AlcoholsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the alcohol homologous series?
Alcohols are a family of compounds (a homologous series) that all contain the functional group −OH. The first three members are:
| Name | Formula |
|---|---|
| Methanol | CH3OH |
| Ethanol | C2H5OH |
| Propanol | C3H7OH |
Each member differs from the next by a −CH2− unit, and all members share similar chemical properties because they share the same functional group.
Section 2
What are the properties of methanol, ethanol and propanol?
Methanol, ethanol and propanol share these properties:
- They dissolve in water to form a neutral solution
- They react with sodium to produce hydrogen gas
- They burn in air (combustion), releasing energy
- They are used as fuels and solvents
Ethanol is the alcohol found in alcoholic drinks and is widely used as a solvent and fuel additive.
Adding a small piece of sodium to ethanol produces fizzing as hydrogen gas is released, similar to (but less vigorous than) sodium reacting with water.
Section 3
How do alcohols react with sodium?
When an alcohol reacts with sodium metal, hydrogen gas is released and a solid compound forms (a sodium alkoxide). This is similar to the reaction between sodium and water, but generally less vigorous.
General word equation: alcohol + sodium → sodium alkoxide + hydrogen
You can be asked to describe this reaction as evidence for the presence of the −OH group, since sodium reacts with −OH groups in a way it does not with simple hydrocarbons.
Section 4
What happens when ethanol is oxidised?
Ethanol can be oxidised to form ethanoic acid, a carboxylic acid. This oxidation can happen by:
- Using chemical oxidising agents (e.g. in the laboratory)
- Microbial action — bacteria in the air oxidise ethanol in wine or cider, which is why alcoholic drinks left open can turn sour (this is how vinegar is made)
Ethanoic acid is the main acid found in vinegar.
Do not confuse fermentation (yeast converting sugar to ethanol) with the oxidation of ethanol to ethanoic acid — they are different processes involving different organisms and different products.
Must Know
- Alcohols contain the functional group −OH; methanol, ethanol and propanol are the first three members
- All three dissolve in water to give a neutral solution
- All three react with sodium to release hydrogen gas
- All three burn in air and are used as fuels and solvents
- Ethanol is the alcohol found in alcoholic drinks
- Ethanol can be oxidised (by chemical agents or microbial action) to form ethanoic acid, the acid in vinegar
That's the notes covered.
Carry on to the next subtopic.