Polymers and crackingIB MYP Chemistry: Revision notes

Crude oil contains many long-chain alkanes, but there is more demand for shorter molecules such as petrol, and for alkenes to make plastics. Cracking breaks long hydrocarbon molecules into shorter, more useful ones.

Cracking uses a high temperature (about 400 °C to 700 °C) and a catalyst, such as zeolite or aluminium oxide. The products are a shorter alkane and an alkene:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

To balance a cracking equation, the number of carbon atoms and hydrogen atoms must be the same on both sides.

Alkenes have the general formula CₙH₂ₙ, for example ethene C₂H₄ and propene C₃H₆. They are unsaturated because they have a C=C double bond, and each carbon in the double bond could form another bond. This makes alkenes more reactive than alkanes, which are saturated (only single bonds).

The double bond is what allows alkenes to join together to make polymers.

In addition polymerisation, many small monomers join to form a long chain called a polymer. The C=C bond in each monomer opens, and the monomers add to each other. Only one product forms.

n CH₂=CH₂ → (-CH₂-CH₂-)ₙ

ethene (the monomer) → poly(ethene) (the polymer)

The letter n stands for a very large number, often thousands. The part in brackets is the repeat unit. Poly(propene) is made in the same way from propene, CH₂=CHCH₃.

Poly(ethene) is made from ethene. It is cheap, light, flexible and waterproof, so it is used for carrier bags, bottles and food wrap.

Poly(propene) is made from propene. It is strong and tough, so it is used for rope, crates and food containers.

Plastics are very useful because they can be moulded into shapes, do not rot and are cheap to make from crude oil.

Most addition polymers are not biodegradable, so microorganisms cannot break them down and they stay in the environment for hundreds of years. This causes problems:

• Litter and wildlife: animals can swallow plastic or become trapped, and plastic collects in rivers and oceans
• Landfill: plastic takes up space and does not rot
• Burning: incineration can release carbon dioxide and toxic gases
• Resources: plastics are made from crude oil, which is non-renewable

Ways to reduce the harm: reduce the amount used, reuse items, recycle plastics, and use biodegradable alternatives. Each option has costs, so a good evaluation weighs benefits against drawbacks and reaches a justified conclusion.