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GCSE Chemistry · GCSEChapter 15 · Crude oil, alkanes and alkenes
GCSE · Chapter 15

Crude oil, alkanes and alkenes

By the end of this chapter you will be able to

  • Explain how fractional distillation separates crude oil into fractions
  • Name and draw the first alkanes, and use the general formula
  • Write equations for complete and incomplete combustion
  • Explain why cracking is needed, and identify an alkene by the bromine water test
  • [H] Write word equations for addition reactions of alkenes

You should already know

  • Covalent bonds and shared pairs (Chapter 5)
  • Intermolecular forces (Chapter 5)
  • Balancing equations (Chapter 7)

Where this leads

  • Alcohols, carboxylic acids and polymers (Chapter 16)
  • Fuels and the atmosphere (Chapter 18)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe crude oil as a mixture of hydrocarbons, and explain fractional distillation and the uses of the fractions, Describe the alkanes as a homologous series, with the general formula CnH2n+2, and the trends in properties, Describe complete and incomplete combustion of hydrocarbons, Explain cracking, and describe alkenes (general formula CnH2n) and the bromine water test, [H] Describe addition reactions of alkenes with hydrogen, steam and halogens.

Exercise P  Do you remember?

1
FHow many shared pairs does a carbon atom make in methane, CH4? Write the number.
2
FWhat type of bond joins the atoms in a molecule of ethane? Write the word.
3
FWhat is 2 x 5 + 2? Write the number.
4
FWhat is the relative formula mass of CO2? Write the number.
5
FWhich gas turns limewater milky? Write the name.
15.1

Crude oil and fractional distillation

GCSE
Describe crude oil as a mixture of hydrocarbons, and explain fractional distillation and the uses of the fractions
Video lesson: Crude oil and fractional distillationComing soon
Context. A pupil asks why petrol, diesel and bitumen all come from the same black liquid. “Are they separate substances?” asks the pupil. Crude oil is a mixture of many hydrocarbons, and fractional distillation separates them by boiling point.

Crude oil is a mixture of hydrocarbons: compounds made only of carbon and hydrogen. Its hydrocarbons have a range of chain lengths and boiling points. In fractional distillation, crude oil is heated to a vapour and enters a tall column. The column is hotter at the bottom and cooler at the top. Each hydrocarbon condenses at the height where its boiling point is reached, so the mixture is separated into fractions.

Fractional distillation of crude oil (simplified)refinery gases (lowest boiling)petrolkerosene (jet fuel)diesel oilfuel oilbitumen (highest boiling)crude oil heated to vapour at the bottomhotcooler
Fractional distillation: the fractions condense at different heights (simplified, not to scale)
FractionChain length (about)Use
refinery gasesC1 to C4bottled gas, fuel
petrol (gasoline)C5 to C10fuel for cars
keroseneC11 to C15jet fuel
diesel oilC15 to C19fuel for lorries and trains
bitumenC70 and largerroad surfacing
The main fractions of crude oil and their uses

Key ideas

  • Crude oil is a mixture of hydrocarbons with different chain lengths.
  • Fractional distillation separates the mixture by boiling point.
  • Short chains boil at low temperatures and condense near the top; long chains condense near the bottom.
  • Fractions are used as fuels, and bitumen is used for road surfacing.
Worked example 15.1.1
Which fraction has the lowest boiling point, and is collected at the top of the column? Write the name.

Method. Compare the boiling points.

  1. Look. Short chains have the lowest boiling points.
    short chains
  2. Decide. So refinery gases are collected at the top.
    refinery gases
  3. Write the answer.
    refinery gases

Check. Refinery gases include bottled gas. ✓

Worked example 15.1.2
Which fraction has the longest chains, and is used for road surfacing? Write the name.

Method. Recall the use of each fraction.

  1. Recall. Bitumen has the longest chains and is used to surface roads.
    bitumen
  2. Decide. So the answer is bitumen.
    bitumen
  3. Write the answer.
    bitumen

Check. It is collected at the bottom of the column. ✓

Worked example 15.1.3
Which fraction is used as jet fuel? Write the name.

Method. Recall the use of each fraction.

