unit 3: organic practical techniques Flashcards
synthesis of compound techniques
heating under reflux, vacuum filtration
purification of compound techniques
distillation, solvent extraction, recrystallisation
determination of purity techniques
melting point analysis, mixed melting point analysis, thin layer chromatography (TLC)
heating under reflux
heats a reaction over an extended period of time without any vapours escaping
heating under reflux equipment
condenser, round bottom flask, heating mantle
anti bumping granules
used to ensure smooth boiling
vacuum filtration
separates solids and liquids and is much faster than gravity filtration
vacuum filtration equipment
buchner funnel connected to a vacuum flask with an arm at the side connected to a water pump
purifying
removes any impurities and side products formed during the synthesis
recrystallisation
involves adding product to hot solvent to dissolve it and letting the product recrystallise as it cools down, any soluble impurities will stay dissolved in the solvent
things to consider when choosing a solvent
product must be soluble in solvent at high temperature and minimally soluble at room temperature, it must not react with the products, the solubility of the impurities
recrystallisation technique
add minimal volume of hot solvent to your synthesised product, filter the hot mixture to get rid of any insoluble impurities, let the solvent slowly cool down which will crystallise the desired product, filter off the product which will remove any soluble impurities left in the solvent
melting point analysis
works out the purity of the sample by finding it’s melting point and comparing it to a known melting point value, impurities in the sample lower and broaden the melting point since they disrupt the intermolecular bonding in the crystal lattice
melting point range
the temperature range over which it starts to melt to when it finishes melting
mixed melting point analysis
mix the sample with a pure sample of what we meant to synthesis, if the melting point is at the correct value and the range is narrow, it is pure. if a different product was made the pure sample will act as an impurity and lower the melting point
thin layer chromatography
separates compounds by their polarity using a film of silica or aluminium oxide as the stationary phase, sample is spotted on the base line and a solvent carries the compound up, a developing agent of UV light visualises the spots and the Rf value is calculated
Rf
distance travelled by the sample / distance travelled by the solvent
if there is only 1 spot in TLC
there are no impurities
distillation
separates compounds by boiling point, more volatile substances with lower boiling points will pass through first. can identify compounds by comparing boiling points or purify substances by only collecting within the target boiling point, any collected outside will be impurities
distillation equipment
burner, distilling flask, thermometer, condenser, receiving flask
when to use solvent extraction
if the product is in an aqueous solution we can use a second solvent to extract it leaving behind impurities
choosing a second solvent in solvent extraction
immiscible with original solvent, desired substance is more soluble in it, volatile to allow the desired product to be easily separated, unreactive with the substance
solvent extraction
done in a separatory funnel. when the extraction is carried out two layers are formed in the funnel, the substance being extracted will dissolve in both solutions and equilibrium is established. the ratio of solute dissolved in both solvent is determined by K. once settled they are run off into speared containers and process is repeated
K in solvent extraction
= [substance in upper]/[substance in lower]
improving solvent extraction
using smaller volumes of solvent more often
back titration is used to
work out the quantity of a substance in a solid with a low solubility (meaning a standard solution cannot be made), can also be carried out if the direct titration is too slow or there is no suitable indicator
back titration
the substance being analysed (A) is reacted with an excess volume of a reactant of known concentration (B). the resulting mixture is titrated with a chemical that reacts with B (C). this allows us to calculate the number of moles of B in excess and therefore the number of moles of A which reacted with it
volumetric analysis
uses a standard solution to determine the concentration of another solution, usually done using a titration
concordant results
within 0.1cm^3 of each other
complexometric titrations
formation of a complex between a metal ion and a complexing agent e.g. EDTA, they react in a 1:1 ratio
indicator in EDTA titrations
murexide indicator which turns purple at the end point
standard solution
a solution of accurately known concentration
preparing a standard solution
by accurate dilution of an existing solution or from a primary standard
preparing standard solution from primary standard
accurately weight out primary standard and dissolve in a small volume of solvent in a beaker, transfer with rinsing to volumetric flash and make up to graduation mark with solvent, stopper and invert to mix
preparing standard solution by dilution
pipette required volume of standard solution into volumetric flask, make up to the graduation mark, stopper and invert to mix
characteristics of primary standard
high purity, stable as a solid in air and in solution, soluble in water, reasonably high formula mass
common primary standards
EDTA, silver nitrate, oxalic acid
sodium hydroxide
is not a primary standard as it absorbs moisture from the air and is unstable as a solid and in solution
if sodium hydroxide must be used in a titration
a solution of approximate concentration can be prepared and standardised by carrying out another titration first to find the accurate concentration of NaOH
stoichiometry
the study of mole relationships involved in chemical reactions
yield is reduced by
mechanical losses, purification of product, side reactions, equilibrium position
experimental errors
human error (scale reading), methodology (side reactions), uncertainties (pipettes/burettes)
classes of equipment
different classes have different levels of accuracy, class A glassware is more accurate than class B
weight accurately approximately 1g
the mass should be around 1g but with an accurate reading
outline how to prepare a standard solution
dissolve desired reactant in deionised water and transfer with rinsings, make up to the mark in a volumetric flask
why would you test a filtrate with standard solution
to check the reaction is complete
why would volumetric analysis rather than gravimetric analysis be more appropriate
titration can be used with lower concentrations
desiccator
prevent any water from the air reacting with the chemical
what analytical technique could be used to determine if a specific element is present in a substance
absorption/emission spectra
in TLC what affects the Rf value for an individual component
the solvent used
% by mass
mass in 100cm^3
