chem topic 6

• What is the general formula for alkanes?

  C$n$H$2n +$$2$
• Why are alkanes considered saturated hydrocarbons?

  Because they contain only single bonds between carbon atoms
• What are two primary uses of alkanes?

  As fuels and as starting materials for organic molecules
• What is crude oil formed from?

  The remains of plants and animals over millions of years
• Why is crude oil considered non-renewable?

  Because it is used faster than it can form
• What is the main process used to separate alkanes from crude oil?

  Fractional distillation
• What type of forces attract alkane molecules?

  London forces
• How does the carbon chain length affect the boiling point of alkanes?

  Longer chain alkanes have a higher boiling point than shorter chain alkanes
• Why do longer chain alkanes have higher boiling points?

  They have a greater surface area for the formation of London forces
• At what temperature does octane boil?

  126 degrees Celsius
• What happens to octane when it is heated to its boiling point?

  It converts from a liquid to a gas
• What occurs when gas octane is cooled below its boiling point?

  It condenses back to liquid form
• What are the key points to understand about fractional distillation of alkanes?

  • Longer chain alkanes have a higher boiling point than shorter chain alkanes.
  • Gaseous alkanes condense to liquid when cooled below their boiling point.
• What are the stages of fractional distillation of crude oil?

  1. Crude oil is heated in a furnace to boil alkanes.
  2. Vapors and liquids enter the fractionating column.
  3. The column is hotter at the bottom and cooler at the top.
  4. Alkanes condense at different levels based on boiling points.
  5. Shorter chain alkanes are collected at the top; longer chain alkanes at the bottom.
  6. Very long chain alkanes form bitumen at the bottom.
• What is collected from the top of the fractionating column?

  Very short chain alkanes such as methane and ethane
• Does fractional distillation separate each individual alkane?

  No, it separates fractions containing alkanes with similar boiling points
• What would be required to separate each individual alkane?

  Further rounds of fractional distillation
• What are the two methods of cracking alkanes mentioned in the video?

  Thermal cracking and catalytic cracking
• Why is cracking of alkanes economically beneficial?

  It converts long-chain hydrocarbons into shorter chain hydrocarbons which are in greater demand
• What is the main product of the petrol naphtha fraction?

  Petrol for vehicles
• What is the relationship between chain length and boiling point in alkanes?

  Shorter chain alkanes have lower boiling points than longer chain alkanes
• What is a major feedstock produced from cracking alkanes?

  Alkenes
• What are the conditions required for thermal cracking?

  High temperature (450-900 degrees Celsius) and high pressure (around 70 atmospheres)
• What types of products are formed during thermal cracking?

  Shorter chain alkanes, alkenes, and hydrogen
• What is the benefit of thermal cracking in terms of alkene production?

  It produces a high percentage of alkenes, which are useful due to their high reactivity
• What happens to covalent bonds during thermal cracking?

  A covalent bond between two carbon atoms splits to form intermediate molecules
• What are free radicals?

  Molecules with one unpaired electron formed during the splitting of covalent bonds
• What is the temperature requirement for catalytic cracking?

  Around 450 degrees Celsius
• What is the pressure requirement for catalytic cracking?

  1 to 2 atmospheres
• What catalyst is used in catalytic cracking?

  A zeolite catalyst containing aluminium oxide and silicon dioxide
• Why are branched chain alkanes produced in catalytic cracking useful?

  They combust very efficiently
• What types of hydrocarbons can catalytic cracking produce besides alkanes?

  Cyclic alkanes and aromatic hydrocarbons such as benzene
• What are the benefits of cracking alkanes?

  • Converts long-chain hydrocarbons into shorter chain hydrocarbons
  • Produces alkenes, which are highly reactive and useful in the chemical industry
• Compare thermal cracking and catalytic cracking in terms of conditions and products.

  Thermal Cracking:

  • High temperature (450-900 °C)
  • High pressure (70 atmospheres)
  • Produces shorter chain alkanes and alkenes

  
  

  Catalytic Cracking:

  • High temperature (around 450 °C)
  • Low pressure (1-2 atmospheres)
  • Produces branched chain alkanes, cyclic alkanes, and aromatic hydrocarbons
• What are the main components of petrol and diesel?

  A mixture of alkanes, hydrocarbons, impurities, and additives
• Why are straight chain alkanes more likely to explode in an engine?

  They are more likely to explode rather than combust
• What is the effect of knocking on combustion efficiency?

  Knocking makes combustion less efficient
• What are the products of reforming straight chain alkanes?

  • Branched alkanes (e.g., octane → 2,5-dimethylhexane)
  • Cycloalkanes (e.g., hexane → cyclohexane)
• What is an example of a branched alkane produced from octane?

  2,5-dimethylhexane
• What is the chemical transformation of hexane to cyclohexane?

  CH3CH2CH2CH2CH2CH3 → C6H12 + H2