C24: Cracking and Alkenes
Why cracking is needed to break long-chain alkanes into shorter, more useful hydrocarbons, the structure and reactions of alkenes, and testing for unsaturation.
Why Cracking is Needed
Crude oil contains more long-chain hydrocarbons than are needed and not enough short-chain hydrocarbons. Cracking breaks long-chain alkanes into shorter, more useful molecules including short-chain alkanes and alkenes.
The demand for short-chain hydrocarbons (like petrol) is higher than the supply from fractional distillation. Cracking converts the surplus long-chain fractions (like fuel oil and bitumen) into more valuable short-chain products.
- Short-chain alkanes are useful as fuels
- Alkenes are useful as feedstock for making polymers and other chemicals
Cracking is needed because supply of short-chain hydrocarbons from fractional distillation does not meet demand, and there is a surplus of long-chain fractions. Cracking solves both problems at once.
Types of Cracking
Catalytic Cracking
Catalytic cracking uses a silica-alumina catalyst at a moderate temperature (approximately 450 °C). The vapourised long-chain alkane is passed over the hot catalyst, which breaks it into shorter molecules.
Steam Cracking
Steam cracking uses a higher temperature (approximately 800 °C) with steam. The long-chain alkane is mixed with steam and heated very briefly, then rapidly cooled (quenched) to prevent further reactions.
| Property | Catalytic Cracking | Steam Cracking |
|---|---|---|
| Temperature | ~450 °C (moderate) | ~800 °C (high) |
| Catalyst | Silica-alumina | None |
| Steam used | No | Yes |
| Main products | Short alkanes, branched alkanes, some alkenes | More alkenes (including ethene) |
| Conditions | Vapour passed over hot catalyst | Vapour mixed with steam at high temperature |
Cracking Equations
Cracking produces a shorter alkane and an alkene. The number of carbon and hydrogen atoms must be the same on both sides of the equation.
Worked Example: Writing cracking equations
Question: Dodecane (C₁₂H₂₆) is cracked to produce ethene and one other product. Write the equation.
Answer: C₁₂H₂₆ → C₂H₄ + C₁₀H₂₂
Check: C: 12 = 2 + 10 ✓ H: 26 = 4 + 22 ✓
The other product is decane (C₁₀H₂₂), an alkane.
Worked Example: Cracking with two products
Question: Octane (C₈H₁₈) is cracked to produce one mole of ethene and one mole of butane. Write the equation.
Answer: C₈H₁₈ → C₂H₄ + C₆H₁₄
Check: C: 8 = 2 + 6 ✓ H: 18 = 4 + 14 ✓
Products: ethene (C₂H₄, an alkene) and hexane (C₆H₁₄, an alkane).
Worked Example: Identifying products
Question: C₁₄H₃₀ → C₃H₆ + C₁₁H₂₄. Identify each product as an alkane or alkene.
Answer: C₃H₆ — check against alkane formula CₙH₂ₙ₊₂: 2(3)+2 = 8, not 6. So C₃H₆ fits CₙH₂ₙ, meaning it is an alkene (propene).
C₁₁H₂₄ — check against alkane formula: 2(11)+2 = 24. Yes, C₁₁H₂₄ is an alkane (undecane).
Alkenes
Alkenes are hydrocarbons that contain a carbon-carbon double bond (C=C). They are unsaturated hydrocarbons because they contain fewer hydrogen atoms than the corresponding alkane.
The general formula for alkenes is:
CₙH₂ₙ
The first four members of the alkene homologous series:
| Name | Molecular formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
| Pentene | C₅H₁₀ |
The C=C double bond makes alkenes much more reactive than alkanes. It is the functional group of alkenes and is the site where addition reactions occur.
| Property | Alkanes | Alkenes |
|---|---|---|
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Type of bond | C−C single bonds only | Contains C=C double bond |
| Saturation | Saturated | Unsaturated |
| Reactivity | Relatively unreactive | More reactive (due to C=C) |
| Type of reaction | Substitution | Addition |
| Bromine water test | Stays orange | Turns colourless |
Addition Reactions of Alkenes
In an addition reaction, the C=C double bond opens up and an atom or group is added to each carbon. Only one product is formed.
