2.3.1Energy
Fuels — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 12 parts Lesson 11 of 12
Part 1 of 10Fossil fuels
Fuel
- Fuel a substance that releases heat energy when burned.
Fossil fuels
- Fossil fuels coal, natural gas and petroleum.
The main constituent of natural gas is methane, CH4.
Hydrocarbons are compounds that contain hydrogen and carbon only.
They formed over millions of years from the remains of dead organisms.
They are non-renewable because they are used faster than they form.
Part 2 of 10Petroleum
Petroleum (crude oil)
- Petroleum (crude oil) a mixture of hydrocarbons, also containing natural gas.
It is a thick, sticky, black liquid found under porous rock underground and under the sea.
The mixture itself is of little use, but each part, called a fraction, has its own uses.
Fractions are groups of hydrocarbons of similar chain length with similar properties and boiling points, mostly alkanes.
Part 3 of 10Fractional distillation
- Method
- The crude oil is heated and vaporises.
- The vapours enter the fractionating column, which is very hot at the bottom and cooler at the top.
- Vapours with high boiling points condense low in the column.
- Vapours with low boiling points rise and condense higher up.
- The fractions are collected at different heights.
- The fractions are separated according to their boiling points, which depend on the number of carbon atoms in the chain.
- Fractional distillation only separates the oil and does not crack it.
- Each fraction is still a mixture of hydrocarbons, because the separation is physical.
Part 4 of 10Trends from bottom to top
1
Chain length decreases going up.
2
Boiling point decreases going up.
3
Volatility increases going up, where volatility is the tendency of a substance to vaporise.
4
Viscosity decreases going up, where viscosity is the ease of flow of a liquid and long chains make a thicker liquid.
5
Larger molecules have greater intermolecular attraction, so more energy is needed to separate them.
Part 5 of 10Fractions and their uses
| Fraction | Carbon atoms | Boiling point range (°C) | Use |
|---|---|---|---|
| Refinery gas | 1–4 | Below 25 | Heating and cooking |
| Gasoline (petrol) | 4–12 | 40–100 | Fuel for cars |
| Naphtha | 7–14 | 90–150 | Chemical feedstock |
| Kerosene (paraffin) | 12–16 | 150–240 | Jet fuel |
| Diesel (gas oil) | 14–18 | 220–300 | Diesel engines |
| Fuel oil | 19–25 | 250–320 | Ships and home heating |
| Lubricating oil | 20–40 | 300–350 | Lubricants, waxes and polishes |
| Bitumen | More than 70 | More than 350 | Surfacing roads |
Part 6 of 10Combustion of fuels
Complete combustion
- Complete combustion fuels burn in excess oxygen to give carbon dioxide and water.
The heat released can be transferred into electricity.
Equation
- Equation hydrocarbon + oxygen → carbon dioxide + water, and the reaction is exothermic.
CH4 + 2O2 → CO2 + 2H2O (methane).
C3H8 + 5O2 → 3CO2 + 4H2O (propane).
C2H5OH + 3O2 → 2CO2 + 3H2O (ethanol).
Balancing
- Balancing balance carbon first, then hydrogen, then oxygen last.
Part 7 of 10Incomplete combustion
Incomplete combustion
- Incomplete combustion in limited oxygen the products are carbon monoxide and/or carbon (soot) as well as water.
2CH4 + 3O2 → 2CO + 4H2O.
CH4 + O2 → C + 2H2O.
Carbon monoxide
- Carbon monoxide it is poisonous because it combines with haemoglobin and stops it carrying oxygen.
Soot
- Soot it gives a yellow smoky flame and deposits on surfaces.
Part 8 of 10Pollutants and their effects
| Pollutant | Source | Effect |
|---|---|---|
| Carbon dioxide | All combustion | Greenhouse gas, global warming |
| Carbon monoxide | Incomplete combustion | Toxic gas |
| Sulfur dioxide | Burning fuels that contain sulfur | Dissolves in rain to form acid rain |
| Oxides of nitrogen | Nitrogen and oxygen of the air reacting in hot engines | Acid rain, breathing problems |
| Particulates (soot) | Incomplete combustion | Breathing problems |
| Unburnt hydrocarbons | Engines | Smog |
Greenhouse gases
- Greenhouse gases carbon dioxide and methane absorb infrared radiation emitted by the Earth and warm the atmosphere,
- and higher concentrations lead to climate change.
