Fuels — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

Section 1 of 23Fuels and fossil fuels

Fuel

Fuel a substance that releases heat energy when it is burned

Fossil fuels

• Fossil fuels coal, natural gas and petroleum
• The heat released can be transferred into electricity.

Natural gas

• Natural gas its main constituent is methane, CH₄
• Methane is a hydrocarbon, which means it contains hydrogen and carbon atoms only.

Section 2 of 23Petroleum

Petroleum (crude oil)

• Petroleum (crude oil) a mixture of hydrocarbons that also contains natural gas
• It is a thick, sticky, black liquid found under porous rock underground and under the sea.

Fraction

Fraction a group of hydrocarbons of similar chain length with similar boiling points

• The mixture itself is not very useful, but each fraction has its own uses.
• Most fractions contain mainly alkanes, which have only single bonds.

Section 3 of 23Fractional distillation of petroleum

1

The crude oil is heated until it vaporises.

2

The vapours enter a fractionating column that is very hot at the bottom and cooler at the top.

3

Hydrocarbons with high boiling points condense low down in the column.

4

Hydrocarbons with lower boiling points rise higher up the column before they condense.

5

The liquid fractions are collected at different heights.

• Basis of separation
  • the fractions are separated by their different boiling points

Section 4 of 23Trends in the properties of the fractions

Boiling point

• Boiling point increases going down the column
• Larger molecules have stronger intermolecular attractions, so more heat is needed to separate them.

Viscosity

• Viscosity increases going down the column
• Longer chains tangle and attract more, so the liquid flows less easily.

Volatility

• Volatility decreases going down the column
• Volatility is the tendency of a substance to vaporise, and it falls as the molecules get larger.

Section 5 of 23Fractions and their uses

Fraction	Carbon atoms	Use
Refinery gas	1–4	Heating and cooking
Gasoline (petrol)	4–12	Fuel for cars
Naphtha	7–14	Raw material for making chemicals
Kerosene (paraffin)	12–16	Jet fuel
Diesel (gas oil)	14–18	Fuel for diesel engines
Fuel oil	19–25	Fuel for ships and home heating
Lubricating oil	20–40	Lubricants, polishes and waxes
Bitumen	More than 70	Surfacing roads

Section 6 of 23More from the specification

More from the specification

Steam reforming and synthetic carboxylic acids

• Steam reforming and synthetic carboxylic acids passing hydrocarbon feedstocks through high-temperature steam yields organic intermediates oxidised to carboxylic acids.

General formula

• A formula giving the composition of every member of a homologous series.

Homologous series

• A family of compounds with the same functional group and general formula.

Substitution reaction

• A reaction in which one atom or group is replaced by another.

Ultraviolet radiation

• Radiation that provides the activation energy for alkane–halogen reactions.

Functional group

• The atom or group of atoms that determines the reactions of a series.

Fossil fuels and hydrocarbons

• Petroleum/crude oil is a complex mixture of hydrocarbons separated into fractions by fractional distillation based on boiling points.

Complete combustion of hydrocarbons

• Burning fuels in excess oxygen produces carbon dioxide and water alongside thermal energy release.

Incomplete combustion

• Occurs with limited oxygen supply, forming toxic carbon monoxide (CO) and soot particles.

Alkene addition reactions

• Ethene reacts with aqueous bromine in an electrophilic addition reaction, decolourising orange bromine water.

Combustion

• Burning in oxygen

Catalyst

• A substance that speeds up a reaction and is not used up.

Carboxylic acid

• A compound containing –COOH with a name ending in -oic acid.

Carbon dioxide

• A gas that turns limewater cloudy.

Bromine water

• An orange solution that is decolourised by alkenes.

Addition reaction

• A reaction in which atoms add across a C=C bond to give one product.

Displayed formula

• A formula showing all atoms and all bonds.

Covalent bond

• A shared pair of electrons between two atoms, giving noble gas configurations.

Alkene

• A hydrocarbon with a C=C bond and a name ending in -ene.

Formula

• Symbols and numbers showing the atoms in a compound.

Alcohol

• A compound containing –OH with a name ending in -ol.

Section 7 of 23Catalytic cracking

Why cracking is needed

• Why cracking is needed the amount of long-chain hydrocarbons from distillation is far greater than the amount needed
• Long molecules are broken into shorter, more useful ones.

