Organic - Unit 4 - Year 10 Chemistry Revision 2024

• Hydrocarbon


  a molecule made up of carbon and hydrogen atoms only
• Homologous Series


  a family of compounds with a general formula, similar chemical properties and a trend in physical properties
• Functional Group


  an atom or group of atoms that give a compound its chemical properties (e.g. -OH functional group in alcohols)
• Saturated


  molecules containing only single covalent bonds
• Unsaturated


  molecules containing at least one double or triple bond
• General Formula


  expresses the molecular formula of all the members of a homologous series
• Isomerism


  when two or more compounds have the same molecular formula but different structural formula
• Molecular Formula


  shows the number and type of each atom present, but tells you nothing about the bonding within the compound

  
• Empirical Formula


  the simplest ratio of atoms of each element present in the compound

  
• Molecular Formula vs Empirical Formula


  Molecular = exact numbers of each element in a molecule (Benzene: C6H6)

  
  

  Empirical = the ratio of each element to the others (Benzene: CH, for every Carbon atom there is one Hydrogen atom)

  
• Structural Formula


  shows the structure of the molecule, but not all bonds are necessarily shown

  
  

  E.g. Butane

  H3C -- CH2 -- CH2 -- CH3

  
  

  Condensed Structural Formula:

  CH3CH2CH2CH3
• Displayed Formula


  shows all the atoms and all the bonds within the molecule

  
• Crude Oil


  A complex mixture of many compounds. Most of these are hydrocarbons. Hydrocarbon molecules in crude oil vary in size and can be separated into smaller mixtures (fractions) by fractional distillation. Each fraction contains molecules with a similar number of carbon atoms and similar boiling points.

  
• Fractional Distillation of Crude Oil


  1. The crude oil is heated and the vapour passes into a fractionating column.

  2. The top of the tower is cooler than the bottom.

  3. As the hydrocarbons rise up the column, they cool down. Eventually, they reach a height where it is cool enough to condense

  4. The most volatile (how easily a compound evaporates) components reach the top.

  5. Larger molecules with higher boiling points will condense below their boiling point (as they travel up the column) and so are tapped off and collected at different heights.
• Crude Oil Fractions

  
• Refinery Gases (or LPG)

  domestic heating, cooking
• Gasoline (Petrol)


  car fuel
• Naphtha


  petrochemicals, cracking
• Kerosene


  aircraft fuel, domestic heating oil
• Diesel Oil


  lorry fuel, central heating fuel
• Fuel Oil


  fuel for ships, industrial heating
• Bitumen


  tar for roads/roofing
• Properties of Fractions


  As number of carbon atoms increases in a hydrocarbon:

  1. Volatility decreases (how easily it evaporates)

  2. Viscosity decreases (the less easily it flows)

  3. Flammability decreases (the less easily it ignites)

  
  

  Longer chain length results in more surface area meaning there is stronger intermolecular forces between molecules making it more difficult for molecules to be pulled away from or slide over each other.
• Why is cracking necessary?


  The demand outweighs supply for shorter chain alkenes and alkanes and supply outweighs demand for long chain alkanes.

  
  

  Supply is lower for short chain as crude oil contains fewer long chain alkanes than short chain + demand is lower for long chain as larger molecules are less flammable so are worse as fuels.

  
  

  Long chain alkanes are broken down into short chain alkanes and alkenes.
• Cracking of Octane


  A temperature of 600-700°C and a catalyst containing aluminium oxide or silicon dioxide (alumina or silica).

  
  

  C8H18 → 2C2H4 + C4H10

  C8H18 → C3H6 + C5H12

  C8H18 → C5H10 + C3H8

  C8H18 → C7H14 + CH4
• Cracking Method


  1. Large alkanes (e.g. naphtha) are heated to vaporise them.

  2. The vapours are passed over a hot catalyst (silicon dioxide or aluminium oxide).

  3. Gases (by displacement) and liquids are collected.

  4. The larger molecules are broken up into smaller more useful alkanes (used as fuels) and alkenes (used to make plastics).
• Combustion of Alkanes


  Complete (plentiful supply of air - blue flame):

  CH4 + O2 → CO2 + 2H2O

  
  

  Incomplete (restricted supply of air - orange flame):

  2CH4 + 3O2 → 2CO + 4H2O

  CH4 + O2 → C + 2H2O
• Carbon Monoxide Poisoning


  A colourless, odourless and tasteless gas.

