Ch. 12: Separations and Purifications Flashcards
what are the 4 methods that use solubility characteristics to separate compounds from a mixture?
- extraction
- wash
- filtration
- recrystallization
defn + func: extraction
one of the simplest ways to separate out a desired product
the transfer of a dissolve compound (the desired product) from a starting solvent into a solvent in which the product is more soluble
what is the fundamental concept that extraction is based on?
like dissolves like (polar substances will associate with other polar substances, nonpolar with nonpolar)
defn: immiscible
form two layers that do not mix, like water and oil
should the two solvents be immiscible when we perform extraction or not?
yes, they should be
process (6): extraction
- the two layers are temporarily mixed by shaking so that solute can pass from one solvent to the other
- the water (aqueous) and ether (organic) phases will separate on their own, given time to do so
- in order to isolate these two phases, we use a piece of equipment called a separatory funnel
- gravitational forces cause the denser layer to sink to the bottom of the funnel, where it can be removed by turning the stopcock at the bottom
- once we drain the aqueous layer from the separatory funnel, we repeat the extraction several times
- additional water is added to the separatory funnel, it is shaken and allowed to settle, and the aqueous layer is once again drained off (multiple extractions with fresh water are more effective for obtaining the most prodcut, rather than a single etraction with a larger volume of water)
align nonpolar and polar with aqueous phase/layer and organic phase/layer
nonpolar –> organic phase
polar –> aqueous phase
which layer is more common to be up top? what is the position of the layers determined by?
it is more common for the organic layer to be on top, but the opposite can also occur
the position of the layers is determined by their relative densities
func + name: rotovap
rotary evaporator
once the desired product has been isolated in the solvent, we can obtain the product alone by evaporating the solvent usually using the rotovap
defn + func: wash
a small amount of solvent is used to extract and remove impurities, rather than the compound of interest
another way to take advantage of solubility properties, the reverse of the extraction
defn: filtration
isolates a solid from a liquid
process (2): filtration
- pour a liquid-solid mixture onto a paper filter that allows only the solvent to pass through, like a coffee filter
- at the end of filtration, one is left with the solid (the residue) and the flask full of liquid that passed through the filter (filtrate)
how can filtration be modified?
depending on whether the substance of interest is the solid or is dissolved in the filtrate
defn + what is the product of interest: gravity filtration
the solvent’s own weight pulls it through the filter
more commonly used when the product of interest is in the filtrate
why is hot solvent often used in gravity filtration?
to keep the product dissolved in liquid
defn + process: recrystallization
a method for further purifying crystals in solution
dissolve or product in a minimum amount of hot solvent and let it recrystallize as it cools
what type of solvent should be chosen for recrystallization? why?
one in which the product is only soluble at high temperatures
so that when the solution cools, only the desired product will recrystallize out of the solution, excluding the impurities
why does distillation come in handy?
when the product itself is a liquid that is soluble in the solvent
defn + process (3): distillation
takes advantage of differences in the boiling point to separate two liquids by evaporation and condensation
- the liquid with the lower boiling point will vaporize first, and the vapors will rise up the distillation column to condense in a water-cooled condenser
- this condensate then drips down into a vessel
- the end product is called the distillate
why is the heating temperature kept low in distillation?
so that the liquid with the higher boiling point will not be able to boil and therefore will remain liquid in the initial container
defn + condition: simple distillation
the least complex version of distillation
should only be used to separate liquids that boil below 150 deg C and have at least a 25 deg C difference in boiling points
what do the restrictions placed on simple distillation allow for?
prevent the temperature from becoming so high that the compounds degrade and provide a large enough difference in boiling points that the second compound won’t accidentally boil off into the distillate
apparatus (3): simple distillation
- a distilling flask containing the combined liquid solution
- a distillation column consisting of a thermometer and a condenser
- a receiving flask to collect the distillate
what 3 additional pieces of equipment may be introduced into distillation to break surface tension and prevent superheating?
- boiling chip
- ebulliator
- magnetic strirer
when does superheating occur?
when a liquid is heated to a temperature above its boiling point without vaporization
occur when gas bubbles within a liquid are unable to overcome the combination of atmospheric pressure and surface tension