Cracking and Reforming

Question 1

which fractions of hydrocarbons are most in demand?

Answer 1

fractions with shorter C chains

Question 2

how is demand for shorter C chain fractions met?

Answer 2

longer hydrocarbons are cracked - breaking long-chain alkanes into smaller hydrocarbons, involves breaking C-C bonds

Question 3

what are the two useful results of cracking?

Answer 3

• shorter, more useful chains produced, especially petrol
• some of the products are alkenes, which are more reactive than alkanes

Question 4

what is petrol made up of?

Answer 4

mixture of mainly alkanes containing between 4 and 12 carbon atoms

Question 5

what are alkenes commonly used for?

Answer 5

chemical feedstock - converted into other compounds such as polymers and products such as paint

Question 6

why are harsh conditions needed to break down alkanes with cracking?

Answer 6

because they are very unreactive (strong C-C and C-H bonds)

Question 7

Give 2 conditions of thermal cracking

Answer 7

• high temps (700-1200K)
• high pressure (up tp 7000kPa)

Question 8

What does thermal cracking produce?

Answer 8

high proportion of alkanes and alkenes

Question 9

what happens in thermal cracking?

Answer 9

• C-C bonds break and one electron from covalent pair goes to each C
• produces 2 free radical intermediates
• intermediated react in variety of ways to produce variety of shorter chain molecules
• not enough H atoms to produce 2 alkanes so one alkane and one alkene produced

Question 10

how is decomposition
(to carbon and hydrogen) prevented in thermal cracking?

Answer 10

alkanes are kept under cracking conditions for very short time, typically 1 second

Question 11

Name 3 conditions of catalytic cracking

Answer 11

• Uses zeolite catalyst
• lower pressure
• 450°C / 700K - lower temperatures

Question 12

why is a zeolite catalyst used in catalytic cracking?

Answer 12

needed to compensate for less harsh conditions (lower pressure and temperature)
also speeds up reactions, saving time and money

Question 13

What does catalytic cracking normally produce?

Answer 13

• branched alkanes
• cycloalkanes
• aromatic compounds

Question 14

Why doesn’t thermal cracking produce 2 alkanes?

Answer 14

Since there isn’t enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C

Question 15

How are the products obtained from cracking separated?

Answer 15

fractional distillation

Question 16

what is reforming?

Answer 16

straight chain hydrocarbons converted to branched chain alkanes and cyclic alkanes

Question 17

why are hydrocarbons reformed?

Answer 17

to burn more efficiently and used in petrol for cars