C3 Topic 5

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A homologous pair is a group of compounds with the same general formula which describes all the compounds of that group. Therefore all the compounds in that group will have the same chemical properties and react in a similar way. The physical properties will vary gradually as the compounds increase in size. Take for example, the boiling points.. In a homologous series the boiling point increases as the size of the molecules increases. 

Alkanes: They are a chain of carbon atoms link with a single bond. They are saturated hydrocarbons because they have no spare bonds and this is why they don't turn bromine water colourless because they have no spare bonds. They won't form any polymers as they have no spare bonds. The general formula is CnH2n+2. 

Examples of Alkanes: 

Methane. It has a formula of CH4. Ethane has a formula of C2H6. Propane has a formula of C3H8. Butane has a formula of C4H10. 

Alkenes: These are chains of carbon atoms with one or more double bonds. They are unsaturated hydrocarbons because they have spare bonds.  They decolourize bromine water because they bind with the bromine atoms. They form polymers by opening their bonds to have a long chain. Alkenes general formula is: CnH2n

Examples of alkenes are: Ethene C2H4. Properne which is C3H6 and Butene which is C4H8. 

The properties of an organic molecule are predominately determined by the properties of the functional group in that compound. Functional group are atoms or a combination of atoms attached to a carbon chain. 

Alcohols: The carbon chains ends with an OH. The general formula of an alcohol is CnH2n+1OH. The basic naming system applies, but it replaces the end with 'ol'. 

Properties of alcohol: 

  • Short chains alcohols are soluble in water. This is due to the hydroxyl group of alcohol interacting with the water molecules. 
  • As the chain length increases, they become less soluble as the hydrocarbon chain cannot form intermolecular bonds with the water molecules. 

Carboxylic Acids: The carbon chain ends with COOH. The general formula is CnH2n+1COOH. The acids end with 'anoic acid'. In the diagrams the carbon at the end has a double bond with the oxygen then a single bond with the OH. 

Properties of carboxylic acids: 

  • Have relatively high melting and boiling point. 
  • Are very soluble in water.
  • Are weak acids
  • Have a characteristic smell. 

Examples of everyday carboxylic acids: 

  • citric acid (found in citrus fruit)
  • Lactic acid (found in tired muscles)
  • malic acid (found in sour and tart food)
  • ethanoic acid is in vinegar: This occurs when the ethanol is oxidised to ethanoic acid. The reaction is used for commercial production of vinegar and it is used to flavouring and preserving foods. 

Carboxylic acids react like any other acid. Therefore acids react with metal to form a metal salt and water. Acids react with a base to produce salt and water. Acids react with carbonates to produce salt, water and carbon dioxide. Ethanoic acids are similar indicators as acids for example, it will…


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