Crude oil and hydrocarbons

Crude oil is a finite resource found in rocks — the remains of an ancient biomass (mainly plankton) buried in mud. It is a mixture of a very large number of compounds.

Most of these compounds are hydrocarbons — molecules made of hydrogen and carbon atoms only. Most of the hydrocarbons in crude oil are alkanes.

Watch out: a hydrocarbon contains only C and H. A molecule containing oxygen (like an alcohol) is not a hydrocarbon, even though it is organic.

Naming and counting alkanes

Each alkane has one more CH₂ than the last. Check them against C_nH_2n+2:

Tip: to check a formula is an alkane, double the carbons and add 2. Butane: 2×4 + 2 = 10 hydrogens. C₄H₁₀ ✓

Spot the alkane

Splitting crude oil into fractions

Crude oil is separated into fractions — each containing molecules with a similar number of carbon atoms — by fractional distillation. The oil is heated so it evaporates, then rises up a column that is hot at the bottom, cooler at the top. Each fraction condenses where the column matches its boiling point.

Trends with chain length

Three properties depend on the size of the molecule, and you must recall how each changes as the chain gets longer:

Why? Longer molecules have stronger forces between molecules, so more energy is needed to separate them — boiling points and viscosity rise together.

How do you sort a liquid of many molecules?

Burning hydrocarbons

Combustion of a hydrocarbon fuel releases energy: the carbon and hydrogen are oxidised. In complete combustion (plenty of oxygen) the only products are:

Watch out: the carbon ends up as carbon dioxide (CO₂) and the hydrogen as water (H₂O). Carbon monoxide and soot come from incomplete combustion, not complete.

Predict the products

Top or bottom of the column?

Tap a fraction, then tap whether it leaves near the top (small molecules, low boiling point) or near the bottom (large molecules, high boiling point).

What is the hydrogen-short molecule for?

Cracking: bigger into smaller

There is a high demand for short-chain fuels but a surplus of long-chain fractions. Cracking breaks down larger hydrocarbons into smaller, more useful molecules.

• Catalytic cracking — vapour passed over a hot catalyst.
• Steam cracking — vapour mixed with steam and heated to a high temperature.

The products are alkanes (useful as fuels) and alkenes — a new type of hydrocarbon used to make polymers and as starting materials for many other chemicals.

Alkenes and the bromine test

Alkenes are unsaturated — they contain a carbon-carbon double bond (C=C). That double bond makes them more reactive than alkanes, which gives us a test:

Misconception: bromine water goes orange → colourless with an alkene, not "clear" or "blue". "Clear" describes transparency, not colour — say colourless.

Read the test

What is the hydrogen-short molecule for?

The alkene family

Alkenes have the functional group C=C and the general formula:

They are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbons (compare C₂H₆ ethane with C₂H₄ ethene).

Note: there is no "methene" — you need at least two carbons to have a C=C bond, so the series starts at ethene (C₂H₄).

Alkane or alkene?

Tap the correct family for each formula or name. Use C_nH_2n+2 (alkane) vs C_nH_2n (alkene).

Addition reactions of alkenes

The C=C double bond lets alkenes do addition reactions: atoms add across the double bond, which becomes a single C–C bond. You need three:

• + Hydrogen (H₂, nickel catalyst, ~150 °C) → the matching alkane. e.g. ethene + hydrogen → ethane.
• + Water (steam, ~300 °C, high pressure, phosphoric acid catalyst) → an alcohol. e.g. ethene + steam → ethanol.
• + Halogens (chlorine, bromine, iodine) → a dihalogenoalkane. e.g. ethene + bromine → dibromoethane.

Also: alkenes burn in air but with smoky flames (incomplete combustion) because they are relatively carbon-rich.

Why do only some polymers give off water?

Alcohols

Alcohols contain the functional group –OH. The first four are methanol, ethanol, propanol and butanol.

The first four alcohols all:

• dissolve in water to give a neutral solution;
• react with sodium (giving off hydrogen);
• burn in air (combustion);
• react with an oxidising agent to make carboxylic acids.

Fermentation: aqueous ethanol is made when sugar solutions are fermented using yeast — warm conditions (~25–35 °C), in the absence of air (anaerobic).

Uses: ethanol is the alcohol in drinks and is used as a fuel and a solvent.

Match the organic family

Tap a name on the left, then its matching functional group / fact on the right.

Carboxylic acids

Carboxylic acids have the functional group –COOH. The first four are methanoic, ethanoic, propanoic and butanoic acid (ethanoic acid, CH₃COOH, is the acid in vinegar).

They:

• react with carbonates → carbon dioxide (fizzing), a salt and water;
• dissolve in water to make acidic solutions;
• react with alcohols (with an acid catalyst) to make esters — e.g. ethyl ethanoate.

Higher only: carboxylic acids are weak acids — they only partially ionise in water, so for the same concentration they have a higher pH than a strong acid like hydrochloric acid.

Addition polymerisation

Alkenes make polymers such as poly(ethene) and poly(propene). In addition polymerisation many small monomers (which contain C=C) join to form one very large polymer:

Misconception: addition polymerisation produces only the polymer — there is no small by-product (that is condensation polymerisation, which loses water).

Identify the repeating unit

Condensation polymerisation

Condensation polymerisation uses monomers with two functional groups. The simplest case uses two different monomers, each with two of the same group. As they join, a small molecule (usually water) is lost each time — that's why it is called condensation.

Compare: addition = one C=C monomer, no by-product. Condensation = two functional groups, loses water.

Why do only some polymers give off water?

Amino acids

Amino acids have two different functional groups in one molecule (an –NH₂ amine group and a –COOH acid group). They react by condensation polymerisation to make polypeptides.

For example, glycine (H₂NCH₂COOH) polymerises to a polypeptide, losing water at each link. Combining different amino acids in one chain produces proteins.

Link: because amino acids have two different functional groups, a single type of monomer can polymerise on its own — unlike the polyester, which needs two different monomers.

DNA and other natural polymers

DNA (deoxyribonucleic acid) is a large molecule essential for life — it encodes the genetic instructions for organisms and viruses. Most DNA is two polymer chains made from four different monomers called nucleotides, in a double helix.

Other naturally occurring polymers important for life — and the monomers they're made from:

• DNA → made from nucleotides
• Proteins → made from amino acids
• Starch and cellulose → made from sugars (glucose)

Name the monomer

The key ideas to know

You've covered the key ideas in AQA 4.7 — from crude oil and alkanes through cracking and alkenes to the Chemistry-only alcohols, acids and polymers. Press Finish to see your score.