Carbon Compounds and Fuels
Carbon compounds and fuels covers hydrocarbons, functional groups, soaps and combustion. Questions are formula-based and usually ask you to distinguish two compounds by a test.
Carbon and its bonds
Carbon forms four covalent bonds, giving chains and rings - the reason for the huge number of carbon compounds. Allotropes: diamond (hard, tetrahedral), graphite (layers, conducts) and fullerene.
Hydrocarbons
Saturated hydrocarbons (alkanes) have only single C–C bonds, general formula CnH2n+2, and burn with a clean blue flame. Unsaturated ones have double (alkenes, CnH2n) or triple bonds (alkynes) and burn with a sooty flame; they decolourise bromine water, the standard test for unsaturation.
Functional groups and series
Alcohol –OH, aldehyde –CHO, ketone –CO–, carboxylic acid –COOH, halide –X. A homologous series is a family with the same functional group and general formula, in which successive members differ by CH₂. Ethanol (C₂H₅OH) is oxidised by alkaline KMnO₄ to ethanoic acid (CH₃COOH), a weak acid that turns blue litmus red and gives CO₂ with sodium hydrogencarbonate; ethanol does not.
Soaps and detergents
Soap is a sodium or potassium salt of a long-chain fatty acid. Its molecule has a hydrophilic head and a hydrophobic tail that traps oil in micelles. In hard water soap forms a scum with Ca²⁺ and Mg²⁺ and loses cleaning power; detergents do not, so they work in hard water, though some are non-biodegradable.
Fuels and combustion
Combustion needs fuel, oxygen and ignition temperature. Calorific value is the energy released per unit mass; LPG, CNG, biogas and hydrogen are common fuels. Incomplete combustion gives carbon monoxide, a poisonous gas.
Worked example
To tell ethanol from ethanoic acid, add a pinch of sodium hydrogencarbonate. Only ethanoic acid gives brisk effervescence of carbon dioxide; ethanol gives no reaction.
Traps
Saying soap lathers well in hard water; calling ethanoic acid a strong acid; forgetting that unsaturated hydrocarbons give a sooty flame.