9.4 Cracking Hydrocarbons

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why and how large hydrocarbons are broken down, the alkenes they make, the bromine water test and balancing cracking equations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe cracking and explain why oil companies need to crack large hydrocarbons
• Describe the conditions for catalytic cracking and steam cracking
• Recognise alkenes as unsaturated hydrocarbons with the general formula CnH₂n
• Describe the bromine water test for alkenes
• Balance cracking equations and work out a missing product

Key words

• Cracking: Breaking down large, less useful hydrocarbon molecules into smaller, more useful ones. It is a thermal decomposition reaction.
• Catalytic cracking: Passing hydrocarbon vapour over a hot, powdered catalyst such as aluminium oxide or a zeolite.
• Steam cracking: Mixing hydrocarbon vapour with steam and heating to a very high temperature.
• Alkene: An unsaturated hydrocarbon with a carbon–carbon double bond, C=C. General formula CnH₂n.
• Unsaturated: Contains at least one C=C double bond, so more atoms can be added.
• Bromine water: An orange solution used to test for alkenes. It turns colourless with an alkene.

Why crack hydrocarbons?

Fractional distillation gives more of the long-chain fractions than people need, and not enough of the short-chain fractions such as petrol. Long-chain hydrocarbons are also poor fuels: they are viscous and do not ignite easily.

Cracking turns the surplus long-chain hydrocarbons into smaller alkanes, which are in demand as fuels, and alkenes, which are used to make polymers and as starting materials (feedstock) for other chemicals such as solvents.

Key fact: Modern life depends on hydrocarbons: fuels for transport, heating and electricity, plus solvents, lubricants, detergents and plastics made by the petrochemical industry. Cracking helps make the right amounts of each.

How cracking is done

• Catalytic cracking; Conditions heat the long-chain hydrocarbon to vaporise it; pass the vapour over a hot, powdered catalyst (aluminium oxide or a zeolite) at about 500–550 °C; Notes the catalyst speeds up the reaction; most petrol is made this way
• Steam cracking; Conditions mix the hydrocarbon vapour with steam and heat it to a very high temperature (about 800–900 °C); no catalyst; Notes gives a high proportion of alkenes such as ethene

Both methods are thermal decomposition reactions. Heat is needed because strong covalent C–C bonds inside the large molecules must be broken: the molecules have to gain enough energy to split.

You can crack liquid paraffin in the lab. The paraffin is warmed so its vapour passes over strongly heated aluminium oxide. The small gaseous molecules made are collected over water.

Safety: Before you stop heating, lift the delivery tube out of the water. Otherwise cold water is sucked back into the hot tube and can crack the glass (suck-back). Throw away the first tube of gas: it is mostly air from the apparatus.

Alkenes and the bromine water test

Cracking always makes at least one alkene, because there are not enough hydrogen atoms for every product to be an alkane. Alkenes have a C=C double bond, so they are unsaturated and much more reactive than alkanes. Their general formula is CnH₂n, for example ethene C₂H₄ and propene C₃H₆.

Alkene names end in -ene (ethene, propene, butene); the first part still gives the number of carbon atoms. Because two carbon atoms share a double bond, an alkene has two fewer hydrogen atoms than the alkane with the same number of carbon atoms.

Alkenes also burn in air, but less cleanly than the small alkanes used as fuels: they tend to give a smokier flame. They are far more useful as feedstock than as fuels.

• Shake with orange bromine water; Alkane (e.g. hexane) stays orange; Alkene (e.g. hexene) turns colourless (decolourised)

Exam tip: Say “colourless”, not “clear”. Bromine water is clear already: it is a clear orange solution.

Cracking equations

Atoms are not created or destroyed, so the numbers of C and H atoms on each side must match.

Worked example: finding a missing product

Complete: C₁₅H₃₂ → C₉H₂₀ + 2 ......
Carbon left: 15 − 9 = 6, shared between 2 molecules → 3 each.
Hydrogen left: 32 − 20 = 12, shared between 2 molecules → 6 each.
Each molecule is C₃H₆ (propene). It fits CnH₂n, so it is an alkene.

Answer: C₁₅H₃₂ → C₉H₂₀ + 2C₃H₆

Worked example: three products

Complete: C₁₄H₃₀ → C₈H₁₈ + C₂H₄ + ......
Carbon left: 14 − 8 − 2 = 4. Hydrogen left: 30 − 18 − 4 = 8.
C₄H₈ fits CnH₂n, so it is another alkene (butene).

Answer: C₁₄H₃₀ → C₈H₁₈ + C₂H₄ + C₄H₈

Key fact: A big molecule can crack in many different ways, so one alkane gives a mixture of products. For example C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄ is just one possibility.

Common mistakes

• Saying cracking makes only alkenes. It makes smaller alkanes and alkenes.
• Saying bromine water goes “clear” with an alkene. It goes colourless.
• Forgetting the catalyst for catalytic cracking, or adding a catalyst to steam cracking.
• Not counting the large number in front of a product, e.g. 2C₂H₄ contains 4 C atoms and 8 H atoms.
• Calling cracking a combustion reaction. No oxygen is involved: it is thermal decomposition.
• Giving an alkene the same number of hydrogen atoms as the alkane. Propene is C₃H₆, not C₃H₈.

Revision questions

Try these, then check your answers below each one.

Question 1. Which catalyst is used in catalytic cracking? (Choose from: Iron, Nickel, Aluminium oxide or a zeolite, Platinum.)

Answer: Aluminium oxide or a zeolite

Working: The hydrocarbon vapour is passed over a hot powdered catalyst of aluminium oxide or a zeolite. Iron is used in the Haber process and nickel for adding hydrogen to alkenes.

Question 2. An alkene is shaken with orange bromine water. What colour does the bromine water turn?

Answer: colourless

Working: Alkenes decolourise bromine water: it turns from orange to colourless. An alkane leaves it orange.

Question 3. Complete the cracking equation by finding the number of hydrogen atoms in the missing alkane: C₁₂H₂₆ → C₂H₄ + C₁₀H?

Answer: 22

Working: 26 − 4 = 22, so the other product is C₁₀H₂₂ (decane). It fits CnH₂n₊₂.

Related lessons

• Previous lesson: 9.3 Burning Hydrocarbon Fuels
• Next lesson: 10.1 Reactions of the Alkenes
• In this chapter: 9.1–9.2 Hydrocarbons and Fractional Distillation of Oil

Continue in the full course

This lesson is section 9.4 (AGCCH0904) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.