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Edexcel Separate Sciences · Chemistry · Paper 2

SC23 · Alcohols and carboxylic acidsTopic 9 — Separate chemistry 2

Organic functional groups, alcohol combustion and carboxylic-acid reactions

Revise the key ideas

Alcohol structures and reactions

  • Alcohols contain the –OH functional group attached to carbon, called a hydroxyl group. A functional group is the part of a molecule responsible for its characteristic reactions. The alcohol’s covalently bonded –OH is different from a free hydroxide ion, OH⁻, in an alkali.
  • The first four straight-chain primary alcohols here are methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH and butan-1-ol CH₃CH₂CH₂CH₂OH. Show all atoms and bonds when asked for displayed formulae.
    Displayed primary alcoholsDisplayed structures showing every atom and bond: Methanol, Ethanol, Propan-1-ol, Butan-1-ol. Each carbon has four covalent bonds.MethanolCOHHHHEthanolCHHHCOHHHPropan-1-olCHHHCHHCOHHHButan-1-olCHHHCHHCHHCOHHH
    The O–H group is bonded to the final carbon; all covalent bonds are shown.
  • These members belong to a homologous series: they share the –OH group, have similar reactions and differ successively by CH₂. Their boiling points generally rise with chain length.
  • Alcohols burn in oxygen. Complete combustion produces carbon dioxide and water; ethanol obeys C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Insufficient oxygen can produce carbon monoxide or soot.
  • Alcohols can be dehydrated to form alkenes by removing water. Ethanol forms ethene: C₂H₅OH → C₂H₄ + H₂O. Dehydration is removal of the elements of water, not simply evaporation of solvent.
    Ethanol dehydrationWater is removed to make an alkene.CH₃–CH₂–OH → CH₂=CH₂ + H₂OWater is removed to make an alkene.
    The carbon count is unchanged; an alkene double bond forms.
  • Ethanol can be oxidised to ethanoic acid. Other suitable primary alcohols can similarly form the carboxylic acid with the same carbon count; the required oxidising reagents are beyond the recall scope here.
  • Methanol is toxic, and alcohols are flammable. Do not infer that every alcohol has the uses or safety profile of ethanol.

Alcohol combustion: core practical

  • Compare ethanol, propanol, butanol and pentanol by burning them in spirit burners beneath a known mass of water. Measure initial and final water temperatures and weigh each burner before and after heating.
    Alcohol combustion calorimetryA spirit burner heats water in a container; a thermometer measures temperature rise and weighing measures fuel burned.Known water massThermometerEnergy transferred upwardsWeigh burner before / after
    Keep the separation between flame and water container constant.
  • Keep the water mass, burner-to-container distance, apparatus, starting temperature and mixing method consistent. Stir the water for a representative temperature and use a lid or draught shield where appropriate.
  • Water energy gain = mass × specific heat capacity × temperature rise. With water mass in g and c = 4.2 J/(g °C), the result is joules; energy per gram burned = energy gained ÷ fuel mass burned.
  • For comparison per mole, calculate fuel moles from mass/Mᵣ, then divide measured energy by the amount burned. Per-gram and per-mole comparisons answer different questions.
  • Heat loss to the surroundings and container, incomplete combustion, evaporation of fuel and changes in flame conditions reduce accuracy. A simple apparatus usually captures less than the true combustion energy.
  • Use repeats and means, identify anomalies and avoid concluding that a larger temperature rise means a better fuel unless the amounts burned and conditions are comparable.
  • Wear eye protection, keep spare fuel away from flames, extinguish burners safely with their caps and allow hot apparatus to cool. Never refill a burning or hot spirit burner.

Carboxylic-acid structures and properties

  • Carboxylic acids contain the –COOH functional group: a carbon with a C=O bond and an –OH group. The first four are methanoic acid HCOOH, ethanoic acid CH₃COOH, propanoic acid CH₃CH₂COOH and butanoic acid CH₃CH₂CH₂COOH.
    Displayed carboxylic acidsDisplayed structures showing every atom and bond: Methanoic acid, Ethanoic acid, Propanoic acid, Butanoic acid. Each carbon has four covalent bonds.Methanoic acidCOHOHEthanoic acidCHHHCOHOPropanoic acidCHHHCHHCOHOButanoic acidCHHHCHHCHHCOHO
    The carboxyl carbon has a C=O bond and a C–O–H group. It counts in the chain length.
  • The carbon in the –COOH group counts towards the molecule’s total carbon number. Ethanoic acid has two carbons, while methanoic acid has one; do not omit the functional-group carbon.
  • Solutions of carboxylic acids show acidic properties: they turn blue litmus red and react with suitable metals, bases, alkalis and carbonates. Many simple carboxylic acids are weak acids because only some molecules ionise in water.
  • A carboxylic acid and an alkali form a salt and water; ethanoic acid with sodium hydroxide gives sodium ethanoate and water. The salt name changes the acid ending to -oate.
  • With a carbonate, a carboxylic acid forms a salt, carbon dioxide and water. Confirm carbon dioxide by its ability to turn limewater cloudy; bubbles alone do not identify a gas.
  • With a suitable reactive metal, a carboxylic acid forms a salt and hydrogen. Reaction rates depend on the acid and metal; do not claim all metals react equally.
  • Members of the carboxylic-acid series have similar reactions because they share –COOH. Use the same pattern to predict a reaction of another member.

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