Cracking of alkanes

Chemistry · Cracking

One molecule, lots of ways to snap

Pick where the decane molecule breaks. Read the products at the end, then try another break point.

Where it breaks

On Decane, C₁₀H₂₂. 5 branches to choose from.

Decane, C₁₀H₂₂, is a long alkane. Heat it so it cracks and it can break in different places. Choose a break point and check the atoms at the end.

Cracking in the lab: follow the alkane

Follow one alkane molecule from the ceramic wool to the test tube.

1. 1SoakThe larger alkane is soaked into ceramic wool.
2. 2HeatThe ceramic wool is heated, so the alkane vaporises.
3. 3Over the catalystThe vapour passes over broken pottery, which acts as the catalyst.
4. 4CrackThe larger alkane cracks into smaller molecules.
5. 5TravelThe cracked products pass through a delivery tube.
6. 6CollectThe products are collected in an inverted test tube by displacing water.+

What you need to know

• Refineries often have too many large hydrocarbons to sell and too few small ones to meet demand.
• Small hydrocarbons make better fuels: they are more volatile, have lower boiling points and are more flammable.
• Have a goYour mate reckons a huge molecule would make better petrol than a tiny one, 'because it's more powerful'. Using the fuel properties above, what do you tell them?

  Small molecules are more volatile, have lower boiling points and are more flammable.

  Evaporating easily and igniting easily are what make a small hydrocarbon a useful fuel, and all three properties go with smaller molecules.

• Cracking is the fix. It's a thermal decomposition reaction: heat breaks larger alkanes into smaller, more useful molecules.
• Fractional distillation is a different job: it separates crude oil by boiling point, while cracking changes the molecular formulae.
• The products are shorter alkanes (useful as fuels) and alkenes (the starting material for polymers and other chemicals).
• Alkanes are saturated, with only single bonds between carbons. Alkenes are unsaturated, with at least one C=C double bond.
• Have a goA hydrocarbon has a C=C double bond somewhere in its carbon chain. Saturated or unsaturated, and alkane or alkene?

  Unsaturated, so it's an alkene.

  Alkenes are unsaturated because they contain at least one carbon–carbon double bond; alkanes have only single bonds.

• A long alkane can break at different points, so cracking the same alkane can give different products.
• No atoms are gained or lost, so the carbon and hydrogen in the products add back to the alkane you cracked.
• Have a goA classmate confidently writes C₁₀H₂₂ → C₈H₁₈ + C₃H₆ and says 'Done, next question!' Check the atoms. Are they right?

  No. Carbon: 8 + 3 = 11, not 10. Hydrogen: 18 + 6 = 24, not 22.

  No atoms are gained or lost, so the products must add back to exactly C₁₀H₂₂; this pair has one carbon and two hydrogens too many.

• To find a product: the alkane follows CₙH₂ₙ₊₂, and the alkene is whatever carbon and hydrogen is left over.

The big picture

Cracking breaks larger alkanes into smaller, more useful molecules: a shorter alkane and an alkene. It's a chemical reaction, not a separation, and because no atoms are gained or lost you can work out any missing product from the alkane you started with.

⚠ Watch out

Using 2n instead of 2n + 2 for the alkane, so C₈H₁₆ instead of C₈H₁₈. That slip makes every leftover wrong too. Always finish by checking that the carbon and hydrogen in the products add back to the alkane you cracked.

Memory hook

Snap a stick of spaghetti in two: nothing vanishes, and the pieces add up to the whole stick. Cracking works the same way with atoms. The analogy stops there, though: the real pieces are a shorter alkane and an alkene.

Check yourself

Cover the page: why isn't cracking just separating crude oil? Then crack C₁₅H₃₂ into an alkane with 8 carbons and an alkene. Write both formulae and check the atoms.