Cracking and Alkenes

Fractional distillation produces more of some fractions than we can sell and less of others than we need. There is far more demand for small-molecule fuels such as petrol than distillation supplies, while the heavy fractions of long-chain hydrocarbons are produced in excess.

Cracking solves this mismatch by breaking the large, less useful molecules into smaller, more useful ones.

Cracking breaks large hydrocarbon molecules into smaller ones. It converts an excess of long-chain hydrocarbons into the short-chain molecules that are in high demand as fuels.

Cracking is a thermal decomposition reaction: heat splits one big molecule into two or more smaller molecules. Unlike fractional distillation, it is a genuine chemical change, because covalent bonds within the molecules are broken and new molecules are formed.

There are two industrial methods, and you need to describe the conditions of each in general terms.

Method	Conditions
Catalytic cracking	Heat the long-chain hydrocarbon to vaporise it, then pass the vapour over a hot catalyst.
Steam cracking	Vaporise the hydrocarbon, mix it with steam and heat to a very high temperature.

In both methods the starting hydrocarbon must first be heated until it turns into a vapour. Catalytic cracking uses a hot catalyst to speed up the breakdown; steam cracking mixes the vapour with steam at a very high temperature. You are not expected to give exact temperatures or the name of the catalyst, only the general conditions above.

Cracking a large alkane produces a mixture of smaller molecules. These products include:

• smaller alkanes (saturated, general formula CₙH₂ₙ₊₂), useful as fuels, and
• alkenes (unsaturated, containing a C=C double bond, general formula CₙH₂ₙ).

An alkene contains a carbon–carbon double bond, written C=C. Because a double bond can open up and add new atoms, alkenes are more reactive than alkanes.

Alkenes are unsaturated: they contain a C=C double bond, so they have two fewer hydrogen atoms than the alkane with the same number of carbons. This double bond makes them more reactive than the saturated alkanes.

Alkenes are extremely useful. They are used to make polymers (such as poly(ethene)) and as starting materials for other chemicals, so cracking is valued as much for producing alkenes as for producing extra fuel.

In a cracking equation, the atoms on the right must add up to the large molecule on the left, because cracking only rearranges atoms. A useful check: the total carbons and total hydrogens must be the same on both sides.

Worked example — a molecule of decane, C₁₀H₂₂, is cracked to make octane, C₈H₁₈, and one alkene. Find the alkene.

Carbons: 10 − 8 = 2 carbons left for the alkene. Hydrogens: 22 − 18 = 4 hydrogens left for the alkene.

So the alkene is C₂H₄ (ethene). The balanced equation is:

${\text{C}}_{10}{\text{H}}_{22}\to {\text{C}}_8{\text{H}}_{18}+{\text{C}}_2{\text{H}}_4$

Check: left has 10 C and 22 H; right has $8+2=10$ C and $18+4=22$ H. Balanced.

Notice the alkene fits CₙH₂ₙ (C₂H₄ has 2 carbons and 4 hydrogens), while the alkane fits CₙH₂ₙ₊₂. Cracking a saturated molecule always leaves at least one unsaturated alkene among the products.

Worked example — octane, C₈H₁₈, is cracked to give hexane, C₆H₁₄, and one other product. Identify it and write the equation.

Carbons: 8 − 6 = 2. Hydrogens: 18 − 14 = 4. The other product is C₂H₄, ethene.

${\text{C}}_8{\text{H}}_{18}\to {\text{C}}_6{\text{H}}_{14}+{\text{C}}_2{\text{H}}_4$

Check: 8 C and 18 H on the left; $6+2=8$ C and $14+4=18$ H on the right. Balanced.

Sometimes more than one small molecule is produced. For example, one long chain might crack into a shorter alkane plus two alkene molecules. Whatever the split, count carbons and hydrogens on both sides and make sure they match before writing the answer.

Because alkenes have a reactive C=C double bond and alkanes do not, a simple test tells them apart: bromine water.

Bromine water is an orange solution. When it is shaken with an alkene, the double bond reacts with the bromine in an addition reaction, and the orange colour disappears. When it is shaken with an alkane, nothing happens and it stays orange.

Bromine water test: an alkene turns orange bromine water colourless; an alkane leaves it orange (no change). The colour change from orange to colourless is the positive result for a C=C double bond.

Substance	Bromine water result
Alkene (e.g. ethene)	orange → colourless
Alkane (e.g. ethane)	stays orange (no change)

Learn the exact colour change. Writing "clear" instead of "colourless" is a common slip, because orange bromine water is already a clear (see-through) solution; the change that matters is the loss of the orange colour.

1. Describing bromine water going "clear" instead of colourless

The orange bromine water is already clear (transparent). The positive result for an alkene is that it goes colourless, meaning it loses its orange colour. Say colourless, not clear.

2. Getting the bromine water result the wrong way round

The alkene decolourises bromine water; the alkane leaves it orange. If asked to identify which tube contains the alkene, it is the one that turned colourless.

3. Saying cracking makes only fuels

Cracking produces both smaller alkanes (fuels) and alkenes. The alkenes are just as valuable, because they are used to make polymers and other chemicals.

4. Producing a cracking equation that does not balance

Every carbon and hydrogen in the large molecule must reappear in the products. Check that total C and total H are equal on both sides; a product cannot simply be dropped.

5. Confusing cracking with fractional distillation

Fractional distillation is a physical separation that makes no new molecules. Cracking is a chemical reaction that breaks bonds and forms new, smaller molecules including alkenes.