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Chemistry - Year One > Catalysis > Flashcards

Catalysis Flashcards

1
Q

What is a catalyst?

A

A catalyst speeds up a reaction without affecting the equilibrium position

In practice, they can degrade over time

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2
Q

What are some expectations of catalysts?

A

Stable

Cheap

Not corrosive

Active in low quantities

Recoverable and reusable

Minimal toxic waste

Soften the reaction conditions

Increase yield

High purity

High selectivity

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3
Q

What is the Eyring equation?

A

k = ((boltzmann constant x T) / h) x exp(-delta G / RT)

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4
Q

How can we plot the Eyring equation in a line where y=mx+c?

A

We can plot y = ln(k/T) and x = 1/T

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5
Q

What does delta H dagger tell us about in a reaction?

A

It yields information about the bond strength

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6
Q

What does delta S dagger tell us about in a reaction?

A

It yields us information about the changes in order

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7
Q

What is the equation for turnover frequency (TOF)?

A

TOF = number of cycles (turnovers) / number of centres x time

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8
Q

What is the equation for turnover number (TON), and what factors does this take into account?

A

TON = Moles of product / moles of catalyst

This takes into account deactivation, and reflects the maximum yield attainable from a catalyst

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9
Q

What are the positives of heterogenous catalysts?

A

Easy to regenerate, recover (filter) and reuse

Thermodynamically and mechanically stable

Often metals or metal oxides

Activity is linked to surface area

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10
Q

What are the negatives of heterogenous catalysts?

A

Selectivity is often low

Reaction control problems due to diffusion and heat transport

Can degrade over time due to leaching, poisoning, swelling or sintering

Reproducible preparations can be difficult

Hard to control the shape or size of the catalyst at the molecular level

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11
Q

What are the positives of homogeneous catalysts?

A

Very active leading to high rates

Selective and easily modifiable

Mild working conditions

Often acids, bases or transition metal complexes

Ligand and solvent variations can be used positively

Mechanistic information is easier to obtain

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12
Q

What are the negatives of homogeneous catalysts?

A

Low thermal stability

Hard to separate

Recovery of catalyst can be costly

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13
Q

What are the positives of enzyme catalysis?

A

Classified by function

Highly specific

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14
Q

What are the negatives of enzyme catalysis?

A

Subject to regulation by inhibitors and activators

Catalyse biological transformations

Sensitive to the environment e.g. pH

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15
Q

What is specific acid catalysis?

A

The hydroxonium ion is the catalyst, with the H+ being fully transferred before the RDS

Rate is proportional to [RH+]

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16
Q

What is general acid catalysis?

A

Undissociated acid, HA, is the proton donor

Proton transfer from the undissociated acid is a slow step; this changes the RDS

17
Q

How can we spot a general acid catalysis?

A

We can obtain a constant pH via a buffer, and change the buffer concentration which will change the rate

18
Q

How can we catalytically crack hydrogen carbons to form ethene?

A

Hydrocarbons are mixed with zeolites and blown through a reaction chamber at 500 degrees

A strong acid can protonate the hydrocarbon

The hydrocarbon breaks down randomly to form a mixture of smaller hydrocarbons

19
Q

What are the key steps for catalytic cracking?

A

Carbenium ions are formed when Bronsted acid sites react with alkene or Lewis acid sites abstracts H-

The carbenium ion reacts with another hydrocarbon chain by H- abstraction

C-C bond beta to carbenium ion breaks heterolytically to yield an alkene and a carbenium ion

Oligomerisation is possible if an alkene and a carbenium ion combine

20
Q

What is beta-scission?

A

The breaking of a beta C-C bond

The rate depends on the stability of the reactant and product carbenium ions

21
Q

What is the Ziegler Natta polymerisation reaction?

A

AlEt₃ + TiCl₃ + Ethene —–> linear alkane polymer

22
Q

What are the molecular weights of the products from Ziegler Natta polymerisation?

A

10,000 to 2,000,000

23
Q

What metals polymerise alkenes nowadays?

A

M⁴⁺ ions, d⁰ ions - these lead to a fast migration as there is no back bonding to the alkene

Ti«Hf<Zr

24
Q

What are three ways TMs can activate hydrogen to break the H-H bond?

A

Oxidative addition

Hydrogenolysis

Heterolytic cleavage

25
Q

What are three heterogeneous supported catalyst composition?

A

Active phase

Promoter

Support/Carrier

26
Q

What is the active phase?

A

This is where the reaction occurs (active site)

27
Q

What is the purpose of the promoter?

A

Makes a catalyst more active

Electronic modifier

Poison-resistant promotor, or to introduce selectivity

28
Q

What is the purpose of the support/carrier?

A

Increases the mechanical strength

Increases the surface area

They may or may not be catalytically active

29
Q

What is the mechanism of heterogeneous hydrogenation?

A

The molecules are absorbed to the surface where the bond is broken and then the reaction occurs on the surface

30
Q

What is the adsorption energy comparable to for chemisorption?

A

Comparable to the energy of a chemical bond

31
Q

What happens in physisorption?

A

We get an elastic collision (no energy transfer) when we hit the surface

Atoms and molecules can ‘stick’ to the surface however

32
Q

What is the equation for the sticking probability?

A

Number of molecules that stick / number of molecular impacts

33
Q

How do we understand if chemisorption will occur?

A

We need to consider the energy needed to break the bond and the energy formed when 2 new M-X bonds are formed

34
Q

What is needed for the chemisorption to be exothermic?

A

E X-M > D X-X / 2

35
Q

How can we measure adsorption?

A

Have two setups where we allow gas to pass into another container, one with a surface on. The differences in pressure can be measured

36
Q

How can we measure the surface coverage?

A

Number of occupied surface sites / overall number of surface sites

37
Q

What does surface coverage depend on?

A

Concentration (pressure)

Temperature

38
Q

What are the three key assumptions of the Langmuir Adsorption Isotherm?

A

(1) The surface has a fixed number of identical sites (monolayer only)

(2) The adsorption enthalpy is independent of coverage

(3) Adsorbates do not interact

Chemistry - Year One (16 decks)

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