Conditions

• Conditions vaporised hydrocarbons at about 600–700 °C pass over a hot alumina or silica catalyst
• Covalent bonds break at random, which is thermal decomposition.

Products

• Products a mixture of shorter alkanes and alkenes, and sometimes hydrogen
• Cracking always produces at least one alkene.
• Alkenes are used to make polymers and hydrogen is used to make ammonia.

Example

C₁₀H₂₂ → C₈H₁₈ + C₂H₄, giving octane for petrol and ethene for making ethanol

Section 8 of 23Alkenes

• Alkenes
  • unsaturated hydrocarbons with a C=C double bond
  • They form a homologous series with general formula C_nH_2n.
  • The double bond makes them more reactive than alkanes.

Alkene	Formula
Ethene	C2H4
Propene	C3H6
But-1-ene	C4H8
Pent-1-ene	C5H10

Section 9 of 23Test to distinguish alkenes from alkanes

Reagent

• Reagent bromine water, which is orange

Alkane result

• Alkane result the solution stays orange because there is no C=C bond

Alkene result

• Alkene result the solution turns colourless
• Bromine adds across the C=C bond and is no longer in solution.
• The result is colourless, not clear.

Section 10 of 23Addition reactions of alkenes

Addition reaction

• Addition reaction atoms of a small molecule add across the C=C double bond
• The C=C bond breaks, new single bonds form and only one product is made.

With bromine

• With bromine a dibromoalkane forms and the orange colour is lost
• C₂H₄ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane).

With hydrogen

• With hydrogen an alkane forms, using a nickel catalyst
• C₂H₄ + H₂ → C₂H₆.

With steam

• With steam an alcohol forms, using a phosphoric acid catalyst
• C₂H₄ + H₂O → C₂H₅OH.
• This addition of water is also called hydration.

Section 11 of 23Alcohols

1Alcohols

• Alcohols a homologous series containing the hydroxyl group –OH
• The –OH group is responsible for their characteristic reactions.
• The general formula is C_nH_2n+1OH and each member differs by one CH₂.

2First members

• First members methanol CH₃OH, ethanol C₂H₅OH and propanol C₃H₇OH

3Ethanol as a fuel and solvent

• Ethanol as a fuel and solvent it burns completely in excess oxygen
• C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Section 12 of 23Manufacture of ethanol

	Hydration of ethene	Fermentation of glucose
Equation	C2H4 + H2O → C2H5OH	C6H12O6 → 2C2H5OH + 2CO2
Conditions	Steam, phosphoric acid catalyst, about 300 °C, 60 atm	Yeast, 25–35 °C, no oxygen
Type of process	Continuous	Batch
Raw material	Ethene from crude oil (non-renewable)	Sugar or starch (renewable)
Rate	Fast	Very slow, several days
Product	Pure ethanol	Dilute solution needing further processing

1Why fermentation is a batch process

• Why fermentation is a batch process the yeast are killed when the alcohol concentration reaches about 15%
• The vessel is emptied and the process is restarted.

2Temperature in fermentation

• Temperature in fermentation too low gives a slow rate and too high denatures the enzymes in yeast

Section 13 of 23Reactions of carboxylic acids

With metals

• With metals a salt and hydrogen form

With bases

• With bases a salt and water form

With carbonates

• With carbonates a salt, water and carbon dioxide form
• 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.

Salt names

• Salt names they end in -anoate
• Methanoic acid forms methanoates and ethanoic acid forms ethanoates.

Section 14 of 23More from the specification

More from the specification

Fractional Distillation of Crude Oil

• Petroleum is separated into simpler fractions in a fractionating column based on the continuous gradient of boiling points of its constituent hydrocarbons.

Trends in Hydrocarbon Fractions

• Descending the fractionating column, carbon chain length increases,
• which increases boiling point and viscosity while decreasing flammability.

Complete and Incomplete Combustion

• In the presence of plentiful oxygen, hydrocarbons burn completely to form CO₂ and H₂O;
  • incomplete combustion produces toxic carbon monoxide and soot.

Oxidation of Alcohols to Carboxylic Acids

• Alcohols derived from petroleum cracking or fermentation can undergo oxidation in the presence of chemical oxidising agents to form carboxylic acids.

