Kinetics

Question 1

What is the steady state approximation (SSA) ?

Answer 1

𝑑[𝐼]/𝑑𝑡

≈ 0

Question 2

What is the zero order rate law?

Answer 2

𝑣 = − 𝑑[A]/𝑑𝑡 = k

Question 3

What is the first order rate law?

Answer 3

𝑣 = − 𝑑[A]/𝑑𝑡 = k[A]

Question 4

What is the second order rate law?

Answer 4

𝑣 = − 𝑑[A]/𝑑𝑡 = k[A]^2

Question 5

What is the integrated rate law for a zero order reaction?

Answer 5

[A] = [A]o − 𝑘t

Question 6

What is the integrated rate law for a first order reaction?

Answer 6

𝑙𝑛 ([A]/[A]o) = −𝑘𝑡
or
[A] = [A]o𝑒^−𝑘t

Question 7

What is the integrated rate law for a second order reaction?

Answer 7

1/[A] − 1/[A]o = 𝑘𝑡
or
1/[A] = 𝑘𝑡 + 1/[A]o

Question 8

What is the Arrhenius Equation?

Answer 8

𝑘 = 𝐴𝑒^−𝐸𝑎⁄𝑅T

A is the pre-exponential factor
Ea is the activation energy
R is the ideal gas constant (8.314 J K^-1 mol^-1)

Question 9

What is kinetics important for?

Answer 9

Kinetics is important for:
• Medicine - appropriate drug dosage
• Environment - catalysis of ozone breakdown into oxygen by CFCs
• Industry - catalytic cracking of crude oil

Question 10

What factors affect rate of reaction?

Answer 10

• Temperature
  
  • Catalysts
  • Concentrations of reactants
  • Surface area of a solid reactant
  • Pressure of gaseous reactants and/or products

Question 11

Define ‘rate of reaction’.

Answer 11

The increase in molar concentration of a product per unit time
OR
The decrease in molar concentration of a reactant per unit time

Question 12

Define ‘activation energy, Ea’.

Answer 12

minimum kinetic energy that reactants need in order to form products

Question 13

Define ‘pre-exponential factor, A’

Answer 13

a measure of the rate at which collisions occur irrespective of their energy

Question 14

What is the Arrhenius equation if you take a natural logarithm of it?

Answer 14

ln⁡𝑘 = ln⁡𝐴 −𝐸_𝑎/𝑅𝑇

Question 15

How can the Ea and A be determined experimentally?

Answer 15

Plot ln𝑘 vs. 1/T

y intercept = lnA
slope = -Ea/R

Question 16

What can cause deviations from Arrhenius behaviour?

Answer 16

Often caused by the activation energy changing with temperature

Viscous liquids (such as glass)
Heterogeneous catalysis on solid surfaces

Question 17

What is collision theory?

Answer 17

Simple collision theory attempts to rationalise the Arrhenius equation by observing that reactions occur through collisions

Question 18

When do collisions occur?

Answer 18

whenever two molecules come within a certain distance (d) of each other
d - The collision diameter.

Question 19

What does the collision theory assume?

Answer 19

Molecules can be treated as ‘hard spheres’ (not deformable)
All molecules have kinetic energy  varies with temperature

Kinetic energy = ½mv2

Also need to know the average speed the molecules travel at
Obtained from the Maxwell distribution
If molecules do not have enough KE they do not react!
Non-reactive collisions are assumed to be elastic
Also dependent on:
Frequency of collisions
Molecular diameter
Pressure, concentration

Question 20

How does rate relate to collision rate?

Answer 20

Rate = collision rate x fraction of collisions with sufficient energy to react.

Question 21

What is the Kinetic theory of gases?

Answer 21

The kinetic theory of gases can be used to find the number of collisions per unit time between two hard spheres

The basic postulates of the model are:
The gas consists of identical particles of mass m which undergo random motion
Each particle has kinetic energy = ½mv2
Particles are modelled as solid spheres with very small, but finite, diameters
There are no long-range forces between particles
Particles collide elastically with one another and the walls of the container

Question 22

What is the Maxwell Boltzmann Distribution?

Answer 22

Predicts the likelihood that a particular particle will have a given speed
Dependent on both molecular mass and temperature

Question 23

How can the most probable speed on a Maxwell distribution be calculated?

Answer 23

𝑐^∗=(2𝑅𝑇/𝑀)^(1/2)

Question 24

How can the mean speed on a Maxwell distribution be calculated?

