Crude Oil and Fuels

Question 1

Why does crude oil need to be seperated?

Answer 1

It contains many different compounds that boil at different temperatures. These burn under different conditions so crude oil needs to be seperated to make useful fuels.

Question 2

What process can be used to seperate a mixture of liquids?

Answer 2

Distillation

Question 3

What does simple distillation of crude oil produce?

Answer 3

Liquids that boil within different temperature ranges. These are called fractions.

Question 4

What are most of the compounds in crude oil (and what are these)?

Answer 4

Hydrocarbons - molecules containing only hydrogen and carbon. Many of these hydrocarbons are alkanes.

Question 5

What is the general formula for alkanes?

Answer 5

C_n+H_2n+2

Question 6

What are alkanes?

Answer 6

Saturated hydrocarbons - they contain as many hydrogen atoms as possible in each molecule (i.e. bonded to each carbon atom). They do not contain a double bond.

Question 7

What is a displayed formula?

Answer 7

A represntation of molecules that shows how the atoms are bonded together

Question 8

What is a molecular formula?

Answer 8

A formula showing the number of each type of atom in each molecule.

E.g. C₂H₆ - ethane

Question 9

How is crude oil separated at refineries?

Answer 9

Using fractional distillation

Question 10

Describe the process of fractional distillation.

Answer 10

• The boiling point of each hydrocarbon depends on the size of the molecule. The larger the molecule, the higher the boiling point.
• Crude oil is vapourised and fed into the fractionating column. It is hotter at the bottom and cooler further up the column.
• The vapours travel up the column, cooling as they do so. The hydrocarbons condense to liquids when they reach the level that is at their boiling point and are collected on one of the trays at the different levels and siphoned off.
• The smallest molecules have the lowest boiling points so are collected at the top of the column. The fractions collected at the bottom have the highest boiling point.
• A very thick residue is left at the bottom of the column due to the fact that it has a boiling point higher than that of the bottom of the fractionating column.

Question 11

What are the properties of fractions with low boiling ranges?

Answer 11

• Low viscocity, so are runny liquids
• Very flamable and ignite easily
• Burn with clean flames, producing little smoke

This makes them very useful as fuels

Question 12

What are the properties of fractions with high boiling ranges?

Answer 12

• Very thick and sticky - very viscous
• Extremely high boiling points
• Can’t be seperated easily
• Not as flammable

Used in making roads and flat roofs

Question 13

What happens when pure hydrocarbons are burnt completely?

Answer 13

They are oxidised to produce CO₂ and water

Question 14

What is produced by incomplete combustion?

Answer 14

Carbon monoxide may be produced

Carbon/soot may be produced

Some hydrocarbons may not burn

The latter two are what form particulates

Question 15

What are particulates?

Answer 15

Small solid particles given off by motor vehicles etc. after incomplete combustion

Question 16

What do most fossil fuels contain and what happens when this is burnt?

Answer 16

Sulfur compounds - produces sulfur dioxide when burnt.

Sulfur dioxide causes acid rain

Question 17

What might also happen at the high temperatures produced when fuels burn?

Answer 17

Oxygen and nitrogen in the air may combine, producing nitrogen oxides

These also cause acid rain

Question 18

What is global dimming?

Answer 18

The reduction in the amount of sunlight that reaches the Earth’s surface caused by particulates in the atmosphere reflecting sunlight away

Question 19

How is acid rain produced?

Answer 19

Sulfur dioxide and nitrogen oxides dissolve in water droplets and react with oxygen in the air

Question 20

How can we remove harmful substances from waste gasses before they are released into the atmosphere?

Answer 20

• Sulfur dioxide can be removed from the waste gasses produced by power stations before these enter the atmosphere.
• Sulfur can also be removed from fuels before they are burnt
• Exhaust systems of cars are fitted with catalytic converters to remove carbon monoxide + nitrogen oxides
• Filters can remove particulates

Question 21

What is smog and what causes it?

Answer 21

A mixture of SMoke and fOg

Caused by lots of cars in a small area and the right weather conditions

Question 22

What are biofuels?

Answer 22

Renewable fuels made from plant or animal products

Question 23

What is biodiesel?

Answer 23

A biofuels made from vegetable oils extracted from plants.

It is used to power cars (as an alternative to diesel)

Question 24

State an advantages of using biodiesel

Answer 24

It’s carbon neutral

Question 25

State a disadvantage of using biodiesel

Answer 25

The plants that are grown for biodiesel use large areas of farmland

Question 26

Give an example of another biofuel and how it can be used + produced

Answer 26

Ethanol made from sugar cane or sugar beet

It is a liquid so can be stored and distributed like other liquid fuels

It can also be mixed with petrol

Question 27

What are the advantages and disadvantages of using hydrogen as a fuel?

Answer 27

• It only produces water when it is burned
• However, it is a gas so takes up a large volume. This makes it difficult to store in the quantities needed for combustion in engines
• It can be produced from water by electrolysis, but this requires large amounts of energy