12.3- INDUSTRIAL CRACKING

Question 1

Is the naphtha fraction from the fractional distillation of crude oil in huge demand?

Answer 1

yes

Question 2

Why is the naphtha fraction in huge demand?

Answer 2

for petrol and by the chemical industry

Question 3

What are longer chain fractions described as, as they’re not as useful?

Answer 3

they’re of lower value economically

Question 4

What happens to meet the demand for shorter chain hydrocarbons?

Answer 4

many of the longer chain fractions are broken into shorter lengths (cracked)

Question 5

What are the useful results of cracking? (2)

Answer 5

shorter, more useful chains produced, especially petrol

some of the products are alkenes, which are more reactive than alkanes

Question 6

What is petrol a mixture of?

Answer 6

mixture of mainly alkanes containing between 4-12 carbon atoms

Question 7

What are alkenes used as?

Answer 7

as chemical feedstock and are converted into a huge range of other compounds including polymers + variety of products from paint to drugs

Question 8

What is probably the most important alkene?

Answer 8

ethene

Question 9

Why is ethene perhaps the most important alkene?

Answer 9

starting material for poly(ethene) and wide range of other everyday materials

Question 10

Why are harsh conditions needed to break down alkanes?

Answer 10

because they’re very unreactive

Question 11

Two different types of cracking?

Answer 11

thermal cracking

catalytic cracking

Question 12

What does thermal cracking involve?

Answer 12

heating alkanes to a high temperature, under high pressure

Question 13

What high temperature is used for thermal cracking?

Answer 13

700-1200 K

Question 14

What high pressure is used for thermal cracking?

Answer 14

up to 7000 kPa

Question 15

What happens to carbon-carbon bond in thermal cracking?

Answer 15

carbon-carbon bond breaks in such way that one electron from the pair in the covalent bond goes to each carbon atom

Question 16

So initially what is produced when the carbon-carbon bond is broken? (cracking)

Answer 16

initially two shorter chains are produced, each ending in a carbon atoms with an unpaired electron

Question 17

What are the fragments ending in a carbon atom with unpaired electrons called?

Answer 17

free radicals

Question 18

How reactive are free radicals and in what way do they react?

Answer 18

highly reactive intermediates + react in number of ways to form a variety of shorter chain molecules

Question 19

What is produced in thermal cracking?

Answer 19

alkane and alkene

Question 20

Why is an alkene produced in thermal cracking?

Answer 20

there’s not enough hydrogen atoms to produce 2 alkanes

Question 21

How many carbon-carbon bonds may break and does it have to be in the middle in thermal cracking?

Answer 21

any number of carbon-carbon atoms may break

chain does not necessarily have to break in the middle

Question 22

What may be also produced in thermal cracking?

Answer 22

hydrogen

Question 23

What does thermal cracking tend to produce a high proportion of?

Answer 23

high proportion of alkenes

Question 24

What is done to avoid too much decomposition (ultimately to carbon and hydrogen) after thermal decomposition?

Answer 24

alkanes are kept in thermal cracking conditions for a very short time, typically one second

Question 25

At what conditions is catalytic cracking carried out? (3)

Answer 25

lower temperature- about 720K

lower pressure- but more than atmospheric

zeolite catalyst- consisting of silicon dioxide and aluminium oxide

Question 26

What is the structure of zeolite like?

Answer 26

honeycomb structure with enormous surface area

Question 27

Is zeolite acidic or alkaline?

Answer 27

acidic

Question 28

What is catalytic cracking mainly used for?

Answer 28

mainly to produce motor fuels

Question 29

What are the products of catalytic cracking mostly like? (3)

Answer 29

mostly branched alkanes, cycloalkanes (rings), and aromatic compounds

Question 30

How are the products obtained from catalytic cracking separated?

Answer 30

by fractional distillation

Question 31

In a lab what can be the catalyst for catalytic cracking?

Answer 31

lumps of aluminium oxide

Question 32

What are the products mostly in catalytic cracking in a lab?

Answer 32

mostly gases- shows they have chain lengths of less than C5

Question 33

What does the mixture from catalytic cracking in a lab do to bromine solution?

Answer 33

decolourises bromine solution

Question 34

What is the bromine solution a test for?

Answer 34

test for carbon-carbon double bond showing the products contain alkenes