Exam 3 CH 302

Question 1

rate law

Answer 1

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

Question 2

reaction rates

Answer 2

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

Question 3

factors affecting rate

Answer 3

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

Question 4

empirical rate laws

Answer 4

rate laws for an overall chemical reaction

rate = k[A]

Question 5

what can assume to be ignored in integrated rate laws?

Answer 5

any “backward” reaction

Question 6

first order reactions

Answer 6

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

Question 7

first order equation

Answer 7

ln [Ai]/[A] = kt

Question 8

first order half life equation

Answer 8

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

Question 9

zero order reactions

Answer 9

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

Question 10

zero order reaction equation

Answer 10

[A] = [Ai] - kt

-yields a linear plot

Question 11

Second Order reactions

Answer 11

result of bimolecular steps occurring in a reaction

rate = k[A]^2

Question 12

second order equation

Answer 12

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

Question 13

pseudo-first order

Answer 13

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

Question 14

zero order half life

Answer 14

[Ai]/2k = t 1/2

directly proportional

Question 15

second order half life

Answer 15

1/k[Ai] = t 1/2

indirectly proportional

Question 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

Answer 16

1. 6 hours

0. 17 M

Question 17

Reaction Mechanisms

Answer 17

• 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

Question 18

elementary steps

Answer 18

• 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

Question 19

unimolecular

Answer 19

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

Question 20

bimolecular

Answer 20

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

Question 21

Collision Theory

Answer 21

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

Question 22

Effective Collisions

Answer 22

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

Question 23

Rate Limiting Step

Answer 23

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

Question 24

intermediate

Answer 24

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

Question 25

Kinetics

Answer 25

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

Question 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?

Answer 26

+0.018 M/s +0.006 M/s

Question 27

Arrhenius Theory

Answer 27

- 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

Question 28

Arrhenius Equation

Answer 28

k = Ae^(-Ea/RT)

Question 29

reaction coordinate

Answer 29

a path that links the reactant molecules and product molecules

Question 30

transition state

Answer 30

- 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

Question 31

activation energy

Answer 31

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

Question 32

Catalysts

Answer 32

- 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

Question 33

enzymes

Answer 33

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