Section 1 of 5Three Routes
Combustion
- Ethanol burns in air to form carbon dioxide and water, releasing heat energy.
Aerobic oxidation
- Acetobacter bacteria use oxygen from the air to oxidise ethanol to ethanoic acid, which is why wine turns sour when left open.
Oxidising agent
- Heating ethanol with acidified potassium dichromate(VI) oxidises it to ethanoic acid, and the solution turns from orange to green.
Meaning of oxidation
- Oxidising an alcohol adds oxygen to it or removes hydrogen from it, and a fully oxidised alcohol becomes a carboxylic acid.
Vinegar
- Vinegar is a dilute aqueous solution of ethanoic acid, about 5% by volume.
Section 2 of 5Equations and Conditions
1
The mixture of ethanol, acidified dichromate and water is heated under reflux for about an hour.
2
A condenser is fitted above the flask so that volatile liquids condense and return rather than escaping.
3
The reaction is slow, so the mixture must be heated for a long time to give a useful amount of product.
4
The colour change from orange dichromate(VI) to green chromium(III) shows that the oxidising agent has been reduced.
| Route | Equation | Observation or condition |
|---|---|---|
| Combustion | C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O | Burns with an almost invisible blue flame |
| Aerobic oxidation | C₂H₅OH + O₂ → CH₃COOH + H₂O | Acetobacter bacteria, with air present |
| Oxidising agent | CH₃CH₂OH + [O] → CH₃COOH + H₂O | Heat under reflux with K₂Cr₂O₇ in H₂SO₄; orange to green |
Section 3 of 5Practical Points
Practical Points
Product
- Full oxidation of ethanol gives ethanoic acid, a weak carboxylic acid containing the –COOH group.
Vinegar
- Vinegar is made commercially by the same aerobic oxidation of ethanol, using acetobacter bacteria.
Acidic conditions
- Sulfuric acid is needed to provide the acidic conditions for the dichromate oxidation.
Reagent safety
- Potassium dichromate(VI) is toxic and an oxidising agent, so gloves and eye protection must be worn.
Section 4 of 5Worked Example
Question Balance the equation C₂H₅OH + O₂ → CH₃COOH + H₂O.
- Atoms on the leftC 2, H 6, O 1 + 2 = 3
- Atoms on the right with one O₂ and one each of CH₃COOH and H₂O: C 2, H 4 + 2 = 6, O 2 + 1 = 3
- All atoms balance with a coefficient of 1 for O₂
Answer C₂H₅OH + O₂ → CH₃COOH + H₂O, already balanced as written.
Section 5 of 5Exam Checklist & Tips
4.5.2 – Key Points for the Exam
- Oxidation A reaction in which oxygen is added to a substance or hydrogen is removed from it.
- Aerobic oxidation Oxidation that needs oxygen from the air, here carried out by bacteria.
- Potassium dichromate(VI) An orange oxidising agent that is reduced to green chromium(III) ions.
- Reflux Heating a reaction mixture while condensing its vapour so that it returns to the flask.
- Ethanoic acid The carboxylic acid CH₃COOH formed by oxidising ethanol.
Common Mistakes
- Forgetting oxygen as a reactant when balancing the combustion of ethanol.
- Stating that the dichromate solution turns from green to orange; it turns from orange to green.
- Omitting the condenser, which means volatile liquids can escape from the reaction.
- Saying that aerobic oxidation happens without oxygen; the word aerobic means oxygen is needed.