Kinetics

Question 1

What factors affect the rate of reaction

Answer 1

Concentration
Temperature
As both increase, rate of reaction increases, but not linearly

Question 2

Transitions states occur at…

Answer 2

The energy maximum

- they are unstable and cannot be isolated

Question 3

Intermediates occur at…

Answer 3

The energy minima

- In principle can be detected, and can be isolated under certain conditions

Question 4

Rate =

Answer 4

• d[reactant] /dt and/or d[product] / dt
  For A + B –> P:
  -d[A]/dt = d[P]/dt = k[A]^m[B]^n
  Rate DOES NOT depend on P

Question 5

Ways of measuring reaction rates, advantages and disadvantages

Answer 5

Stopwatch method
- Measure concentration of a product or reactant at certain times

Continuous flow

• 2 reagents mixed and passed along a flow-tube, concentration probed at different points on the tube
• Plot of [reactant] or [product] as a function of time
• Wasteful in reagents but useful in industry

Stopped flow

• 2 reagents mixed and passed via an observation cell into a third syringe
• Reagents flow through until receiving syringe is full and hits an end-stop, starting data collection
• Transmission of light through the observation cell monitored as a function of time
• Plot of [reactant] or [product] as a function of time
• Much less wasteful, fast mixing times can be achieved

Relaxation methods

• Disturb equilibrium and monitor changed in concentration spectrophotometrically
• Can measure reactions on very small timescales

Flash Photolysis

• For reactions brought about by light
• A absorbs light and converts into compound B (which is highly coloured)
• [B] determined by measuring characteristic absorption band as concentration ∝ absorbance

Question 6

Rate law can only be determined…

Answer 6

experimentally, not from stoichiometry of reaction

Question 7

Methods for determining the rate law

A + B –> P

Answer 7

Isolation method

• If B is in large excess compared to A, [B] is almost constant throughout reaction
• Rate = k’[A]^m, k’ = k[B]^n

Method of initial rates

• Measure initial rate to determine k’ because [A]0 is known
• Initial rate r0 = k’([A]0)^m
• Take logs: log(r0) = log(k’) + mlog([A0])
• Gives a straight line
• Can be very inaccurate and is wasteful in reactants

Question 8

Integrated rate laws

Answer 8

Zeroth order:

• d[A]/dt = k
• ->[A] = [A]0 -kt

First order:

• d[A]/dt = k[A]
• ->ln [A] = ln[A]0 -kt

Second order:

• d[A]/dt = k[A]^2
• -> 1/[A] = (1/[A0]) + kt

Question 9

How to determine what order reaction is w.r.t to A

Answer 9

Plot for 0, 1st and 2nd order and see which gives the straightest line

Question 10

What to do if give [P] rather than [A]

Answer 10

Do [P] = [A]0 - [A]

Always plot [A] not [P]

Question 11

Definition of half-life

Answer 11

The half-life, t(1/2) of a reactant id the time taken for its concentration to halve
For a first order reaction t(1/2) = ln2 / k (so constant)
For a second order reaction t(1/2) = 1/([A]0k)

Question 12

For elementary steps in a reaction…

Answer 12

can directly infer rate law

eg A + B –> P, rate = k[A][B]

Question 13

What is steady-state approximation?

Answer 13

Applies to reacts such as:
A + B –> I
I –> P
Where I is a very unstable intermediate, so can assume d[I]/dt = 0
can determine Is concentration w.r.t other reactants and so determine the rate law

Question 14

What is kinetic control?

Answer 14

The major product is from the fastest reaction i.e. C the kinetic product
(irreversibile)

Question 15

What is thermodynamic control?

Answer 15

The major product is the more stable one, i.e. D the thermodynamic product
(reversible)

Question 16

Competitive enzyme inhibition

Answer 16

Substrate cannot bind to enzyme at same time as inhibitor
Equilibrium for dissociation of EI:
EI --> E + I, KI = [E][I] / [EI]
v = [S]vmax / αkM + [S]
α = 1 + ([I] / KI)

Question 17

Non-competitive enzyme inhibition

Answer 17

Inhibitor does not prevent enzyme from binding substrate but prevents formation of the product
Equilibrium for dissociation of enzyme-substrate-inhibitor complex IES:
IES –> I + ES, KI = [I][ES] / [IES]
v = [S]vmax / α(kM + [S])
α = 1 + ([I] / kI)

Question 18

Enzymes: no inhibitor present

Answer 18

v = [S]vmax / (kM + [S])