OxfordAQA A-Level Chemistry: Alkanes (9620)

Here you will learn section 3.3.2 Alkanes from the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. It covers all four points: 3.3.2.1 on fractional distillation of crude oil, 3.3.2.2 on cracking, 3.3.2.3 on combustion and 3.3.2.4 on the chlorination of methane. The whole section is International AS, so none of it is International A-level only. It belongs to Unit 2 (Organic 1 and Physical 1), and Unit 5 (Practical and synoptic) can draw on any content.

When you have worked through this page, condense it with the alkanes revision notes and test yourself on the alkanes practice questions. Every 9620 topic sits on the course hub, and the printable checklist has a line for each point. A free 10-minute diagnostic will show you which areas need the most work.

Background you will use: van der Waals forces from shapes, polarity and intermolecular forces, moles and pV = nRT from amount of substance, and enthalpy of combustion from energetics.

What this unit covers

Point (all International AS)	You should be able to
3.3.2.1	State that alkanes are saturated hydrocarbons
3.3.2.1	Describe petroleum as a mixture, mainly of alkanes, that fractional distillation separates
3.3.2.2	State that cracking breaks C–C bonds in alkanes
3.3.2.2	Give the conditions and main products of thermal and of catalytic cracking (no mechanisms)
3.3.2.2	Explain the economic reasons for cracking alkanes
3.3.2.3	Write equations for complete and incomplete combustion of alkanes and other organic compounds
3.3.2.3	Name the engine pollutants NOₓ, CO, carbon and unburned hydrocarbons, and say how catalytic converters remove the gases
3.3.2.3	Link sulfur in fuels to SO₂; say why calcium oxide or calcium carbonate can strip SO₂ from flue gases
3.3.2.4	Explain methane + chlorine as free-radical substitution: initiation, propagation, termination

Section 3.3.2 lists no required practical activity.

Crude oil and fractional distillation (3.3.2.1)

Alkanes are the main constituent of crude oil, an important raw material for the chemical industry.

Two words define an alkane. Hydrocarbon: made of carbon and hydrogen atoms and nothing else. Saturated: every carbon–carbon bond is a single bond. Chain alkanes fit CₙH₂ₙ₊₂, so the twelve-carbon alkane is C₁₂H₂₆ and the nine-carbon one is C₉H₂₀.

Petroleum (crude oil) is a mixture made up mainly of alkanes with a wide spread of chain lengths. Fractional distillation splits it into fractions: groups of hydrocarbons whose boiling points fall within a narrow range.

What happens in the column

• Crude oil is heated in a furnace, and the hot mixture of vapour and liquid is fed into the base of a tall column.
• The column is hot at the base and gets steadily cooler towards the top.
• As vapour moves upward it cools. A compound turns back to liquid at the first level that is cooler than its boiling point, and trays at that level draw it off.
• Small molecules (low boiling points) come off near the top; large ones condense near the base. The heaviest material never boils and leaves as a residue.

Why bigger alkanes boil higher. Alkane molecules are non-polar, so the only forces between them are van der Waals forces. Each extra CH₂ group adds electrons and increases the area over which two molecules touch. Stronger induced-dipole attractions take more energy to overcome, so the boiling point rises with chain length. Nothing inside a molecule breaks when it boils. Distillation is a physical separation, and that is what sets it apart from cracking.

Cracking (3.3.2.2)

Cracking splits large alkane molecules into smaller ones by breaking C–C bonds. A long alkane does not hold enough hydrogen for all the fragments to be alkanes. So a cracking reaction gives at least one alkene (CₙH₂ₙ), usually alongside a shorter alkane.

The specification describes two types. Learn the wording for each.

• Thermal cracking: high pressure and high temperature. It gives a high percentage of alkenes.
• Catalytic cracking: a slight pressure, high temperature and a zeolite catalyst. It is used mainly to make motor fuels and aromatic hydrocarbons.

Mechanisms are not required for either type.

Worked example 1: writing and completing cracking equations

(a) Dodecane, C₁₂H₂₆, is cracked. Each molecule gives one molecule of hexane and propene only. Write the equation.

