how to use a separating funnel
- shake mixture and leave to settle in the separating funnel
- open the tap to separate the layers
drying agents
- anhydrous magnesium sulfate (MgSO4)
- anhydrous calcium chloride (CaCl2)
how could you use TLC to monitor the progress of a reaction?
- place spots of controls on TLC plate
- place a spot of the reaction mixture at regular intervals alongside controls
- compare Rf values
what controls would you use when measuring the progress of a reaction using TLC?
- solution of reactant
- solution of product
purpose of reflux
prevent evaporation of reactants or products OR ensure complete reaction (check with Ed)
how could you separate/purify an organic liquid?
- separate layers with a separating funnel
- add drying agent to the organic layer (to remove any traces of water)
- redistill collecting product at the desired product’s boiling point (to separate any organic substances)
how could you separate/purify an organic solid?
- completely dissolve the solid in the minimum volume of hot solvent
- cool the solution in an ice bath (scratch bottom of beaker with stirring rod to encourage crystal growth)
- filter under reduced pressure
- rinse filtrate (organic solid in buchner flask) with cool solvent
- allow the organic solid to dry
how would you estimate the melting/boiling point of an impure substance?
- range must be greater than that of pure
- melting/boiling point must be lower than the pure one (highest temp of impure lower than lowest temp of pure)
what happens to the impurities during recrystallisation?
- organic solid crystallises as it is only soluble in hot solvent
- impurities stay in solution as they are soluble in the hot and cool solvent
how would you choose a solvent for recrystallisation?
- impurities must be more soluble in the solvent than the organic product
- e.g. if the impurities are non-polar and the organic product is polar, use a non-polar solvent so the organic solid only dissolves at high temperatures
limitations of TLC
- there may not be a known Rf value for some molecules
- similar molecules may have similar Rf values
- molecules usually shows smudges rather than clear dots
adsorption (in the context of TLC)
process by which the solid silica holds the different substances in the mixture to its surface
how are molecules separated in TLC?
relative absorptions of substances with the stationary phase
when is gas chromatography used?
separating an identifying volatile organic compounds
stationary phase in gas chromatography
high boiling liquid or solid adsorbed onto an inert solid support
mobile phase in gas chromatography
inert carrier gas such as helium or neon
how are compounds separated in gas chromatography?
- separated by solubility if stationary phase is liquid
- separated by adsorption if stationary phase is solid
how are the Rt values measured in gas chromatography?
- each compound spends a different length of time dissolved in the liquid stationary phase
- each compound reaches the detector at different times
- the compound retained in the column for the shortest time has the lowest retention time and is detected first (least soluble)
retention time
time taken for each component to travel through the column in gas chromatography
how are gas chromatograms interpreted?
- retention times are used to identify compounds present by comparing them to known retention times in data books
- peak integrations determine the concentrations of components in the sample
how can you determine concentrations of components in gas chromatography?
- make standard solutions of known concentrations of the compound
- gas chromatograms must be obtained for each standard solution
- plot a calibration curve of peak area against concentration
- use the calibration curve to measure the unknown concentration of the compound
test for a haloalkane
- add silver nitrate and ethanol
- warm to 50°C in a water bath
- chloroalkane: white ppt
- bromoalkane: cream ppt
- iodoalkane: yellow ppt
test for phenol
- pH indicator paper (weak acid)
- won’t react with Na2CO3
- mix with bromine water and it decolorises it (from orange colour)
- undergoes electrophilic substitution reaction with bromine at room temperature
