22. FUELS
8.1 Recall that hydrocarbons are compounds that contain carbon and hydrogen only
Hydrocarbons are compounds that contain hydrogen and carbon atoms only.
8.2 Describe crude oil as:
· complex mixture of hydrocarbons
· containing molecules in which carbon atoms are in chains or rings (names, formulae and structures of specific ring molecules not required)
· an important source of useful substances (fuels and feedstock for the petrochemical industry)
· a finite resource
Crude oil
Crude oil is a natural resource that is a complex mixture of hydrocarbons. The carbon atoms in these molecules are joined together in chains and rings. In the ball and stick models of simple hydrocarbons below, carbon atoms are black and hydrogen atoms are white.
Some examples of hydrocarbon molecules
Crude oil is a finite resource because it forms extremely slowly over millions of years and cannot quickly be replaced. However, crude oil is an important source of:
· fuels
· feedstock for the petrochemical industry
A feedstock is a raw material used to provide reactants for an industrial reaction. A petrochemical is a substance made from crude oil using chemical reactions. For example, ethene is produced from crude oil. It is used as feedstock to make poly(ethene), a polymer.
8.3 Describe and explain the separation of crude oil into simpler, more useful mixtures by the process of fractional distillation
Fractional distillation is used to separate crude oil into simpler, more useful mixtures. This method can be used because different hydrocarbons have different boiling points.
Fractional distillation
During the fractional distillation of crude oil:
· heated crude oil enters near the bottom of a tall fractionating column, which is hot at the bottom and gets cooler towards the top
· vapours from the oil rise through the column
· vapours condense when the temperature of the column is cool enough
· liquids are led out of the column at different heights
Some hydrocarbons have low boiling points. They do not condense but leave the column as gases. Some hydrocarbons have high boiling points. They leave the column as hot liquid bitumen.
Crude oil fractions
The different, useful mixtures separated from crude oil are called fractions. This is because they are only part of the original crude oil.
Crude oil fractions and their uses
Non-renewable fuels
Fuels obtained from crude oil or natural gas are called fossil fuels. They include:
· petrol, kerosene and diesel oil from crude oil
· methane from natural gas
These fuels are non-renewable - they are being used up faster than they are being formed. This means that they will run out one day if we carry on using them.
8.4 Recall the names and uses of the following fractions:
· gases, used in domestic heating and cooking
· petrol, used as fuel for cars
· kerosene, used as fuel for aircraft
· diesel oil, used as fuel for some cars and trains
· fuel oil, used as fuel for large ships and in some power stations
· bitumen, used to surface roads and roofs
One way to remember the names of the fractions is 'Good Penguins Keep Diving For Bass'.
8.5 Explain how hydrocarbons in different fractions differ from each other in:
· the number of carbon and hydrogen atoms their molecules contain
· boiling points
· ease of ignition
· viscosity
and are mostly members of the alkane homologous series
Properties of fractions
Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. The alkanes in each fraction have similar (but not identical):
· numbers of hydrogen and carbon atoms in their molecules
· boiling points
· ease of ignition (how easily they are set on fire)
· viscosity (a measure of how they flow)
For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These have boiling points below room temperature. They are very flammable and have a low viscosity because they are gases.
The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have boiling points well above room temperature. They are very difficult to ignite and have a high viscosity (they flow with difficulty).
8.6 Explain a homologous series as a series of compounds which:
a have the same general formula
b differ by CH2 in molecular formulae from neighbouring
compounds
c show a gradual variation in physical properties, as exemplified by their boiling points
d have similar chemical properties
Alkanes as a homologous series
A homologous series is series of compounds which:
· have the same general formula
· differ by CH2 in the molecular formula of neighbouring compounds
· show a gradual variation in physical properties, such as their boiling points
· have similar chemical properties
The table shows four alkanes, their molecular formulae and their structures.
The general formula for the alkanes is CnH2n+2 (where n stands for the number of carbon atoms). As the number of carbon atoms increases:
· the intermolecular forces between alkane molecules become stronger
· more energy is needed to overcome these forces
· the boiling point increases
The molecules C2H6, C8H18 and C15H32 are all alkanes because they fit the general formula CnH2n+2.
Question
Describe how you can tell from their molecular formulae that ethane, propane and butane are neighbouring compounds in the same homologous series.
answer
Their molecular formulae differ by CH2 from one compound to the next. For example, ethane is C2H6 but propane is C3H8 (and the next one, butane, is C4H10).
8.7 Describe the complete combustion of hydrocarbon fuels as a reaction in which:
· carbon dioxide and water are produced
· energy is given out
Combustion of hydrocarbon fuels
Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.
Complete combustion
Complete combustion of a hydrocarbon fuel happens when there is a good supply of air (excess).