Answer 24

𝑐̅ =(8𝑅𝑇/𝜋𝑀)^(1/2)

Question 25

How can the relative mean speed on a Maxwell distribution be calculated?

Answer 25

𝑐 ̅rel =((8𝑘_𝐵 𝑇)/𝜋𝜇)^(1/2) or 𝑐 ̅rel = 2^1/2 𝑐 ̅

Question 26

How can the volume of a collision tube be determined?

Answer 26

Over the length of time delta t, a molecule travelling at relative mean speed 𝒄 ̅_𝒓𝒆𝒍, will sweep out a collision tube of volume: σ 𝑐 ̅rel Δt

Question 27

How can the collision frequency for a single molecule A in a gas of other molecules be determined?

Answer 27

𝑧=𝜎𝑐 ̅_𝑟𝑒𝑙 𝒩_𝐴

Question 28

What is the collision density for like molecules at concentration [A]?

Answer 28

𝑍_𝐴𝐴 = 4𝜎((𝑘_𝐵 𝑇)/(𝜋𝑚_𝐴 ))^(1∕2) 𝑁_𝐴^2 [𝐴]^2

Question 29

What is 𝒩_𝐴?

Answer 29

the number density of A molecules: 𝒩_𝐴=𝑁_𝐴 [𝑐𝑜𝑛𝑐]

Question 30

What are the energy and steric considerations?

Answer 30

- The probability that a collision occurs with sufficient energy - The relative orientations of the colliding species

Question 31

What is the rate constant taking into account sterics and energy requirements?

Answer 31

𝑘 = 𝑃𝜎((8𝑘_𝐵 𝑇)/𝜋𝜇)^(1∕2) 𝑁_𝐴 𝑒^ −𝐸𝑎 ∕ 𝑅𝑇)

Question 32

What is a complex reaction?

Answer 32

Reactions where the mechanism cannot be described by a simple one-term rate equation. E.g reversible, concurrent and consecutive reactions.

Question 33

Define 'irreversible reaction'.

Answer 33

Reactions that proceed in one direction only and where the reactants are completely converted into products

Question 34

Define ' reversible reaction'.

Answer 34

Reactions that can proceed both forwards and backwards and do not go to completion

Question 35

Define 'rate determining step'.

Answer 35

One step that is slower than the others. Lower k value for slowest step.

Question 36

Define 'chain carriers'.

Answer 36

The intermediates in chain reactions | Typical chain carriers include radicals and atoms.

Question 37

Define 'chain reaction'.

Answer 37

Reactions where an intermediate produced in one step generates an intermediate in a subsequent step, which itself then generates another intermediate etc.

Question 38

What are the steps in a chain reaction?

Answer 38

Initiation, propagation, inhibition, termination.

Question 39

Define 'catalyst'.

Answer 39

A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change

Question 40

What are enzymes?

Answer 40

Enzymes are substances produced by living organisms that act as a catalyst to bring about a specific biochemical reaction

Question 41

For an enzyme (E) catalysed reaction S --> P what is the rate law?

Answer 41

(d[P])/d𝑡=(𝑘[E]o[S])/([S]+𝐾_𝑀 ) 𝐾_𝑀 is te Michaelis constant.

Question 42

What is 𝐾_𝑀, the Michaelis Constant?

Answer 42

The substrate concentration at half of the maximum reaction velocity  the point at which exactly half of the enzyme active sites are filled with substrate

Question 43

What is the Harpoon Mechanism?

Answer 43

The Harpoon mechanism postulates that electron transfer occurs when K is sufficiently close to Br2 (electron is harpooned across). The resulting coulombic attraction between the ions move them closer together leading to reaction and the resulting products (KBr + Br).

Question 44

What must there be for a successful collision?

Answer 44

- A Collision! - Correct geometry - Sufficient energy (> Ea)

Question 45

What is an initiation step in a chain reaction?

Answer 45

Where the reactant generates one or more intermediates.

Question 46

What is a propagation step in a chain reaction?

Answer 46

Where an intermediate generates another intermediate through reaction with reagents (and products can also be formed)

Question 47

What is an inhibition step in a chain reaction?

Answer 47

Where intermediates react with the product, reagents are reformed and the number of intermediates remain the same.

Question 48

What is a termination step in a chain reaction?

Answer 48

Where intermediates combine, resulting in fewer intermediates to undergo reaction.