  1. Look. Kerosene is a middle fraction, between petrol and diesel.
    middle fraction
  2. Decide. It is used as jet fuel for aircraft.
    kerosene
  3. Write the answer.
    kerosene

Check. Kerosene condenses above diesel in the column. ✓

Watch out

Watch out: crude oil is a mixture, not a compound. The fractions are not pure substances; each one is still a mixture of hydrocarbons with a range of chain lengths.

Exam tip

Exam tip. Link the boiling point to the height in the column: higher boiling points condense lower down.

Exercise 15A  Crude oil and fractional distillation

1
FWhat is crude oil? Write the words.
2
FWhat are compounds made only of carbon and hydrogen called? Write the word.
3
FWhich process separates crude oil into fractions? Write the name.
4
FIn a fractional distillation column, is it hotter at the top or the bottom? Write the word.
5
FWhich fraction is used as jet fuel? Write the name.
6
FWhich fraction is used as fuel for lorries and trains? Write the name.
7
FWhich fraction is collected at the top of the column? Write the name.
8
FWhich fraction has the highest boiling point? Write the name.
9
PSWhy does a fraction with longer chains have a higher boiling point? Write one reason.
10
RExplain why crude oil is separated into fractions rather than used as it is, in one sentence. Accept any sentence that says the fractions have different uses, such as fuels and road surfacing.
11
RExplain why the fractions condense at different heights in the column, in one sentence. Accept any sentence that says each fraction condenses where its boiling point is reached, which depends on the temperature.
15.2

Alkanes and their properties

GCSE
Describe the alkanes as a homologous series, with the general formula CnH2n+2, and the trends in properties
Video lesson: Alkanes and their propertiesComing soon
Context. A student builds models of hydrocarbons with plastic balls and sticks. “Each one has two more hydrogen atoms than twice the carbons, plus two,” she says. That pattern is the general formula of the alkanes.

The alkanes are a homologous series: a family of compounds with the same general formula and similar chemical properties, where each member differs from the next by one CH2 unit. Their general formula is CnH2n+2. They are saturated: all their bonds are single bonds. The names are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane and decane.

Structures of the first alkanes (saturated, single bonds only)CH<sub>4</sub>methaneC<sub>2</sub>H<sub>6</sub>ethaneC<sub>3</sub>H<sub>8</sub>propaneC<sub>4</sub>H<sub>10</sub>butanegeneral formula: C<sub>n</sub>H<sub>2n+2</sub>
The first four alkanes
PropertyAs chain length increasesWhy
boiling pointincreasesstronger forces between longer molecules
viscosity (how runny)increaseslonger molecules tangle more
flammabilitydecreaseslarger molecules are harder to ignite
colour and state at room temperaturegas, then liquid, then solidmore energy needed to separate molecules
Trends in the properties of alkanes as the chains get longer

Key ideas

  • General formula of the alkanes: CnH2n+2.
  • The prefix gives the number of carbons: meth- (1), eth- (2), prop- (3), but- (4), pent- (5).
  • Boiling point, viscosity increase with chain length; flammability decreases.
  • Alkanes are saturated: all single bonds.
Worked example 15.2.1
How many hydrogen atoms are in a molecule of an alkane with 5 carbon atoms? Write the number.

Method. Use the general formula CnH2n+2 with n = 5.

  1. Substitute. H = 2 x 5 + 2.
    2 x 5 + 2
  2. Answer. H = 12, so the formula is C5H12.
    12
  3. Write the answer.
    12

Check. Pentane is C5H12. ✓

Worked example 15.2.2
What is the molecular formula of propane? Write the formula.

Method. Propane has 3 carbons.

  1. Carbons. Propane is a 3-carbon alkane.
    C3
  2. Hydrogens. H = 2 x 3 + 2 = 8.
    H8
  3. Formula. So propane is C3H8.
    C3H8
  4. Write the answer.
    C3H8

Check. The prefix prop- means 3. ✓

Worked example 15.2.3
Which alkane has the higher boiling point: butane or octane? Write the name.

Method. Compare the chain lengths.

