Combustion
Carbon and its compounds burn in oxygen to give carbon dioxide, water, heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light; C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat + light.
- Saturated hydrocarbons usually burn with a clean blue flame.
- Unsaturated hydrocarbons burn with a yellow, sooty flame (more carbon, incomplete burning).
- If air is limited, even a saturated fuel gives a sooty flame, which is why a stove whose air holes are blocked blackens pots.
Coal and petroleum contain nitrogen and sulphur, so burning them also releases oxides of sulphur and nitrogen, which cause acid rain.
Oxidation
Combustion is a kind of oxidation, but alcohols can also be oxidised in a controlled way. Alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇) are oxidising agents: they give oxygen to other substances.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (on heating with alkaline KMnO₄).
The pink colour of permanganate disappears while it is being used up; once all the ethanol is oxidised, extra KMnO₄ stays pink.
Addition reaction
Unsaturated compounds add atoms across their double or triple bond. With hydrogen and a catalyst (nickel or palladium), ethene becomes ethane: CH₂=CH₂ + H₂ → CH₃–CH₃.
A catalyst changes the speed of a reaction without being used up.
Hydrogenation of oils
Vegetable oils have long unsaturated chains. Adding hydrogen with a nickel catalyst turns them into solid saturated fats (vanaspati). Unsaturated oils are considered healthier; saturated fats in excess are linked to heart problems.
Substitution reaction
Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine replaces their hydrogen atoms one by one: CH₄ + Cl₂ → CH₃Cl + HCl. With more chlorine, CH₂Cl₂, CHCl₃ and CCl₄ can form.
Addition vs substitution: addition needs a double/triple bond and gives one product; substitution swaps an atom and gives two products.
Ethanol (ethyl alcohol)
Ethanol C₂H₅OH is a colourless liquid at room temperature, mixes with water in all proportions and is a good solvent (tinctures, cough syrups). It is the active part of alcoholic drinks; even small amounts affect the body and brain, and methanol (sometimes mixed in illegally) can cause blindness and death. Industrial alcohol is denatured by adding methanol and a blue dye so that it cannot be drunk.
Reactions of ethanol
- With sodium: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂↑ (sodium ethoxide + hydrogen).
- Dehydration: heating with excess concentrated H₂SO₄ at 443 K removes water: CH₃CH₂OH → CH₂=CH₂ + H₂O.
Ethanol mixed with petrol is used as a cleaner fuel (ethanol blending from sugarcane).
Ethanoic acid (acetic acid)
Ethanoic acid CH₃COOH is a weak acid (it only partly ionises). Pure ethanoic acid freezes at about 290 K (17 °C), so it looks like ice in cold weather: glacial acetic acid. Vinegar is a 5–8% solution in water.
- Esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (with conc. H₂SO₄). The ester, ethyl ethanoate, smells sweet and fruity.
- Saponification: an ester + NaOH → alcohol + sodium salt of the acid. This is how soap is made.
- With a base: CH₃COOH + NaOH → CH₃COONa + H₂O.
- With carbonates/hydrogencarbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ (turns lime water milky).
Key formulas and definitions
- Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
- Oxidation: CH₃CH₂OH → CH₃COOH (alk. KMnO₄ or acidified K₂Cr₂O₇, heat)
- Addition: CH₂=CH₂ + H₂ → CH₃CH₃ (Ni catalyst)
- Substitution: CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)
- 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
- C₂H₅OH → C₂H₄ + H₂O (hot conc. H₂SO₄, 443 K)
- CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (acid catalyst)
- CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa (saponification)
- CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
Worked examples
1. Write the balanced equation for complete combustion of ethane.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + heat + light. (Balance C first: 4, then H: 12 → 6H₂O, then O: 8 + 6 = 14 atoms = 7O₂.)
2. Why does a saturated hydrocarbon give a clean flame but naphthalene gives a sooty one?
Naphthalene is unsaturated and rich in carbon. There is not enough oxygen to burn all the carbon, so unburnt carbon particles make a yellow, sooty flame.
3. Which is an addition reaction: ethene + H₂ or methane + Cl₂? Why?
Ethene + H₂ is addition: two H atoms add across the C=C and one product (ethane) forms. Methane + Cl₂ is substitution: Cl swaps with H and two products (CH₃Cl + HCl) form.
4. A compound X, C₂H₆O, reacts with sodium to give a gas that burns with a pop. On oxidation with alkaline KMnO₄ it gives Y, C₂H₄O₂. Identify X, Y and the gas.
X is ethanol (C₂H₅OH), the gas is hydrogen (pop test), and Y is ethanoic acid (CH₃COOH).
5. How would you tell ethanol from ethanoic acid using baking soda?
Add NaHCO₃. Ethanoic acid fizzes and gives CO₂ (turns lime water milky); ethanol shows no fizz.
6. Y (C₂H₄O₂) is warmed with X (C₂H₆O) and a few drops of conc. H₂SO₄. A sweet smell is noticed. Name the reaction and product, then show how the product can give X back.
Esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O; product ethyl ethanoate. Heating it with NaOH (saponification) gives ethanol + sodium ethanoate.
Common mistakes
- Saying saturated hydrocarbons undergo addition. Only unsaturated compounds (with C=C or C≡C) add; saturated ones substitute.
- Forgetting the conditions: addition needs a Ni/Pd catalyst, substitution needs sunlight, esterification needs an acid catalyst.
- Writing the ethanol + Na gas as oxygen. It is hydrogen, which burns with a pop.
- Calling ethanoic acid a strong acid. It is weak: only a small fraction ionises in water.