Carbon and hydrogen atoms in the fuel react with oxygen in an exothermic reaction:
· carbon dioxide and water are produced
· the maximum amount of energy is given out
In general:
Hydrocarbon + oxygen → carbon dioxide + water
Here are the equations for the complete combustion of propane, used in bottled gas:
Propane + oxygen → carbon dioxide + water
C3H8 + 5O2 → 3CO2 + 4H2O
Question
Write a balanced equation for the complete combustion of methane, CH4, found in natural gas.
answer
CH4 + 2O2 → CO2 + 2H2O
8.8 Explain why the incomplete combustion of hydrocarbons can produce carbon and carbon monoxide
Incomplete combustion
Incomplete combustion happens when the supply of air or oxygen is poor insufficient oxygen. Water is still produced, but carbon monoxide and carbon are also produced. Less energy is released than during complete combustion.
For example, here is one possible equation for the incomplete combustion of propane:
Propane + oxygen → carbon monoxide + carbon + water
C3H8 + 3O2 → 2CO + C + 4H2O
Notice that fewer oxygen molecules are needed to balance the equation than are needed for the complete combustion of propane.
Soot
The carbon is released as fine black particles. We see this in smoky flames and it is deposited as soot. Soot can cause breathing problems and it blackens buildings. It may block boilers and other appliances, or cause a fire.
8.9 Explain how carbon monoxide behaves as a toxic gas
Carbon monoxide
Carbon monoxide is a toxic gas. It is absorbed in the lungs and binds with the haemoglobin in the red blood cells. This reduces the capacity of the blood to carry oxygen. Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die.
It is colourless, odourless and tasteless, making it difficult to detect.
8.10 Describe the problems caused by incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuels
- The main products of incomplete combustion are carbon monoxide and soot although some H2O and CO2 are inevitably produced as well.
- Incomplete combustion occurs in some appliances such as boilers and stoves as well as in internal combustion engines, where space is cramped.
- Incomplete combustion of gasoline in car engines is a major source of CO and C:
C8H18 + 8.5O2 → 8CO + 9H2O
C8H18 + 4.5O2 → 8C + 9H2O
- The carbon particles released clump together to form soot which gradually fall back to the ground.
- Soot causes respiratory problems and covers buildings and statues, making them look unclean.
8.11 Explain how impurities in some hydrocarbon fuels result in the production of sulfur dioxide
Atmospheric pollutants
Carbon and soot are not the only atmospheric pollutants produced by the combustion of hydrocarbon fuels. Sulfur dioxide and oxides of nitrogen may be produced too.
Sulfur dioxide
Many hydrocarbon fuels naturally contain sulfur impurities. When the fuels are burned, the sulfur oxidises to form sulfur dioxide gas:
Sulfur + oxygen → sulfur dioxide
S(s) + O2(g) → SO2(g)
Sulfur dioxide dissolves in water in the clouds to form sulfurous acid:
SO2(g) + H2O(l) → H2SO3(aq)
Oxygen in the air reacts with sulfurous acid to form sulfuric acid:
H2SO3(aq) + ½O2(g) → H2SO4(aq)
The mixture of the two acids falls from the clouds as acid rain.
Question
Explain why sulfurous acid is oxidised when it forms sulfuric acid.
answer
Sulfurous acid reacts with oxygen and gains oxygen in the reaction.
8.12 Explain some problems associated with acid rain caused when sulfur dioxide dissolves in rain water
Effects of acid rain
Acid rain damages the natural and built environment. For example, it:
· reacts with metals and rocks such as limestone, weakening and damaging buildings and statues
· damages plants and trees, making them lose their leaves and die
· makes rivers and lakes too acidic for some aquatic life to survive
These stone carvings have been damaged by acid rain
8.13 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
Oxides of nitrogen
High temperatures are reached when fuels are burned in engines. At these high temperatures, nitrogen and oxygen from the air can react together to produce oxides of nitrogen. For example:
Nitrogen + oxygen → nitrogen monoxide
N2(g) + O2(g) → 2NO(g)
Nitrogen monoxide gas can be oxidised further in air to produce nitrogen dioxide gas, NO2.
These two oxides of nitrogen are together represented by the formula NOx. They are atmospheric pollutants. They can react in sunlight with other substances to produce a hazy, harmful smog. Nitrogen dioxide is toxic. It can cause bronchitis and other lung diseases. It also dissolves in water in the clouds, forming an acidic solution that contributes to acid rain.
Question
Write a balanced equation for the reaction between nitrogen monoxide and oxygen, forming nitrogen dioxide. Include state symbols in your answer.
answer
Sulfur dioxide is produced because of impurities in the fuel, but oxides of nitrogen are produced because of the high temperatures in engines.
8.14 Evaluate the advantages and disadvantages of using hydrogen, rather than petrol, as a fuel in cars
Hydrogen versus petrol
For more than a century most cars have been fuelled by petrol or diesel. Hydrogen is another fuel that can be used for cars. It has advantages over petrol but it has disadvantages too.