Fractional Distillation of Crude Oil

• Petroleum is separated into simpler fractions in a fractionating column based on the continuous gradient of boiling points of its constituent hydrocarbons.

Trends in Hydrocarbon Fractions

• Descending the fractionating column, carbon chain length increases,
• which increases boiling point and viscosity while decreasing flammability.

Complete and Incomplete Combustion

• In the presence of plentiful oxygen, hydrocarbons burn completely to form CO₂ and H₂O;
  • incomplete combustion produces toxic carbon monoxide and soot.

Oxidation of Alcohols to Carboxylic Acids

• Alcohols derived from petroleum cracking or fermentation can undergo oxidation in the presence of chemical oxidising agents to form carboxylic acids.

Substitution reaction

• A reaction in which one atom or group is replaced by another.

Ultraviolet radiation

• Radiation that provides the activation energy for alkane–halogen reactions.

Functional group

• The atom or group of atoms that determines the reactions of a series.

Fraction

• A group of hydrocarbons with similar chain length and boiling points.

Boiling

• Liquid to gas throughout the liquid at the boiling point.

Combustion

• Burning in oxygen

Carboxylic acid

• A compound containing –COOH with a name ending in -oic acid.

Fractional distillation

• Separating a mixture of liquids by their boiling points.

Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

Displayed formula

• A formula showing all atoms and all bonds.

Section 15 of 23Making ethanoic acid from ethanol

Making ethanoic acid from ethanol

Oxidation

• Oxidation ethanol is oxidised to ethanoic acid by one of two methods

Microbial oxidation (fermentation)

• Microbial oxidation (fermentation) acetobacter bacteria use oxygen from the air to oxidise ethanol
• C₂H₅OH + O₂ → CH₃COOH + H₂O.
• It gives a weak solution of ethanoic acid called vinegar.

Chemical oxidation

• Chemical oxidation ethanol is heated with acidified potassium manganate(VII)
• CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
• The oxidising agent is written as [O].

Section 16 of 23Why wine turns to vinegar

Cause

• Cause acetobacter bacteria in the air oxidise the ethanol in an opened bottle of wine

Result

• Result ethanoic acid gives the sour, vinegary taste

Section 17 of 23Heating under reflux

Reflux

• Reflux heating the mixture in a vessel with a condenser fitted to the top
• The condenser returns volatile ethanol and ethanoic acid to the flask so that they are not lost.

Colour change

• Colour change the purple potassium manganate(VII) turns colourless as it is reduced

Section 18 of 23Esterification

Esterification

• Esterification the reaction of an alcohol with a carboxylic acid to form an ester and water
• The reaction is reversible.

Ester

Ester a compound containing the group R–COO–R

• Esters are sweet-smelling, oily liquids used in food flavourings and perfumes.

Catalyst

• Catalyst concentrated sulfuric acid is required
• It speeds up the reaction and absorbs the water produced.

Example

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O, forming ethyl ethanoate

Section 19 of 23Naming esters

1First part

• First part comes from the alcohol, shows the length of its carbon chain and ends in -yl

2Second part

• Second part comes from the carboxylic acid, shows the length of its carbon chain and ends in -oate

Alcohol	Carboxylic acid	Ester
Ethanol	Ethanoic acid	Ethyl ethanoate
Methanol	Propanoic acid	Methyl propanoate
Propanol	Methanoic acid	Propyl methanoate
Methanol	Butanoic acid	Methyl butanoate

• Splitting an ester formula at the –COO– link
  • the part on the single-bonded oxygen is from the alcohol
  • The part containing C=O is from the acid.

Section 20 of 23Sequence of functional groups

1

Ethanol (–OH) is oxidised to ethanoic acid (–COOH)

2

Ethanoic acid reacts with an alcohol to give an ester (–COO–) and water

Section 21 of 23More from the specification

More from the specification

Industrial steam reforming of hydrocarbon fuels

• Industrial steam reforming of hydrocarbon fuels heating gaseous hydrocarbons with steam produces synthetic precursors subsequently oxidised into organic carboxylic acids.

Fractional Distillation of Hydrocarbons

• Crude oil is separated into useful fractions containing hydrocarbon molecules of similar chain lengths and boiling points,
• where smaller alkanes boil at lower temperatures than heavy fractions.

