Alkanes

• What are alkanes?

  Saturated hydrocarbons.
• What is the general formula of alkanes?

  CnH2n+2
• What are hydrocarbons?

  Molecules that contain carbon and hydrogen only.
• What are structural isomers?

  Molecules with the same molecular formula but a different structural formula.
• What are chain isomers?

  Molecules with the same molecular formula but their structures have their carbon chain arranged differently.
• Are C-H bonds polar?

  No- C and H have very similar electronegativity so the bonds are non-polar. This means that all alkane molecules will also be non-polar.
• What forces are between molecules in an alkane?

  Induced dipole dipole forces.
• Are alkanes soluble in water?

  No- they don’t form hydrogen bonds with water so aren’t soluble.
• What happens to the boiling point of the alkane as the length of the carbon chain increases?

  Boiling point increases because the molecules are getting larger and the induced dipole dipole forces between the molecules get stronger and require more energy to be broken.
• As the number of branches on an alkane increases, what happens to the boiling point?

  Decreases because there are fewer points of contact between the molecules and the surface area of the molecules decreases. This means the induced dipole dipole forces between the molecules get weaker and needs less energy to be broken.
• What is a fraction?

  A group of molecules with a boiling point within a similar range due to a similar number of carbon atoms.
• What is the first step in separating crude oil?

  Crude oil is vaporised
• Where is the vaporised crude oil introduced in the fractioning column?

  Near the bottom of the fractioning column
• What happens to the vapours in the fractioning column?

  They rise up the column
• What is the temperature gradient in the fractioning column?

  Cooler at the top and hotter at the bottom
• Why do fractions condense at different levels in the column?

  Because they have different boiling points
• What determines the boiling point of hydrocarbons?

  The size of the hydrocarbon
• Where are hydrocarbons with lower boiling points collected?

  As gases at the top
• Where are hydrocarbons with higher boiling points collected?

  At the bottom as a thick residue
• What is meant by the term cracking?

  When longer less useful molecules are broken down into smaller, more useful molecules of higher value and higher demand.
• What are the 2 types of cracking?

  Thermal and catalytic.
• Why is thermal cracking done at a high temperature?

  Strong C-C and H-H bonds need to be broken.
• What does thermal cracking produce?

  Alkanes and a high percentage of alkenes.
• What does catalytic cracking produce?

  • Cycloalkanes
  • Branched alkanes
  • Aromatic hydrocarbons
• Why are branched alkanes used as motor fuels?

  They burn more smoothly than straight chain alkanes.
• At what temperature does thermal cracking occur?

  400 - 900 ‘C
• What pressure does thermal cracking occur at?

  High- up to 7000kPa
• Are catalysts used in thermal cracking?

  No.
• Which mechanism is involved in thermal cracking?

  Free radical.
• What are the products of thermal cracking?

  High proportion of alkenes.
• At what temperature does catalytic cracking occur?

  450 ‘C
• What pressure does catalytic cracking occur at?

  Slightly above 100kPa
• Which catalyst is used in catalytic cracking?

  Zeolites.
• What mechanism is involved in catalytic cracking?

  Carbocation intermediate.
• What are the products of catalytic cracking?

  Motor fuels (branched alkanes) and aromatic hydrocarbons.
• What are the products of complete combustion (plentiful supply of oxygen)?

  Carbon dioxide and water.
• What are the products of incomplete combustion (limited supply of oxygen)?

  Carbon monoxide and water.
• What are the products of further incomplete combustion (very limited supply of oxygen)?

  Soot (carbon) and water.
• Why does carbon dioxide contribute to global warming?

  Carbon dioxide is a greenhouse gas.
• What is carbon monoxide?

  Colourless, odourless, toxic gas