Polymers, Alcohols and Carboxylic Acids
Alkanes and Alkenes
Hydrocarbons are compounds containing only hydrogen and carbon atoms. Alkanes are saturated — every carbon atom is bonded to as many hydrogen atoms as possible. Alkenes are unsaturated, containing at least one carbon—carbon double bond.
The distinction matters because the double bond in alkenes is chemically reactive. You can test for an alkene using bromine water: alkenes decolourise the orange solution to colourless because bromine adds across the double bond. Alkanes do not react, so the bromine water stays orange.
Alkanes are saturated (single C—C bonds) and do not decolourise bromine water. Alkenes are unsaturated (contain a C=C double bond) and do decolourise bromine water from orange to colourless.
Addition Polymerisation
Addition polymerisation occurs when alkene monomers react together to form a polymer. The double bond in each monomer opens up to join the chain. No other product is formed. A simple example is ethene (\(\text{CH}_2{=}\text{CH}_2\)) polymerising to form poly(ethene):
$$n\, \text{CH}_2{=}\text{CH}_2 \rightarrow \text{---}(\text{CH}_2\text{---}\text{CH}_2)_n\text{---}$$
The monomer is written inside brackets with a double bond; the polymer is shown with the repeat unit inside brackets and the subscript n indicating many thousands of units. Other addition polymers include poly(propene) and poly(chloroethene) (PVC).
To draw an addition polymer from a monomer: (1) remove the double bond; (2) draw a single bond on each side of the two carbons that formed the double bond; (3) add a bracket around the repeat unit with bonds crossing each bracket; (4) add subscript n. To identify the monomer from a polymer: reverse the process — add the double bond back between the two carbons in the repeat unit.
Condensation Polymerisation
Condensation polymerisation links monomers that each have two functional groups. Each time two monomers join, a small molecule — usually water — is eliminated. Unlike addition polymerisation, the monomers do not need a double bond.
Two important condensation polymers are polyesters and polyamides. Polyesters form ester links (—COO—) between a diol and a dicarboxylic acid. Polyamides, such as nylon, form amide links (—CONH—) between a diamine and a dicarboxylic acid. Polyamides are useful background: this course only asks about polyesters.
Addition polymerisation: monomers have C=C double bonds; no other product. Condensation polymerisation: monomers have two functional groups; small molecule (usually water) released at each link. Polyesters and nylon are condensation polymers.
Alcohols
Alcohols form a homologous series. All alcohols contain the hydroxyl (—OH) functional group. The first four members are methanol, ethanol, propanol, and butanol. Ethanol (C₂H₅OH) is the most important for GCSE: it is the alcohol in drinks, and it is widely used as a fuel and solvent.
Ethanol can be made in two ways: by reacting ethene with steam in the presence of an acid catalyst (hydration — industrial process, uses a finite resource; background only on this course), or by fermentation of sugars using yeast.
In fermentation, yeast converts glucose to ethanol and carbon dioxide under anaerobic (oxygen-free) conditions and at an optimum temperature of around 25–35 °C:
$$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$$ $$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$
The ethanol concentration cannot rise above about 12–15% because higher concentrations kill the yeast. To obtain pure ethanol, the mixture is distilled. Fermentation uses a renewable resource (sugar) and happens at room temperature, making it lower energy than industrial hydration.
Carboxylic Acids
Carboxylic acids contain the —COOH functional group. They are weak acids — they partially ionise in water. The first members are methanoic acid, ethanoic acid (CH₃COOH), propanoic acid, and butanoic acid. Ethanoic acid is found in vinegar.
Carboxylic acids react with carbonates to produce a salt, water, and carbon dioxide. They also react with alcohols in the presence of an acid catalyst to form esters and water. Esters have fruity smells and are used as food flavourings and perfumes. Esters are background only: this course does not test how they are made.
In this core practical, a spirit burner containing a known mass of alcohol is used to heat a measured volume of water in a calorimeter. The temperature rise of the water is recorded. Key variables to control: mass of water, draught conditions, distance between flame and calorimeter. The energy released is calculated using \(Q = mc\Delta T\), where \(m\) is the mass of water, \(c\) is the specific heat capacity of water (4.18 J/g°C), and \(\Delta T\) is the temperature change. The mass of alcohol burned (from the change in mass of the spirit burner) gives the energy per gram.
Limitations include heat loss to surroundings, incomplete combustion, and evaporation of alcohol. These mean the experimental value is lower than the standard enthalpy of combustion. Repeats and averaging reduce random error.
Recycling Polymers
Most addition polymers are non-biodegradable, meaning they persist in the environment for centuries. This creates significant waste problems. Polymers can be recycled mechanically (sorting, melting, re-moulding) or by chemical methods such as cracking back into monomers. Some condensation polymers (e.g. polyesters) can be chemically recycled more easily than addition polymers.
Alternatively, some polymers can be used as a fuel by incineration, recovering some energy. However, this releases carbon dioxide and may release harmful gases if chlorine-containing plastics like PVC are burned. Biodegradable plastics (bioplastics made from plant materials) offer a more sustainable future but currently have higher production costs and limited mechanical strength.
Non-biodegradable polymers persist in landfill and oceans for centuries. Solutions include mechanical recycling, chemical recycling (cracking), incineration (energy recovery but CO₂ released), and biodegradable bioplastics (sustainable but costly).