8.1 Fuels Flashcards
Crude oil
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).
Chains and rings
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.
Use of crude oil
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.
Crude oil is a finite resource
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.
Fractional distillation of crude oil
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.
Main fractions of crude oil
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
Number of hydrogen and carbon atoms in crude oil
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.
Boiling point of crude oil
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.
Viscosity of crude oil
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.
Ease of ignition of crude oil
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.
Homologous series
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.
Complete combustion
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
Fossil fuels
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.
Incomplete combustion
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
Carbon monoxide
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.
Sulphur dioxide
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)
Nitrogen oxides
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.
Acids rain
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.
Hydrogen fuel
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
Advantages and disadvantages of hydrogen fuel
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.
Cracking
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.
Catalytic cracking
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.
Steam cracking
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.
Why is cracking necessary
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.