Saturated Alkanes vs. Unsaturated Alkenes

• Alkanes are saturated hydrocarbons containing only single covalent C-C bonds and C-H bonds,
• whereas alkenes are unsaturated hydrocarbons possessing a carbon-carbon double bond (C=C).
  • (such as ethene, C₂H₄)

Electrophilic Addition Across the C=C Bond

• Alkenes undergo addition reactions where the double bond opens up to bond two new atoms;
  • for instance, ethene reacts with bromine molecules to produce 1,2-dibromoethane.

Chemical Test Distinguishing Alkanes from Alkenes

• Adding orange aqueous bromine water to an alkene results in rapid decolourisation from orange to colourless as bromine adds across the double bond,
• whereas saturated alkanes do not react without UV light.

General formula

• A formula giving the composition of every member of a homologous series.

Homologous series

• A family of compounds with the same functional group and general formula.

Substitution reaction

• A reaction in which one atom or group is replaced by another.

Ultraviolet radiation

• Radiation that provides the activation energy for alkane–halogen reactions.

Functional group

• The atom or group of atoms that determines the reactions of a series.

Combustion

• Burning in oxygen

Distillation

• Separating a liquid from a solution by boiling and condensing the vapour.

Carbon dioxide

• A gas that turns limewater cloudy.

Fractional distillation

• Separating a mixture of liquids by their boiling points.

Bromine water

• An orange solution that is decolourised by alkenes.

Addition reaction

• A reaction in which atoms add across a C=C bond to give one product.

Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

Displayed formula

• A formula showing all atoms and all bonds.

Hydrogen

• A gas that gives a squeaky pop with a lighted splint.

Alkene

• A hydrocarbon with a C=C bond and a name ending in -ene.

Alkane

• A saturated hydrocarbon with a name ending in -ane.

Molecule

• Two or more atoms held together by covalent bonds.

Hydrocarbon

• A compound containing hydrogen and carbon only.

Fraction

• A group of hydrocarbons with similar chain length and boiling points.

Boiling

• Liquid to gas throughout the liquid at the boiling point.

Section 22 of 23Key terms

Term	Meaning
Fuel	A substance that releases heat energy when it is burned.
Fossil fuel	Coal, natural gas or petroleum formed from ancient living things.
Petroleum	A mixture of hydrocarbons, also called crude oil.
Fraction	A group of hydrocarbons with similar chain lengths and boiling points.
Fractional distillation	Separation of a mixture into fractions by differences in boiling point.
Volatility	The tendency of a substance to vaporise.

Term	Meaning
Cracking	Breaking long-chain alkanes into shorter alkanes and alkenes using heat and a catalyst.
Addition reaction	A reaction in which atoms add across a C=C double bond to give one product.
Hydroxyl group	The –OH functional group of alcohols.
Fermentation	Conversion of glucose to ethanol and carbon dioxide by yeast.
Batch process	A process carried out in separate lots rather than continuously.
Hydration	Addition of water across a C=C double bond.
Homologous series	A family of compounds with the same functional group and general formula.

Term	Meaning
Vinegar	A dilute solution of ethanoic acid.
Acetobacter	Bacteria that oxidise ethanol to ethanoic acid using oxygen from air.
Ester	A compound containing the group R–COO–R.
Esterification	Reaction of an alcohol with a carboxylic acid to form an ester and water.
Oxidation	Addition of oxygen or loss of electrons.
Reflux	Heating with a condenser fitted so that vapours return to the flask.

Section 23 of 23Exam technique

1

Saying the heaviest fractions rise to the top — fractions with high boiling points condense at the bottom

2

Saying petroleum is a pure compound — it is a mixture of hydrocarbons

3

Thinking the process is chemical — fractional distillation is a physical separation by boiling point

4

Stating that viscosity decreases down the column — it increases as the chains get longer

1

Saying bromine water becomes clear — it becomes colourless

2

Thinking alkanes decolourise bromine water — there is no reaction and it stays orange

3

Forgetting that cracking always produces an alkene

4

Mixing up the conditions of hydration and fermentation

5

Writing the salt of ethanoic acid as ethanoic — it ends in -anoate

1

Naming the ester with the acid part first — the alcohol part comes first

2

Leaving out water as a product of esterification

3

Omitting the acid catalyst in esterification

4

Saying wine goes sour because it is heated — acetobacter oxidise the ethanol

5

Forgetting reflux — without a condenser the volatile ethanol escapes