Hydrocarbons and Fuels

A fuel is a substance that burns to release energy . Chemical reactions that release energy are known as exothermic reactions.

Fire can be explained using a fire triangle.   The triangle has three sides: fuel, oxygen and heat. Each of these sides represents a key element for fire to occur. If one of them was removed, the triangle would collapse and the fire would go out

Fossil fuels Fossil fuels were formed over millions of years from the decaying remains of once living things Fossil fuels are finite resources View video

Crude oil is a thick black liquid which is made up of a mixture of substances called hydrocarbons Hydrocarbons are compounds which are made up of the elements carbon and hydrogen only. Hydrocarbon fuels burn (combust) when they react with oxygen in the air producing the same two products. http://www.twigonglow.com/films/glossary/combustion-410/ http://www.twigonglow.com/films/oxygen-and-combustion-1449/

Water Pump air Lime water methane cobalt chloride paper

Carbon dioxide – which turns lime water cloudy Water – which turns blue cobalt chloride paper to pink

Copy and complete this paragraph in your jotter :- The two products of combustion are _________ and __________ ___________. All hydrocarbons burn to produce these products.   methane + oxygen - _________ + _______ Now change this word equation into a formula equation and balance it.

CH4 + 2O2  CO2 + 2H2O

If there is insufficient air for complete combustion, incomplete combustion takes place. Hydrogen is still oxidised to water, but carbon monoxide forms instead of carbon dioxide. Carbon monoxide is a toxic gas, so adequate ventilation is important when burning fuels.

Several waste products are also released when fuels burn Several waste products are also released when fuels burn. These do not just disappear and they can harm the environment by contributing to global warming and acid rain. Carbon dioxide dissolves in water in the atmosphere to form a weakly acidic solution. This means that rainwater is naturally slightly acidic. However, some of the products from burning fuels make rainwater more acidic than normal. This is acid rain.

Most hydrocarbon fuels naturally contain some sulphur compounds. When the fuel burns, the sulphur it contains is oxidised to sulphur dioxide: sulphur + oxygen → sulphur dioxide This gas dissolves in water to form an acidic solution. It is a cause of acid rain

Acid rain reacts with metals and rocks such as limestone, causing damage to buildings and statues. Acid rain damages the waxy layer on the leaves of trees. This makes it more difficult for trees to absorb the minerals they need for healthy growth and they may die. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

If the video does not start from the slide there is a copy of the video on the server .

The hydrocarbon fractions in crude oil need to be separated out to make them useful. This is done using a special type of distillation called Fractional Distillation. view link

The smallest hydrocarbons (which have the lowest boiling points) boil first.   These vapours go straight to the top of the tower and are collected first.   As the size of the hydrocarbons increase, the boiling points increase, and these hydrocarbons are collected further down the column. Eventually, the last fraction consists of a very thick, black substance.

Flammability is how easy it is for something to burn Viscosity is how thick something is. Ease of Evaporation is how easily it is for something to turn into a gas.

Copy and complete the following table to show how the properties of different fractions change with molecular size

Which of these is a true statement about the changes that occur as hydrocarbon molecules get larger? Boiling point decreases. Viscosity increases. Flammability increases. Transparency increases.

Uses of each fraction Burned in the refinery to fuel the distillation process, sold as LPG, purified and sold as bottled camping gas Fuel gas Petrol / gasoline Fuel for cars and motorcycles, also used to make chemicals. Naphtha Used to make chemicals. Paraffin / Kerosine Fuel for greenhouse heaters and jet engines, manufacture of chemicals. Diesel fuel Fuel for lorries, trains. Fuel and lubricating oil Fuel for the heating systems of large buildings, fuel for ships, lubricating oil. Bitumen Roofing, and road surfaces.

Alkanes

The alkanes are a family of hydrocarbons that share the same general formula. This is: CnH2n+2 The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. The alkanes form part of what is known as a homologous series. A homologous series is a series of compounds that have similar properties and the same general formula.

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from burning or combustion, which is their reaction with oxygen in the air.

There are many members of the alkanes, but for the purposes of this course, we only need to know the first 8 members. These are: - Methane - CH4 Ethane - C2H6 Propane - C3H8 Butane - C4H10 Pentane - C5H12 Hexane - C6H14 Heptane - C7H16 Octane - C8H18

Using a mnemonic can make learning the names easier.   Methane CH4 Monsters Ethane C2H6 Eat Propane C3H8 Pupils Butane C4H10 But Pentane C5H12 Prefer Hexane C6H14 Hairy Heptane C7H16 Haggis Octane C8H18 Occasionally

Remember that :- e.g C3H8 is the molecular formula for propane and is the structural formula for propane

  The structural formula is shown as a 2D representation but remember that you can draw a 3D representation using a flying wedge projection e.g. methane can be shown in 2D as And can be shown on 3D as This allows the tetrahedral arrangement to be seen more clearly

Isomers

Isomers are molecules with the same chemical formula but different structural formula. This means that they contain the same atoms but how the atoms are joined together is different. Making them different compounds

View video Don’t worry about naming the alkanes . We will return to this later .

