Chem unit 2.5

• Why are fossil fuels considered non-renewable resources?

  They are used faster than reformed
• How were fossil fuels formed?

  From the remains of organisms
• What will be the future trend regarding our dependence on energy sources?

  Depend much more on other sources
• What are the disadvantages of using fossil fuels?

  • Non-renewable – because they take millions of years to form and we are using them faster than new ones can form. Basically the supply is running out.
  • Combustion of fossil fuels produces Carbon dioxide, which is a greenhouse gas.
  • Acid rain
• What are the advantages of using fossil fuels?

  • Widely available all the time (not dependent on the weather)
  • Found in many different forms e.g. solid coal and natural gas is used in power stations, petroleum (crude oil) can be separated into fractions that have many uses e.g. medicines, cosmetics, fuels.
• What effect does carbon dioxide have on the Earth's temperature?

  It absorbs infrared radiation and reflects it back
• What environmental problems are linked to climate change caused by carbon dioxide?

  Rising sea levels and crop failure
• How much more effect on temperature does methane have compared to carbon dioxide?

  34 times more effect
• What element do fossil fuels contain that leads to the formation of acid rain?

  Sulfur
• What is the chemical formula of sulfurous acid?

  H₂SO₃
• What two substances react inside car engines to form oxides of nitrogen?

  Nitrogen and oxygen
• What acid is formed when oxides of nitrogen react with water?

  Nitric acid
• What are the effects of acid rain on the environment?

  Damage to buildings, kills fish, kills trees
• What are the products of complete combustion of fossil fuels?

  Carbon dioxide and water
• What is produced in addition to carbon dioxide and water during incomplete combustion?

  Carbon monoxide
• Why is carbon monoxide (CO) poisonous?

  It blocks oxygen transport in blood
• How does carbon monoxide interfere with oxygen transport in the body?

  It bonds to haemoglobin
• What are two safety precautions mentioned to prevent carbon monoxide poisoning?

  CO detectors and air vents
• What is the visible sign of incomplete combustion in engines?

  Black smoke from exhaust
• What is the general formula for alkanes?

  $C_nH_{2n+2}$
• What type of compounds are alkanes?

  Saturated
• How does each member of an alkane homologous series differ from the next?

  By a $CH_2$ group
• How does the boiling point of alkanes change as the molecules increase in size?

  Boiling point increases
• What are the two main reactions of alkanes?

  Combustion and halogenation
• What are the products of complete combustion of a hydrocarbon?

  Carbon dioxide and water
• What are the products of incomplete combustion of a hydrocarbon?

  Carbon monoxide and water
• Why are alkanes relatively unreactive?

  Strong C-H and C-C bonds
• What are the bond enthalpies of C-C and C-H bonds in alkanes?

  C-C = 348 kJmol-¹, C-H = 413 kJmol-¹
• Why is there no polarity in C-H bonds in alkanes?

  Little electronegativity difference
• Why is an attack by molecules unlikely on alkanes?

  Nothing to attract nucleophile/electrophile
• What is the name of the process by which alkanes split when attacked?

  Homolytic bond fission
• What is the name of the reaction that changes an alkane into a halogenoalkane by adding a halogen?

  Halogenation
• What products are formed from reacting methane with chlorine?

  Chloromethane and hydrogen chloride
• What are three uses of halogenoalkanes?

  Solvents, refrigerants, pesticides
• What type of reaction is the chlorination of methane?

  Substitution reaction
• What is the final product of the complete chlorination of methane?

  Tetrachloromethane ($CCl_4$)
• What type of reaction mechanism is involved in the halogenation of alkanes?

  Chain reaction
• What are the three steps in the reaction mechanism for the halogenation of alkanes?

  • Initiation
  • Propagation
  • Termination
• What is required to start the initiation step in the halogenation of alkanes?

  UV light energy
• What type of bond fission occurs in the initiation step?

  Homolytic bond fission