Addition of Hydrogen (Hydrogenation)
Alkene + hydrogen → alkane
Conditions: 60 °C, nickel catalyst
Worked Example: Hydrogenation
C₂H₄ + H₂ → C₂H₆
Ethene + hydrogen → ethane
The C=C double bond opens and one hydrogen atom bonds to each carbon. The product is a saturated alkane.
Addition of Halogens
Alkene + halogen → dihalogenoalkane
Worked Example: Addition of bromine
C₂H₄ + Br₂ → C₂H₄Br₂
Ethene + bromine → 1,2-dibromoethane
The reddish-brown bromine is decolourised as it reacts with the alkene. This is the basis of the bromine water test.
Addition of Steam (Hydration)
Alkene + steam → alcohol
Conditions: high temperature, high pressure, phosphoric acid catalyst
Worked Example: Hydration of ethene
C₂H₄ + H₂O → C₂H₅OH
Ethene + steam → ethanol
This is an industrial method for producing ethanol.
Addition of Hydrogen Halides
Worked Example: Addition of HBr
C₂H₄ + HBr → C₂H₅Br
Ethene + hydrogen bromide → bromoethane
The Bromine Water Test
Bromine water is an orange-brown solution. When it is shaken with an alkene, the C=C double bond reacts with the bromine and the solution turns colourless. With an alkane, the bromine water stays orange-brown.
This test distinguishes between saturated and unsaturated hydrocarbons:
- Alkene (unsaturated): bromine water is decolourised (orange → colourless)
- Alkane (saturated): bromine water stays orange
Worked Example: Identifying unknown hydrocarbons
Question: Two colourless liquids, A and B, are tested with bromine water. Liquid A decolourises bromine water. Liquid B does not. Which is an alkene?
Answer: Liquid A is the alkene because it decolourises bromine water. The C=C double bond in A reacts with bromine in an addition reaction, removing the orange colour. Liquid B is an alkane because it does not react with bromine water.
Always state both observations when using the bromine water test: "The alkene decolourises bromine water (orange to colourless) while the alkane does not." Do not just say "it changes colour."
Complete Combustion of Alkenes
Alkenes undergo complete combustion in oxygen to produce carbon dioxide and water, just like alkanes. However, alkenes produce a smokier flame than alkanes because they have a higher carbon-to-hydrogen ratio.
CₙH₂ₙ + 3n/2 O₂ → nCO₂ + nH₂O
Worked Example: Combustion of ethene
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
Practice Questions
1. Explain why cracking is necessary in the oil industry. (3 marks)
There is a surplus of long-chain hydrocarbons from fractional distillation but a high demand for short-chain hydrocarbons such as petrol. Cracking converts long-chain alkanes into shorter, more useful alkanes and alkenes. The alkenes produced are also valuable as chemical feedstock for making polymers.
2. Write an equation for the cracking of C₁₄H₃₀ that produces ethene and one other product. (2 marks)
C₁₄H₃₀ → C₂H₄ + C₁₂H₂₆. The other product is dodecane (an alkane).
3. Describe how you would use bromine water to distinguish between hexane and hexene. (3 marks)
Add bromine water to each hydrocarbon and shake. Hexene is an alkene with a C=C double bond, so it decolourises the orange bromine water (turns colourless). Hexane is a saturated alkane with no C=C bond, so the bromine water stays orange.
4. Write the equation for the addition reaction between propene (C₃H₆) and hydrogen. Name the product and state the conditions. (3 marks)
C₃H₆ + H₂ → C₃H₈. The product is propane. Conditions: 60 °C with a nickel catalyst.