  1. Compare. Octane has 8 carbons, butane has 4.
    8 and 4
  2. Decide. The longer chain has the higher boiling point.
    octane
  3. Write the answer.
    octane

Check. Longer chains have stronger forces. ✓

Watch out

Watch out: the general formula gives the number of hydrogens, not the number of carbons. For pentane, n = 5 and the formula is C5H12.

Exam tip

Exam tip. Learn the prefixes meth-, eth-, prop-, but-, pent-, hex- and their numbers: they tell you the number of carbons.

Exercise 15B  Alkanes and their properties

1
FWhat is the general formula of the alkanes? Write the formula.
2
FWhat is the formula of methane? Write the formula.
3
FWhat is the formula of ethane? Write the formula.
4
FWhat is the name of the alkane with 6 carbon atoms? Write the name.
5
FWhat is the molecular formula of hexane? Write the formula.
6
FWhich alkane has 10 carbon atoms? Write the name.
7
FAre alkanes saturated? Write yes or no.
8
FAs the chain length increases, does the boiling point increase or decrease? Write the word.
9
FAs the chain length increases, does flammability increase or decrease? Write the word.
10
PSWhat is the formula of an alkane with 7 carbon atoms? Write the formula.
11
PSAn alkane has the formula C8H18. Check it fits the general formula, and write yes or no.
12
RExplain why alkanes with longer chains are more viscous, in one sentence. Accept any sentence that says longer molecules tangle and resist flow more.
13
RExplain why the boiling point of an alkane increases with chain length, in one sentence. Accept any sentence that says longer molecules have stronger intermolecular forces, which need more energy to overcome.
15.3

Combustion of hydrocarbons

GCSE
Describe complete and incomplete combustion of hydrocarbons
Video lesson: Combustion of hydrocarbonsComing soon
Context. A camper uses a gas stove in a small tent. The flame turns yellow and sooty, and a headache starts. “Why does the flame change?” asks a pupil. With limited oxygen, the fuel does not burn completely, and carbon monoxide and soot are made.

When a hydrocarbon burns completely in plenty of oxygen, it forms carbon dioxide and water. When there is limited oxygen, the combustion is incomplete: it forms carbon monoxide, which is poisonous, and carbon (soot), as well as water. Incomplete combustion is dangerous in a closed space.

TypeEquation (methane)Products
complete (plenty of oxygen)CH4 + 2O2 -> CO2 + 2H2Ocarbon dioxide and water
incomplete (limited oxygen)2CH4 + 3O2 -> 2CO + 4H2Ocarbon monoxide (toxic) and water; soot may form
Complete and incomplete combustion of methane

Key ideas

  • Complete combustion: hydrocarbon + oxygen -> carbon dioxide + water.
  • Incomplete combustion: limited oxygen; carbon monoxide and soot form.
  • Carbon monoxide is poisonous because it binds to haemoglobin in the blood.
  • Soot is carbon particles; it causes a yellow or smoky flame.
Worked example 15.3.1
Write the word equation for complete combustion of ethane. Write the products.

Method. Hydrocarbons burn completely to carbon dioxide and water.

  1. Reactants. Ethane and oxygen.
    ethane + oxygen
  2. Products. Carbon dioxide and water.
    carbon dioxide and water
  3. Write the answer.
    carbon dioxide and water

Check. Each C makes one CO2 and each 2 H make one H2O. ✓

Worked example 15.3.2
Which poisonous gas is formed in incomplete combustion? Write the name.

Method. Recall the gas.

  1. Recall. Incomplete combustion forms carbon monoxide.
    carbon monoxide
  2. Decide. It is poisonous, so the answer is carbon monoxide.
    carbon monoxide
  3. Write the answer.
    carbon monoxide

Check. Carbon monoxide is colourless and odourless. ✓

Worked example 15.3.3
In complete combustion of methane, how many oxygen molecules are needed for each methane molecule? Write the number.

Method. Balance the equation.

  1. Equation. CH4 + 2O2 -> CO2 + 2H2O.
    2O2
  2. Answer. So 2 O2 molecules are needed.
    2
  3. Write the answer.
    2

Check. Four O atoms on each side. ✓

Watch out

Watch out: incomplete combustion does not mean the fuel is not burning. It burns, but it does not burn fully, so harmful products such as carbon monoxide are made.