Hexane is the alkane:   C6H14
Carbon left over:       12 − 6 = 6, shared as 2 × C3H6 (propene)
Hydrogen check:         14 + 2 × 6 = 26   matches C12H26

C12H26 → C6H14 + 2C3H6

(b) Tridecane, C₁₃H₂₈, cracks to give heptane, ethene and one other product, X. Find X.

C13H28 → C7H16 + C2H4 + X
Carbon in X:    13 − 7 − 2 = 4
Hydrogen in X:  28 − 16 − 4 = 8
X = C4H8, which fits CnH2n, so X is butene

Subtract carbon and hydrogen separately, then check each formula against CₙH₂ₙ₊₂ or CₙH₂ₙ.

Economic reasons for cracking

1. Distillation produces fixed proportions of each fraction, set by the crude oil used.
2. Those proportions do not match what customers buy. There is more of the heavy, long-chain fractions than the market takes, and not enough of the lighter fractions used as motor fuels.
3. Light fractions sell for more because demand for them is high. Cracking turns surplus heavy material into more of these higher-value products.
4. Cracking also gives alkenes. They react far more readily than alkanes, so they are valuable feedstocks for polymers and other chemicals.
5. Catalytic cracking in particular yields motor fuels and aromatic hydrocarbons, both of which are in demand.

Combustion of alkanes (3.3.2.3)

Alkanes serve as fuels. Alkanes and other organic compounds may burn completely or incompletely.

• With enough oxygen, combustion is complete: all the carbon becomes CO₂ and all the hydrogen becomes H₂O.
• With too little oxygen, combustion is incomplete: some carbon ends up as carbon monoxide (CO) or as carbon particles (soot). Water still forms.

Worked example 2: balancing combustion equations

Write equations for (a) complete combustion of nonane, (b) incomplete combustion of nonane to carbon monoxide and water, and (c) complete combustion of propanone, C₃H₆O.

Order: carbon, then hydrogen, then oxygen.

(a) C9H20 → 9CO2 + 10H2O
    O atoms needed: 9 × 2 + 10 = 28, so 14 O2
    C9H20 + 14O2 → 9CO2 + 10H2O

(b) C9H20 → 9CO + 10H2O
    O atoms needed: 9 + 10 = 19, so 9½ O2
    C9H20 + 9½O2 → 9CO + 10H2O

(c) C3H6O → 3CO2 + 3H2O
    O atoms in products: 6 + 3 = 9; the fuel supplies 1, so 8 come from O2
    C3H6O + 4O2 → 3CO2 + 3H2O

Part (c) shows the trap with oxygen-containing compounds: subtract the oxygen already in the molecule.

Worked example 3: masses and gas volumes from combustion

3.20 g of nonane burns completely. Calculate (a) the mass of water formed and (b) the volume of carbon dioxide formed at 100 kPa and 300 K. (Take R = 8.31 J K⁻¹ mol⁻¹ and relative atomic masses H 1.0, C 12.0, O 16.0.)

Mr(C9H20) = 9 × 12.0 + 20 × 1.0 = 128.0
n(C9H20)  = 3.20 ÷ 128.0 = 0.0250 mol

(a) n(H2O) = 10 × 0.0250 = 0.250 mol
    mass   = 0.250 × 18.0 = 4.50 g

(b) n(CO2) = 9 × 0.0250 = 0.225 mol
    V = nRT ÷ p = 0.225 × 8.31 × 300 ÷ 100 000
      = 5.61 × 10⁻³ m³ = 5.61 dm³

Pressure in pascals gives the volume in m³; multiply by 1000 for dm³.

What engines emit

Learn where each of the four engine pollutants comes from.

• NOₓ (NO and NO₂). The nitrogen comes from the air, not the fuel. At the very high temperature in the cylinder, N₂ and O₂ combine: N₂ + O₂ → 2NO. Nitrogen monoxide then oxidises further in air: 2NO + O₂ → 2NO₂.
• Carbon monoxide. Incomplete combustion when oxygen runs short in parts of the cylinder. It is toxic.
• Carbon. Soot particles, also from incomplete combustion.
• Unburned hydrocarbons. Fuel that leaves the engine without burning.

