1/2) is constant
for 1st order only. 7

8. M∙s–1 s–1 M–1∙s–1 Summary 0th Order 1st Order 2nd Order [A] ln[A] 1
Rate Law
Rate = k
Rate = k[A]
Rate = k[A]2
Integrated Rate Law
[A] = –kt + [A]0
ln[A] = –kt + ln[A]0
Linear plot
[A]
ln[A]
k & slope of line
Slope = –k
Slope = k
M∙s–1
s–1
M–1∙s–1
Half-Life
depends on [A]0
y = mx + b
y = mx + b 1
[A] t t t
M = ? s
M = ?∙M s
M = ?∙M2 s
Units of k