Ethylene

• What is ethylene?

• What is cracking? Why do we do it?

Ethylene

• Ethylene is an unsaturated hydrocarbon.

• Structure: ?

• Formula: ?

• Shape: ?

It is obtained from petroleum. (crude oil)

Fractional Distillation

Products of fractional distillation• Crude oil contains mostly large molecules.

Q – Why is this a problem?

Fuel gas PetrolNaphtha KerosineDiesel Fuel Oil and bitumen

Big molecules

Small molecules

Medium molecules

P• The large proportion of long chains is a problem

because there is little demand for the long chain fractions (eg:____ ) but a HUGE demand short chains (eg:_____ )

Fuel gas PetrolNaphtha KerosineDiesel Fuel Oil and bitumen

Big molecules

Small molecules

Medium molecules

Products of fractional distillation

CrackingOne solution to this is to “crack” the long chain

molecules into short chains.

Two options are available:

1) Thermal cracking (using very high temperatures to break the bonds)

2) Catalytic cracking (using a catalyst to break the bonds at lower temperature)

Cracking

• Imagine you own a cracking plant.• Thinking about time, safety, energy and cost,

justify the preferred use of catalytic cracking over thermal cracking.

• 5 mins

Catalytic Cracking• Large hydrocarbons are broken into smaller

molecules using heat and a catalyst.

• The small molecules produced are then separated by distillation.

CatalyticcrackerHeat to

vaporise

Distillationtower

pressure

Big Molecules

Sm

alle

r m

ole

cule

s

Molecules break up

Catalytic Cracking• Using molymod kits, perform the cracking of

decane to make ethylene. • Complete the missing gaps in the following slide.• You should add a balanced symbol equation.

Catalytic Cracking• In the catalytic cracker long chain molecules are

split apart or ‘cracked’. An example of such a reaction is:

C CH

H

H

H+

ethylene

Used to make 1) _________2)__________

Heat pressure

catalyst

Used as a ______

decane

______

Reasons for catalytic cracking

Q) Evaluate the importance of catalytic cracking on the petroleum and polymer industries.

(5 Minutes)

Reasons for catalytic cracking

A) You should have mentioned:

1) production of short chain alkanes (useful)

2) reduction of surplus long chain fractions.

3) production of useful alkenes, used to make polymers.

What catalyst is used?Imagine you work for a chemical company.In groups your task is to design a catalyst to

crack some alkanes.

1. Chemically, how should it behave?2. What physical properties must it have?3. Thinking microscopically, what kind of

shape would be the most effective (think surface area) ?

What catalyst is used?

The catalyst should:• be inert (unreactive) yet form partial

bonds with alkanes.• have a high melting point.• have a large surface area.

Electron microscope “pictures” of the catalyst used

Zeolites

• Aluminosilicate compounds (Al, Si and O.)

• Honeycomb structure (huge surface area) for alkanes to be adsorbed on to.

• Circulated as powders in the cracker.