Exam tip

Exam tip. When you name products of combustion, say complete (carbon dioxide and water) or incomplete (carbon monoxide, soot and water).

Exercise 15C  Combustion of hydrocarbons

1
FWhat are the two products of complete combustion of a hydrocarbon? Write the names.
2
FWhat poisonous gas is formed in incomplete combustion? Write the name.
3
FWhat solid is formed in incomplete combustion, giving a smoky flame? Write the name.
4
FIs a limited oxygen supply associated with complete or incomplete combustion? Write the word.
5
FWrite the balanced equation for complete combustion of methane, using whole numbers. Write the number in front of O2.
6
PSWhich gas is produced in both complete and incomplete combustion? Write the name.
7
PSIn incomplete combustion of methane, the equation is 2CH4 + 3O2 -> 2CO + 4H2O. How many carbon monoxide molecules are made from 2 methane molecules? Write the number.
8
RExplain why carbon monoxide is dangerous to people, in one sentence. Accept any sentence that says it binds to haemoglobin in the blood, so less oxygen is carried.
9
RExplain why a gas heater should be well ventilated, in one sentence. Accept any sentence that says ventilation brings in fresh air, so combustion is more complete and less carbon monoxide builds up.
15.4

Cracking and alkenes

GCSE
Explain cracking, and describe alkenes (general formula CnH2n) and the bromine water test
Video lesson: Cracking and alkenesComing soon
Context. There is more demand for petrol than for the heavy fraction in crude oil. “How can the heavy fraction be turned into petrol?” asks a pupil. The answer is cracking, which breaks long hydrocarbon chains into shorter, more useful molecules.

Cracking breaks long-chain alkanes into shorter alkanes and alkenes, using heat and often a catalyst. The alkenes are useful for making polymers and other chemicals. Alkenes have the general formula CnH2n and contain a C=C double bond, so they are unsaturated. Ethene (C2H4) and propene (C3H6) are the first two.

decane C<sub>10</sub>H<sub>22</sub>heat + catalystoctane C<sub>8</sub>H<sub>18</sub>+ethene C<sub>2</sub>H<sub>4</sub>
Cracking decane into octane and ethene
Alkenes: a C=C double bondethene C<sub>2</sub>H<sub>4</sub>propene C<sub>3</sub>H<sub>6</sub>general formula: C<sub>n</sub>H<sub>2n</sub>
Alkenes have a double bond
TestResult with an alkeneResult with an alkane
bromine water (orange to colourless)decolourises: the solution goes colourlessno change: stays orange
The bromine water test (teacher demonstration only)

Key ideas

  • Cracking: long alkane -> shorter alkane + alkene; heat and a catalyst.
  • Alkenes: general formula CnH2n; contain a C=C double bond.
  • Test for an alkene: bromine water goes from orange to colourless.
  • Alkenes are unsaturated; alkanes are saturated.
Worked example 15.4.1
Decane (C10H22) is cracked to give octane and one other product. What is the other product? Write the formula.

Method. Use the carbon and hydrogen numbers.

  1. Carbon. Decane has 10 C; octane has 8 C, so the other product has 2 C.
    2 C
  2. Hydrogen. Decane has 22 H; octane has 18 H, so the other product has 4 H.
    4 H
  3. Answer. The other product is C2H4, ethene.
    C2H4
  4. Write the answer.
    C2H4

Check. C10H22 = C8H18 + C2H4. ✓

Worked example 15.4.2
What is the general formula of the alkenes? Write the formula.

Method. Recall the formula.

  1. Recall. Alkenes are CnH2n.
    CnH2n
  2. Check. Ethene (n = 2) is C2H4, which fits.
    C2H4
  3. Write the answer.
    CnH2n

Check. Alkanes have 2n + 2 hydrogens; alkenes have 2n. ✓

Worked example 15.4.3
Bromine water is added to an alkene. What is seen? Write the words.

Method. Think about the test.

  1. Look. Bromine water is orange.
    orange
  2. Decide. An alkene decolourises bromine water, so the solution goes colourless.
    colourless
  3. Write the answer.
    it goes from orange to colourless

Check. An alkane does not change the colour. ✓

Watch out

Watch out: the double bond in an alkene is what reacts with bromine water. An alkane has only single bonds, so it does not decolourise bromine water.

