3.3.2 - Alkanes

• State the conditions for catalytic cracking:

  • 450 °C
  • Slight pressure
  • Zeolite catalysts
• Give the conditions for thermal cracking:

  • High pressure → 7000 kPa
  • High temperature → 700 degrees c
• What is petroleum? A mixture consisting of mainly alkane hydrocarbons
• What is a petroleum fraction? A mixture of hydrocarbons with a similar chain length and similar boiling point
• Cracking:

  • conversion of large hydrocarbons to smaller, more useful hydrocarbon molecules
  • by breaking C-C bonds
• Catalytic Cracking:

  • slight/moderate presure, high temperature, zeolite catalyst    
  • produces branched, cyclic alkanes 
  • used mainly to produce motor fuels and aromatic hydrocarbons
• What are the two types of cracking? Catalytic and Thermal
• What are fuels? Compounds that release heat energy when burnt
• What are alkanes used as? Fuels
• Alkanes are used as fuels because they readily burn in the presence of oxygen.
• What can soot cause? Global dimming - reflection of the sun's light
• Internal combustion engines produce the following pollutants:

  • NOx 
  • CO
  • unburned hydrocarbons
  • SOx
• Coal is high in sulfur content, and large amounts of sulfur oxides are emitted from power stations.
• SO2 can be removed from waste gases produced from furances through ... flue gas desulfurisation
• How are NOx compounds produced inside car engines? High temperatures and spark in the engine provide sufficient energy to break strong N2 bond.
• Catalytic converters: A device in cars that removes CO, NOx and unburned hydrocarbons from exhaust gases, turning them into CO2, N2 and H2O.
• What do catalytic converters contain?

  • A ceramic honeycomb coated
  • with a thin layer of a catalyst metals, platinum, palladium, rhodium
  • to give them a large surface area.
• What are the 3 main greenhouse gases?

  1. Carbon Dioxide
  2. Methane
  3. Water Vapour
• What are the 3 steps of free radical substitution?

  • Initiation
  • Propagation
  • Termination
• Initiation Step of Free Radical Substitution:

  • In the precense of UV light, the halogen is broken down. 
  • This is an example of homolytic fission - each atom gets one electron from the covalent bond. 
• Describe the Propagation step of Free Radical Substitution:

  • A hydrogen is replaced and the Cl free radical is reformed as a catalyst. 
• The Termination step of Free Radical Substitution:

  • Two radicals join to end the chain reaction
  • and form a stable product.
• What is a free radical? A reactive species with possess an unpaired electron
• How are halogenoalkanes classified?

  • According to how many carbon atoms are attached to the C-H group. 
  • Primary = 1
  • Secondary = 2 
  • Tertiary = 3
• Nucleophile: An electron pair donator
• Substitution: Swapping a halogen atom for another atom/groups of atoms
• Nucleophilic Substitution reaction process:

  1. Nucleophiles attack positive carbon atom
  2. Carbon atom has a delta positive charge due to electronegativity difference between the halogens and carbon
  3. nucleophile replaces the halogen, to form a new compound + halogen free radical 
• Why is the naturally occuring ozone layer beneficial? It filters out much of the sun's UV radiation
• Why is the C-F bond not affected by UV? It is stronger than the C-Cl bond
• What have policymakers done to combat ozone depletion? banned the use of CFCs and developed alternative chlorine-free compounds
• Write the equation for the reaction of fluorine with methane to form trifluoromethane:

  
  

  CH4 + 3F2 →CHF3 + 3HF
• What is an alkane? A saturated hydrocarbons containing C-H bonds only
• What is the general formula of an alkane? C n H2n+2
• Alkane bonds are nonpolar because carbon and hydrogen have similar electronegativities
• Alkanes only have van der Waals forces of attraction - bonds are non-polar
• Alkanes are insoluble because hydrogen bonds in water are stronger than alkanes’ van der Waals forces of attraction
• How reactive are alkanes? Very unreactive
• Which reactions will alkanes undergo? Combustion and reaction with halogens
• Crude oil:

  • Mixture of fractions (hydrocarbons with similar boiling points and properties)
  • Formed at high temperatures and pressures deep below earth’s surface over millions of years → therefore non-renewable
• Fractional distillation process:

  • Crude oil heated until mostly vaporised
  • Passed into a fractionating tower that is cooler at the top than the bottom
  • Liquid fractions are piped off at the bottom
  • Vapours rise up the column and - via trays and bubble caps - condense when temperature < their boiling point
  • Shortest chain hydrocarbons condense at the top as they have the lowest boiling points