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Flashcards in Exam 3 CH 302 Deck (33):
1

rate law

a means by which we can relate the rate of a chemical reaction to concentrations of the reactants

2

reaction rates

a measure of the change in the concentration of the chemical species as a function of time

3

factors affecting rate

1. medium or nature of the reactants (physical state and particle size)
2. concentration
3. temperature
4. a catalyst

4

empirical rate laws

rate laws for an overall chemical reaction
rate = k[A]

5

what can assume to be ignored in integrated rate laws?

any "backward" reaction

6

first order reactions

-most important cases
-when reaction is overall first order, the rate is proportional to the amount of that reactant
-exponential graph. (negative)

7

first order equation

ln [Ai]/[A] = kt

8

first order half life equation

(ln2)/k =. t1/2
a quantity related to the rate constant but not concentration
-when ratio equals 0.5
1/s

9

zero order reactions

-the rate of reaction is independent of all the concentration of the reaction
-rate of reaction will not change
rate = k
M/s

10

zero order reaction equation

[A] = [Ai] - kt
-yields a linear plot

11

Second Order reactions

result of bimolecular steps occurring in a reaction
rate = k[A]^2

12

second order equation

1/[A] - 1/[Ai] = kt

13

pseudo-first order

when a reaction is 2nd order overall but 1st order w/respect to two reactants
rate = [A][B]

14

zero order half life

[Ai]/2k = t 1/2
directly proportional

15

second order half life

1/k[Ai] = t 1/2
indirectly proportional

16

A -> 2B
k = 1.5 x 10^-3 M-1s-1
How long does it take 0.15 M A to fall to 0.065 M?
t= _____ hours

What is [B] at this time?
____ M

1.6 hours
0.17 M

17

Reaction Mechanisms

-made up of steps involved in chem run
-a breakdown of what actually happens during the course of a reaction
-will have a predicted rate law

18

elementary steps

-mechanism is typically written as a series of elementary steps
-steps characterized by molecularity
-sum of steps = overall reaction
-almost always either unimolecular or bimolecular

19

unimolecular

-one reactant makes products
-A -> products
rate = k[A]

20

bimolecular

-two reactants make products
-A + B -> products
rate = k[A][B]
-2A -> products
rate = k{A]^2
-requires a collision

21

Collision Theory

-molecules must collide in order to react (2nd order)

22

Effective Collisions

lead to product formation
two factors:
1. must have enough energy (how hard they hit)
-thermal energy (increase T, increase energy)
2. orientation of molecules

23

Rate Limiting Step

-the slowest step in the mechanism is the rate determining step/rate limiting step

24

intermediate

a chem species involved in a mechanism that doesn't appear in the overall reaction

25

Kinetics

rates of reactions
change in conc/change in time
units: M/s or other time units (M/min, M/hour, M/day)

26

2A + B -> 3C +2D
A is reacting @ a rate = -0.012 M/s what is the rate of C?

what is the overall rate of the reaction?

+0.018 M/s

+0.006 M/s

27

Arrhenius Theory

-not instantaneous and is typically slowed by the fact that in order to convert from reactants and products, system must first pass through transition state
-size of the energy barrier controls the temp dependence of the rate constant
-to speed up rate, raise temp or lower barrier

28

Arrhenius Equation

k = Ae^(-Ea/RT)

29

reaction coordinate

a path that links the reactant molecules and product molecules

30

transition state

-high energy point btw 2 minimal along the run coordinate
-each step in a mechanism have a transition state
-highest energy point along lowest energy path

31

activation energy

height of barrier along the rxn pathway
change in H = Ea(f) - Ea(r)

32

Catalysts

-a substance that speeds up the rate of a chem rxn but is not consumed in the reaction
-homogenous: catalyst in same place
-heterogenous: catalyst in different places
-catalysts are selective
-lower activation energy by changing mechanism
-highest barrier will be significantly lowered

33

enzymes

-biological catalysts
-proteins that fold into conformations
-substrates and enzymes bind to form complexes
-active site: wehre substrate binds to enzymes