  
  

  1. Combines with haemoglobin in the blood when breathed in.

  2. Haemoglobin has 4 binding sites, so if CO binds to it, less O2 can bind to it + CO binds to haemoglobin 200 to 300x as strongly as O2.

  3. Body is deprived of oxygen.
• Nitrogen Oxides in Cars


  In petrol engines, sparks are used to ignite the petrol air mixture to power the car. These sparks have enough energy to react oxygen and nitrogen (from the air) in the engine. When these nitrogen oxides dissolve in water droplet in clouds, acid rain forms.
• Combustion of Impurities in Fuel


  Sulphur dioxide (SO2) is produced when fuels that contain sulphur compounds burn. It dissolves in water droplets in clouds to make rain more acidic, creating problems:

  
  

  1. Acid rain reacts with metals and rocks (e.g. limestone) causing damage to buildings and statues.

  2. Acid rain damages the waxy layer on leaves to make absorbing minerals for growth more difficult, killing trees.

  3. Rivers and lakes become to acidic for some aquatic life to survive.
• Alkanes


  a homologous series of saturated hydrocarbons with the general formula C(n)H(2n+2)
• Isomers of Alkanes


  Isomers of alkanes have similar chemical properties but different physical properties. For isomers, as the length of carbon chain increases, intermolecular forces increase (due to larger surface are between molecules), resulting in lower boiling point (more energy required to break attractions).
• Naming of Isomers of Alkanes


  1. Count the number of carbons in the longest continuous chain and use the corresponding ending (e.g. 4 carbons is butane).

  2. Count the number of methyl (CH3) groups and use the corresponding prefix (e.g. 2 methyls is dimethyl).

  3. Count the number of ethyl (CH2CH3) groups and use the corresponding prefix.

  4. Write the position of the methyl or ethyl groups in terms of which carbon of the longest continuous chain they are bonded to. This should be the smaller number rather than the larger number.

  
  

  E.g.

  2-methylhexane

  2,3-dimethylpentane

  3-ethylpentane

  4-ethyl-2,3-dimethylhexane
• Halogenation of Alkanes


  Apart from taking part in combustion reactions, alkanes are very unreactive. Halogenation reactions turn alkanes into more reactive and useful 'haloalkanes'. This reaction requires UV light.

  
  

  When an alkane reacts with a halogen, a substitution reaction takes place. For every hydrogen atom that is substituted, one diatomic molecule of halogen is required and one molecule of hydrogen halide is produced.
• Substitution Reaction


  a reaction in which an atom or group of atoms is replaced with a different atom or group of atoms (alkanes undergo substitution reactions)
• Halogenation of Methane


  In UV light, the halogen reacts with methane so fast that it can explode. A mixture of products is produced as further substitution takes place.

  
  

  methane + bromine → bromomethane + hydrogen bromide

  CH4 + Br2 → CH3Br + HBr

  bromomethane + bromine → dibromomethane + hydrogen bromide

  CH3Br + Br2 → CH2Br2 + HBr

  
  

  Methane can also be reacted with bromine to form tribromomethane (CHBr3) and tetrabromomethane (CBr4).
• Alkenes


  a homologous series of unsaturated hydrocarbons (i.e. contain one carbon-carbon double bond) with the general formula C(n)H(2n)
• Naming Alkenes


  1. Count the number of carbons so that that double bond has the lowest possible number.

  2. Choose the correct prefix (e..g 4 carbons is but-)

  3. Add the "ene" ending.

  4. Add the number and position of any methyl groups at the start.
• Halogenation of Alkenes


  alkene + halogen → haloalkane (addition reaction)
• Addition Reaction


  a reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule (alkenes undergo addition reactions)