Fuels & Earth science

Question 1

Crude oil

Answer 1

Crude oil is a complex mixture of lots of different hydrocarbon compounds of different sizes.
Hydrocarbons are compounds that contain hydrogen and carbon atoms only.
It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea).

Question 2

Chains and rings

Answer 2

The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms.
The mixture contains molecules with many different ring sizes and chain lengths.

Question 3

Use of crude oil

Answer 3

Crude oil is the main source of hydrocarbons which are used for producing fuels such as petrol and diesel.
It is also a main source of raw materials (called feedstock) for the petrochemical industry.

Question 4

Crude oil is a finite resource

Answer 4

Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals.
Since it is being used up much faster than it is being formed crude oil is a finite resource.
The petrochemical industry is hugely important for modern society and development.
The fuels that are used in most modern methods of transport (cars, trains, airplanes etc.) are all based on oil products.
Polymers, lubricants, solvents, detergents and adhesives are all products that are obtained from crude oil.

Question 5

Fractional distillation of crude oil

Answer 5

Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications.
Each fraction consists of groups of hydrocarbons of similar chain lengths.
The fractions in petroleum are separated from each other in a process called fractional distillation.
The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain.
The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains.
Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.

Question 6

Main fractions of crude oil

Answer 6

The array of fractions in crude oil and the huge range of compounds we can produce from them all stem from carbon’s ability to form multiple strong covalent bonds with itself leading a huge number of organic compounds
The main fractions and their uses:
Liquified petroleum gas - Domestic heating & cooking
Petrol - Fuel for cars (gasoline)
Kerosene - Jet fuel (paraffin)
Diesel - Diesel engines (gas oil)
Heavy fuel oil - Ships & power stations
Bitumen - Surfacing roads and roofs

Question 7

Number of hydrogen and carbon atoms in crude oil

Answer 7

The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains.
Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.

Question 8

Boiling point of crude oil

Answer 8

As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them.
This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point.

Question 9

Viscosity of crude oil

Answer 9

Viscosity refers to the ease of flow of a liquid.
High viscosity liquids are thick and flow less easily.
Viscosity also increases with increasing chain length.
This is also due to the increased intermolecular forces of attraction as molecular size increases.
Increased viscosity means that higher alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts.

Question 10

Ease of ignition of crude oil

Answer 10

Molecular size again influences the ease of ignition or flammability of hydrocarbons.
Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules.
This makes them very useful as fuels, releasing large amounts of energy when they burn.

Question 11

Homologous series

Answer 11

Homologous series are families or groups of organic compounds that have similar features and chemical properties due to them having the same functional group.
All members of a homologous series have:
The same general formula.
The difference in the molecular formula between one member and the next is CH2.
Gradation in their physical properties.
Same functional group.
Similar chemical properties.
Gradation in the physical properties of a homologous series can be seen in the trend in boiling points of the alkanes.

Question 12

Complete combustion

Answer 12

A fuel is a substance which releases energy in an exothermic reaction.
Complete combustion occurs when there is excess oxygen.
If you burn hydrocarbons, the carbon and hydrogen react with oxygen from the air to form carbon dioxide and water releasing energy. This makes hydrocarbons great fuels. When there’s plenty of oxygen, the only products are carbon dioxide and water - this is called complete combustion.
Hydrocarbon + oxygen —— water + carbon dioxide
For propane: C3H8 + 5O2 → 3CO2 + 4H2O

Question 13

Fossil fuels

Answer 13

The combustion of fossil fuels is the major source of atmospheric pollution
Fossil fuels include: coal, oil, natural gas, oil shales and tar sands.
Non-renewable fossil fuels are obtained from crude oil by fractional distillation.
There are finite amounts of fossil fuels and they all contribute to pollution and global warming.
All these fuels contain carbon, hydrogen and small quantities of sulphur.
The burning of fossil fuels releases the gases carbon dioxide, carbon monoxide, oxides of nitrogen and oxides of sulphur.
The main constituent of natural gas is methane, CH4.

Question 14

Incomplete combustion

Answer 14

Incomplete combustion occurs when there is insufficient oxygen to burn.
It occurs in some appliances such as boilers and stoves as well as in internal combustion engines.
In addition incomplete combustion of the fuels gives rise to unburned hydrocarbons and carbon particulates.
Hydrocarbon + oxygen ——- carbon + carbon monoxide + water
For methane: 2CH4 + 3O2→ 2CO + 4H2O or CH4 + O2→ C + 2H2O

Question 15

Carbon monoxide

Answer 15

Carbon monoxide is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death.
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide.

