Kinetics

Question 1

Explain features of the reaction

Answer 1

• from the thermodynamic viewpoint, DG is negative (–ve) and very large (so equilibrium lies well to the right →)
• BUT the reaction is extremely slow (thus, the reaction mechanism, and particularly the activation energy, is very important)

1. Reactions have a series of events (elementary steps), each involving a transition-state
2. Reaction can only proceed if reactants have at least this E
3. Details of the rate determining step (RDS) is known as the intimate mechanism

Question 2

Describe the first order reaction

Answer 2

1. The rate is proportional to the concentration raised to the power of 1 (hence “first” order)
2. The rate equation, and the reaction it describes, is said to be first order in A

Question 3

Write rate units of first order reaction

Answer 3

Question 4

Describe the second order reaction

Answer 4

1. If B = A, the rate is is proportional to the concentration raised to the power of 2 (hence “second” order) (rate = k[A]^2)

Question 5

What are the rate units of the second order reaction?

Answer 5

Question 6

Explain pseudo-first order reactions

Answer 6

1. Can be used to study a second order reaction under pseudo-first order conditions
2. If [B]&raquo_space; [A] then [B] ~ constant

Question 7

What are the limitations of kinetics?

Answer 7

1. We can not conclude a mechanism from a rate law
   e.g.complicated by:
   - involvement of solvent (pseudo-first order behaviour)?
   - complex reaction with only one rate limiting step?
2. At best a rate law can only be consistent with a mechanism, it can never prove it

(remember – you can only disprove a mechanism, you can never prove a mechanism)

Question 8

What types of terms are “stable” and “unstable”?

Answer 8

Thermodynamic

Question 9

What types of terms are “inert” and “labile”?

Answer 9

Kinetic

Question 10

What is the 1/2 life for inert species and how do we study it?

Answer 10

1. Half-life (t1/2) for inert species > 1 minute
   (study with classic ‘static’ techniques – light absorbtion, pH measurements etc.)

Question 11

What is the 1/2 life for labile species and how do we study it?

Answer 11

1. Half-life (t1/2) for labile species ~ 1 minute – 1 ms
   (study with stop-flow measurements, rapid mixing, fast spectroscopy etc.)

Question 12

Define “stoichiometric mechanism”

Answer 12

Sequence of elementary steps by which the reaction takes place

Question 13

Define “intimate mechanism”

Answer 13

Details of a mechanism on the molecular scale
i.e. the activation process and the energetics of formation of an activated complex in the rate determining step