Transition state theory

Question 1

Rate of reaction (for A+B -> P =

Answer 1

change in P over time = k[A][B]

Question 2

k is the

Answer 2

rate coefficient/constant that defines the kinetics of the reaction

Question 3

Arrhenius equation describes the

Answer 3

temperature dependence of the rate coefficient

Question 4

pre exponential factor and activation energy can be determined from the experimental measuremnt of

Answer 4

k as a funciton of T (lnk vs T^-1 is a straight line)

Question 5

Activation energy is a barrier over which

Answer 5

reactants must pass before products are formed

Question 6

In the arrhenius equation T tend to infinity…

Answer 6

exponential tends to 1 so k tends to A

Question 7

In Arrhenius equation as T tend to 0

Answer 7

exponential tends to 0 so k tends to 0

Question 8

exp(-E_a/RT) is the fraction of

Answer 8

reactants that have enough energy to react

Question 9

pre-exponential factor, A, is the

Answer 9

maximum possible rate for the reaction

Question 10

if A and B do not collide there can be no

Answer 10

reaction

Question 11

A potential energy surface is a multidimensional surface of

Answer 11

potential energy as a function of all inter-nuclear bond distances and/or bond angles

Question 12

For A+BC -> AB+C when AB distance is large the potential energy is that of the

Answer 12

diatomic molecule BC

Question 13

For A+BC -> AB+C when BC distance is large the the potential energy is that of the

Answer 13

diatomic molecules AB

Question 14

For A+BC -> AB+C, along reaction coordinate as AB distance becomes smaller then

Answer 14

AB bond is formed as BC is broken (distance AB and BC changes) and the potential energy of the system goes up

Question 15

Minimum energy path from reactants to products is shows from a plot of

Answer 15

energy vs reaction coordinate

Question 16

When energy is maximum along the minimum energy path this point is called the

Answer 16

transition state (double dagger ++)

Question 17

The structure of the complex at the transition state is called the

Answer 17

activation complex

Question 18

The transition state is close to the reactants in energy then the activated complex will have a

Answer 18

similar structure to the reactants (and same for products)

Question 19

For exothermic reaction the reactants and transition state are close in

Answer 19

energy and the activated complex is ‘reaction like’ (in contrast for endothermic ts close to products and AC is product like)

Question 20

A potential energy surface for a reaction can be constructed from

Answer 20

Experiments (spectroscopy, crossed molecular beam)
Theory (ad initio electronic structure calculations)
Semi-empirical - mixture of above 2 (density functional theory and maths ad initio calculation to experiment)

Question 21

An accurate Potential energy surface allows one to determine

Answer 21

the energy level of reactants, AC and spectroscopic constants for AC (moment of inertia from mass/bond distances and vibrational frequencies)
Value of E_0
structure of AC and dynamics

Question 22

E_0 is the difference between

Answer 22

zero-point energies in the reactants and transition state

Question 23

Activated is at maximum in energy along reaction coordiante so it is

Answer 23

highly unstable and short-lived

Question 24

In transition state theory there exists an equilibrium between

Answer 24

reactants and activated complex

Question 25

Once the activated complex is formed it can sponteneously form

Answer 25

products or reactants (because AC is at maximum along reaction coordinate)

Question 26

K_c is the

Answer 26

concentration equilibrium constant between the reactants and the activated complex in terms of molar concentrations

Question 27

2 assumptions in transition state theory are

Answer 27

reaction coordinate over the barrier is separable from all other modes on the activated complex reaction cannot re-cross the transition state once it has passed it (non-recrossing rule)

Question 28

v^double dagger is a

Answer 28

vibration

Question 29

motion leading to reaction is a specific

Answer 29

vibration of the activated complex

Question 30

The specific mode v^doubledagger leading to reaction corresponds to a vibration of a very

Answer 30

weakly bound complex as bond are being made and broekn

Question 31

v^doubledagge is very smaller and hv^double dagger <<

Answer 31

k_BT

Question 32

Because assumed that reactive mode is spereable from all other modes in the activated complex we can take out the

Answer 32

reactive vibration from total partition function

Question 33

In gas-phase atom +atom -> product the AC is a

Answer 33

diatomic molecules

Question 34

if A and B are monatomic gases they can translate but do not

Answer 34

rotate or vibrate

Question 35

For two atoms reacting transition state theory reduces to

Answer 35

simple collision theory

Question 36

In gas-phase unimolecular reaction A-> P the molecular partition function ratio of A/AC is

