Cracking

Section 1 of 7Alkanes and alkenes

Saturated alkanes

• Saturated alkanes Alkanes contain only single carbon–carbon bonds, so each carbon holds as many hydrogen atoms as it can.
• General formulaC_nH_2n+2, for example hexane C₆H₁₄.

Unsaturated alkenes

• Unsaturated alkenes Alkenes contain a C=C double bond, so they hold fewer hydrogen atoms than the alkane with the same number of carbons.
• General formulaC_nH_2n, for example ethene C₂H₄.

Why the difference matters

• Why the difference matters Short-chain alkanes are runny and burn easily, so they make good fuels.
• Alkenes are far more reactive than alkanes and are the starting point for making polymers and alcohols.

Section 2 of 7Why cracking is needed

1Crude oil is fractionated

• Crude oil is fractionated Fractional distillation separates crude oil into fractions, but the proportions are fixed by the oil's composition.

2Supply does not match demand

• Supply does not match demand There is more of the long-chain, heavy fractions (fuel oil, bitumen) than customers want to buy.
• There is less of the small, light fractions (petrol, kerosene) than the market demands.

3Cracking converts surplus

• Cracking converts surplus Large molecules are broken into smaller, more useful ones, so the surplus is turned into high-demand products.

4Extra useful products

• Extra useful products Alkenes and sometimes hydrogen are made as well, supplying the chemical industry.

Section 3 of 7Two methods of cracking

Feature	Catalytic cracking	Steam cracking
Conditions	Hydrocarbon vapour at roughly 450–700 °C passed over a hot catalyst	Hydrocarbon vapour mixed with steam and heated to a very high temperature
Catalyst	Powdered aluminium oxide (alumina) or silica	No solid catalyst; the steam dilutes the hydrocarbon
Type of reaction	Thermal decomposition: covalent bonds break on the catalyst surface	Thermal decomposition at a higher temperature
Typical products	A mixture of shorter alkanes and alkenes	Mainly alkenes (and hydrogen)

Section 4 of 7Carrying out cracking in the laboratory

1

Soak mineral wool in a liquid alkane such as paraffin and place it at one end of a hard-glass test tube.

2

Place broken porcelain or aluminium oxide pieces in the middle of the tube and clamp the tube almost horizontally.

3

Heat the catalyst strongly, then warm the mineral wool so that vapour passes over the hot catalyst.

4

Collect the gaseous products over water in a test tube; the delivery tube must be removed from the water before heating stops, otherwise water sucks back.

5

Test the gas with bromine water: it turns from orange to colourless if an alkene has formed.

Section 5 of 7Writing cracking equations

Rule

• Rule Count atoms on both sides: the total carbon and hydrogen must be equal, and the products must be an alkane plus one or more alkenes.

Example 1

• Example 1 Decane to octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Example 2

• Example 2 Hexane to butane and ethene: C₆H₁₄ → C₄H₁₀ + C₂H₄.

Energy

• Energy Cracking is endothermic: energy is needed to break the strong covalent bonds.

Section 6 of 7Key terms

Term	Meaning
Cracking	The thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules.
Saturated	A hydrocarbon with only single carbon–carbon bonds.
Unsaturated	A hydrocarbon containing at least one C=C double bond.
Catalyst	A substance that speeds up a reaction but is not used up.
Fraction	A mixture of hydrocarbons with similar boiling points separated from crude oil.
Thermal decomposition	A compound breaking down into simpler substances when heated.

Section 7 of 7Exam technique

1

Saying cracking removes carbon atomsthe atoms are only rearranged, so the equation must balance for both C and H.

2

Writing a cracking equation with two alkanes as products: one product must be an alkene (C_nH_2n).

3

Calling cracking exothermicit requires a continuous energy supply, so it is endothermic.

4

Describing bromine water as turning 'clear' for an alkene: it decolourises from orange to colourless, and a plain alkane leaves it orange.