Exam tip

Exam tip. For the bromine water test, give the colour change for the alkene and say the alkane shows no change.

Exercise 15D  Cracking and alkenes

1
FWhat is cracking? Write the words.
2
FWhy is cracking needed? Write the words.
3
FWhat is the general formula of alkenes? Write the formula.
4
FWhat is the formula of ethene? Write the formula.
5
FWhich type of bond makes a molecule an alkene? Write the words.
6
FWhat is the colour of bromine water before it reacts with an alkene? Write the colour.
7
FWhat colour does bromine water become with an alkene? Write the colour.
8
FIs an alkene saturated or unsaturated? Write the word.
9
PSWhat are the products of cracking C8H18 into C4H8 and another product? Write the formula of the other product. (Carbon 8, hydrogen 18.)
10
PSDecane is cracked into an alkene with 6 carbon atoms and an alkane. Name the alkane formed. Write the formula.
11
RExplain why alkenes are more reactive than alkanes, in one sentence. Accept any sentence that says the C=C double bond can open up to react with other atoms.
12
RA pupil says “cracking makes alkanes from alkenes.” Explain why this is wrong, in one sentence. Accept any sentence that says cracking breaks long alkanes to make shorter alkanes and alkenes.
15.5

Addition reactions and structure [H]

GCSE
[H] Describe addition reactions of alkenes with hydrogen, steam and halogens
Video lesson: Addition reactions and structure [H]Coming soon
Context. A chemist adds hydrogen to ethene in the presence of a nickel catalyst, and the product is a saturated alkane. “What happened to the double bond?” asks a pupil. In an addition reaction, the double bond opens and atoms are added across it, with no other product.

[H] Higher tier only. In an addition reaction, a small molecule adds across the C=C double bond, so the product is saturated. Examples: ethene + hydrogen -> ethane (nickel catalyst); ethene + steam -> ethanol (high temperature and pressure, catalyst); ethene + bromine -> 1,2-dibromoethane.

Key ideas

  • Addition: atoms add across the C=C double bond; no other product.
  • Ethene + hydrogen -> ethane (nickel catalyst).
  • Ethene + steam -> ethanol (catalyst, high temperature and pressure).
  • Ethene + bromine -> 1,2-dibromoethane; this is also the bromine water test.
Worked example 15.5.1
[H] Write the word equation for ethene reacting with hydrogen. Write the product.

Method. Add H2 across the double bond.

  1. Reactants. Ethene and hydrogen.
    ethene + hydrogen
  2. Product. The product is ethane, C2H6.
    ethane
  3. Write the answer.
    ethane

Check. C2H4 + H2 gives C2H6. ✓

Worked example 15.5.2
[H] Write the formula of the product of ethene reacting with steam. Write the formula.

Method. Add H2O across the double bond.

  1. Adding. C2H4 + H2O.
    C2H4 + H2O
  2. Product. The product is ethanol, C2H5OH, which is C2H6O.
    C2H6O
  3. Write the answer.
    C2H6O

Check. Ethanol is made by hydration of ethene. ✓

Worked example 15.5.3
[H] What is the name of the product of ethene and bromine? Write the name.

Method. Add Br2 across the double bond.

  1. Adding. C2H4 + Br2.
    C2H4 + Br2
  2. Product. The product is 1,2-dibromoethane.
    1,2-dibromoethane
  3. Write the answer.
    1,2-dibromoethane

Check. It is a colourless liquid. ✓

Watch out

Watch out: addition products are saturated. Do not write a double bond in the product of an addition reaction.

Exam tip

Exam tip. For an addition question, add the small molecule to each carbon of the double bond.

Exercise 15E  Addition reactions and structure [H]

1
F[H] What is the product when ethene reacts with hydrogen? Write the name.
2
F[H] What catalyst is used in the hydrogenation of ethene? Write the name.
3
F[H] What is the product of ethene and steam? Write the name.
4
F[H] What is the product of ethene and bromine? Write the name.
5
F[H] Is the product of an addition reaction saturated or unsaturated? Write the word.
6
PS[H] Write the formula of the product when ethene adds hydrogen. Write the formula.
7
PS[H] Write the formula of 1,2-dibromoethane. Write the formula.
8
R[H] Explain why an addition reaction makes a saturated product, in one sentence. Accept any sentence that says the double bond is replaced by single bonds with the added atoms.
9
R[H] Explain why ethanol is made from ethene by addition rather than from ethane, in one sentence. Accept any sentence that says ethene has a double bond that can react with steam, while ethane cannot add water.

