Alkanes

• What is a hydrocarbon ?

  A compound containing carbon an hydrogen only.
• What are sigma bonds ?

  The overlap of orbitals between bonding atoms.
• What type of bonds are sigma bonds ?

  covalent
• What shape and bond angle do alkanes have ?

  Shape = tetrahedral

  Bond Angle = 109.5
• What is the general formula for alkanes ?

  CnH2n+2
• What is fractional distillation and what is it used for ?

  It is boiling and condensing at different temperatures to separate crude oil
• Is there a difference in electronegativity between C and H ?

  No, about the same
• What intermolecular forces do alkanes have ?

  London forces
• Explain why the boiling point increases when the chain length increases in terms of intermolecular forces.


  Increasing the chain length increases the boiling point as there is greater surface points of contact for the london forces, therefore more london forces between the molecules, leading to stronger forces that need more energy to break.
• Explain why the boiling point decreases when branching increases in terms of intermolecular forces.

  As the number of branches increases the boiling point decreases because the surface points of contact decrease, leading to weaker London forces between the molecules so less energy is required to break them.
• Why do alkanes have a tetrahedral shape ?

  The 4 bonding pairs of electrons around the carbon repel each other equally, to get as far apart as possible and hence decrease repulsion. This results in a tetrahedral shape. The bond acts as an axes around the atoms which can rotate freely so these shapes are not rigid.
• Give 3 reasons for the lack of reactivity of alkanes.

  1. C-C and C-H sigma bonds are strong
  2. C-C bonds are non polar
  3. C and H have similar electronegativities
• In complete combustion, what are the products ?

  Carbon dioxide and water.
• Short alkanes are valuble for what ?

  Fuel
• Write the equation for the complete combustion of methane.

  CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (l)
• When does incomplete combustion occur ?

  When there is a limited supply of oxygen
• Write the equation for the incomplete combustion of methane.

  CH4 (g) + 3/2 O2 (g) --> CO (g) + 2H2O (l)
• What is the difference between complete and incomplete combustion ?

  Complete: CO2 is produced

  Incomplete: CO is produced
• Why is carbon monoxide highly toxic ?

  Because it prevents haemoglobin in red blood cells from binding with oxygen so the body's tissues become starved.
• What is a covalent bond ?

  A strong electrostatic force of attraction between a shared pair of electrons and the nuclei of the bonded atoms.
• What is breaking a bond called ?

  Bond fission
• What is the difference between Heterolytic bond fission and Homolytic bond fission ?

  Heterolytic: When both electrons go to one of the bonded atoms and a positive and negative ion is formed. Homolytic: When one electron goes to each bonded atom and forms two radicals.
• Define radical.


  A species with an unpaired electron
• What type of reaction do alkanes undergo ?

  Substitution Reactions
• Which group of elements do alkanes undergo substitution reactions with ?

  Halogens
• What conditions are required for alkanes to form haloalkanes ?

  UV light
• CH4 + Cl2 -->

  HCl + CH3Cl
• What is a substitution reaction ?


  When an atom or group of atoms is replaced with a different atom or group of atoms
• What is the name of the mechanism for an alkane to form a haloalkane ?

  Free Radical Substitution
• What are the three steps of radical substitution ?

  Initiation, propagation and termination
• Why is the propogation step known as a "chain reaction" ?

  The halogen radical is regenerated so it can react with a further molecule
• What does a Cl radical act as in a propogation step ?

  catalyst as it is never used up
• What are the 3 limitations of radical substitution ?

  1. More than one termination step
  2. Mixture of further substituted products
  3. Halogen can react at different positions of the carbon chain
• What type of bond fission is present in free radical substitution ?

  Homolytic bond fission