AQA Chemistry AS Unit 1

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5 Organic Chemistry


  • Carbon can form rings and long chains that can be branched:
  • Carbon has four electrons in its highest energy level so forms 4 covalent bonds
  • C-C bonds are relatively strong and are non-polar
  • C-H bond is strong

Bonding in Carbon Compounds

  • In all stable carbon compounds carbon forms 4 covalent bonds and has 8 electrons in its highest energy level


  • Displayed formula shows all atoms and bonds
  • Structural shows atoms  involved but not the bonds
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5.2 Nomenclature

  • The first six roots are: meth, eth, prop, but, pent and hex
  • Side chains of hydrocarbon chains can be: methyl, ethyl, propyl or butyl

Functional Groups

  • An atom or group of atoms which is responsible for characteristic reactions of a molecule
  • Functional group families: alkanes, alkenes, haloalkanes, alcohols, aldehydes, ketones and carboxylic acids
  • Functional groups are placed in alphabetical order
  • If you have more than one of the same functional groups you add: di, tri or tetra

Homologous Series

  • Have same functional group but different carbon chain lengths
  • Each member differs from the next by CH2
  • The Carbon chain length has little effect on chemical reactivity
  • As chain length increases so does MP and BP as intermolecular forces increase
  • Chain branching reduces melting points as molecules pack less well
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5.3 Isomerism

  • Isomers: molecules that have the same molecular formula but whose atoms are arranged differently

Structural Isomers

  • Have the same molecular formula but different structural formula, there are:
  • Positional Isomers: same functional groups attached to main chain at different points
  • Functional Group Isomers: have different functional groups
  • Chain Isomers: different arrangement of hydrocarbon chain e.g. branched
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6 Alkanes

  • Are saturated hydrocarbons so contain C-C single bonds and C-H bonds
  • Used as fuels and lubricants, main source is crude oil

General Formula: CnH2n+2

  • Unbranched chain: Carbon chain angle is 109.5 degrees

Physical Properties

  • Polarity: alkanes are non-polar so only have weak van der Waals
  • Boiling Point: Increase as chain length does since intermolecular forces strength increases as molecule gets larger. Branched alkanes have lower BP as can't pack closely together
  • Solubility: Insoluble in water as has strong hydrogen bonds

How React

  • React with halogens and burn in oxygen to form CO2 and H2O
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6.2 Fractional Distillation

  • Crude oil is a mixture of alkanes, can contain S so when burnt produce SO2 which causes acid rain

Fractional Distillation

  • Done in a fractionating tower
  • Crude oil heated in furnace so vapourised
  • It is passed through the tower that is cooler at the top than bottom
  • The vapour travels up the tower till reaches their boiling point were they condense
  • Shorter chains condense near the top as they have lower boiling points
  • Thick residue at the bottom is called tar and is used for road surfacing
  • Up the tower: tar, fuel oil (ships/power stations), lubricating oil (candles/ engine oil), diesel oil (lorries/ taxis), kerosene (jet fuel), gasoline (cars) and gases (fuel on site)
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6.3 Industrial Cracking

  • Longer chain fractions from fractional distillation are not as useful as short chain and are in lower demand than shorter chain
  • Shorter chain produces are economically more valuable so long chains are broken down into shorter in cracking which produces:
  • Shorter more useful chains e.g. petrol (mainly alkanes between 4 and 12 carbon atoms)
  • Alkenes that are more reactive than alkanes, used to supply industries and make products like paints and plastics (polyethene)

Thermal Cracking

  • Involves high temperature: 700-1200K and high pressure up to 7000kPa
  • C-C bonds broken so one electron from the pair in covalent bond goes to each carbon atom
  • Initially 2 shorter chains produced ending in carbon atom with unpaired electron, these fragments are called free radicals
  • Free radicals are highly reactive and react to form shorter chains
  • In thermal cracking alkenes and alkanes are produced, hydrogen can also be produced 
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Industrial Cracking Cont

Catalytic Cracking

  • Lower temperature of 720K and low pressure
  • Used zeolite catalyst made up of silicon dioxide and aluminium oxide
  • Zeolite has honeycomb structure with large surface area and are acidic
  • This cracking used to produce motor fuels
  • Products are branched alkanes, cycloalkanes and aromatic compounds
  • Products obtained are separated by fractional distillation
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6.4 Combustion of Alkanes


  • Complete combustion, with oxygen, of an alkane produces carbon dioxide and water
  • Combustion reactions give out heat so have a large negative enthalpy of combustion
  • The more carbon present the great the heat output
  • Alkanes store large amounts of energy for small weight
  • Alkane fuels used: methane, propane (camping gas), butane (calor gas), petrol and paraffin

Incomplete Combustion

  • With limited oxygen supply carbon monoxide is produced and with even less oxygen carbon (soot) is produced
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Combustion of Alkanes Cont

Polluting the Atmosphere

  • Crude oil polluting produces:
  • CO: poisonous gas
  • NO, NO2, N2O4: formed when enough energy to combine oxygen and nitrogen in the air, it happens in petrol engine as high temperature. The oxides can react with water vapour and oxygen to form nitric acid so contribute to acid rain
  • SO2 contributes to acid rain and is produced from crude oil impurities, it can combine with water vapour and oxygen to form sulfuric acid
  • Carbon particles: particulates that cause cancer and worsen asthma
  • Unburnt hydrocarbons: contribute to smog and are greenhouse gases
  • CO2: greenhouse gas and its level is rising causing climate change
  • Water vapour: greenhouse gas

Removing Sulfur

  • Flues in power stations use CaO or CaCO3 to absorb SO2 to produce gypsum (CaSO4) which is used for plaster: Flue Gas Desulfurisation
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Combustion of Alkanes Cont

Catalytic Converters

  • Reduce the output of CO, NO and unburnt hydrocarbons in exhaust gas
  • It is a honeycomb made of ceramic material coated in platinum and rhodium
  • It has a large surface area due to honeycomb shape
  • Carbon Monoxide and Nitrogen Oxide react to form Nitrogen and Carbon Dioxide
  • Hydrocarbons e.g. C8H18 and Nitrogen Oxide react to form Nitrogen, Carbon Dioxide and Water

Global Warming

  • Carbon dioxide traps the sun's infra-red radiation so the Earth heats up which is could in small quantities as otherwise the earth would be too cold
  • CO2 levels have been rising causing the Earth's temperature to rise too
  • If temperature rises water vapour (greenhouse gas) will increase causing more global warming: could cause offset to cloud formation
  • Methane and Water vapour at moment have not much change in level so is less effect
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