3. synthesis, purification, determination of purity Flashcards
name the two synthesis methods
heating under reflux, vacuum filtration
name the three purification methods
distillation, solvent extraction, recrystallisation
name the two determination of purity methods
melting point analysis and thin layer chromatography
what is heating under reflux?
a way of heating a reaction over an extended period of time without any vapours escaping
what is the purpose of a condenser?
to condense any reactant that has evaporated and escaped the flask
what is added to the round bottom flask and why?
anti-bumping granules - to ensure smooth boiling
drawing a diagram of heating under reflux
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what is vacuum filtration?
a way of separating solids and liquids that is faster than gravity filtration
why is vacuum filtration faster than gravity filtration?
due to reduced pressure in the vacuum flask
what is one way of creating a vacuum?
using a water pump
draw a labelled diagram of vacuum filtration
:)
describe the 4 steps of recrystallisation
- add a minimal volume of hot solvent to your product
- filter the hot mixture to get rid of any insoluble impurities
- let the solvent cool slowly to recrystallise the desired product
- filter off the product, leaving soluble impurities in the solvent
what must be considered when choosing a solvent for recrystallisation?
- product must be soluble at high temperature, but insoluble at low temperature
- solvent must not react with products
- solubility of impurities
how does melting point analysis help to determine the purity of a substance?
by determining the melting point and comparing it to a known melting point from literature
what is the melting point range?
the temperature range over which a substance starts to melt to when it finishes melting
how does the presence of impurities affect the melting point range and why?
it lowers and brodens it
they disrupt the intermolecular bonding in the original lattice
how does mixed melting point analysis determine the purity of a substance?
our sample is mixed with a pure sample of what we synthesised
if the melting point is at the correct literature value and the range is narrow, we have succeeded
if we have not succeeded, the pure sample will act as an impurity and lower the melting point
how does chromatography separate compounds?
based on their polarity
what does TLC use as the stationary phase?
a film of silica or aluminium oxide
which variables can affect how far the sample travels?
- solvent used
- temperature of TLC platr
how are the spots on the chromatogram visualised?
using UV light or a developing agent
how does one calculate the Rf?
distance travelled by sample/distance travelled by solvent
how does TLC tell us that we have a pure compound?
there is only 1 spot in our sample
the Rf value matches that of the pure compound
what is distillation?
the process of separating out compounds by their boiling point
what can distillation be used for an why?
to identify compounds, as the b.p. of the fraction collected can be compared to literature values
how can distillation be used to purity substances?
by only collecting any distillate within a certain range of the target b.p.
what is solvent extraction used to do?
extract desired product from an aqueous solution, often leaving behind any impurities
in which piece of apparatus is solvent extraction usually carried out?
separatory funnel
which qualities must the second solvent in solvent extraction have?
1) immiscible with original solvent
2) one which desired substance is more soluble in
3) volatile to allow desired product to be easily separated from solvent
4) unreactive with substance
what is formed while solvent extraction is being carried out?
two layers, with the substance being extracted dissolving in both. this causes an equilibrium to be established
how is K worked in solvent extraction?
K = substance in upper/substance in lower
the greater the value of K…
the more substance present in the upper layer
what happens once the flask has been shaken and left to settle into layers?
the lower layer is run off into a container and the upper a second container. the process is repeated to maximise the quantity of substance extracted
how is the process of solvent extraction improved?
by using smaller volumes of solvent more often
