Topic 8 – Fuels and Earth Science

Question 1

What is a hydrocarbon

Answer 1

● Hydrocarbons are compounds that contain carbon and hydrogen only

Question 2

Describe crude oil

Answer 2

● A complex mixture of hydrocarbons
● Containing molecules in which carbon atoms are in chains or rings
● An important source of useful substances (fuels and feedstock for the petrochemical industry)
● A finite resource

Question 3

Describe and explain the separation of crude oil into simpler, more useful mixtures by the process of fractional distillation

Answer 3

Crude oil is a mixture of different hydrocarbons, different hydrocarbons have different boiling points (longer chain hydrocarbons have higher boiling points)
● The crude oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures.
● The fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporised oil rises up the column and the various fractions are
constantly tapped off at the different levels where they condense.
● The fractions can be processed to produce fuels and feedstock for the petrochemical industry.

Question 4

Recall the names and uses of the following fractions

Answer 4

● Gases: domestic heating and cooking
● Petrol: fuel for cars
● Kerosene: fuel for aircraft
● Diesel oil: fuel for some cars and trains
● Fuel oil: fuel for large ships and in some power stations
● Bitumen: surface roads and roofs

Question 5

Explain how hydrocarbons in different fractions differ from each other

Answer 5

● Some properties of hydrocarbons depend on the size of their molecules. These properties influence their use as fuels.
● Shorter the molecules, the less viscous it is. (more runny) and the longer the molecules, the more viscous it is.
● The shorter the molecules, the lower the temperature at which that fraction evaporates or condenses – and the lower its boiling point.
● The shorter the molecules – the more flammable it is, so the easier it is to ignite

Question 6

What is a homologous series

Answer 6

Aseries of compounds which:
● Have the same general formula
● Differ by CH2 in molecular formulae from neighbouring compounds
● Show a gradual variation in physical properties, as exemplified by their boiling points
● Have similar chemical properties

Question 7

Describe the complete combustion of hydrocarbon fuels

Answer 7

A reaction in which:
● The hydrocarbon reacts with O2 and CO2 and H2O are produced
● Energy is given out (exothermic)

Question 8

Explain why the incomplete combustion of hydrocarbons can produce carbon and carbon monoxide

Answer 8

● If there’s not enough oxygen, some of the fuel doesn’t burn – this is partial combustion. Here, solid particles of soot (carbons) and unburnt fuel are released.
● Carbon monoxide (CO) is also released when there isn’t enough oxygen to produce CO2 instead

Question 9

Explain how carbon monoxide behaves as a toxic gas

Answer 9

● carbon monoxide is a colourless and odourless gas which, if breathed in, prevents red blood cells carrying oxygen around your body, which leads to death

Question 10

Describe the problems caused by incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuel

Answer 10

● Carbon monoxide causes health problems

● Soot causes global dimming

Question 11

Explain how impurities in some hydrocarbon fuels result in the production of sulfur dioxide

Answer 11

● Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur
● when the fuels are burnt in oxygen, this sulfur can react to form sulfur dioxide

Question 12

Explain some problems associated with acid rain (caused when sulfur dioxide dissolves in rain water)

Answer 12

● Damages buildings and statues (made of limestone)
● Reduce the growth of or kill trees and crops
● Lower pH of water in lakes, killing fish

Question 13

Explain why, when fuels are burned in engines, oxygen and nitrogen can react together at high temperatures to produce oxides of nitrogen, which are pollutants

Answer 13

● Nitrogen and oxygen from the air combine to produce nitrogen monoxide
● When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide
● nitrogen monoxide and nitrogen dioxide are pollutants

Question 14

Advantages and disadvantages of using hydrogen, rather than petrol, as a fuel in cars

Answer 14

● Advantages
○ Use of hydrogen – petrol is from crude oil, a finite resource
○ Only produces water – no CO2 produced which contributes to global warming
● Disadvantages
○ Expensive
○ Difficult to transport and store hydrogen
○ Dangerous – hydrogen can be explosive

Question 15

Recall that petrol, kerosene and diesel oil are…

Answer 15

● Non-renewable fossil fuels obtained from crude oil

● And methane is a non-renewable fossil fuel found in natural gas

Question 16

Explain how cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)

Answer 16

● Hydrocarbons can be cracked to produce smaller, more useful molecules. This process involved heating the hydrocarbons to vaporise them.
● The vapours are:
○ Either passed over a hot catalyst
○ Mixed with steam and heated to a very high temperature so that thermal decomposition reactions can occur.
● The products of cracking include alkanes and unsaturated hydrocarbons called alkenes

Question 17

Alkenes

Answer 17

○ Alkenes have the general formula CnH2n
○ The first 2 alkenes are ethene and propene.
○ they are unsaturated because they have a double bond

Question 18

Why is cracking necessary?

Answer 18

● Demand for smaller chained alkanes is much greater than that for longer chained alkanes
● shorter chained hydrocarbons ignite more easily and so are more useful as fuels

Question 19

What gases produced the Earth’s early atmosphere?

Answer 19

● There was intense volcanic activity that released gases that formed the early atmosphere
○ At the start of this period, the atmosphere may have been like the atmospheres of Mars and Venus today, mainly CO2 with little or no O2(g)
○ Volcanoes also produced nitrogen which gradually built up in the atmosphere & there may have been small proportions of methane (CH4) and NH3

Question 20

Describe that the Earth’s early atmosphere was thought to

contain

Answer 20

● Little or no oxygen
● A large amount of CO2
● Water vapour
● Small amounts of other gases

Question 21

Explain what the oceans were formed from?

Answer 21

● Water vapour condensed to form the oceans – H2O (g) -> H2O (l)

Question 22

Explain how the amount of CO2 in the atmosphere was decreased when CO2 dissolved as the oceans formed

Answer 22

● CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere

Question 23

Explain how the growth of primitive plants changed the composition of gases in the atmosphere?

Answer 23

● Algae & plants produced the O2 that is now in the atmosphere by photosynthesis:
carbon dioxide + water –(light)-> glucose + oxygen
● Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere
● Over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolve
● Algae and plants decreased the % CO2
in the atmosphere by photosynthesis

Question 24

Describe the chemical test for oxygen (O2)

Answer 24

● Uses a glowing splint inserted into a test tube of the gas

o Splint relights in oxygen

Question 25

What gases are involved in the greenhouse effect?

Answer 25

methane
water vapour
carbon dioxide.

Question 26

What is the greenhouse effect?

Answer 26

Some gases in the Earth’s atmosphere stop heat radiating into space from the Earth. This is called the greenhouse effect and the gases involved are called greenhouse gases.

Question 27

Explain the greenhouse effect

Answer 27

● Electromagnetic radiation at most wavelengths from the sun passes through the Earth’s atmosphere
● The Earth absorbs some radiation and thus warms up (essential for life on Earth). But some heat is radiated from the Earth as infrared radiation.
● Some of this IR radiation is absorbed by greenhouse gases in the atmosphere
● Atmosphere warms up leading to the greenhouse effect and global warming

Question 28

Examples of human activity include which increase levels of CO2 & CH4

Answer 28

● Activities increase levels of CO2 & CH4
● Examples of human activity include:
o Driving (CO2)
o Consuming electricity (CO2)
o Raising livestock (cows – CH4)
o Decay of organic waste in landfill sites (CH4)

Question 29

The potential effects on the climate of increased levels of CO2 and CH4

Answer 29

● Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth’s atmosphere to increase at the surface
and that this will result in global climate change

Question 30

Describe the effects on the climate may be mitigated

Answer 30

o It is difficult to model such complex systems as global climate change.
o This leads to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence and which may be biased.