Crude Oil And Fuels

Question 1

Why do hydrocarbons with short chains make good fuels?

Answer 1

They easily ignite and burn well as they have lower boiling points. Very flammable

Question 2

What is the test for an unsaturated hydrocarbon

Answer 2

Turns orange bromine water colourless
(Alkanes do not react with bromine water so this test can distinguish between an alkene and an alkane)

Question 3

What is a hydrocarbon

Answer 3

A substance of only hydrogen and carbon

Question 4

Fuel reacts with oxygen to release energy
Name the type of reaction that releases energy from fuel

Answer 4

Combustion (exothermic)

Question 5

Why is carbon monoxide produced

Answer 5

There is incomplete/partial combustion (of fuel)
Or
Because there is insufficient oxygen to burn fuel

Question 6

Describe and explain how petrol is separated from the mixture of hydrocarbons in crude oil

Answer 6

• the process is called fractional distillation
  -heat crude oil to about 350* C
• most of the hydrocarbons evaporate/ vaporise, some remain as liquids because their boiling point is very high
• vapours rise up the fractionating column, which gets cooler towards the top (temperature gradient)
• once a hydrocarbon has reached its condensing point, it condenses into a liquid and flows out a pipe in the fraction
• some gases flow out through the top because there condensing point is very low

Question 7

What is the general formula of alkanes

Answer 7

C H
n 2n+2

Question 8

Carbon monoxide

Answer 8

CO
Colourless and odourless gas, your red blood cells pick up this gas and carry it around in your blood instead of oxygen

Question 9

Products of combustion

Answer 9

Carbon dioxide and water

Question 10

Crude oil

Answer 10

Mixture of hydrocarbons

Question 11

Alkanes

Answer 11

Saturated hydrocarbons
All carbon-carbon bonds are single covalent bonds

Question 12

Methane, ethane, propane and butane formulae

Answer 12

Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10

Question 13

Properties of short chain hydrocarbons

Answer 13

Low boiling point
Volatility (tendency to turn into gas)
Viscosity - very runny
High flammability

Question 14

Properties of long chain hydrocarbons

Answer 14

High BP
Low volatility
Viscosity - thick
Lower flammability

Question 15

What happens in complete combustion

Answer 15

The carbon and hydrogen in the fuel are oxidised completely when they burn.

Question 16

How to test gases formed in complete combustion of a hydrocarbon

Answer 16

CO2 turns limewater cloudy
Water turns blue cobalt paper pink

Question 17

Incomplete combustion

Answer 17

When there is not enough oxygen. CO is formed instead of CO2 which is toxic gas

Question 18

Why crack hydrocarbons

Answer 18

Cracking produces saturated hydrocarbons (used as fuels) and unsaturated hydrocarbons
The heavier fractions in fractional distillation of crude oil are not in demand these are made up of large molecules.
The larger, less useful hydrocarbon molecules in crude oil can be broken down in to smaller, more useful ones by cracking.

Question 19

Cracking down process

Answer 19

In a cracker, a heavy fraction distilled from crude oil is heated to vaporise the hydrocarbons.
Then Vapour is then either:
-passed over a hot catalyst, or
- mixed steam and heated to a very high temperature
Hydrocarbons are cracked as thermal decomposition reactions take place.

Question 20

What is an alkene

Answer 20

Unsaturated molecules which contain at least one double bond between their carbon atoms.

Question 21

Tests on the gaseous products from experiment for alkenes

Answer 21

Burn in air (but not as well as small alkanes)
React with bromine water