RP10 - Preparation of a pure organic solid and test of its purity Flashcards
what’s the method for recrystallisation
1) dissolved the impure compound in a minimum volume of hot solvent
2) hot filter solution through filter paper quickly
3) cool then filtered solution by inserting beaker in ice
4) suction filtrate with a büchner flask to separate out crystals
5) wash the crystals with distilled water
6) dry the crystals between absorbent paper
what is an appropriate solvent to be using in recrystallisation
It’s on which will dissolve both compound and impurities when hot and one in which the compound itself does not
why do u have to hot filter solution through filter paper quickly in recrystallisation
this step will remove any insoluble impurities and heat will prevent crystals reforming during filtration
why do u have to cool the filtered solution in recrystallisation
To increase amount of crystals formed
why do u have to suction the filtrate with a büchner flask to separate out crystals in recrystallisation
the water pump connected to büchner flask reduced the pressure and speeds up filtration
why do you have to wash the crystals with distilled water in recrystallisation
to remove soluble impurities
what is recrystallisation
purifying an organic solid - used for purifying aspirin
what is a loss of yield in the process of recrystallisation
- crystals lost when filtering or washing
- some product stays in solution after recrystallisation
- other side reactions occurring
what happens to the mass if the crystals are not dried properly in recrystallisation
the mass will be larger than expected which can lead to a percentage yield > 100%
in recrystallisation , why should you avoid naked flames
ethanol is flammable
why would soluble impurities remain in solution form
they are present in small quantities so solution is not saturated . Ice will increase the yield of crystals
EQ : describe recrystallisation process used to purify aspirin (6)
- Dissolve crude product in hot solvent
- of minimum volume
- filter (hot)
- cool to recrystallise
- filter under reduced pressure / with a buchner apparatus
- wash with cold solvent and dry
why is a minimum volume of hot solvent used
dissolve well when cold .
The minimum volume is used to obtain saturated solution and to enable crystallisation on cooling
EQ : explain the purpose of each underlined point (5)
Minimum volume :
- Enable crystallisation / increase yield
Hot water :
- to keep solution saturated
Filtered hot :
- To remove insoluble impurities
Cooled in ice :
- to increase amount of crystals formed
Washed with cold water :
- To remove soluble impurities
EQ : assume that the reaction goes to completion . Suggest two practical reasons why the percentage yield for this reaction may not be 100% . (2)
- product left in beaker / glassware
- sample was still wet
- sample was lost during recrystallisation
