Alkanes

• What are alkanes?

  Saturated hydrocarbons.

  Non-polar, insoluble in water, low boiling points (increase with chain lengths), branched alkanes have lower bp as less surface area so weaker Van Der Waals forces.
• What are alkanes used for?

  Flammable= fuels.

  Lubricants

  Very unreactive as c-c bonds are strong.
• What is crude oil?

  Mixture of alkanes.

  Bigger alkanes= higher boiling point.

  Smaller alkanes= weaker boiling point.
• Fractional distillation method?

  Crude oil is heated and enters column as a vapour.

  Vapour rises up column, cooling as it does.

  Different alkane lengths condense at different boiling boiling points.

  Fractions are collected.
• Shorter alkanes are?

  Lower boiling points, more volatile, more flammable, less viscous, less coloured.
• What is cracking?

  Decomposition reaction.

  Breaks down large alkanes into shorter.

  Shorter alkanes make more money eg used for petrol.
• Thermal cracking?

  1000 degrees.

  7000KPa.

  Produces alkenes.
• Catalytic cracking?

  450 degrees.

  Zeolite catalyst.

  Produces aromatic compounds, fuels, cycloalkanes.
• Combustion?

  Exothermic reaction.

  Larger alkanes= sootier flame.

  eg CH4 (g) + 2O2(g) > CO2(g) + 2H2O (l)
• Incomplete combustion?

  Insufficient O2

  eg C3H8(g) + 3.5O2(g) > 3CO(g) + 4H2O(l) Toxic

  C3H8(g) + 2O2(g) > 3C(s) + 4H2O(l) Soot
• Pollutants?

  CO= toxic gas

  Carbon particulates= asthma, cancer

  CO2= greenhouse gas

  SO2= acid rain

  NO= acid rain, toxic

  Unburned hydrocarbons= Photochemical smog, global dimming.
• Flue gas desulfurization?

  Remove sulfur dioxide using CaO or CaCo3.

  CaO(s) + SO2(g) > CaSo3(s)

  CaCo3(s) + SO2(g) > CaSo3(s) + Co2(g)
• Catalytic converter?

  Removes nitrogen oxide caused by high temps/pressure in car engines.

  Rhodium metal used in converter to use less metal and increase surface area.

  Produced:

  N2 + O2 > 2NO AND 2NO + O2 > 2NO2

  Removed:

  2NO > N2 + O2 AND 2NO + 2CO > N2 + 2CO2
• Global warming?

  Sun emits electromagnetic rays. Earth reflects rays as I-R radiation and gets hotter.

  CO2 in (photosynthesis) = CO2 out (combustion)
• Why is SO2 found in car exhaust fumes?

  Fractions contain sulfur.

  Environmental effect? Acid rain.
• What is meant by the term fuel?

  Releases energy when burned.
• How does a catalyst work?

  Alternative pathway that lowers the activation energy.
• Why is a thin layer of metal used in a catalytic converter?

  To reduce amount of metal needed and increase the surface area.
• Equations Cl radicals reacting with ozone?

  Cl. + O3 > .ClO + O2

  .ClO + O3 > 2O2 + .Cl
• Why does 1,1,1-trifluoroethane not lead to ozone depletion?

  Does not contain Cl
• State the meaning of saturated and hydrocarbon?

  Saturated- Single bonds only

  Hydrocarbon- Contains hydrogen and carbon atoms only
• Why should sulfur compounds be removed from fractions?

  As SO2 is produced which causes acid rain
• How do oxides of nitrogen form?

  N2 and O2 react at high temps.
• Overall equation for decomposition of ozone into oxygen?

  2O3 > 3O2
• Meaning of structural isomers?

  Same molecular formula but different structural formula.
• How does NO and CO react in a catalytic converter?

  2CO + 2NO > 2CO2 + N2
• Displayed formula of chloropentafluoroethane?

  CFC

  
• Calculate the no. molecules in 1.41kg of CF3CH3

  Moles of CF3CH3= 16.79 moles

  Molecules= Avogardos constant x 16.79= 1.01x10^25
• Why do bonds in molecules such as CO2 and other greenhouse gases absorb IR?

  As the bonds vibrate and are polar.
• Which substance doesn't contribute to greenhouse effect?

  Nitrogen
• CHF3 + Br2 > CBrF3 + HBr

  Br2 > 2Br.

  CHF3 + Br. > .CF3 + HBr

  .CF3 + Br2 > CBrF3 + Br.

  2Br. > Br2

  .CF3 + .CF3 > C2F6

  .CF3 + Br. > CBrF3
• Why are Bromine atoms more likely to be produced over Chlorine in the upper atmosphere?

  C-Br bond is weaker than C-Cl bond.
• Bromine decomposing ozone?

  Br. + O3 > BrO. + O2

  BrO. + O3 > Br. + 2O2
• Catalyst used in catalytic cracking?

  Zeolite
• Give the name of the position isomer of but-1-ene

  But-2-ene
• Chain isomer of but-1-ene?

  2-methylpropene
• Functional group isomer of but-1-ene?

  Cyclobutane

  
• Is C3H8 or C4H10 liquified more easily?

  C4H10 as it is a bigger molecule, so forms stronger VDW forces.
• Why does CaO/CaCO3 react with SO2?

  As it acts as a base and neutralises the SO2
• Why do branched chain isomers have lower bp than straight?

  Because the molecule has less surface area, so VDW forces between molecules are weaker.