Aspirin Flashcards
how do you decide which solvent to use for recrystallisation
desired product dissolves readily at high temperatures
but only slightly soluble at room temperature
why is the solvent chosen for recrystallisation
solubility of product drops with cooling so it recrystallises from solution at lower temps`
why is minimum volume of hot solvent used used when recrystallising
ensure as much solute is obtained as possible
too much lowers yield
hot solution is saturated so crystals form on cooling
why do repeat crystallisation
to obtain more product
why is slow crystallisation preferred
too fast some impurities are trapped
why use ice on cooling
allows more crystals to form due to reduced solubility at that temperature
why wash crystals with ice-cold solvent
removes aqueous impurities but doesn’t allow the crystals to dissolve
things to remember when finding melting point
range of values found
larger range means more impure sample
pure sample has one specific melting point small range
heat slowly
why ensure product is completely dry when checking melting point
if wet it contains impurities such as water
why might you not get 100% yield
- product lost during purification
i. e. solid may still be dissolved during crystallisation - product lost when transferring vessels
why use water for aspirin
dissolves readily at higher temperatures
but only slightly soluble at room temperature
