Alkanes

• Define Alkanes:

  A hydrocarbon chain that contains only singly bonded carbon atoms
• Each carbon atom can form 4 covalent bonds, these are sigma bonds
• σ-bonds are called sigma bonds
• Define Sigma bond (σ-bond):

  The strongest type of covalent bond formed by the direct overlapping of atomic orbitals between bonding atoms which allows the free rotation of the bonds
• Are alkane bonds (C-C and C-H) polar or nonpolar?Give all the reasons why?

  Nonpolar

  C-C = the same atom has the same electronegativity, no difference in electronegativity, no net pull one way or the other, the bond is not polarised, purely covalent

  C-H = carbon and hydrogen have similar electronegativities, small difference in electronegativity
• The 3D arrangement around each carbon atom in an alkane has the bond angle of...

  109.5
• The 3D arrangement around each carbon atom in an alkane is...

  Tetrahedral
• What is the functional group for alkane?

  Alkanes do not have a functional group
• Alkanes have no centre where reactions tend to happen. Why is that?

  C-C and C-H bonds are strong (high bond enthalpy), not easily broken, and non-polar, not very attracted to other reactants
• What type of intermolecular forces do alkanes have?

  london forces
• Alkanes are the main components of what two things?

  natural gas and crude oil
• What are alkanes mainly used as?

  Fuels
• Alkanes have a relatively low reactivity
• What method is used to separate crude oil into fractions?

  Fractional distillation
• What makes fractional Distillation possible in separating crude oil?

  Different boiling points of alkanes
• The different boiling points of alkanes are dependent on...

  • chain length
  • branching
• Name all the alkanes that are gas at RTP.

  • methane
  • ethane
  • propane
  • butane
• What is the alkane that is first to be a liquid at room temperature?

  Pentane
• Outline why the length of the alkane chain determines its boiling point.

  • as the chain length increases
  • Mr increases - larger molecule
  • surface area increases
  • more points of surface contact between molecules
  • molecules are closer together
  • increase in the number of london forces - induces a greater attraction
  • stronger london forces
  • more energy is required to break these london forces
  • higher boiling point
• Outline why branching in an alkane chain determines its boiling point.

  • surface area decreases
  • fewer points of surface contact
  • molecules are further apart
  • induces a weaker attraction
  • weaker london forces
  • less energy is required to break these london forces
  • lower boiling point
• What kind of bonds are C-C and C-H?

  covalent/sigma
• Are alkanes soluble or insolube in water?

  Insoluble
• Outline why are alkanes insoluble in water?

  • alkanes are nonpolar molecules
  • water is a polar solvent
  • nonpolar bonds are soluble in nonpolar solvents
  • the hydrogen bonding in water is much stronger than the london forces in alkanes
  • the hydrogen bonding is too strong to be broken for the molecules of alkanes to dissolve in
• What type of reaction is complete combustion?

  Exothermic and Oxidation
• Define Complete Combustion in terms of alkanes:

  A reaction where an alkane is burnt in a plentiful supply of oxygen resulting in the products carbon dioxide and water
• Define Incomplete Combustion in terms of alkanes:

  A reaction when an alkane is burnt in a limited supply of oxygen resulting in the products water and carbon monoxide or soot
• Which kind hydrocarbons are most likely to undergo incomplete combustion?

  longer chain alkanes
• When balancing a combustion equation, there is an order for which elements to balance first. The order goes from carbon, hydrogen, then oxygen
• What is the products for complete combustion?

  carbon dioxide and water
• What is the environmental impact of CO2?

  Its a greenhouse gas, increasing levels of CO2 are thought to be responsible for increasing global temperatures
• What is the environmental impact of CO?

  its a toxic gas
• Explain why carbon monoxide is dangerous?

  it is a toxic gas that is colourless and odourless.

  It binds to the haemoglobin in red blood cells preventing the transport of oxygen around the body
• What's the environmental/ social impact of soot (carbon)?

  Air pollution and respiratory problems
• Oxides of nitrogen and sulfur are also produced as a by-product of alkane combustion, along with carbon particulates from unburnt fuel in incomplete combustion
• Name two oxides of sulfur:

  • sulfur dioxide
  • sulfur trioxide
• Name two oxides of nitrogen:

  • nitrogen monoxide
  • nitrogen dioxide
• What type of reaction are halogenoalkanes formed from alkanes?

  Radical Substitution
• In the presence of what does alkanes react with halogens?

  Ultraviolet (UV) light
• What are the three steps in Radical Substitution Mechanism:

  1. Initiation
  2. Propagation
  3. Termination
• Alkanes react with halogens, specifically chlorine and bromine