Production of hydrogen and petrol
Petrol
Petrol is a non-renewable fossil fuel. It is made from crude oil, a finite resource. Some of the substances found in petrol are made by cracking crude oil fractions, a process which uses large amounts of energy.
Hydrogen
Hydrogen is manufactured in several ways. These include:
· reaction of methane from natural gas with steam
· cracking of crude oil fractions (hydrogen is a by-product of this process)
· electrolysis of water
Methane is a non-renewable fossil fuel made from natural gas, a finite resource. The production of hydrogen from natural gas or from crude oil offers no advantage over petrol.
Question
The electrolysis of water needs large amounts of electrical energy. Explain an advantage and a disadvantage of producing hydrogen by the electrolysis of water.
answer
Hydrogen can be renewable if the electricity is generated from renewable resources, such as sunlight and wind. However, most electricity worldwide is generated from coal, oil or natural gas. Hydrogen produced this way will be non-renewable.
Combustion products
Petrol
Incomplete combustion happens in cars fuelled with petrol. This produces:
· carbon particles, which appear as smoke and soot
· carbon monoxide, a toxic gas
Question
Carbon dioxide is produced during the combustion of petrol. Describe a problem this causes to the environment.
answer
Carbon dioxide is a greenhouse gas. Increasing amounts of this gas are linked to global warming and climate change.
Hydrogen
Hydrogen usually provides the fuel for hydrogen-oxygen fuel cells in cars. However, it can also be used in some internal combustion engines (the type of engines found in petrol cars). However, water is the only product of combustion when hydrogen is used.
There are far fewer hydrogen filling stations than petrol filling stations. Depending on where you live, this could make it more difficult to run a car fuelled by hydrogen.
Filling a car at a filling station
Other features
The table shows some other features of hydrogen and petrol as fuel for cars.
Hydrogen | Petrol | |
Ease of ignition | Very flammable - ignites easily | Very flammable - ignites easily |
Energy released per kg of fuel (MJ) | 142 | 46 |
State at room temperature and pressure | Gas | Liquid |
Question
Suggest an explanation for why hydrogen is usually liquefied by cooling or stored as a gas under pressure.
answer
Hydrogen is a gas at room temperature and pressure. Its molecules are spread far apart so not very much can be stored in a given volume. If it is liquefied or pressurised, more can be stored.
8.15 Recall that petrol, kerosene and diesel oil are non-renewable fossil fuels obtained from crude oil and methane is a nonrenewable fossil fuel found in natural gas
8.16 Explain how cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)
Cracking
Cracking is a reaction in which larger hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:
· the original starting hydrocarbons are alkanes
· the products of cracking include alkanes and alkenes, members of a different homologous series
For example, hexane can be cracked to form butane and ethene:
Hexane → butane + ethene
C6H14 → C4H10 + C2H4
The larger alkanes are heated to around 650°C and their vapours are passed over a hot catalyst containing aluminium oxide. This causes covalent bonds to break and reform. The slideshow describes this process.
8.17 Explain why cracking is necessary
Reasons for cracking
Cracking is important for two main reasons:
1. it helps to match the supply of fractions with the demand for them
2. it produces alkenes, which are useful as feedstock for the petrochemical industry
Supply and demand
The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Fractional distillation of crude oil often produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.
Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
Alkenes
Alkanes and alkenes both form homologous series of hydrocarbons, but:
· alkanes are saturated (their carbon atoms are only joined by C-C single bonds)
· alkenes are unsaturated (they contain at least one C=C double bond)
As a result, alkenes are more reactive than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a polymer.
Glossary
1. acid
Substance producing more hydrogen ions than hydroxide ions when dissolved in water.
2. acid rain
Rain that contains dissolved acidic gases such as nitrogen oxides and sulfur dioxide.
3. acidic
Having a pH lower than 7.
4. alkane
Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds.
5. alkene
Unsaturated hydrocarbon with a double bond between the carbon atoms.
6. atom
The smallest part of an element that can exist.
7. boiling point
The temperature at which a substance rapidly changes from a liquid to a gas.
8. bronchitis
Inflammation of the bronchi and bronchioles.
9. carbon dioxide
A gaseous compound of carbon and oxygen, which is a by-product of respiration, and which is needed by plants for photosynthesis.
10. carbon monoxide
Poisonous gas produced during incomplete combustion.
11. catalyst
A substance that changes the rate of a chemical reaction without being changed by the reaction itself.
12. chemical properties
A description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc.
13. combustion
The process of burning by heat.
14. complete combustion
Burning in a plentiful supply of oxygen or air. Complete combustion of a hydrocarbon produces water vapour and carbon dioxide.
15. compound
A substance formed by the chemical union of two or more elements.
