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Welcome to GCSE Edexcel Science revision.

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Unit S C 20: Fuels.

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A hydrocarbon contains only carbon and hydrogen.

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Methane, C H 4, is a hydrocarbon; ethanol is not, because it also contains oxygen.

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Crude oil is a complex mixture of hydrocarbons with different molecular sizes and chain or ring structures.

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It is not one pure compound and most constituents are not polymers.

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Crude oil supplies fuels and starting materials (feedstock) for the petrochemical industry.

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These are used to make other chemicals and materials, including polymers.

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Crude oil and fossil natural gas are finite, non-renewable resources on human timescales.

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Natural gas is mainly methane and is used in cooking and heating.

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Petrol, kerosene and diesel are fossil fuels obtained from crude oil.

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Hydrocarbons can also be produced by other routes, so not every hydrocarbon molecule is necessarily fossil-derived.

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A fraction is a mixture of hydrocarbons with a similar range of boiling points, not normally a single chemically pure substance.

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Heat crude oil so much of it vaporises, and pass vapour into a fractionating column.

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The column is hot at the bottom and cooler at the top.

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Hydrocarbons condense at different levels according to their boiling points: longer-chain,

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higher-boiling molecules condense lower down;

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shorter-chain,

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lower-boiling ones travel higher.

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Very low-boiling refinery gases can leave the top as gases rather than condensing as liquids there.

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High-boiling residue such as bitumen is collected at the bottom.

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Fractional distillation is a physical separation.

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It does not break carbon chains or change the identity of the hydrocarbons; cracking does that separately.

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The main fractions in order from top towards bottom are gases, petrol, kerosene, diesel oil, fuel oil and bitumen.

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Schematic column; top gases need not condense there and bitumen is bottom residue.

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Gases are used for domestic heating and cooking;

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petrol for cars;

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kerosene for aircraft;

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diesel for some cars and trains;

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fuel oil for large ships and some power stations;

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bitumen for roads and roofs.

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Fractions contain mixtures, not one pure alkane each.

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As chain length generally increases, boiling point and liquid viscosity increase and ease of ignition decreases.

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Larger molecules have stronger intermolecular attractions.

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Compare similar hydrocarbons under the same conditions.

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Viscosity describes resistance to flow: a more viscous liquid flows less readily under comparable conditions.

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Compare temperature as well as composition.

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Alkanes are saturated hydrocarbons: they have only single carbon to carbon bonds.

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Alkanes without rings, called acyclic alkanes, have general formula C subscript n, H subscript two n plus two.

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Ring-shaped hydrocarbons do not necessarily follow this formula.

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The first four alkanes are methane, C H four; ethane, C two H six; propane, C three H eight; and butane, C four H ten.

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Acyclic saturated alkanes follow C subscript n, H subscript two n plus two.

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A homologous series has the same general formula, similar chemical properties and a gradual change in physical properties.

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Neighbouring members differ by C H 2.

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For an acyclic alkane with five carbons, 2n plus 2 equals 12 hydrogen atoms, giving C 5 H 12.

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Check that you are using the correct series before applying the formula.

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Complete combustion with sufficient oxygen produces carbon dioxide and water and releases energy: hydrocarbon plus oxygen produces carbon dioxide plus water.

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For methane: C H 4 plus 2 O 2 produces C O 2 plus 2 H 2 O.

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Balance carbon, hydrogen and oxygen atoms without changing the substance formulae.

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Incomplete combustion occurs with insufficient oxygen and can produce carbon monoxide and or carbon particles (soot), together with water and sometimes carbon dioxide.

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Example C O-producing equation: 2 C H 4 plus 3 O 2 produces 2 C O plus 4 H 2 O.

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A soot-producing example is C H 4 plus O 2 produces C plus 2 H 2 O.

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Real incomplete combustion can produce mixtures.

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Incomplete combustion can yield carbon monoxide and or carbon, rather than one fixed product mix.

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Incomplete combustion releases less useful energy from a given amount of fuel than full oxidation to C O 2, and creates hazards and pollution.

