Modification of alkanes by cracking — AQA A-Level Chemistry
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Modification of alkanes by cracking explained
Fractional distillation delivers more long chain alkane than anyone wants and less petrol than everyone wants, so the long chains are broken up.
Read the full explanation
Cracking snaps C–C bonds in a large alkane to give smaller molecules, and because the fragments have to share out the hydrogen that was there, at least one product is normally unsaturated. Equations must balance exactly: C₁₄H₃₀ → C₈H₁₈ + C₆H₁₂ is one route, and C₁₄H₃₀ → C₈H₁₈ + 2C₃H₆ is another that a mark scheme accepts for the same starting material. The short alkanes go into motor fuel, while the alkenes are the raw material for poly(ethene), poly(propene) and much of the rest of the organic chemical industry.
Thermal cracking takes place at high pressure and high temperature and produces a high percentage of alkenes (mechanism not required).
Thermal cracking uses heat and pressure alone: temperatures around 1000 K and pressures up to about 7000 kPa. The severe conditions break C–C bonds homolytically, so the mechanism is free-radical, although the specification does not require you to write it out. What matters is the product mix, because thermal cracking gives a high percentage of alkenes, especially ethene and propene, which feed the polymer industry. The residence time is deliberately short, around one second, because the products are more reactive than the feedstock and would otherwise react further. Contrast this with catalytic cracking, which uses a much lower pressure and a zeolite catalyst and is aimed at motor fuels rather than at alkenes.
Catalytic cracking takes place at a slight pressure, high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons (mechanism not required).
Catalytic cracking runs at about 720 K and only a slight pressure over a zeolite catalyst, an aluminosilicate honeycomb with a huge surface area and acidic sites. The mechanism goes through carbocations rather than radicals, though you are not required to write it. The products differ in character from those of thermal cracking: branched alkanes, cycloalkanes and aromatic hydrocarbons, all of which burn smoothly in an engine, so this route is aimed at motor fuels. A mark scheme accepts C₁₄H₃₀ → C₈H₁₈ + C₆H₁₂ for cracking tetradecane into octane and a cycloalkane. Because the catalyst permits a lower temperature and a far lower pressure, less energy is needed and the plant is cheaper to run.
Your focus
- Write a balanced equation for the cracking of a named long chain alkane.
- Explain the economic reason for cracking in terms of the supply of and demand for different fractions.
- Identify which bonds break during cracking and why an unsaturated product is usually formed.
Show all 9 objectives
- State the conditions used for thermal cracking with appropriate units.
- Write a balanced equation for a thermal cracking reaction that produces an alkene.
- Compare thermal with catalytic cracking in terms of conditions and product mix.
- Explain why the catalytic route is chosen to make motor fuel, naming the branched alkanes, cycloalkanes and aromatics it produces.
- Write a balanced equation for the catalytic cracking of a named alkane into a fuel-range alkane and a cycloalkane.
- Explain why the catalytic route is cheaper to operate than the thermal route.
Modification of alkanes by cracking exam tips
Marking Points
- one mark for a balanced equation in which carbon and hydrogen atoms tally on both sides
- one mark for at least one unsaturated product where the hydrogen count requires it
- one mark for identifying C–C bonds as the bonds that break
- one mark for an economic reason, such as converting a fraction in low demand into fuels and alkenes in high demand
- one mark for a high temperature of the order of 1000 K
- one mark for a high pressure, up to about 7000 kPa
- one mark for a high percentage of alkenes among the products
- one mark for a balanced equation that shows an alkene as a product
- one mark for a zeolite catalyst together with a slight or moderate pressure
- one mark for a high temperature of about 720 K
- one mark for motor fuels and aromatic hydrocarbons as the main products
- one mark for a balanced equation such as C₁₄H₃₀ → C₈H₁₈ + C₆H₁₂
Examiner Tips
- 💡Balance carbon first, then hydrogen, and let the leftover hydrogen tell you whether an alkene is needed.
- 💡More than one correct equation usually exists, so any balanced answer with sensible products will score.
- 💡Say which product goes to fuel and which to polymers if the question asks about usefulness.
- 💡Learn the two sets of conditions as a contrasting pair so you attach the right one to the right process.
- 💡Quote units with every condition, because a number alone is not a condition.
- 💡Name ethene or propene as the alkene when asked what thermal cracking is used to produce.
- 💡Attach zeolite and slight pressure to catalytic cracking every time, because those two words identify the process.
- 💡Check a cycloalkane product against CₙH₂ₙ before declaring the equation balanced.
- 💡Mention lower energy costs if the question asks for an advantage of the catalytic route.
Common Mistakes
- writing an equation that does not balance, most often by dropping hydrogen atoms
- producing only alkanes from an alkane, which cannot balance unless hydrogen is also a product
- saying C–H bonds are broken in cracking rather than C–C bonds
- confusing cracking with combustion because both involve high temperatures
- quoting the low pressure of catalytic cracking for thermal cracking
- adding a zeolite catalyst to thermal cracking, which by definition uses none
- giving the main products as aromatic hydrocarbons or motor fuels rather than alkenes
- converting carelessly between °C and K when quoting the temperature
- giving high pressure for catalytic cracking, which is the thermal cracking condition
- naming the catalyst as platinum or nickel rather than a zeolite
- stating alkenes as the main product, which is the thermal cracking answer
- writing an unbalanced equation by forgetting that a cycloalkane has the formula CₙH₂ₙ