Catalysis

Question 1

What is a catalyst

Answer 1

Speeds up reaction without effecting eqm position and is not consumed in the process
However in practice catalyst degrades overtime and activity will change

Question 2

What properties would an ideal catalyst have?

Answer 2

Stable, cheap
Accelerate reaction
Active in low quantities
Recoverable and reusable
High yield

Question 3

Arrhenius equation

Answer 3

k = Ae^(-Ea/RT)
lnk = lnA - Ea/RT

Question 4

Gibbs Free energy equation

Answer 4

G = H - TS

Question 5

Eyring Equation

Answer 5

k = kbT/h e^(ΔS/R) e^(-ΔH/RT)

Question 6

How to assess catalyst reactivity

Answer 6

Rate
Turnover frequency (TOF)
Turnover Number (TON)

Question 7

Equation for rate

Answer 7

converted amount of substrate/ catalyst volume or mass x time

Question 8

Equation of TOF

Answer 8

number of cycles/ number of centres x time

Question 9

Equation of TON

Answer 9

moles of product/ moles of catalyst

Question 10

Equation for selectivity

Answer 10

quantity of desired products x 100 / amount of reactants consumed

Question 11

Equation for efficiency

Answer 11

m.w of desired product x100 / m.w of all products from reaction

Question 12

What is syngas? and what are some sources?

Answer 12

Mixture of H2, CO and CO2
Natural gas, coal, biomass, liquid petroleum gas

Question 13

Homogeneous

Answer 13

Catalyst is in the same phases as the reactants

Question 14

Heterogeneous

Answer 14

Catalyst is in a different phase to the reactants e.g. solid catalyst in liquid reactants

Question 15

Advantages of heterogeneous catalyst

Answer 15

easy to recover and reuse
Thermodynamically and mechanically stable

Question 16

Disadvantages of heterogeneous catalyst

Answer 16

Mechanism not always known
often low selectivity
Can degrade over time - poisoning
Hard to control shape/ size of catalyst at molecular level

Question 17

Advantages of homogeneous catalyst

Answer 17

Very active so high rates
Often selective and easily modifiable
Work at low temp and conc
well defined reaction sites = obtaining mechanistic info easier

Question 18

Disadvantages of homogeneous catalyst

Answer 18

Hard to separate from reactants
Low thermal stability means temp below 200°c often used

Question 19

Advantages of enzyme catalyst

Answer 19

Highly specific
homogeneous

Question 20

Disadvantages of enzyme catalyst

Answer 20

Subject to regulation by inhibitors and activators
Sensitive to their environment

Question 21

What are solvent effects?

Answer 21

When the solvent used in a chemical reaction interacts with the solute to influence outcomes

Question 22

Why is proton transfer in acid-base catalysis involving water so fast?

Answer 22

Because protons in hydrogen bonded structures e.g. water move rapidly bc their mass is low

Question 23

What is specific acid catalysis

Answer 23

In specific acid catalysis, protonated solvent is the catalyst. The reaction rate is proportional to the concentration of the protonated solvent molecules SH+ and therefore is proportional to the pH

Question 24

What are the reaction steps in specific acid catalysis

Answer 24

First is fast protonation of molecule via strong dissociated acid
Then nucleophilic attack by e.g. H2O

Question 25

What is general acid catalysis?

Answer 25

All species able to donate a proton contribute to reaction rate Proton transfer is from a now undissociated acid HA. This is much slower than dissociated H+

Question 26

What are the reaction steps for general acid catalysis

Answer 26

Slow protonation by a weak undissociated acid in solution at same time as nucleophilic attack

Question 27

Difference between general and specific acid catalysis?

Answer 27

Specific catalysis is detected by taking rate measurements at constant pH but different buffer concentrations. If the rate of the reaction is independent of the concentration of the buffer, specific acid catalysis is involved.

Question 28

How do enzymes stabilise reaction transition states

Answer 28

Amino acids on enzymes control the properties e.g. potential acid or base and can stabilise reaction transition states increasing the reactions rate (via general acid/base catalysis mechanism) Both acid and base effects simultaneously to lower E barrier

Question 29

Example of acidic amino acid that can be used to stabilise T.S

Answer 29

Protonated His

Question 30

Example of basic amino acid that can be used to stabilise T.S

Answer 30

Unprotonated Asp, Glu or His

Question 31

What are zeolite catalysts?

Answer 31

Microporous crystalline aluminosilicates composed of SiO4 and AlO4 tetrahedra SiO4 neutral AlO4 neg charged Charge is compensated by extra framework cations e.g. Na+

Question 32

Zeolite formation is a synthetic process, this means we can control the pore diameter what effect does this have on reactivity?

Answer 32

Only molecules that are smaller than the pore diameter fit and undergo reaction

Question 33

What occurs during catalytic cracking

Answer 33

Hydrocarbons mixed with zeolite (strong acid) Mixture blown through reaction chamber at 500°c Hydrocarbons break down randomly producing a mixture of smaller hydrocarbons some w C=C bonds

Question 34

How do carbenium ions form during catalytic cracking with zeolite?

Answer 34

Carbenium ions form when bronsted acid sites reacts with alkene or lewis acid site abstracts H-

Question 35

Which type of carbenium ion is most stable and how does this effect rate of reaction

Answer 35

Tertiary carbenium ion Rate depends on the stabilities of product and reactant carbenium ions e.g. 2 tertiary ions fast

Question 36

How can carbenium ions rearrange to achieve more stable configuration?

Answer 36

Rearrange via H- and CH3- movement

Question 37

How can zeolites be used as molecular sieves?

Answer 37

1) reactant selectivity - only certain size molecule will fit into zeolite pores eg. non branched alkane 2) Product selectivity - only some products can leave the cage/ pore for example only para subs benzene ring not meta or ortho 3) Restricted T.S selectivity - cage/ pore shape restricts size/shape of intermediates

Question 38

What is Ziegler Natta Polymerisation

Answer 38

TiCl3 and AlEt3 combine to produce heterogeneous ethene polymerisation catalyst Produces linear polymer

Question 39

What is an isotactic polymer?

Answer 39

alternating chiral centres along back bone all have same geometry

Question 40

What is syndiotactic polymer?

Answer 40

Alternating chiral centres along back bone have alternating geometry

Question 41

What is an atactic polymer?

Answer 41

Alternating chiral centres along polymer backbone have random geometries

Question 42

Methods of termination of a polymerisation sequence off a metal centre

Answer 42

1) -βH 2) +H2 3) +HX e.g. H2O

Question 43

How do we control the reactivity of catalysts used in polymer formation

Answer 43

Modify the catalyst precursor e.g. tether the 2 Cp rings together using Me2Si which changes bond angle and orbital overlap changing reactivity leading to high isotacticity polymers