Ethanol and ethanoic acid
Ethanol (C₂H₅OH) is the alcohol used in medicines and sanitisers. Ethanoic acid (CH₃COOH) is the acid in vinegar. Together they form a sweet-smelling ester.
Reactions of ethanol
| Reaction | Equation | What to note |
|---|---|---|
| With sodium | 2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂ | Hydrogen gas is given off; sodium ethoxide forms |
| With hot concentrated sulphuric acid, at 443 K | CH₃CH₂OH → CH₂=CH₂ + H₂O | Concentrated H₂SO₄ removes water (it is a dehydrating agent), giving ethene |
| Oxidation | CH₃CH₂OH → CH₃COOH | Write alkaline KMnO₄ + heat, or acidified K₂Cr₂O₇ + heat, over the arrow; these oxidising agents add oxygen |
| Combustion | CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O | Burns with a clean flame, giving heat and light |
Uses and dangers of ethanol
- A good solvent: used in tincture of iodine, cough syrups and many tonics.
- Used in hand sanitisers, and as a fuel mixed with petrol. Ethanol can be made by fermenting molasses from sugarcane.
- Even a small amount of pure ethanol (absolute alcohol) can be lethal. Drinking alcohol over a long time causes serious health problems.
- Denatured alcohol is ethanol made unfit to drink by adding poisonous methanol and a blue dye.
Reactions of ethanoic acid
| Reaction | Equation | Product |
|---|---|---|
| Esterification: with ethanol and an acid catalyst | CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O | Ethyl ethanoate, a sweet-smelling ester used in perfumes and flavours |
| Saponification: the ester with sodium hydroxide | CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa | Gives back the alcohol and the sodium salt of the acid; this kind of reaction is used to make soap |
| With a base | NaOH + CH₃COOH → CH₃COONa + H₂O | Sodium ethanoate (sodium acetate) and water |
| With a carbonate | 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ | Carbon dioxide turns lime water milky |
| With a hydrogencarbonate | CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ | Brisk effervescence |
Activity: properties of ethanoic acid
- 1Odour: gently waft the vapour towards your nose.It smells of vinegar
- 2Solubility: add 1 mL of ethanoic acid to 5 mL of water and shake.It dissolves completely
- 3Litmus: put a drop on blue litmus paper.Blue litmus turns red, so it is an acid
- 4★Sodium hydrogencarbonate: add a pinch to 2 mL of ethanoic acid, and pass the gas into lime water.Brisk fizzing; the lime water turns milky, so the gas is CO₂
- 5Esterification (shown by the teacher): warm 1 mL of ethanol with 1 mL of glacial acetic acid and a few drops of concentrated H₂SO₄. Keep it in a water bath for 5 minutes, then pour it into water.A sweet, fruity smell shows that an ester has formed
★ key step
Ethanol vs ethanoic acid
Pure ethanoic acid freezes at 290 K (about 17 °C), so in cold weather it looks like ice. That is why it is called glacial acetic acid.
Vinegar helps keep pickles from spoiling, so it is used as a preservative. Fruity esters give many sweets and soft drinks their flavour.
2 marks: “Two unlabelled bottles hold colourless liquids: one is ethanol and the other is ethanoic acid. Using only baking soda and lime water, how will you tell which is which?” Add a pinch of baking soda (NaHCO₃) to each. Ethanoic acid gives brisk effervescence of CO₂, which turns lime water milky; ethanol shows no reaction.
3 marks: “What is esterification? Write the equation for the reaction of ethanoic acid with ethanol, and give one use of the product.” An acid and an alcohol react to give an ester and water, with an acid catalyst. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. Esters are used in perfumes and flavourings.
3 marks: “What happens when ethanol is heated with excess concentrated sulphuric acid at 443 K? Write the equation and state the role of the acid.” Ethene is formed: CH₃CH₂OH → CH₂=CH₂ + H₂O. Concentrated H₂SO₄ is a dehydrating agent: it removes water from ethanol.
Acid + alcohol = ester + water (sweet smell). Ester + NaOH = alcohol + salt (soap-making). Ethanoic acid freezes at 290 K, so it is glacial.