Organic Prep & Analysis Practicals: RP10a, RP10b & RP12 (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about the organic practicals: Required Practical 10a, Required Practical 10b, testing aldehydes and ketones, and Required Practical 12.

Each of these practicals makes or separates an organic substance, and then proves what it is by comparing a measured value against a known one.

These four practicals share the same shape. An organic substance is made or separated, and then its identity or purity is confirmed by comparing a measured value with a known one. Required Practical 10a produces a pure organic solid and tests its purity by melting point. Required Practical 10b produces a pure organic liquid and uses its boiling point as evidence of identity. Tollens' reagent distinguishes an aldehyde from a ketone by what forms in the test tube. Required Practical 12 separates a mixture and identifies each component by its Rf value against reference standards.

The video takes them in that order: the solid, then the liquid, then the functional group test, then the separation.

The first practical is Required Practical 10a, the preparation of a pure organic solid and a test of its purity, a common example is the synthesis of aspirin. In recrystallisation, a hot solvent dissolves both the organic solid and the impurities, and as the solution cools the solid crystallises out, leaving the impurities behind in the solution. The key is using the minimum amount of solvent, so that product is not lost. The crystals are recovered by filtration, which is faster using Buchner apparatus, where filtration happens under reduced pressure. The melting point is then measured, and that is what proves the purity. Impurities lower the melting point and broaden the melting range, so an impure sample might melt across five degrees where a pure one melts across one.

The second practical is Required Practical 10b, the preparation of a pure organic liquid, and ethyl ethanoate is a suitable choice. Ethanol, glacial ethanoic acid and a few drops of concentrated sulfuric acid are heated under reflux for about fifteen minutes. The mixture is then poured into a saturated sodium carbonate solution, which neutralises any unreacted acids and helps the ester separate as an oily layer. The layers are then separated in a separating funnel. The ester is purified by distillation: esters do not form hydrogen bonds like alcohols and carboxylic acids, so ethyl ethanoate has a lower boiling point than the reactants. The temperature at which the distillate comes over is recorded, and that boiling point is the evidence of what has been made.

The third practical is the Tollens' test, which distinguishes an aldehyde from a ketone. Tollens' reagent contains the silver(I) complex ion, and is also known as ammoniacal silver nitrate. It is formed when aqueous ammonia is added to a solution of silver nitrate, and the solution is colourless. A few drops of the suspected aldehyde are added and the tube is gently warmed. The aldehyde is oxidised to a carboxylic acid, and in being oxidised it reduces the silver ions to solid metallic silver, which forms a silver mirror on the inside of the tube. That is the positive result. A ketone cannot be oxidised by Tollens' reagent, so no reaction takes place and the solution stays colourless. Here the known value being matched is an expected observation rather than a number.

The last practical is Required Practical 12, the separation of species by thin-layer chromatography. Separation depends on a balance between each substance's solubility in the mobile phase, the solvent moving up the plate, and its retention on the stationary phase, the coating on the plate. A baseline is drawn in pencil near the bottom, the samples are spotted onto it, and the plate stands in a covered beaker with the solvent level below the spots. When the solvent has run, the solvent front is marked and the spots are viewed under a UV lamp. The Rf value is the distance travelled by the component divided by the distance travelled by the solvent. Each spot is identified by matching its Rf against reference standards run alongside.

Four things the examiners look for. Always quote a melting point as a range, and reference a data book value if you have one. Draw the TLC baseline and the spots in pencil, not pen, because ink would travel up the plate. For a negative result with a ketone, say 'no visible change' or 'the solution stays colourless', and never write 'no observation', which examiners penalise. And esters have characteristic pleasant odours, but smell alone is not sufficient to identify them: use the boiling point of the ester, or the melting point of the carboxylic acid it is hydrolysed to.

A melting point proves a pure organic solid. A boiling point proves a pure organic liquid. A silver mirror means an aldehyde, and no visible change means a ketone. An Rf value matched against standards identifies each component of a mixture. In all four practicals the substance is made or separated first, and then confirmed by comparing a measured value with a known one.