ALKANES AND PETROLEUM

Question 1

Crude oil

Answer 1

crude oil is a mixture composed mainly of straight and branched chain alkanes.
- it aslo includes lesser amount of cycloalkanes and arenes. and impurities like sulphur compounds.

Question 2

petroleum

Answer 2

petroleum is a mixture which mostly contains alkane hydrocarbons that can be separated by fractional distillation.

Question 3

alkanes

Answer 3

group of compounds having the general formula cn h2n+2

Question 4

cycloalkanes

Answer 4

special type of compounds whuch are still saturated but has the general formula cnh2n.

Question 5

fractional distillation

Answer 5

fractional distillation can be defined a s aprocess by which mixture of different compounds can be separated based on different boililng points in a fractionating column.

Question 6

process of fractional distillation.

Answer 6

• oil is heated upto about 370 C and is pumped into the bottom of a tall tower called the fractionating column.
• the fractionating column is hot at the bottom but cooler at the top. as the oil vapourizes it rises up and condenses.
• larger molecules have a higher boiling point so they condense near the bottom of the column amd exits through the bottom of the column.
• lower molecules have a lower boiliing point and condenses further up and exits through the top.
• there is a temperature gradient in the column.

Question 7

boiling point of alkkanes

Answer 7

• the bigger the molecules, the higher the boiling point of alkanes. alkanes have van der waals force because they are non polar.

the boiling point of straight chain alkanes increases with the chain lenth. as the chain length increases, increases its surface area and there is strong van der waals interaction between molecules.
however branched chain alkanes have a lower boiling point because the chains are not packed close together. so there are fewer contact points and the vander waals forces are weaker.

Question 8

cracking

Answer 8

cracking is a process by which long chain alkanes are broken down into short chain alkanes, alkenes and hydrogen.

Question 9

importance of cracking

Answer 9

1. cracking increases the amount of gasoline and other economically important fractions.
2. cracking produces alkenes which is an imp feedstock for chemicals.
3. cracking produces branching in chains which is imp for petrol..

Question 10

what is involved in cracking

Answer 10

breaking of C-C bonds.

Question 11

cracking using a catalyst

Answer 11

• fractions with larger hydrocarbon molecules are heated at a high temperature to vaporise.
• they are then passsed over a hot catalyst or mixed with steam and heated to a very high temperature.
• chemical bonds between the molecules break.
• as a result alkenes and smaller alkanes are produced.

Question 12

thermal cracking

Answer 12

-takes place at a high temperature (1000 c) and high pressure (70 atm)
- produces a lot of alkenes
- alkenes used to make heaps of valuable products like the polymer. eg polyethene produced from ethene.

Question 13

catalytic cracking

Answer 13

catalytic cracking uses something called a zeolite catalyst ( hydrated alumino silicate) at high temperature ( about 450c ) and slight pressure.

• produces aromatic hydrocarbons and alkanes necessary for motor fuels.

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Question 14

why is catalyst used?

Answer 14

• reaction can be carried out at a lower temperature and pressure.
• speeds up the chemical reaction so speeds up the time and saves money as well.

Question 15

thermal vs catalytic cracling

Answer 15

catalytic is better because
1. produces a higher proportion of branched chain alkanes which burns easily than the straight chain alkanes and therefore an imp component of petrol.
2. low temp and low pressure so saves time and minimises cost.
3. produces a huge proportions of arenes which are an imp feed stock for chemicals.