SQA National 5 Chemistry Area 2 Nature's Chemistry: a complete overview of the homologous series, alcohols, carboxylic acids and energy from fuels

Area 2, Nature's Chemistry, is the study of carbon compounds and the energy they release. It runs on two big ideas: the functional group that names and controls each family, and combustion, the reaction that turns fuels into energy. This guide pulls the four key areas together; each has its own key-area page with worked questions.

The homologous series

A homologous series is a family of carbon compounds with the same general formula, similar chemical properties, and each member differing from the next by $\text{CH}_2$.

• Alkanes ($\text{C}_n\text{H}_{2n+2}$) are saturated, with only single carbon to carbon bonds. The first eight are methane, ethane, propane, butane, pentane, hexane, heptane, octane.
• Cycloalkanes ($\text{C}_n\text{H}_{2n}$) are saturated rings, such as cyclopropane and cyclobutane.
• Alkenes ($\text{C}_n\text{H}_{2n}$) are unsaturated, containing a carbon to carbon double bond, such as ethene and propene.

Isomers are compounds with the same molecular formula but a different structural arrangement, such as butane and 2-methylpropane.

Saturation and the bromine test

A saturated molecule has only single carbon to carbon bonds; an unsaturated molecule has a double bond. The double bond lets alkenes take part in addition reactions, where a small molecule adds across it. This gives the test for unsaturation: bromine solution is quickly decolourised by an alkene but not by an alkane or cycloalkane.

Alcohols

Alcohols contain the hydroxyl functional group, -OH, and are named from the matching alkane with the -ol ending: methanol, ethanol, propanol. The -OH group makes the lower alcohols mix freely with water and raises their boiling points above those of alkanes of similar size. Alcohols are flammable and are used as solvents and fuels; ethanol is the alcohol in drinks and is blended into petrol.

Carboxylic acids

Carboxylic acids contain the carboxyl functional group, -COOH, and are named with the -oic acid ending: methanoic acid, ethanoic acid, propanoic acid. Because the carboxyl group releases hydrogen ions, these compounds are acids and react with bases, alkalis, metal oxides, metal hydroxides and metal carbonates to form a salt and water (and carbon dioxide with carbonates). Ethanoic acid is the acid in vinegar.

Energy from fuels

Fossil fuels (coal, oil, natural gas) are finite resources. Burning them is combustion, an exothermic reaction that releases energy:

• Complete combustion (plenty of oxygen) gives carbon dioxide and water.
• Incomplete combustion (too little oxygen) gives the poisonous gas carbon monoxide and soot.

The energy a fuel transfers to water is calculated with:

where $c$ is the specific heat capacity of water ($4.18 \text{ kJ kg}^{-1}\,{}^\circ\text{C}^{-1}$), $m$ is the mass of water in kilograms, and $\Delta T$ is the temperature rise.

How to revise Area 2

1. Memorise the general formulae and functional groups precisely, because the naming marks turn on exact wording.
2. Be able to state the bromine test result and what it shows.
3. Drill the energy calculation, converting the mass of water to kilograms.
4. Finish with SQA past papers and marking instructions.

For the official course specification, visit sqa.org.uk and always revise from the current specification.