Answer 36

1 for q^trans (mass is same), roughlt 1 for q^rot (moment of inertia similar), one normal mode removed from AC so q^vib <=1 (1 if A is large) so q'/q_A is roughly 1

Question 37

For a bimolecular reaction A+B -> P (A has n_A atoms and B n_B) AC is n_A + n_B atoms. partition functions are

Answer 37

q_A = (q_trans)^3(q_rot)^3(q_vib)^(3n-6) = q_b )3 for degrees of freedom - 3D) q_ts' = (q_trans)^3(q_rot)^3(q_vib)^(3n_a+3n_b-7) (-6-1 because thrown one vib mode away)

Question 38

Simple collision theory overestimates k because it does not account for the molecules

Answer 38

internal structure and steric effects

Question 39

The reactive mode has a very low frequency ,v^doubledagger, because this mode is involved in

Answer 39

making/breaking bonds at transition state

Question 40

Large partition function means lots of states can populate complex molecules, lots of degrees of freedom so rate constant is slow becasue of

Answer 40

entropy

Question 41

when reactants have many modes (complex molecules) A (preexponential factor) is lower because

Answer 41

many more modes in which reactants can exist before reactive mode is accessed

Question 42

To calculate k we need

Answer 42

qa, qb, E0 and q_doubledagger

Question 43

Anion photoelectron spectroscopy

Answer 43

Make a stable anion from TS to directly probe the spectroscopy of the AC

Question 44

Although AC is short-lives and unstable one can probe the reaction by using

Answer 44

ultrafast spectroscopy by intiating very short laser pulse and probing at excited state using a second pulse

Question 45

Crossed molecular beam allows one to look at

Answer 45

product distributions and deduce important parameters of the potential energy surface

Question 46

Primary kinetic isotopic effect is when the bond to the isoptopically substituted atom in the AC is

Answer 46

broken/formed

Question 47

The secondary kinetic isotopic effect is when the AC contains isotopic subsitituted atoms that are

Answer 47

not directly involved in the reaction

Question 48

force constant is determined from the ... and does not depend on

Answer 48

PES, mass

Question 49

there is only one PES for all isotopes of a reaction because the chemical species

Answer 49

do not change, only the number of neutrons which are not directly involved in chemical bonding

Question 50

A heavy isotope has a lower ... and therefore lower ...

Answer 50

vibrational frequency (bigger reduced mass), zpe

Question 51

A strongly bound system (large k) has a higher

Answer 51

zpe (large v)

Question 52

If the transition state is close in energy to the reactants than the AC is reaction like so the AC is ............ and zpe at AC are....

Answer 52

similar to the reactants and bonds are still quite strong, similar to reactants

Question 53

If the transition state is equally separated in energy from the reactants and products, or large barrier, the the transition state is neither like reactants or products so bonds at AC are much ...... so the zpe is ...... and zpe at AC are .... than .......

Answer 53

weaker, smaller, smaller, reactants

Question 54

Determination of PKIE show that an istopic atom transfer forms part of the ......

Answer 54

rate-determining step

Question 55

The magnitude of the PKIE shows the extent of isoptic atom bond

Answer 55

breakage/formation

Question 56

the contributions from partition function in k_H/k_D have a

Answer 56

small effect (because E_0D - E_0H is exponential so has large effect)and ratio of q^trans,q^rot,q^vib is roughly 1

Question 57

If bonding is only partiallybroken in the transition state then k_H/k_D will be reduced so an early/late transition state leads to a lower

Answer 57

PKIE that for a symmteric transition stae

Question 58

SKIE has a much smaller effect than PKIE because no

Answer 58

direct breakage/formation of isotopic bond in the TS

Question 59

In SKIE the further the isotopic atom is away from the disrupted bond, the

Answer 59

smaller the effect

Question 60

if isotopic replacement involves an element other than H, the PKIE effect will be

Answer 60

SIGNIFICANTLY REDUCED

Question 61

for unimolecular gas-phase reaction the change in the number of moles of gas-phase molecules is

Answer 61

0

Question 62

for bimolecular gas-phase reaction the change in the number of moles of gas-phase molecules is

Answer 62

-1

Question 63

unimolecular reactions involve mainly

Answer 63

bonding breaking- high, positive enthalpy change between reactants and AC

Question 64

bimolecular reactions involve bond

Answer 64

breaking and making - lower enthalpy change between reactants and AC

Question 65

For unimolecular reactions entropy change is

Answer 65

small and normally positive as AC is similar to reactants but weaker bonds - slightly more disordered