Alkenes

The second homologous series is the alkenes The second homologous series is the alkenes. Their names all end in –ene for example ethene.

The alkenes all contain a double bond For this reason, alkenes all have 2 less hydrogens than the corresponding alkane Methene does not exist since a molecule must have at least 2 carbon atoms to have a double bond. The general formula for the alkenes is CnH2n

Full Structural Formula Copy and complete the table into your jotter :- Name Molecular Formula Full Structural Formula Ethene C2H4 Propene Butene Pentene

Alkenes are said to be unsaturated hydrocarbons because of the double bond between two carbon atoms in the molecule. Alkanes and alkenes are very alike. What is needed is a chemical test that will tell you which is which. Alkenes will decolourise bromine solution. Alkanes will not View

Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules. There is more demand for shorter alkane molecules and alkenes than for the many of the longer chains formed during fractional distillation. This is where cracking comes in. Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules.

Cracking produces a mixture of smaller alkanes and alkenes Cracking produces a mixture of smaller alkanes and alkenes. This helps to meet the demand for the more useful smaller fractions and to increase profit. Now watch the animation showing the cracking of large hydrocarbons (BBC Bitesize). ANIMATION

Complete the following word and formula equations. Octane cracking to give pentane and propene. Heptane cracking to produce ethane and pentene. (iii) Hexane cracking to produce 3 different products.

Alkene isomers Alkene isomers

View video Don’t worry about naming the alkenes ,we will return to this later

Cycloalkanes Cycloalkanes

Cycloalkanes Another way of putting carbon and hydrogen together is in rings This family is called the cycloalkanes These compounds are isomers of the alkenes i.e they have the same molecular formula (CnH2n) but the atoms are joined together differently.

Cycloalkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane.

Can you suggest a way to tell the difference between these two isomers of propene? – propene C3H6 and cyclopropane C3H6

Naming branched alkanes

Straight chain hydrocarbons have all the carbon atoms linked in a continuous chain 4 3 2 1

Branched chain hydrocarbons have one or more side chains coming from the main chain 5 4 3 1 2

Naming Branched Alkanes 8 6 1 5 4 3 7 2 8 carbon chain so based on octane Name the main chain two CH3 groups so dimethyl Name the side group Number the position of the side group methyl groups are on carbons 2 and 5

H C 8 6 1 5 4 3 7 2 2,5- dimethyloctane

Shortened Structural Formula 5 4 3 1 2 2-methylpentane

2-methylpentane can also be shown by its shortened structural formula CH3CH2CH2CHCH3 CH3

Naming Alkenes H C Number main chain oct-3-ene 8 6 1 5 4 3 7 2 Number main chain oct-3-ene Number position of double bond dimethyl Name side groups Number position of side groups 2,5-dimethyloct-3-ene

Renewable Fuel - Biomass

A renewable form of energy - BIOMASS Have you ever sat by a campfire or fireplace? If so, you've seen biomass energy in action. Biomass is any natural matter like trees, plants, or animal waste that can be used as an energy source. Energy comes from the sun through a process called photosynthesis, and is released when biomass is burned or decomposes. It can be used as a source of biofuels (a renewable energy source), to reduce our dependence on fossil fuels and to help reduce air pollution.

Burning biomass releases carbon dioxide. Having too much carbon dioxide in the atmosphere is contributing to the greenhouse effect. However the plants that produce biomass also use carbon dioxide to grow their leaves flowers, branches, and stems. The use of biomass for energy production is said to be carbon neutral.

The vital difference between biomass and fossil fuels is one of time scale. Biomass takes carbon out of the atmosphere while it is growing, and returns it as it is burned.  If it is managed on a sustainable basis, biomass is harvested as part of a constantly replenished crop. This maintains a closed carbon cycle with no net increase in atmospheric carbon dioxide levels. Fossil fuels such as coal, oil and gas are also derived from biological material, however material that absorbed carbon dioxide from the atmosphere many millions of years ago . Burning fossil fuels therefore adds carbon dioxide to the atmosphere

Examples of Biomass:- Biogas - produced when bacteria break down sewage in a digester . The methane gas produced by the bacteria can be used as a fuel. Bioethanol – yeast breaks down sugar in sugar cane to produce alcohol. Bioethanol is used in Brazil to fuel cars Fast growing timber- fast growing trees such as willow can be burned in power stations. Electricity is generated using renewable biomass instead of fossil fuels .