16. condense
Condensation is a change of state in which gas becomes liquid by cooling.
17. covalent bond
A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons.
18. cracking
The breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vaporising them and passing them over a hot catalyst.
19. crude oil
Mixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms.
20. dissolve
When a substance breaks up and mixes completely with a solvent to produce a solution.
21. electrolysis
The decomposition (breakdown) of a compound using an electric current.
22. energy
The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow.
23. environment
The immediate surroundings in which people live, or the natural world such as the land, air or water.
24. exothermic
Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases.
25. feedstock
A raw material used to provide reactants for an industrial reaction.
26. finite resource
A resource that is no longer being made or which is made extremely slowly, such as crude oil and metal ores.
27. flammable
Able to ignite and burn.
28. fossil fuel
Natural, finite fuel formed from the remains of living organisms, eg oil, coal and natural gas.
29. fraction
In fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other.
30. fractional distillation
In fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures.
31. fractionating column
Piece of apparatus in which mixtures separate during fractional distillation.
32. fuel
Material that is used to produce heat, like coal, oil or gas.
33. fuel cell
Device that produces a voltage continuously when supplied with a fuel and oxygen.
34. general formula
An algebraic formula that sets out a rule or trend which is followed by all members of a homologous series. For example, all non-cyclic alkanes have the general formula CnH2n+2.
35. haemoglobin
The red protein found in red blood cells that transports oxygen round the body.
36. homologous series
A 'family' of organic compounds that have the same functional group and similar chemical properties.
37. hydrocarbon
A compound that contains hydrogen and carbon only.
38. impurity
A substance, usually unwanted, that is present in another substance.
39. incomplete combustion
Burning when there is a limited supply of air or oxygen.
40. intermolecular forces
Weak attractive forces between molecules. When a simple molecular substance melts or boils, it is the intermolecular forces that are broken (not the covalent bonds in each molecule).
41. limestone
A type of sedimentary rock.
42. metal
Shiny element that is a good conductor of electricity and heat, and which forms basic oxides.
43. mixture
Two or more substances that are not joined together. The substances can be elements, compounds, or both.
44. molecular formula
Chemical formula showing the actual number of atoms of each element in a molecule.
45. molecule
A collection of two or more atoms held together by chemical bonds.
46. natural gas
A naturally occurring hydrocarbon gas mixture.
47. natural resources
Minerals that have been made through the formation of the world that can be used for human benefit.
48. non-renewable
A resource that cannot be replaced when it is used up, such as oil, natural gas or coal.
49. oxide
Compound containing oxygen or oxide ions.
50. oxidise
Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons.
51. particle
A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules.
52. petrochemical
Substance made from crude oil using chemical reactions.
53. physical properties
A description of the appearance of a substance or how it acts without involving chemical reactions. For example, state, melting point, conductivity, etc.
54. pollutant
A toxic chemical or object that causes damage to the land, air or water.
55. polymer
A large molecule formed from many identical smaller molecules known as monomers.
56. reactive
The tendency of a substance to undergo a chemical reaction.
57. red blood cell
The blood cell which contains the pigment haemoglobin responsible for the transport of oxygen.
58. saturated
A saturated hydrocarbon contains no carbon-to-carbon double bonds, only single bonds.
59. smog
A type of air pollution that appears as a visible smoky fog which is very harmful to health.
60. soot
The fine black particles, chiefly composed of carbon, produced by incomplete combustion of coal, oil, wood, or other fuels.
61. toxic
Poisonous.
62. unsaturated
An unsaturated compound contains at least one double or triple bond.
63. viscosity
A measure of how difficult it is for a substance to flow - the higher the viscosity, the 'thicker' it is.
Fuels - Edexcel test questions - Edexcel
1
Which of these is a hydrocarbon?
C8H18
C2H5OH
C2H5NH2
2
What is the name of the process which separates crude oil into simpler mixtures of hydrocarbon compounds?
Cracking
Polymerisation
Fractional distillation
3
Where in the fractionating column will the longest hydrocarbons condense?
At the top
In the middle
At the bottom
4
Which fraction is used as a fuel for aircraft?
Gases
Kerosene
Bitumen
5
What term could be used to describe a resource which is being made more slowly than it is being used up?
Finite
Infinite
Sustainable
6
Which of the following is not an alkane?
C2H6
C4H10
C3H6
7
Which of these alkanes has the highest boiling point?
C2H6
C3H8
C4H10
8
During complete combustion of butane, what are the chemical products?
Carbon monoxide and water
Water and carbon dioxide
Carbon and water
9
Which gas produced by the combustion of fossil fuels is generally responsible for producing acid rain?
Nitrogen
Sulfur dioxide
Carbon monoxide
10
Which of the following could be the products formed by the cracking of decane, formula C10H22?
C8H18 + C2H4
C20H42
C5H12 + C5H12
