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CHEM 1220 > Ch. 13: Chemical Kinetics > Flashcards

Ch. 13: Chemical Kinetics Flashcards

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1
Q

the ____ of a chemical reaction is a measure of how fast the reaction occurs

A

rate

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2
Q

fast rate=….?

A

a large fraction of the molecules reacting to form products in a given period of time

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3
Q

slow rate=….?

A

a small fraction of the molecules reacting to form products in a given period of time

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4
Q

How are measurements of rate usually expressed?

A

change in some quantity per unit time

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5
Q

the reaction rate is defined as the _____ of the change in concentration of a reactant divided by the change in time

A

negative

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6
Q

why is the reaction rate negative in terms of the reactants?

A

because reactant concentrations decrease as the reaction proceeds

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7
Q

when rate is defined is it defined in respect to the products or the reactants? why?

A

products because then the reaction rate is naturally a positive number

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8
Q

the reactant concentration ____ with time because…?

A

decreases

reactants are consumed in a reaction

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9
Q

the product concentration _____ with time because…?

A

increases

products are formed in a reaction

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10
Q

what is the equation for average reaction rate when looking at the reactants?

A
  • (stoichiometric coefficient) [(change in concentration) / (change in time)]
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11
Q

as the reactants transform to products, their concentrations _____ and the reaction rate _____

A

decrease

decreases

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12
Q

for most reactions, the rate depends on the ____ of the reactants

A

concentration

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13
Q

how can you find the instantaneous rate of the reaction?

A

the slope of a line tangent to to the curve at that point

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14
Q

what is the most common way to study reaction kinetics?

A

spectroscopy

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15
Q

what common three ways are reaction rates measured in a vessel?

A
  1. spectroscopy
  2. pressure measurement
  3. polarimetry
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16
Q

name three way in which aliquots (samples) that are periodically withdrawn from the reaction vessel and analyzed can be used to determine the progress of the reaction.

A
  • gas chromatography
  • mass spectrometry
  • titration and other wet chemical techniques
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17
Q

how can the removal of aliquots be used to determine the reaction rate?

A

by taking these samples at regular time intervals and determining the relative amounts of reactants and product as a function of time

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18
Q

define rate law

A

a relationship between the rate of the reaction and the concentration of the reactants

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19
Q

what is the rate law?

A

rate= k[A]^n

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20
Q

what is the k in rate law?

A

k is the constant of proportionality called the rate constant

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21
Q

what is the n in rate law?

A

n is the reaction order. it is usually an integer and it reflects how the rate depends on the concentration

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22
Q

what is the order when the rate is independent of the concentration?

A

zero order / n=0

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23
Q

what is the order if the rate is directly proportional to the concentration of A?

A

first order / n=1

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24
Q

what is the order if the rate is proportional to the square of the concentration of A?

A

second order / n=2

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25
Q

what conditions are necessary for the rate of a reaction to be constant (aka for n to equal zero)?

A

the amount of reactant actually available for reaction must be unaffected by the changes in the overall quantity of reactant

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26
Q

How is reaction order determined?

A

experimentally

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27
Q

what is a common way to determine reaction order?

A

method of initial rates

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28
Q

how does the method of initial rates work?

A

the initial rate is measured by running the reaction several times with different initial reactant concentrations to determine the effect of the concentration on the rate.

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29
Q

do the zero, first and second order all have the same units?

A

no

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30
Q

what is the rate law for more than one reactant?

A

Rate= k [A]^m [B]^n

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31
Q

what is the overall order of a rate law for two reactants?

A

the sum of the order for A and the order B

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32
Q

define overall order

A

the sum of the orders of all the reactants in a chemical reaction (all the n’s)

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33
Q

define integrated rate law

A

a relationship between the concentrations of the chemical reactants in a chemical reaction and time

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34
Q

what is the integrated order if the rate is directly proportional to the concentration?

A

first order

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35
Q

what type of graph do the zero, first and second order integrated rate law form?

