required practical 10

prepare and purify a pure organic solid (aspirin) and then test its purity

• weigh out around 6g of salicylic acid into a 100cm3 round-bottomed flask • use a 10cm3 measuring cylinder to add 10cm3 of ethanoic anhydride to the flask and swirl the contents • add 5 drops of concentrated sulfuric acid to the flask and swirl the mixture for a few minutes to ensure thorough mixing • fit the flask with a reflux-water condenser and gently warm using a hot water bath for 20 mins about about 60 degrees • allow the flask to cool and pour its contents into 75cm3 of water in a beaker (stir well to precipitate solid) • filter off the aspirin under reduced pressure (Buchner filtration) and leave to dry

• dissolve the aspirin in a minimum volume of hot (near boiling) solvent (one which dissolves compound and impurities when hot but compound doesn’t dissolve well in when cold) - minimum volume obtains saturated solution and allows crystallisation to occur with cooling, step ensures production of pure aspirin crystals • hot filter the solution through hot filter paper quickly - removes insoluble impurities and heat prevents crystals reforming during filtration • insert a beaker in ice to cool the filtered solution - crystals reform but soluble impurities remain as solution as they are present in small quantities so solution isn’t saturated, ice increases yield of crystals • suction the filtrate with a Buchner flask to separate the aspirin crystals - water pump connected to flask reduces pressure and speeds up filtration • wash aspirin with ice-cold distilled water - removes soluble impurities, and if water is at room temperature some crystals will dissolve which lowers the yield • dry aspirin between absorbent paper - removes excess water which would affect percentage yield

• some crystals may be lost in filtering or washing • some product will stay in the solution after recrystallisation • other side reactions will occur

• crush the aspirin with the back of a spatula onto some filter paper to ensure it is dry and finely powdered • put the aspirin into a capillary tube • strap the capillary tube containing the aspirin to a thermometer • fill the Thiele tube with mineral oil which has a boiling point higher than the melting point of the aspirin and a low flammability • place the thermometer and capillary tube into the Thiele tube and secure in place using a stopper with a wedge cut • heat the Thiele tube and determine the approximate melting point range by recording the temperature that the first signs of melting appear and then the temperature at which the whole sample becomes a liquid • repeat with a slower heating rate to get a more accurate measurement

• if the sample is pure it will have one melting point at the same value quoted in data books • if the sample is impure the melting point will be lowered and the sample will melt over a range of temperatures

• add ethanol followed by iron (III) chloride to the aspirin • if the aspirin is pure there will be no visible change and if it isn’t (salicylic acid present) there will be a colour change to purple

prepare a pure organic liquid (ethyl ethanoate)

• add 25cm3 of ethanol and ethanoic acid to a 100cm3 round-bottomed flask and mix the contents thoroughly by swirling • add 2cm3 of concentrated sulfuric acid slowly while swirling • add a few anti-bumping granules to the flask • set up a condenser in a vertical position and clamp in place • gently boil the mixture with an electric heating mantle for abut 15 minutes, and then leave to cool

• pour the cooled mixture into a beaker containing sodium carbonat solution (reacts with any unreacted acid) • transfer the contents of this beaker to a separating funnel and add a stopper • invert the separating funnel and then open the stopcock to release the pressure - repeat several times • two layers will form - the aqueous bottom layer should be drained and discarded, and the top layer containing the organic product should be run into a clean, dry beaker

• add some anhydrous calcium chloride to the beaker containing the ester to soak up any remaining water • decant the liquid into a round-bottomed flask containing anti-bumping granules • ensure the distillation apparatus is set up correctly (heating mantle, thermometer at T junction connecting condenser to flask, condenser with water entering at the bottom) • distill to collect the pure ester at a temperature of about 80 degrees

• solid at room temperature • sparingly soluble in cold water • much more soluble in hot water