AQA A-level chemistry chemical tests Questions and Answers

AQA A-level chemistry chemical tests Questions and Answers What is the process for purifying an organic liquid (recrystallisation)? Dissolve impure compound in minimum volume of hot solvent Hot filter solution through (fluted) filter paper quickly Cool the filtered solution by inserting beaker in ice Suction filtrate with a Buchner flask to separate out crystals Wash the crystals with distilled water Dry the crystals between absorbent layer What is the purpose of using the minimum volume of hot water in recrystallisation? To obtain a saturated solution and enable crystallisation on cooling What is the purpose of hot filtering through filter paper in recrystallisation? To remove insoluble impurities Heat will prevent crystals reforming What is the purpose of cooling in ice in recrystallisation? To increase amount of crystals that are formed What is the purpose of washing with cold water in recrystallisation? To remove soluble impurities What could cause a loss of yield in recrystallisation? Crystals lost when filtering or washing Some product stays in solution after recrystallisation Other side reactions occur What could cause yield to be greater than 100% for recrystallisation? If the crystals are not dried properly How do you measure melting point for an organic solid? Heat melting tube in oil bath Slowly near melting point Why may the experimental melting point for a pure organic solid be different to the actual melting point? Temperature on thermometer not the same as sample State two observations when measuring the melting point of an organic solid that would indicate it is not pure? Melting range would be wide Melting range below/ before true melting point Draw the apparatus to filter under reduced pressure How do you determine that an organic solid is pure? Measure melting point (by heating melting tube in oil bath) Sharp melting point/ melting point that matches data source value = pure How would you purify an organic liquid? Put the distillate of impure product into separating funnel Wash product by either: Sodium hydrogen carbonate solution, shaking and releasing pressure from carbon dioxide produced Saturated sodium chloride solution Allow the layers to separate in the funnel, and then run and discard the aqueous layer Run the organic layer into a clean, dry conical flask and add drying agent (e.g. anhydrous sodium sulfate or calcium chloride) to dry the organic liquid When dry, the organic liquid should appear clear Carefully decant the liquid into the distillation flask Distill to collect pure product What does sodium hydrogencarbonate do when purifying an organic liquid? Remove any remaining reactant acid