A

a linear graph

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36
Q

define half life (t 1/2)

A

the time required for the concentration of a reactant to fall to one-half its initial value

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37
Q

define lifetime (𝓣)

A

the time for a reactant to decrease to 1/e of the original concentration

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38
Q

what does the lifetime of an chemical represent?

A

the average life expectancy of the chemical entity

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39
Q

what is the equation for decay time for first order reactions?

A

t= ln(n) / k

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40
Q

what are t and n for half-lifes?

A

t= t 1/2

n=2

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41
Q

what are t and n for lifetimes?

A

t=𝓣

n=e

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42
Q

what is the equation for lifetime in a first order reaction?

A

𝓣=ln(e)/k
or
𝓣=1/k

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43
Q

for first order reactions are lifetimes and half-lifes are independent of the initial concentration of the chemical?

A

yes

44
Q

for a second order reaction is the half-life dependent of the concentration?

A

yes

the time for the concentration to reach half is twice as long as the first time due to the depleted concentration

45
Q

how does the decreasing concentration of a second order reaction affect the half -life of the chemical?

A

as the concentration decreases the time it takes for the concentration to reach half the initial increases

46
Q

how does the decreasing concentration of a zero order reaction affect the half -life of the chemical?

A

as the concentration decreases the time it takes for the concentration to reach half the initial decreases

47
Q

what two things does the rate law relate?

A

the rate of the reaction to the concentration of the reactants

48
Q

what two things does the integrated rate law relate?

A

the concentration of the reactants to time

49
Q

half-life, lifetime. and decay time of a first order reaction are ______ of/on the initial concentration.

A

independent

50
Q

the half-lives and decay times of zero-order and second-order reactions _____ of/on the initial concentration.

A

depend

51
Q

define activation energy (Ea)

A

an energy barrier in a chemical reaction that must be overcome for the reactants to be converted into products.

52
Q

define frequency factor (A)

A

the number of times that reactants approach the activation energy per unit time

53
Q

define activated complex

A

a high-energy intermediate state between reactant and product

54
Q

define transition state

A

a high-energy intermediate state between reactant and product

55
Q

the higher the activation energy, the _____ the reaction rate (at a given temperature)

A

slower

56
Q

define exponential factor

A

a number between zero and one that represents the fraction of molecules that have enough energy to make it over the energy barrier on a given approach

57
Q

what does the exponential factor depend on?

A

temperature (T) and the activation energy (Ea)

58
Q

what is the exponential factor?

A

e^-Ea/RT

59
Q

a low activation energy and a high temperature make the negative exponent _____, so that the exponential factor approaches ____

A

small

one

60
Q

if the activation energy is zero, then the exponent is ____, and the exponential factor is _____

A

zero

exactly one

61
Q

a large activation energy and a low temperature make the negative exponent _____, so that the exponential factor is ____

A

large

very small

62
Q

if the temperature approaches 0 K, the exponent approaches an indefinitely _____ number, and the exponential factor approaches ____

A

large

zero

63
Q

how does a small change in temperature effect the number of molecules having enough energy to surmount the activation barrier?

A

it greatly effects the number of molecules that have enough energy to surmount the activation barrier

64
Q

the exponential factor increases with ____ temperature

A

increasing

65
Q

a large activation energy results in a _____ exponential factor

A

small

66
Q

what is the arrhenius equation in the form y=mx+b?

A

ln k= -(Ea/R)(1/T)+ln A

67
Q

define arrhenius equation

A

an equation which relates the rate constant of a reaction to the temperature, the activation energy, and the frequency factor

68
Q

define collision model

A

a model of chemical reactions in which a reaction occurs after a sufficiently energetic collision between two reactant molecules

69
Q

define orientation factor

A

in the collision model, the fraction of sufficiently energetic collisions in which the reactants are in the correct orientation to react

70
Q

define collision frequency

A

the number of collisions that occur per unit time

71
Q

what letter represents the orientation factor?