Acid rain
- Acid rain it damages limestone buildings and kills aquatic life.
Part 9 of 10Reducing pollution and choosing fuels
Catalytic converters
- Catalytic converters they convert CO, nitrogen oxides and unburnt hydrocarbons into CO2, N2 and water.
Removing sulfur
- Removing sulfur taking sulfur from fuels, or treating flue gases with calcium oxide or calcium carbonate, removes SO2.
Alternative energy
- Alternative energy hydrogen, biofuels, wind, solar and hydroelectric power.
- Hydrogen burns to form only water2H2 + O2 → 2H2O.
- Biofuels such as ethanol are renewable.
| Fuel | Advantage | Disadvantage |
|---|---|---|
| Natural gas | Clean, easy to pipe | Non-renewable, produces CO2 |
| Petrol and diesel | Portable, high energy | Pollute, non-renewable |
| Coal | Cheap and plentiful | Releases SO2 and particulates |
| Hydrogen | Only product is water | Hard to store |
| Biofuels (ethanol) | Renewable | Crops compete for land |
Part 10 of 10More from the specification
1Oxidation of steam cracked hydrocarbons
- Oxidation of steam cracked hydrocarbons reacting high-temperature steam with refinery fractions produces oxygenated intermediates converted into carboxylic acids.
2Ultraviolet radiation
- Radiation that provides the activation energy for alkane–halogen reactions.
3Fractional Distillation of Petroleum
- Crude oil is separated into fractions in a fractionating column based on differences in boiling point;
- lighter fractions with shorter carbon chains condense near the top,
- while heavy bitumen collects at the bottom.
4Catalytic Cracking of Long-Chain Hydrocarbons
- Passing vaporised heavy hydrocarbon fractions over a heated alumina/silica catalyst at ~500 °C cracks large alkanes into shorter, more valuable alkanes and unsaturated alkenes like ethene.
5Complete versus Incomplete Hydrocarbon Combustion
- Complete combustion in excess oxygen yields carbon dioxide and water; incomplete combustion under limited oxygen generates toxic carbon monoxide and carbon soot particulates.
- (which binds irreversibly to haemoglobin)
6Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
7Bromine water
- An orange solution that is decolourised by alkenes.
8Addition reaction
- A reaction in which atoms add across a C=C bond to give one product.
9Displayed formula
- A formula showing all atoms and all bonds.
10Covalent bond
- A shared pair of electrons between two atoms, giving noble gas configurations.
11General formula
- A formula giving the composition of every member of a homologous series.
12Homologous series
- A family of compounds with the same functional group and general formula.
13Substitution reaction
- A reaction in which one atom or group is replaced by another.
14Alkene
- A hydrocarbon with a C=C bond and a name ending in -ene.
15Formula
- Symbols and numbers showing the atoms in a compound.
16Alcohol
- A compound containing –OH with a name ending in -ol.
Key terms
| Term | Meaning |
|---|---|
| Fossil fuel | a fuel formed from the remains of living things, such as coal, natural gas and petroleum. |
| Petroleum | a mixture of hydrocarbons, also called crude oil. |
| Fraction | a group of hydrocarbons with similar chain length and boiling points. |
| Viscosity | how easily a liquid flows. |
| Volatility | the tendency of a substance to vaporise. |
| Fractional distillation | separating a mixture of liquids by their boiling points. |
| Complete combustion | burning in plenty of oxygen to give carbon dioxide and water. |
| Incomplete combustion | burning in limited oxygen to give carbon monoxide and/or carbon. |
| Catalytic converter | an exhaust device that converts CO, nitrogen oxides and unburnt hydrocarbons into harmless gases. |
| Greenhouse gas | a gas such as CO2 or methane that absorbs infrared radiation. |
Avoid these mistakes
1
Saying petroleum or its fractions are pure compounds — petroleum is a mixture of hydrocarbons
2
Mixing up the column — long chains condense at the bottom and are more viscous, short chains rise to the top
3
Saying fractional distillation cracks the oil — it only separates by boiling point
4
Mixing up carbon monoxide and carbon dioxide — carbon monoxide forms in limited oxygen and is poisonous
5
Saying nitrogen oxides come from the fuel — they form from nitrogen and oxygen of the air at high temperature
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