Chapter summary

SkillMethod
Crude oilA mixture of hydrocarbons; separated by fractional distillation by boiling point.
AlkanesCnH2n+2; saturated; trends: boiling point and viscosity rise, flammability falls.
CombustionComplete: CO2 and H2O; incomplete: CO and soot.
CrackingLong alkane -> shorter alkane + alkene; heat and catalyst.
AlkenesCnH2n; C=C double bond; decolourise bromine water.
[H] AdditionAtoms add across the C=C double bond; product is saturated.

Can you…?

  • I can describe fractional distillation of crude oil and the uses of the fractions
  • I can use the general formula of the alkanes and describe their trends
  • I can write equations for complete and incomplete combustion
  • I can explain cracking and identify an alkene by the bromine water test
  • [H] I can write word equations for addition reactions of alkenes

Key terms

hydrocarbon
a compound made only of carbon and hydrogen
fraction
a group of hydrocarbons with similar boiling points, separated from crude oil
homologous series
a family of compounds with the same general formula and similar properties
saturated
a hydrocarbon with only single bonds
unsaturated
a hydrocarbon with at least one double bond
cracking
breaking long-chain alkanes into shorter alkanes and alkenes
addition reaction
[H] a reaction in which a small molecule adds across a double bond

Exercise 15Z  Check-up

1
What is the formula of octane? Write the formula.
2
What is the general formula of the alkenes? Write the formula.
3
Name the process that separates crude oil into fractions. Write the name.
4
What is the colour change in the bromine water test for an alkene? Write the words.
5
What gas is formed in incomplete combustion of hydrocarbons that is poisonous? Write the name.
6
Name the process that breaks long-chain alkanes into shorter molecules. Write the word.
7
Write the molecular formula of the product of cracking C10H22 into C5H12 and an alkene with 5 carbons. Write the alkene formula.
8
[H] What is the product of ethene and steam? Write the name.
9
Explain why alkanes have lower boiling points than alkanes with longer chains, in one sentence. Accept any sentence that says shorter molecules have weaker intermolecular forces.
10
Explain why complete combustion produces water as well as carbon dioxide, in one sentence. Accept any sentence that says the hydrogen in the hydrocarbon is oxidised to water.

Total 21 marks. Allow about 21 minutes.

Exercise 15X  Exam-style questions

1
FName the fraction that is used as jet fuel, and state the position it condenses in the column (near the top or near the bottom). Write two answers.[2 marks]
2
FWrite the general formula of the alkanes, and give the formula of an alkane with 4 carbon atoms. Write two formulae.[2 marks]
3
RExplain why incomplete combustion is dangerous in a closed room, in two sentences. Accept any answer that says carbon monoxide is made, which is poisonous and binds to haemoglobin.[3 marks]
4
FDescribe the test for an alkene, including the colour change, in one sentence. Accept any sentence that says orange bromine water turns colourless with an alkene.[2 marks]
5
RExplain why cracking is carried out, in one or two sentences. Accept any answer that says there is more demand for short-chain molecules than the heavy fraction provides, and cracking makes useful alkanes and alkenes.[3 marks]
6
FName the products of complete combustion of a hydrocarbon. Write two names.[2 marks]
7
F[H] Write the word equation for the reaction of ethene with bromine. Write the product name.[2 marks]
8
PSDecane (C10H22) is cracked into two molecules with 5 carbons each. Write the formulae of the two products, alkane first then alkene.[3 marks]
9
RExplain why a pupil would expect the boiling point of pentane to be higher than that of butane, in one sentence. Accept any sentence that says pentane has a longer chain and stronger intermolecular forces.[2 marks]
← Chapter 14: Reversible reactions, equilibrium and the Haber processChapter 16: Alcohols, carboxylic acids and polymers →

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