Catalytic converters in the exhaust remove the gaseous pollutants. Exhaust gases pass over a hot catalyst surface, where they react to give carbon dioxide, nitrogen and water.

2CO + 2NO → 2CO2 + N2
C10H22 + 31NO → 10CO2 + 11H2O + 15½N2

CO and the hydrocarbon are oxidised; NO is reduced to N₂. In the second equation, 10 CO₂ and 11 H₂O need 31 O atoms, all supplied by NO.

Sulfur dioxide

Some hydrocarbon fuels contain sulfur compounds. When they burn, the sulfur is oxidised to sulfur dioxide, which causes air pollution:

S + O2 → SO2

Power stations remove SO₂ from flue gases with calcium oxide or calcium carbonate. The reason is acid–base chemistry. SO₂ is an acidic oxide. Calcium oxide is a basic oxide, and calcium carbonate is a base that reacts with acids. The SO₂ is neutralised and locked up as solid calcium sulfite.

CaO + SO2 → CaSO3
CaCO3 + SO2 → CaSO3 + CO2

Flue-gas mass calculations belong to Group 2 chemistry and are worked on the Group 2 study guide.

Chlorination of methane (3.3.2.4)

In ultraviolet light, methane and chlorine react. A chlorine atom takes the place of a hydrogen atom, so this is substitution. The reactive particles are radicals, species with an unpaired electron, so the mechanism is free-radical substitution.

Overall reaction:   CH4 + Cl2 → CH3Cl + HCl

Section 3.3.1.2 of the specification sets the notation: show the unpaired electron as a dot; curly arrows are not required for radical mechanisms. Each step needs its own balanced equation.

Initiation

The energy in UV light breaks the Cl–Cl bond homolytically: each atom takes one of the two shared electrons.

Cl2 → 2Cl•

Propagation

Each propagation step uses up one radical and makes another, so the sequence keeps itself going.

CH4 + Cl• → HCl + •CH3
•CH3 + Cl2 → CH3Cl + Cl•

The Cl• released in the second step feeds the first, so one initiation can make many product molecules.

Termination

Two radicals meet and pair their electrons. Radicals are lost, and the chain stops.

2Cl• → Cl2
Cl• + •CH3 → CH3Cl
2•CH3 → C2H6

Worked example 4: a mixture of products

A sample made by chlorinating methane contains trichloromethane, CHCl₃, and a trace of chloroethane, CH₃CH₂Cl. Explain how each forms, with equations.

Trichloromethane: Cl• attacks dichloromethane formed earlier
  CH2Cl2 + Cl• → HCl + •CHCl2
  •CHCl2 + Cl2 → CHCl3 + Cl•

Chloroethane: termination between two different carbon radicals
  •CH3 + •CH2Cl → CH3CH2Cl

Substitution does not stop at one chlorine. Any C–H bond still present can be attacked, so the product is a mixture. Chloroethane has two carbons, so it can only come from two carbon radicals joining.

Checking a step: propagation has one radical on each side; termination has two on the left and none on the right; initiation has none on the left.

Common errors

• Saying distillation “breaks down” crude oil. Bonds break only in cracking.
• Linking boiling point to chain length but never naming van der Waals forces.
• Giving catalytic cracking a high pressure, or giving thermal cracking a zeolite.
• Writing the alkene in a cracking equation as CₙH₂ₙ₊₂, so hydrogen will not balance.
• Tracing the N in NOₓ to the fuel rather than to air drawn into the engine.
• Writing that CaO “soaks up” SO₂, with no mention of a base reacting with an acidic oxide.
• Putting the dot on the wrong atom, making H• instead of HCl, or leaving out UV light.

Where to go next

Next, use the alkanes revision notes, then the alkanes practice questions. For chlorine itself, see Group 7(17), the halogens.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. Section 3.3.2 Alkanes, points 3.3.2.1 to 3.3.2.4, headed: Fractional distillation of crude oil; Modification of alkanes by cracking; Combustion of alkanes; Chlorination of alkanes.