Question 16

Sulphur dioxide

Answer 16

Sulphur dioxide is a colourless, pungent smelling gas that is a major air pollutant responsible for acid rain.
The sulphur dioxide released mixes with clouds and readily dissolves in rainwater.
SO2 is a non-metal oxide so it forms an acidic solution in water, hence forming acid rain.
Sources: combustion of fossil fuels - especially coal.
Fossil fuels are often contaminated with small amounts of sulfur impurities
When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulphur dioxide.
S (s) + O2 (g) → SO2 (g)

Question 17

Nitrogen oxides

Answer 17

These compounds (NO and NO2) are formed when nitrogen and oxygen react in the high pressure and temperature conditions of internal combustion engines and blast furnaces.
Nitrogen dioxide gas reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain:
2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq)
Nitrogen oxides are harmful pollutants - they can cause photochemical smog. Photochemical smog is a type of air pollution that can cause breathing difficulties, headaches and tiredness. It often forms in large cities, where there is a lot of traffic.

Question 18

Acids rain

Answer 18

The sulphur dioxide produced from the combustion of fossil fuels dissolves in rainwater droplets to form sulphuric acid.
Sulphuric acid is one of the components of acid rain which has several damaging impacts on the environment.
Nitrogen dioxide produced from car engines reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain.
Acid rain causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms. Pollutes crops and water supplies, irritates lungs, throats and eyes.

Question 19

Hydrogen fuel

Answer 19

Hydrogen is used in rocket engines and in fuel cells to power some cars and buses.
It reacts with oxygen in an exothermic reaction:
2H2 + O2 →2H2O

Question 20

Advantages and disadvantages of hydrogen fuel

Answer 20

Advantages: It releases more energy per kilogram than any other fuel (except for nuclear fuels).
It does not pollute as it only produces water on combustion, no other product is formed.

Disadvantages: Expensive to produce and requires energy for the production process.
Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers).
The production of hydrogen process releases carbon dioxide.

Question 21

Cracking

Answer 21

Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms.
Alkanes are saturated compounds and alkenes are unsaturated compounds.
Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules.
A process called cracking is used to convert them into short chain molecules which are more useful.
Small alkenes and hydrogen are produced using this process.
Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen.
There are two methods used to crack alkanes: catalytic cracking and steam cracking.

Question 22

Catalytic cracking

Answer 22

Catalytic cracking involves heating the hydrocarbon molecules to around 470 – 550°C to vaporise them.
The vapours then pass over a hot powdered catalyst of aluminium oxide
This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions.
The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes.
Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure.

Question 23

Steam cracking

Answer 23

In steam or thermal cracking the process is carried out at slightly higher temperatures to catalytic cracking and produces more ring structures and unsaturated compounds.
The vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces cracking.

Question 24

Why is cracking necessary

Answer 24

Crude oils vary considerably in their composition and some need more refining than others.
Supply is how much of a particular fraction can be produced from refining the crude oil.
Demand is how much customers want to buy.
General the demand for certain fractions outstrips the supply so this is why cracking is necessary to convert surplus unwanted fractions into more useful ones.
This is mostly larger, heavier fractions that are cracked into smaller lighter fractions.

Question 25

Earth’s early atmosphere

Answer 25

The surface of the early Earth was molten for millions of years during which time there was no atmosphere surrounding the planet.
Eventually cooling began to take effect and allow for molten materials to slowly solidify forming land masses.
Volcanoes formed on the land masses and released gases from the Earth’s interior through violent eruptions.
Earth’s gravity prevented the gases from escaping into outer space and they formed the atmosphere.
Analysis of the minerals in the earth’s crust enables scientists to deduce the gases present millions of years ago that helped form those minerals.
It is thought that the atmosphere at that moment in Earth’s history was similar to that of Venus or Mars today which consist mainly of CO2.
During a period of intense volcanic activity, large amounts of carbon dioxide and water vapour were released, as well as nitrogen, hydrogen, ammonia, methane and other gases.
The early atmosphere therefore contained mainly CO2 and water vapour
There was little or no oxygen present.
While the surface of the Earth was still very hot the large quantities of water vapour remained in the gaseous state.
When conditions cooled sufficiently, the water vapour later condensed and fell to the surface of the Earth, forming the oceans.
Carbon dioxide is a water soluble gas and dissolves readily.
When the water vapour condensed large amounts of CO2 dissolved in the oceans.
Carbonate substances were precipitated during this process which later formed sediments.

Question 26

How carbon dioxide decreased

Answer 26

Carbon dioxide is a water soluble gas and dissolves readily.
When the water vapour in Earth’s early atmosphere condensed large amounts of CO2 dissolved in the oceans.
Carbonates were precipitated during this process which later formed sediments on the seabed.
As marine life began to evolve sea creatures began to appear which used up the carbonates to form shells and skeletons.
Green plants and algae began to evolve and absorbed considerable amounts of carbon dioxide during photosynthesis.
When these organisms died, their remains locked the atmospheric carbon into the rocks.
This is how the large amounts of carbon dioxide in the early atmosphere were reduced.

Question 27

How oxygen increased

Answer 27

Primitive plants and algae began photosynthesising which used up carbon dioxide from the atmosphere and released oxygen.
Through these processes over a long period of time, the amount of O2 in the atmosphere increased and the amount of CO2 decreased.
Algae first evolved around 2.7 billion years ago and during the next billion years or so small green plants began to appear.
As more and more plants began to appear the levels of oxygen began to increase which allowed for more complex life forms to evolve.
This trend continued until around 200 million years ago the composition of the atmosphere reached similar characteristics as today: around 20% oxygen, 80% nitrogen and tiny amounts of other gases.
Marine algae are thought to be responsible for about 90% of all the atmospheric oxygen produced.

