Reaction Kinetics

Question 1

Steady state approximation

Answer 1

If there are reactive intermediates, their concentration will increase to a point, then remain constant in time

Question 2

What does the steady state approximation allow us to assume

Answer 2

d[I]/dt = 0

Question 3

Chain length

Answer 3

The rate of propigation divided by the rate of termination

Question 4

If there are two possible equations for chain length, which is used

Answer 4

The smallest value answer

Question 5

If a reaction rate is pressure dependent, what does that mean practically

Answer 5

It is dependent on [M]

Question 6

Rate equation for the formation of products in a unimolecular reaction

Answer 6

d[products]/dt = k1k3[A][M]/(k2[M] + k3)

Question 7

Change in uni molecular rate at high pressure

Answer 7

At high pressure, [M] is large and so, k3[M] + k3 is approximately equal to k3[M] as k3 is relatively so small

Question 8

Change in unimolecular rate at low pressure

Answer 8

[M] is low and so k3[M] + k3 is equal to k3 as k3 is relatively so much larger

Question 9

Overall changes in unimolecular reaction rate with pressure

Answer 9

First order at high pressure, second order at low pressure

Question 10

k(infinity) definition

Answer 10

Hugh pressure limiting my rate coefficient - k at infinite pressure

Question 11

k0 definition

Answer 11

The low pressure limiting rate coefficient

Question 12

Plot to check validity of the unimolecular rate model

Answer 12

Plot 1/kobs against 1/[M]. Intercept is 1/k(infinity) and gradient is 1/k1

Question 13

Experimental flaws with the Lindemann mechanism

Answer 13

A high pressure, kobs increases more rapidly than the theory predicts

Question 14

Assumptions made my the Lindemann model

Answer 14

Assumed that any excited reactant will undergo the same reaction with the same value of k. Generally, the excited energy must be in a degree of freedom related to the reaction e.g. excitation of a vibrational state of the bond due to break

Question 15

Differences in bath gases

Answer 15

Bath gases impart energy in reagents depending on their degrees of freedom. Translational quanta are small, rotational larger etc. this means the more degrees of freedom, the better the energising properties of the bath gas

Question 16

Sulphur based, high energy bath gas

Answer 16

SF5CF3

Question 17

Association reactions

Answer 17

Two reactive intermediates combine to form an energised complex

Question 18

Rates of association reactions at high and low pressure

Answer 18

2nd order at high pressure, 3rd order at low pressure

Question 19

How does the third order reaction work?

Answer 19

It is not three species colliding simultaneously, but instead two, bimolecular reactions. First to form the reactive intermediate, then to form the products

Question 20

Rates of association reactions with temperature

Answer 20

Often, the observed rate of product formation increases with decreasing temperature

Question 21

Explain a negative activation energy for associative reactions

Answer 21

The rate constant, k is a product of all the rate constants k1 (association), k2 (decomposition) and k3 (products). k2 is the largest, and the denominator of k. It increases and decreases more rapidly that the other two numerator rate constants and so, rate increases with decreasing temp. The arhenius relationship relates k with T

Question 22

Unimolecular mechanism

Answer 22

A + M = A* + M
A* + M = A + M
A* = products

Question 23

Association reaction mechanism

Answer 23

I + I = I2*
I2* = I + I
I2* + M = I2 + M