Core 2: Kinetics

Question 1

What is the general requirement for an branched chain reaction?

Answer 1

When more reactants are produced as a result of the reaction.

Question 2

What 3 predictions does bimolecular theory rely on? How achievable are they?

Answer 2

1. Cross section - achievable
2. Steric factor - difficult
3. Activation energy - impossible with this theory

Question 3

Name the 2 main types of optical analysis methods.

Answer 3

1. Beer-Lambert law absorbances
2. Fluorescence methods by exciting intermediates with lasers and detecting emitted photons

Question 4

How is the steric factor determined for bimolecular collision theory?

Answer 4

A_experimental/A_predicted

Question 5

What is the inital rates method?

Answer 5

Over a short time period at the start of an experiment the inital rate of reaction is measured. These are compared at different reactant concentrations. The order with respect to a reactant can then be calculated.

Question 6

What is the implication when a negative order of reaction is found?

Answer 6

The elementary reactions may be incorrect.

Question 7

What happens to the rate law of a reversable reaction when the time is left to go to infinity?

Answer 7

The steady state approximation can be used and the ratio of the rate constants is equal to the ratio of the concentrations.

Question 8

What is the half life and lifetime for each order of reactions?

Answer 8

1st: t_½=ln2/k τ=1/k
2nd: t_½=1/2k[A]₀ τ=(e-1)/2k[A]₀
0th: t_½=[A]₀/2k τ=[A]₀/k • e-1/e

Question 9

What is the equation for kinetic chain length for how many times a carrier is recycled?

Answer 9

Chain length=rate of propagation/rate of termination, for a specific intermediate

Question 10

How do you predict the rate constant at different temperatures?

Answer 10

Taking the ln of the Arrhenius equation and taking one from another.

Question 11

How can reactions be mixed fast?

Answer 11

Using a continuous flow reactor with double syringe injection of reactants.

Question 12

Why do branching reactions not always spontaneously occur?

How can the rate of the branching reaction be increased?

Answer 12

Radicals often terminate with the walls, this rate depends on the pressure of the reaction.

1. Increasing the vessel size as removal of radicals will decrease
2. Pacifying the wall materials so they are less reactive to radicals
3. Add an inert bath gas to reduce diffusion of radicals

Question 13

How is stochiometry accounted for in the rate law?

Answer 13

rate=1/n x d[R]/dt where n moles of R is produced in the reaction.

Question 14

What is a method of fast initiation?

Answer 14

Flash photolysis where a reactant such as a peroxide starts reacting after energy is provided.

Question 15

Define half life and lifetime

Answer 15

t_½ = Time taken for a reactant to reach half its inital value

τ = Time taken for a reactant to drop to 1/e of its inital value

Question 16

In what situation is the universal frequency factor greater than the A factor?

Answer 16

When the entropy for forming the transition state is negative.

Question 17

What is the steady state approximation?

Answer 17

A reactive intermediate will react quickly after forming so its rate can be estimated to zero.

Question 18

What final factor is added to reach the Eyring equation and what is this equation?

Answer 18

τ is added which represents the collisions that go back to the reactants but it is close to 1.

k_overall=τ(k_bT/h)exp(ΔS/R)exp(-ΔH/RT)

The A factor is τ(k_bT/h)exp(ΔS/R)

Question 19

How do you calculate ΔH and ΔS for going to the TS?

Answer 19

The Saddle point, where the lowest energy path between the reactant and product valleys joins.

The entropy of a system is found orthogonal to the reaction coordinate(perpendicular). The width of the valley shows the entropy of the system as it showed how well defined the position of the atoms are, narrow valley=low entropy=well defined positions.

Question 20

How can the integrated rate equation be used for pressure?

Answer 20

n/V=p/RT

This will accurately determine the rate constant for gases.

Question 21

For each phase, name a way of measuring concentrations.

Answer 21

Gas: pressure when moles of gas changes, use a mercury manometer. p/RT at constant T is concentration

Liquid: measure volume using a capillary tube and using density changes within a molecule.

Solid: measure conductance when ions are released in a reaction and calibrate with know concentrations.

Question 22

What can be found when competing reactions from the same reactant occur?

Answer 22

The ratio of the reactant that goes to a specific product by the ratio of concentrations and the ratio of rate constants.

Question 23

What is the isolation/excess method for finding the order with respect to a reactant?

Answer 23

The concentration of one reactant is so high compared to the other that its concentration is effectivally constant. The order of the other reactant can then be measured from the rate.

Question 24

What is the difference between elementary reactions and bulk reactions?

Answer 24

Elementay reactions occur at atomic/molecular levels and are kinetically simple. Bulk reactions proceeds via many simaltaneous elementary reactions and have very complex kinetics.

Question 25

What is the basis of transition state theory?

Answer 25

The transition state exists in equlibrium with the reactants and the products. The products can have a rate law written from this. Additionally the equlibrium constant for the inital reactions. Then substitute in the relation between G and K and substitute G for H and S.

Question 26

What kind of reaction might have a negative temperature dependance?

Answer 26

Termolecular reactions where an increase in temperature makes an excited intermediate less stable.

Question 27

What is the integrated rate law for 1st, 2nd and 0th rate reactions?

Answer 27

1st: ln[A]_t=ln[A]₀-kt
2nd: 1/[A]_t=1/[A]₀+2kt
0th: [A]_t=[A]₀-kt

Question 28

What is the universal frequency factor?

Answer 28

The rate at which the reaction proceeds from a transition state to the products. It has a frequency of the asymmetrical stretch of the molecule and has a similar value for all molecules, k₂=k_BT/h