Alkanes

Question 1

petroleum fraction

Answer 1

Mixture of hydrocarbons with a similar chain length and boiling point range

Question 2

How does fractional distillation of crude oil work

Answer 2

Oil is pre-heated
• then passed into column. • The fractions condense at different heights
• The temperature of column decreases upwards
• The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the van der waals forces
• Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures
• and big molecules condense at the bottom at higher temperatures.

Question 3

What substance does the fractions represent

Answer 3

20 - Fuel gas 
40 - Petrol/gasoline 
110 - naptha
180 - jet fuel/kerosene 
250 - diesel oil 
300 - fuel oil 
340 - lubricating oils 
Bitumen

Question 4

Explain how the boiling points of branched chain alkanes will be different from straight chain alkanes with the same number of C atoms:

Answer 4

As the chain length increases, the van der waals forces increases. Branched-chain alkanes have a lower boiling point, as the chain cannot pack together and there will be fewer points of contact, therefore the intermolecular forces are weaker

Question 5

Describe and explain the trend in boiling point as you increase chain length? Include polarity in your answer

Answer 5

As the carbon chain increase in length, the boiling point increases. There is an increase in van der waals forces. It is non polar as the electronegativity of hydrogen and oxygen is similar

Question 6

How will the Solubility and reactivity of branched chain alkanes be different from straight chain alkanes

Answer 6

Water is bonded by hydrogen bonding and is stronger than van der waals forces, thus alkanes are insoluble in water. Alkanes are unreactive, but they can undergo combustion and halogenation.

Question 7

Cracking

Answer 7

Conversion of large hydrocarbons to smaller hydrocarbons molecules by breaking C-C bonds

Question 8

Economic reasons for cracking

Answer 8

The petroleum fractions with shorter C chains (e.g. petrol and
naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand
for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more valuable than the starting
materials (e.g. ethene used to make poly(ethene), branched alkanes
for motor fuels, etc.)

Question 9

Conditions and properties of thermal cracking

Answer 9

Conditions: High temperature (800-900), High pressure (7000kPa)

Produced mostly alkenes (high purity)

Mechanism: free radical

Question 10

conditions and properties of catalytic cracking

Answer 10

Condition: High temperature (400-450), Pressure (1-2 atm)
Catalyst (zeolites)

Few alkenes mainly motor fuels, aromatic hydrocarbons, cycloalkanes.

Question 11

Fuel

Answer 11

Releases heat energy when burnt

Question 12

Catalytic converter

Answer 12

? Exhaust emission control device placed in the car