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Carbon monoxide is colourless, odourless and toxic.

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It binds to haemoglobin, reducing the blood's ability to transport oxygen.

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Soot can dirty appliances and obstruct air flow or flues, increasing problems with fuel-burning appliances.

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Carbon particles are also harmful air pollution.

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Complete combustion still releases carbon dioxide, a greenhouse gas.

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Absence of soot does not mean the fuel has no environmental impact.

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Sulfur impurities in some fuels burn to form sulfur dioxide: S plus O 2 produces S O 2.

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Removing sulfur from fuels reduces this source of pollution.

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Sulfur dioxide dissolves in atmospheric moisture to produce acidic solutions; further oxidation can contribute to sulfuric acid formation.

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Do not treat S O 2 plus water alone as directly making H 2 S O 4 without extra oxygen.

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Acid rain can damage plants, acidify lakes and harm aquatic organisms, and react with limestone buildings and monuments.

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An acid-rain carbonate equation is H 2 S O 4 plus C A C O 3 produces C A S O 4 plus H 2 O plus C O 2.

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With a suitable reactive metal: F E plus H 2 S O 4 produces F E S O 4 plus H 2 for dilute acid.

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At high engine temperatures, nitrogen and oxygen from air can react to produce nitrogen oxides, nitrogen oxides.

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Their formation does not require nitrogen to be part of the hydrocarbon fuel.

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Their effects differ: toxic gases, particles, acid rain and greenhouse warming.

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Nitrogen oxides contribute to acidic precipitation and air pollution.

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Catalytic converters reduce several harmful exhaust gases, including converting nitrogen oxides towards nitrogen under suitable conditions.

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Removing sulfur, treating emissions and reducing combustion can help, but different pollutants need different measures.

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C O 2 climate effects differ from S O 2 acid-rain effects.

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Cracking breaks larger saturated hydrocarbon molecules into smaller molecules, including shorter alkanes and unsaturated alkenes, using suitable heating and sometimes catalysts.

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Cracking is useful because demand for short-chain fuels and alkene feedstocks can exceed the supply from simple distillation;

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longer fractions can be converted into more useful products.

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Alkenes are unsaturated hydrocarbons with a carbon to carbon double bond.

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Alkenes without rings and with one C double bonded to C bond have general formula C subscript n, H subscript two n.

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A balanced cracking example is C 10 H 22 produces C 8 H 18 plus C 2 H 4.

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Check totals: ten carbons and twenty-two hydrogens on each side.

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Cracking is chemical change; fractional distillation is physical separation.

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Ethene is a useful feedstock for poly(ethene): monomers join into long-chain molecules.

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Distillation only separates existing molecules; cracking changes them chemically.

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Specific demand and profitability vary by market and time.

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Do not assume petrol is always more profitable than every other fraction.

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Petrol is energy-dense, easy to transport as a liquid and supported by widespread refuelling infrastructure.

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Its combustion emits C O 2 and can produce other pollutants.

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Hydrogen reacts with oxygen to form water: 2 H 2 plus O 2 produces 2 H 2 O.

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The hydrogen fuel itself contains no carbon, so this reaction produces no fuel-derived C O 2.

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Hydrogen is difficult to store and transport compactly and needs suitable tanks or other storage methods and refuelling infrastructure.

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It is flammable and needs appropriate engineering controls.

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Hydrogen production can use fossil feedstocks or electricity, so overall emissions depend on the production method and energy source.

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Tailpipe emissions alone do not describe the full life cycle.

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Burning hydrogen in air at high temperature can form nitrogen oxides; a hydrogen fuel cell operates differently and produces electricity without ordinary combustion.

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No fuel-derived C O 2 at use does not establish zero whole-life emissions.

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Evaluate fuel choices using storage, energy supply, costs, safety and whole-life environmental impacts.

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Avoid saying hydrogen is automatically emission-free in every sense.

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That completes Fuels.

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Revisit the notes and test yourself on the revision website.