Alkanes

Cards (60)

  • What are alkanes?

    - saturated hydrocarbons - only contains C-C bonds or C-H single bonds
    - unreactive but react with halogens
    - used as fuels/lubricants
    - main source of alkanes is crude oil
  • Physical properties of alkanes
    - alkanes are mostly are non-polar because the electronegativity of carbon and hydrogen are similar
    - they have low melting points
    - increasing van der waals due to chain length increasing so more electrons
    - alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms, because the atoms can't PACK CLOSELY TOGETHER so van der waals aren't that strong
    - alkanes are insoluble in water because h2o molecules are held by H-bonds which are stronger than van der waals
    - alkanes mix with other non-polar liquids
  • What is crude oil
    - crude oil is a fossil fuel formed by breakdown of plant and animal remains at the high pressures and temperatures deep below the earth's surface so it is non-renewable
    - crude oil is a mixture of mainly alkanes
  • Define fractions
    - a group of hydrocarbons with similar boiling points
  • Describe the steps of fractional distillation (4)
    - crude oil is vaporised (1)
    - vapour passed into fractionated column (2)
    - top of fractionating column is cooler at the top than the bottom (3)
    - fractions are separated by their boiling points (4)
  • Outline the features of fractional distillation of crude oil that allows the crude oil to be separated into fractions(4)
    - fractions/hydrocarbons have different boiling points (1)
    - boiling points depend on chain length (2)
    - cooler at top of fractionating column (3)
    - high boiling points/larger molecules are at the bottom of the column (4)
  • Define empirical formula
    - the simplest whole number ratio of atoms of each element in a compound
  • Define molecular formula
    - tells us the actual number of atoms of each different element that makes up a molecule of a compound
  • Define isomer
    - one of two with the same molecular formula but a different arrangement of atoms
  • Define homologous series
    - a family or organic compounds with the same functional groups attached but different carbon chain length
  • Describe the features of a homologous series
    - same general formula
    - same chemical reactivity
    - trend in physical properties
  • Define cracking
    - breaking up larger hydrocarbon molecules into smaller more useful ones
    - it is decomposition reaction
  • Define the features of cracking
    - the naptha fraction (C5-C12) from fractional distillation of crude oil is in huge demand for petrol
    - shorter chain products are economically more valuable than longer chains.
    - to meet demands for shorter chain hydrocarbons, the longer chain fractions are broken to shorter lengths
  • Name the 2 useful results of cracking
    - shorter, useful chains are produced, like petrol
    - some products are alkenes, which are more reactive than alkanes
  • What are alkenes used for?
    - alkenes are used as chemical feedstocks and converted to polymers and products like paints and drugs
    - ethene is very important - starting material for polyethene
  • What is petrol made of?
    - a mixture of mainly alkanes
    - 4-12 carbon atoms
  • What are the 2 types of cracking?
    - thermal
    - catalytic
  • Describe thermal cracking
    - involves heating alkanes to high temperatures under high pressures
    - free radicals are highly reactive intermediates and react to for shorter chain molecules
    - there aren't enough hydrogen atoms to produce 2 alkanes so one of the new chains must have a carbon carbon double bond (alkene)
  • Products of thermal cracking
    - mainly alkenes
    - hydrogen may be produced
    - to avoid too much decomposition, alkanes are kept under high pressures and temperatures for a very short amount of time
  • Why are high pressures and temperatures needed? (1)
    - to break the strong C-C bonds or strong C-H bonds
  • Describe catalytic cracking
    - no alkenes produced
    - need high temperatures and slight pressures
    - needs a catalyst called zeolite - consists of silicon dioxide and aluminium oxide
    - zeolites have a honeycomb structure with a large surface area and is also acidic
  • Products of catalytic cracking
    - used to produce motor fuels
    - products are mainly branched alkanes, cycloalkanes and aromatic compounds
  • Define fuel
    - releases heat energy when burnt
    - also stores large amount of energy for a small amount of weight
  • Describe combustion of alkanes
    - shorter chain alkanes burn completely in a good supply of oxygen to produce carbon dioxide and water
    - combustion of alkanes is highly exothermic
    - the more carbon present, the greater the heat output
  • Give examples of alkanes
    - methane
    - propane
    - petrol
    - paraffin
  • Describe incomplete combustion
    - if there is a limited amount of oxygen, then incomplete combustion will take place
    - produces carbon monoxide CO, a poisonous gas
    - can produce water
    - carbon (soot) may also be produced
  • Describe what is produced when sulfur is burnt/combusted
    - fossil fuels contain sulfur
    - when burnt, sulfur dioxide gas is produced which causes acid rain
    - sulfur dioxide combines with oxygen and water in atmosphere to form sulfuric acid
    - SO2 + 1/2O2 + H20 ———-> H2SO4
  • Describe how sulfur can be removed from the waste gases
    - from furnaces like coal fired power stations by flue gas desulfurisation
    - the gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction
    - calcium sulfite is formed which is used to make calcium sulfate
    - SO2 + CaO ————> CaSO3
  • Describe the pollutants and environmental consequences
    - nitrogen oxides: NO is toxic, NO2 is toxic and acidic and forms acid rain
    - carbon monoxide: toxic
    - carbon dioxide: contributes to global warming
    - unburnt hydrocarbons: contributes towards formation of smog
    - soot: global dimming and respiratory problems
  • Describe catalytic converters
    - these remove CO, nitrogen oxides and unburned hydrocarbons from exhaust gases
    - turns them into carbon dioxide, nitrogen, water
    2CO + 2NO ———> N2 + 2CO2
    - hydrocarbons + nitrogen oxides ———> nitrogen + CO2 + H2O
  • Describe the shape of catalytic converters
    - converters have a ceramic honeycomb coated with a thin layer of catalyst metals: platinum, rhodium, palladium
    - to give a large surface area
    - little goes a long way
    - reactions take place on the surface of the catalyst
  • List greenhouse gases
    - water
    - carbon dioxide
    - methane
  • Describe the mechanism of the greenhouse effect
    - UV wavelength radiation is passed through the atmosphere to the earths surface and heats it up
    - earth radiates out infrared long wavelength radiation
    - the C=O bond is polar and absorbs infrared radiation so it doesn't escape from atmosphere.
    - this energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed
  • Why has carbon dioxide levels increased?
    - risen significantly due to increases burning of fossil fuels
    - largely resposnsible for global warming
    - earth is thought to be getting warmer due to increasing amounts of greenhouse gases in atmosphere
  • Both c3H8 and C4H10 can be liquified and used as fuels. Suggest with a reason which of these two fuels is liquified more easily (1)
    - C4H10
    - higher BP/larger molecules/longer chain
  • Explain why complete combustion of C4H10 may contribute to environmental problems (2)
    - carbon dioxide is a greenhouse gas which contributes to global warming
  • Identify a metal used as a catalyst in a catalyst converter and suggest a reason why the catalyst is coated on a ceramic honeycomb (2)
    - platinum/rhodium/pallidium (1)
    - honeycomb shape provides a large surface area (2)
  • Identify one substance used to remove sulfur dioxide and suggest why it is used (2)
    - calcium oxide (1)
    - neutralises the gas (2)
  • Describe nitrogen oxides
    - formed in high temperatures in car engines using oxygen and nitrogen from air
    - causes acid rain
    - remove these using a catalytic converter
  • Describe sulfur dioxide
    - made by combustion of fuels containing sulfur
    - causes acid rain
    - remove by flue gas desulfurisation