7.4 Cracking & Alkenes

• What is crude oil primarily composed of?

  A mixture of different hydrocarbons
• What process is used to separate hydrocarbons in crude oil?

  Fractional distillation
• Why are shorter hydrocarbons considered better fuels?

  They are more flammable
• What happens to longer chain hydrocarbons during cracking?

  They are broken down into shorter, more flammable hydrocarbons
• What type of reaction is cracking?

  Thermal decomposition reaction
• What does 'decomposition' mean in the context of thermal decomposition?

  Breaking down molecules
• What are the two methods of cracking mentioned?

  Catalytic cracking and steam cracking
• What does the term 'thermal' refer to in thermal decomposition?

  Heating
• What is the first step in both catalytic and steam cracking?

  Heat and vaporize long chain hydrocarbons
• What is used as a catalyst in catalytic cracking?

  Hot powdered aluminium oxide
• What happens to long hydrocarbons in catalytic cracking?

  They split apart into two smaller hydrocarbons
• What is mixed with hydrocarbons in steam cracking?

  Steam
• What is the result of heating hydrocarbons in steam cracking?

  Long hydrocarbon chains split apart into shorter ones
• What is the balanced chemical equation for cracking a long chain alkane?

  Long chain alkane → Shorter alkane + Alkene
• If decane is cracked, what is one of the products formed?

  Heptane
• What is the alkene produced when decane is cracked?

  Propene
• Why must the number of carbons and hydrogens be the same on both sides of a balanced equation?

  To satisfy the law of conservation of mass
• What is the structural difference between alkenes and alkanes?

  Alkenes have a double bond, while alkanes have only single bonds
• What does it mean for alkenes to be unsaturated?

  They contain at least one double bond
• What is one important feature of alkenes?

  They are more reactive than alkanes
• What happens when bromine water is added to a solution of alkenes?

  It decolorizes from orange to colorless
• What is the test for alkenes?

  Adding bromine water
• How can alkenes be used in polymer production?

  Their double bonds can break to form new bonds
• What are the steps involved in the cracking process?

  1. Heat long chain hydrocarbons to vaporize them.
  2. For catalytic cracking, pass vapor over hot powdered aluminium oxide.
  3. For steam cracking, mix vapor with steam and heat to high temperature.
  4. Long hydrocarbons split into shorter hydrocarbons.
• What are the key differences between alkanes and alkenes?

  • Alkanes: Saturated, only single bonds, less reactive.
  • Alkenes: Unsaturated, contain double bonds, more reactive.
• What are the uses of alkenes after cracking?

  • Production of polymers
  • Starting materials for other chemicals
• How do you balance a chemical equation for cracking?

  1. Identify reactants and products.
  2. Count the number of carbons and hydrogens.
  3. Adjust coefficients to ensure mass balance.