A

p

72
Q

what letter represents the collision frequency

A

z

73
Q

what is the arrhenius equation in terms of collision model?

A

k=pze ^(-Ea/RT)

74
Q

if p=0.16 how many successful collisions are there out of 100?

A

16

75
Q

why might to molecules colliding with significant energy not react?

A

the must collide in such a way that allows the necessary bond to occur

76
Q

what is the approximate orientation factor of individual atoms and why?

A

approx. 1, because atoms are spherically symmetric and thus any orientation can lead to the formation of products

77
Q

what does a orientation factor of more than one mean?

A

that collisions do not have to occur for chemicals to react

more reactions than collisions

78
Q

what is the process dubbed the harpoon mechanism

A

the transfer of an electron without a collison

the atom basically harpoons a passing molecule with an electron and reels it in through the coulumbic

79
Q

do most reactions occur in a single step?

A

no

80
Q

chemical equations are used to represent a chemical reaction and it usually represents the _____ equation, not a series of individual steps by which the reaction occurs

A

overall

81
Q

what does the overall chemical equation show?

A

the substances present at the beginning of the reaction and the substances formed by the reaction

82
Q

define reaction mechanism

A

a series of individual chemical steps by which an overall chemical reaction occurs

83
Q

define elementary step

A

an individual step in a reaction mechanism

84
Q

can elementary steps be broken down into simpler steps?

A

no

85
Q

define reaction intermediates

A

species that are formed in one step of a reaction mechanism and consumed in another

86
Q

for a valid reaction mechanism the individual steps in the mechanism ____ to the overall reaction

A

add

87
Q

elementary steps are characterized by their _____

A

molecularity

88
Q

define molecularity

A

the number of reactant particles involved in and elementary step

89
Q

what are the most common molecularities?

A

unimolecular and bimolecular

90
Q

what does unimolecular step look like?

A

A —> products

91
Q

what does a bimolecular step look like?

A

A + A —> products
or
A + B —> products

92
Q

define unimolecular

A

describes a reaction that involves only one particle that goes on to form products

93
Q

define bimolecular

A

an elementary step in a reaction that involves two particles, either the same species or different, that collide and go on to form products

94
Q

define termolecular

A

an elementary step of a reaction in which three particles collide and go on to form products

95
Q

how common are termolecular steps? why so?

A

very rare because the probability of three particles simultaneously colliding is small

96
Q

can rate law be deduced from the overall reaction?

A

no

97
Q

can rate law be determined from an elementary step?

A

yes

98
Q

since we know that an elementary step occurs through the collision of the reactant particles, the rate law is proportional to…?

A

the concentration of those particles

99
Q

define rate-determining step

A

the step in a reaction mechanism that occurs much more slowly than any of the other steps

100
Q

the rate determing step determines what?

A

the rate law for the overall reaction

101
Q

for a purposed reaction mechanism to be valid, what two conditions must be met?

A
  1. the elementary steps in the mechanism must sum to the overall reaction
  2. the rate law predicted by the mechanism must be consistent with the experimentally observed rate law
102
Q

is a valid mechanism a proven mechanism?

A

no we can only say that the given mechanism is consistent with the kinetic observations and therefore possible

103
Q

define steady rate approximation

A

the assumption that the rate of formation of an intermediate is equal to its rate of consumption

104
Q

what is the first step in using the steady rate approximation mechanism?

A

determining a preliminary expression for the rate of the reaction

105
Q

how does the steady-state approximation allow for the intermediate concentration of a chemical to be used in determined the rate of the reaction?

A

it assumes that after a short period of time the intermediate will have reached a stead state in which the rate of production of the intermediate is the same as the rate of consumption of the intermediate

106
Q

how can you use steady-state approximation if one of the chemicals is an intermediate and therefore its concentration is unknown to figure out the reaction rate?

A

by determining the equations for the rate for production and consumption of the intermediate and then using substitution and basic algebra to figure out the reaction rate

107
Q

what is RDS?

A

rate determining step

CHEM 1220 (4 decks)

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