Organic Chemistry — AQA Combined Science: Trilogy
Organic chemistry is the chemistry of carbon compounds. In Combined Science this focuses on crude oil, hydrocarbons and how they are processed into useful fuels and materials.
Crude oil and hydrocarbons
Crude oil is a finite resource formed over millions of years from the remains of ancient sea organisms (mainly plankton). It is a mixture of many compounds, most of which are hydrocarbons.
A hydrocarbon is a compound made of hydrogen and carbon only.
Alkanes
The main hydrocarbons in crude oil are alkanes — a homologous series (a family of compounds with the same general formula and similar properties).
- General formula: CₙH₂ₙ₊₂
- The first four: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
- Alkanes are saturated — they contain only single carbon–carbon bonds.
Properties down the series
As the molecules get larger (more carbon atoms):
- boiling point increases
- they become more viscous (thicker)
- they are less flammable (harder to ignite)
Fractional distillation
Crude oil is separated into useful fractions by fractional distillation:
- The oil is heated and evaporates.
- Vapours rise up a fractionating column that is hot at the bottom and cooler at the top.
- Each fraction condenses at a different height depending on its boiling point — larger molecules condense lower down, smaller ones higher up.
Each fraction contains molecules with a similar number of carbon atoms. Useful fractions include refinery gases, petrol, kerosene (jet fuel), diesel, fuel oil and bitumen.
These fractions are used as fuels and as feedstock for the petrochemical industry to make polymers, solvents, lubricants and detergents.
Combustion
When hydrocarbons burn (combust) in plenty of oxygen, complete combustion occurs: hydrocarbon + oxygen → carbon dioxide + water
This is an exothermic reaction that releases energy, which is why hydrocarbons are used as fuels. During combustion, both the carbon and hydrogen are oxidised.
Incomplete combustion (not enough oxygen) produces carbon monoxide (a toxic gas) and soot (carbon) as well as water — a key reason for ensuring good ventilation when burning fuels.
Cracking
Long-chain hydrocarbons are less useful and in lower demand. Cracking breaks them into smaller, more useful molecules:
- It can be done by catalytic cracking (heating with a catalyst) or steam cracking (mixing with steam at high temperature).
- Cracking produces smaller alkanes (useful as fuels like petrol) and alkenes.
Alkenes
Alkenes are hydrocarbons with a carbon–carbon double bond (C=C), so they are unsaturated.
- General formula: CₙH₂ₙ.
- They are more reactive than alkanes.
- Alkenes turn bromine water from orange to colourless — this is the test for an alkene (a C=C double bond).
- Alkenes are used to make polymers and other chemicals.
Balancing supply and demand
Cracking is economically important because it converts surplus long-chain molecules into the short-chain fuels and alkenes that are in high demand, matching supply to what people want to use.
Exam tips
- Define a hydrocarbon precisely: hydrogen and carbon only.
- Learn the alkane general formula CₙH₂ₙ₊₂ and the trends in boiling point, viscosity and flammability.
- Be able to describe fractional distillation in steps, linking the height of condensing to boiling point.
- Remember the bromine water test: orange → colourless indicates an alkene.
- Know why cracking is done (to make smaller, more useful molecules and alkenes) and the products of complete vs incomplete combustion.