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Ethanol Reactions and Oxidation

Distinguish ethanol combustion from oxidation to ethanoic acid, conserve oxygen atoms, and interpret acidified manganate(VII) observations.

  • SEC G3 Pure Chemistry 2027

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Burning ethanol and making vinegar both involve oxidation, but their carbon-containing products are different. Follow the carbon skeleton and count every oxygen source before choosing an equation.

Combustion

Complete combustion of ethanol in sufficient oxygen produces carbon dioxide and water:

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

Water is shown as a liquid after cooling; it is vapour in the hot flame. The products contain seven oxygen atoms. Ethanol already supplies one, so three oxygen molecules supply the other six. Do not balance it as though ethanol were a hydrocarbon with no oxygen.

Oxidation to ethanoic acid

The required example is ethanol to ethanoic acid. The two-carbon skeleton remains while the end functional group changes:

Keep both carbons, change the functional group

In ethanol the second carbon bonds to two hydrogens and OH. In ethanoic acid it has a double bond to oxygen and a single bond to OH. The two-carbon skeleton remains.

Scroll across the graph to read all labels.

Displayed structures of ethanol and ethanoic acid showing the OH-bearing end carbon before and after oxidationDisplayed structures of ethanol and ethanoic acid showing the OH-bearing end carbon before and after oxidation
The comparison shows the organic molecules. The full equations also account for the oxidant and water.
View figure data
Groups on the second carbon
MoleculeMolecular formulaFunctional groupAtoms on the second carbon
ethanolC2H6O-OH2 H and -OH
ethanoic acidC2H4O2-CO2H=O and -OH

Atmospheric oxygen is the oxidant in vinegar production, where bacteria catalyse the conversion:

C₂H₅OH + O₂ → CH₃COOH + H₂O

Exposure to air does not mean every bottle of ethanol instantly becomes an acid. The oxygen participates in an oxidation process under suitable conditions.

Acidified potassium manganate(VII) also oxidises ethanol to ethanoic acid. When a small amount of its purple solution reacts with sufficient ethanol, the purple colour disappears as the manganate(VII) is reduced. Unused excess reagent can leave the solution purple.

Using [O] to represent oxygen supplied by the oxidising agent:

C₂H₅OH + 2[O] → CH₃COOH + H₂O

This is a balanced shorthand for the organic change, not a full equation containing every species from the acidified reagent. The OH position affects alcohol oxidation; do not assume that all the alternative alcohol structures give carboxylic acids in this way.

Balance and explain the changes

Common misconception 1

Combustion equation

Find and correct the mistake

Learner equation

A learner writes C₂H₅OH + 4O₂ → 2CO₂ + 3H₂O for complete combustion of ethanol, with water collected after cooling. Correct the oxygen coefficient and explain the counting error.

Count oxygen on both sides, including ethanol

O atoms in products
Required O2 coefficient

View solution step by step

  1. Balance carbon and hydrogen

    Method

    Use two carbon dioxide and three water molecules.

    Reason

    Ethanol contains two C atoms and six H atoms.

    Working

    C₂H₅OH + O₂ → 2CO₂ + 3H₂O.

  2. Count every oxygen source

    Method

    Use three O₂ molecules.

    Reason

    The products contain seven O atoms; ethanol already supplies one, so O₂ must supply the remaining six.

    Working

    C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

Guided practice 2

Two routes to ethanoic acid

About 6 min

Guided reaction comparison

State two ways to oxidise ethanol to ethanoic acid. For the reagent method, give the expected colour change when a small amount of oxidant is fully used up by sufficient ethanol.

Connect both routes to the same product

Air route
Acidified manganate(VII) colour change
Organic product

Hints

Hint 1: two oxidants
One route uses oxygen in air; the other uses the prescribed purple oxidising agent.
Hint 2: organic change
Both routes follow ethanol → ethanoic acid.
View solution step by step

  1. State the atmospheric route

    Method

    Use atmospheric oxygen, with bacteria catalysing the process.

    Reason

    In vinegar production, bacteria catalyse the oxidation of ethanol using oxygen from air.

    Working

    C₂H₅OH + O₂ → CH₃COOH + H₂O.

  2. State the reagent route

    Method

    Use acidified potassium manganate(VII) and record purple to colourless.

    Reason

    The reagent is reduced while oxidising ethanol to the same organic product.

    Working

    C₂H₅OH + 2[O] → CH₃COOH + H₂O; product: ethanoic acid.

Try without prompts

Mind stretcher 1: Follow the reactions through a processExtension

Glucose solution is fermented with yeast in a warm vessel fitted with an airlock. The ethanol-rich product is later exposed to oxygen with the bacteria used in vinegar production. Identify the organic product at each step and explain why the air conditions differ. In a separate test, a small amount of acidified purple potassium manganate(VII) remains purple: does that observation alone prove there is no ethanol? Explain.

Show answer

First, fermentation makes ethanol and carbon dioxide; the airlock limits incoming oxygen while CO2 escapes. Second, bacteria use oxygen to oxidise ethanol into ethanoic acid, so oxygen is now a reactant. The conditions suit different processes.

A remaining purple colour alone is not proof that ethanol was absent. Some reagent may remain unreacted, for example because it was in excess or the reaction was incomplete. Interpret the observation with the quantities and reaction conditions. When the small amount of reagent is fully used up in the prescribed ethanol oxidation test, purple changes to colourless.

Bring the reactions together

Complete combustion produces carbon dioxide and water; the studied oxidation to ethanoic acid keeps ethanol’s two-carbon skeleton. The oxygen already in ethanol counts in both cases. Use the reacting substances and conditions to interpret a colour change; an observation alone does not identify every possible product.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.

Syllabus and review details

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