21.1-Hydrocarbons

Hydrocarbons only contain hydrogen and carbon atoms

• A hydrocarbon is any compound that is formed from carbon and hydrogen atoms only

• So C10H22 is a hydrocarbons, but CO2 isn’t because it contains oxygen

Alkanes have all C-C single bonds

• Alkanes are the simplest type of hydrocarbon you can get

  • They have the general formula CnH2n+2

  • The alkanes are a homologous series, a group of organic compounds that react in a similar way

  • Alkanes are saturated compounds, each carbon atoms forms four single covalent bonds

  • The first four alkanes are methane, ethane, propane, butane

Hydrocarbon properties change as the chain gets longer

• The shorter the carbon chain the…

  • Less viscous

  • More volatile

  • More flammable

  • Lower boiling point

• The longer the carbon chain is…

  • More viscous

  • Less volatile

  • Less flammable

  • Higher boiling point

Complete combustion occurs when there’s plenty of oxygen

• The complete combustion in oxygen release lots of energy

• Waste products are carbon dioxide and water vapour

  • Hydrocarbon + oxygen - Carbon dioxide + water

• During combustion, both hydrogen and carbon is oxidised

• Hydrocarbons are used as fuels due to the large amount of energy released

21.2-Fractional Distillation

Crude oil is made over a long period of time

• Formed from the remains of plants and animals that died millions of years ago and through heat and pressure turned into crude oil

• Non-renewable fuel

Fractional distillation is used to separate hydrocarbon fractions

• Crude oil is a mixture of lots of different hydrocarbons

• To separate them:

  • Heat until most has turned into a gas, bitumen won’t and will drain straight out

  • Gases enter a fractionating column which has a temperature gradient, hot at the bottom and cooler towards the top

  • Longer hydrocarbons have higher boiling points, so they condense back to liquids early on when they’re near the bottom, shorter hydrocarbon then take longer

  • End with crude oil mixture separated into different fractions

• Fractions in crude oil are:

  • LPG

  • Petrol

  • Kerosene

  • Diesel Oil

  • Heavy fuel oil

  • Bitumen

• Makes:

  • Lubricants

  • Petrol

  • Paraffin

  • Solvents

  • Detergents

21.3-Uses and Cracking of Crude Oil

Crude oil has various uses in modern life

• Uses as fuel for transport

• Petrochemical industry uses some of the hydrocarbons as a feedstock to make new compounds for uses in things like polymers, solvents, lubricants and detergents

• All products are examples of organic compounds

Cracking means splitting up long-chain hydrocarbons

• Short-chain hydrocarbons are more useful than long-chain hydrocarbons as they can make fuels so long-chains are cracked and broken into a short alkane and an alkene

• Alkenes are used as a starting material when making lots of other compounds and polymers

There are different methods of cracking

• Cracking is a thermal decomposition reaction-breaking molecules down by heating them

• Catalytic cracking

  • Heat long-chain hydrocarbons until they are vaporised

  • Vapour is then passed over a hot powered aluminium oxide catalyst

  • Long-chain molecules are split apart on the surface of the specks of catalyst

• Steam cracking

  • Heat long-chain hydrocarbons until they are vapourised

  • Mix them with steam and heat them to a very high temperature

Practice questions:

• Properties of shorter chain hydrocarbons

  • lower viscosity, lower boiling point, higher flammable, more volatile

• What is cracking?

  • A thermal decomposition reaction breaking molecules down by heating them

• How to do catalytic cracking

  • Heat long chain hydrocarbons until they are vapourised and then pas over a catalyst