5. Compare catalytic cracking and steam cracking. Include two differences in your answer. (2 marks)
Catalytic cracking uses a moderate temperature (~450 °C) with a silica-alumina catalyst, while steam cracking uses a higher temperature (~800 °C) with steam and no catalyst. Catalytic cracking produces mainly short alkanes and some alkenes, while steam cracking produces more alkenes including ethene.
Maths Skills
Balancing Cracking Equations
In cracking, a long-chain alkane breaks down into a shorter alkane and an alkene (or sometimes multiple products). The number of atoms of each element must be the same on both sides of the equation.
Step-by-step method:
- Write the formula of the long-chain alkane on the left.
- Write the products you know on the right.
- Count the C and H atoms on each side and balance.
Example: C₁₄H₃₀ → C₈H₁₈ + ?
Left side: 14 C, 30 H. Right side so far: 8 C, 18 H.
Remaining: 14 − 8 = 6 C, 30 − 18 = 12 H.
Ratio H:C = 12:6 = 2:1, so the alkene is C₆H₁₂ (hexene, CₙH₂ₙ).
Full equation: C₁₄H₃₀ → C₈H₁₈ + C₆H₁₂
Check: Left: 14C, 30H. Right: 8+6=14C, 18+12=30H. ✓
Common Misconceptions
Misconceptions About Cracking and Alkenes
Cracking is the same as fractional distillation — both separate mixtures.
Cracking is a chemical reaction that breaks covalent bonds in long-chain alkanes to produce shorter molecules (alkanes and alkenes). Fractional distillation is a physical separation technique that separates existing compounds based on boiling points without breaking any bonds.
Alkenes are saturated because they have more bonds than alkanes.
Alkenes are unsaturated because they contain a C=C double bond — they do not have the maximum possible number of hydrogen atoms. Alkanes are saturated because they only have single bonds and contain the maximum number of hydrogen atoms.
Cracking always produces just one alkane and one alkene.
Cracking can produce various combinations of shorter alkanes and alkenes, and sometimes hydrogen gas is also produced. The specific products depend on the conditions and the starting alkane.
6-Mark Question
Explain why cracking is necessary and describe the differences between alkanes and alkenes.
Cracking is necessary because crude oil contains more long-chain hydrocarbons than are needed but not enough short-chain hydrocarbons. The demand for short-chain alkanes (e.g. petrol) and alkenes (e.g. ethene for polymers) is much higher than the supply from fractional distillation alone. Cracking converts surplus long-chain fractions into useful shorter molecules. Alkanes are saturated hydrocarbons with only single C—C bonds and have the general formula CₙH₂ₙ₊₂. They are relatively unreactive and are mainly used as fuels. Alkenes are unsaturated hydrocarbons containing a C=C double bond with the general formula CₙH₂ₙ. The double bond makes alkenes much more reactive than alkanes — they undergo addition reactions, such as with bromine water (which turns from orange to colourless), and they are used as monomers to make polymers through addition polymerisation. Alkenes can be distinguished from alkanes using the bromine water test.
AO3: Analysis and Evaluation
Comparing Cracking Methods
Catalytic cracking uses a catalyst at ~450 °C. Steam cracking uses high temperatures (~800 °C) with steam and no catalyst.
Evaluate the advantages and disadvantages of each method, considering yield, energy costs, and the types of products formed.
Answer: Catalytic cracking is more energy-efficient because it uses lower temperatures, saving on fuel costs. The catalyst speeds up the reaction, providing a good yield of short alkanes and some alkenes. However, catalysts are expensive and can be poisoned by impurities. Steam cracking produces more alkenes (especially ethene and propene), which are valuable for the polymer industry, but the high temperature means much higher energy costs and more energy is wasted as heat. Steam cracking does not require a catalyst, so there is no risk of catalyst poisoning and no catalyst replacement costs. The choice depends on whether the refinery needs more alkenes (steam cracking) or more short-chain alkanes (catalytic cracking).
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