Question 28

Test for oxygen

Answer 28

The test for oxygen consists of placing a glowing splint inside a test tube of gas.
If the gas is oxygen the splint will relight.

Question 29

Greenhouses gases

Answer 29

When shortwave radiation from the sun strikes the Earth’s surface it is absorbed and re-emitted from the surface of the Earth as infrared radiation.
Much of the radiation, however, is trapped inside the Earth’s atmosphere by greenhouse gases which can absorb and store the energy.
Carbon dioxide, methane and water vapour are gases that have this effect
Increasing levels of carbon dioxide and methane, although present in only small amounts, are causing significant upset to the Earth’s natural conditions by trapping extra heat energy.
This process is called the enhanced greenhouse effect.

Question 30

Source of CO2 and methane

Answer 30

Carbon dioxide - Combustion of wood and fossil fuels, respiration of plants and animals, thermal decomposition of carbonate rocks and the effect of acids on carbonates.
Methane - Digestive processes of animals, decomposition of vegetation, bacterial action in swamps and in rice paddy fields.

Question 31

Global warming

Answer 31

The level of carbon dioxide in the atmosphere has increased because we are adding more CO2 to the atmosphere and less is being removed from it. We are also adding to the amount of other greenhouse gases in the atmosphere, such as methane.
Increased levels of greenhouse gases in the atmosphere enhance the greenhouse effect as more IR radiation is absorbed and radiated back towards Earth, which causes the Earth to get warmer - this is global warming.

Question 32

Increased energy consumption

Answer 32

Human population is increasing and with it the global demand for food, water, consumer goods, housing and energy which are supplied with greater and more widespread industrialisation.
This creates more waste so more landfill sites are needed which increases the amount of methane by decomposition.
The increased energy demands are met in most cases by the burning of fossil fuels which produces CO2:
Fossil fuel + oxygen → energy + H2O + CO2

Question 33

Deforestation

Answer 33

Added to this is the effect of deforestation on the amount of CO2 as large areas of forested land are being destroyed for building and agricultural activities.
Plants and trees remove CO2 during photosynthesis:
6CO2 + 6H2O → C6H12O6 + 6O2
Hence their removal increases the amount of atmospheric CO2 as there are less plants available to remove it during photosynthesis.

Question 34

Effects of global warming

Answer 34

Scientists agree that greenhouse gases are causing the average temperature of the Earth to increase by the enhanced greenhouse effect, which is causing global warming.
Climate change is when there is a shift in worldwide weather patterns and conditions.
Effects of global warming include difficulties in obtaining fresh drinking water, melting of the polar ice caps, flooding and flash fires, and the destruction of ecosystems.

Question 35

Methane and farming

Answer 35

The greenhouse gas methane is also causing problems. Like carbon dioxide, the concentration of methane has also risen a lot in recent years due to increased human activity. For example, in livestock farming, cows produce large amounts of methane. Paddy fields, in which rice is grown, produce a fair bit too. So, the larger the population gets, the more we need to farm to produce food, and the more methane is produced.
Though it’s currently only present in tiny amounts in our atmosphere, the increasing concentration of methane is an issue as it’s a highly effective greenhouse gas.

Question 36

Dealing with climate change

Answer 36

The production of greenhouse gases needs to be reduced drastically to avoid or at least slow climate change.
CO2 emissions can be reduced by using renewable energy supplies such as solar or wind energy instead of burning fossil fuels.
Awareness and being conscientious about reducing energy usage and our carbon footprint is another way to reduce emissions.
Other ways of mitigating the effects of climate change include:
Constructing flood defence systems in areas of low lying land.
Developing new farming methods to include switching to crops that are better adapted to the new climate.
Building more effective irrigation systems to ensure crops can get water during times of drought.

Question 37

Evaluating evidence for global warming

Answer 37

Advances in science and technology mean current levels of CO2 and global temperatures can be determined with a high degree of accuracy.
There are some methods to estimate past climate conditions, which include:
Analysis of the fossil record and tree rings.
Analysis of gas bubbles trapped in ice from hundreds of thousands of years ago.
The complexity of the Earth’s climate and contributing factors make it a difficult task to produce a working model that clearly shows the link between global warming and greenhouse gases.
This and other difficulties have led to hype and speculation in the media in recent times in which some scientists have cast doubts on human activity and climate change.
However, academic surveys have shown that about 97% of climate scientists do believe human activity is causing climate change.

Question 38

Historical data

Answer 38

Historical data is much less accurate due to the lack of accurate instrumentation and methods.
Fewer locations would also have been measured due to the lack of satellites and transport.
Unfortunately, historical methods, while providing at least some data, are not as precise as modern day techniques nor do they provide data which is representative on a global scale.