Cracking

Question 1

What is economical advantage of cracking?

Answer 1

there is an abundance of large chains, and they are not as useful therefore not as valuable economically

naptha and shorter chains or more valuable

therefore to meet the demands for the shorter chains, longer chains have to be cracked

Question 2

What are the two useful results of cracking?

Answer 2

some of the products are alkenes which are more reactive

more useful chains are produced, especially petrol

Question 3

How many carbons do petrol contain?

Answer 3

four to twelve carbons

Question 4

What are the uses of alkenes?

Answer 4

chemical feedstock

converted into compound such a polymers (e.g. polyethene) and drugs and paint

Question 5

What are the conditions for thermal cracking?

Answer 5

700-1200 K

high pressure (up to 7000 KPa)

Question 6

How do C-C bonds break?

Answer 6

one electron from the pair in the covalent bond goes to each carbon atoms

so initially, two shorter chains are produced

each chain ends in a carbon atom with an unpaired electrons

Question 7

What are the fragments? How do they react?

Answer 7

these fragments are free radicals

free radicals are highly reactive intermediates and react in a number of ways to form a variety of short chain molecules

as there are not enough hydrogen atoms to produce two alkanes, one of the new chains must have C=C bond, and is therefore an alkane.

Question 8

How is too much decomposition avoided?

Answer 8

they are only kept in these condition for approx one second

Question 9

What is produced from thermal cracking?

Answer 9

produces a high proportion of alkenes and hydrogen may also be produced

the chain could break at any point

Question 10

What are the conditions for catalytic cracking?

Answer 10

takes place at a lower temp ( 720 K ) and lower pressure (approx atmospheric)

using a zerolite catalyst, consisting of silicon dioxide and albumim oxide

Question 11

What is the structure of the zerolite catalyst?

Answer 11

zeolites have a honeycomb structure with an enormous surface area

they are acidic

Question 12

What is catalytic cracking mainly used for? What are the products?

Answer 12

used mainly to produce motor fuels

products are mainly alkanes, cycloalkanes and aromatic compounds

the products are separated by fraction distilation