Question 66

For bimolecular reactions entropy change is

Answer 66

large and negative as two reactants to one AC is more order

Question 67

ions produce electrostriction of solvents (solvents held tightly) so solvent molecules are more ..... so solutions with ionic reactants/products have .... entropy values

Answer 67

ordered around ions, smaller (compared to neutral)

Question 68

change in entropy for solution phase with large solvation effect is

Answer 68

large,-ve for charge separation, large +ve for charge neutralisation, moderate -ve for charge decolalisation

Question 69

Large change in enthalpy leads to a large ...... and ..... from reaction

Answer 69

activation, temperature dependance

Question 70

negative change in entropy means much smaller

Answer 70

pre exponential factor as A is proportional to Texp(change in antropy/R\0

Question 71

Solvent molecules are more

Answer 71

ordered

Question 72

KIE larger for reactants that are

Answer 72

strongly bound

Question 73

reaction rates can change substantially with the presence of non-reacting salts due to the

Answer 73

ionic strength effects

Question 74

ionic atmospheres lower the overall ..... of the central ion

Answer 74

free energy

Question 75

effect on the activity coefficients of ionic salt added is given from

Answer 75

debye huckel theory limiting law equation on sheet

Question 76

ionic strength is a measure of how many

Answer 76

charges are present in the solution

Question 77

Debye-Huckel limiting law is only valid for low concentrations because at higher concentrations ions begin to

Answer 77

pair-up so that the relative charge is reduced and I is overstimated

Question 78

reactant ions of the same sign k .... with I

Answer 78

increases

Question 79

reactant ions of the opposite sign k .... with I

Answer 79

decreases

Question 80

reactant involve a neutral species k

Answer 80

is not atrered by I

Question 81

Reactantss of the same sign: ionic atmospheres affect on reactants and AC's free energy

Answer 81

reduces both but reduces AC complex to a greater extent as it is highly charged at the transition state. gibbs E_A goes down and rate constant goes up with ionic strength

Question 82

Reactants of opposite sign: ionic atmospheres affect on reactants and AC's free energy

Answer 82

reduces reactants free energy. AC now carries less charge so lowers energy to a lesser extent. Gibbs E_A goes up and rate constant goes down with ionic strength

Question 83

if any neutral species is involved there is no change in rate because reactants and AC are

Answer 83

stabilised to similar extent because charge remains the same. Gibbs energy not change as reactants and products are stabilised to same extent with ionic strength k = k_0

Question 84

generalised solvent coordinate essentially involves all solvent molecules around the

Answer 84

D(donor molecules)A(acceptor moleucle) and D+A- will be different for both

Question 85

change in free energy along generalised solvent coordinate (q) can be treated as being

Answer 85

harmonic. (linear response theory)

Question 86

y_cross is the

Answer 86

change in free energy at transition stae

Question 87

lamda is the ..... and corresponds to the change in ...... to be in a solvent environment associated with the .....

Answer 87

reorganistation energy, free energy for the reactants, products

Question 88

with decreasing (change in _r (G_0)) the rate first ..... and then begins to

Answer 88

increases to a maximum decrease again

Question 89

range in which K_ET increases again for change in _r(G_0) is called th e

Answer 89

inverted region

Question 90

rrate increases when a

Answer 90

non-reacting salt is added

Question 91

gradient of kinetic salt effect is proportional to the

Answer 91

magnitude and sign of the charges

Question 92

generalised solvent coordinate (q)

Answer 92

represents change in geometry of reactants and change in solvation environment. Free energy surface associate varies with q

Question 93

barrier for diffusion limited reactions comes about from the

Answer 93

energy required to break and make intermolecular bonds within a solvent to enable diffusion

Question 94

non-recorssing rule is not always

Answer 94

true

Question 95

can introduce a probability factor on the

Answer 95

Eyring equation

Question 96

assumption that motion along the reaction path is classical on the PES breaks down for

Answer 96

light reactants and cold temperatures

Question 97

at high temperatures the rate is determined by the

Answer 97

porbability of molecules that have suffiecinet energy to react (boltzmann)

Question 98

as temperature approaches 0k, boltmann tends to 0 and rate coefficient is

Answer 98

0 because no molecules have enough energy to overcome barrier

Question 99

if atoms are lights than probability of tunneling through barrier comparable to

Answer 99

boltzmann and can even dominate- so classical TST underestimates the rate constant as the reaction may process by tunelling through the barrier and cutting corner on a PES