Researching chemistry: Practical skill and techniques Flashcards
how can the Rf value be calculated
by dividing the distance travelled by the substance, by the distance travelled by the solvent
what is TLC used for
TLC is used to assess the purity of substances
how are you able to identify a pure substance in TLC
A pure substance, when spotted and developed on a TLC plate, should appear as a single spot (some impurities may not be visible by TLC analysis). The
presence of more than one spot shows that impurities are present
method of TLC
Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic. A small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram. A solvent dissolves the compounds in the spot and carries the compounds up the chromatogram.
what makes the spots travel far
How far the compounds are carried depends on how soluble the compounds are in the chosen solvent and how well they adhere to the plate
what is used to visualise the spots on a chromatogram
A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.
how can the identity of a substance be confirmed by TLC
comparing the experimentally determined
Rf values with a literature or known value determined under the same conditions
making a direct comparison on a TLC plate between the
compound being tested and the pure substance — a co-spot could be used
what is colorimetry
Colorimetry uses the relationship between colour intensity of a solution and the concentration of the coloured species present.
what is used to measure the absorbance of light
A colorimeter or a spectrophotometer is used to measure the
absorbance of light of a series of standard solutions, and this data is
used to plot a calibration graph.
how is the concentration of the solution determined (colorimetry)
The concentration of the solution being tested is determined from its absorbance and by referring to the calibration curve.
The concentration of coloured species in the solution being tested must lie in the straight line section of the calibration graph.
what is distillation used for
Distillation is used for identification and purification of organic
compounds.
how is the boiling point of a compound determined
The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity.
how can distilation be used to purify a compound
Distillation can be used to purify a compound by separating it from
less volatile substances in the mixture.
what is heating under reflux
Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
method of carrying out heating under reflux
When carrying out heating under reflux, the reaction mixture is placed in a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat.
what is vacuum filtration
Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.
how is the solvent of recrystallisation chosen
The solvent for recrystallisation is chosen so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures.
what is solvent extraction
Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.
the technique of recrystalisation
dissolving an impure solid gently in a minimum volume of a hot solvent
hot filtration of the resulting mixture to remove any insoluble impurities
cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
filtering, washing and drying the pure crystals
what happens during a solvent extraction
When carrying out a solvent extraction, the two immiscible solvents form two layers in the separating funnel. The solute dissolves in both solvents and an equilibrium establishes between the two layers.
how is the ratio of solute in each layer determined (solvent extraction)
The ratio of solute dissolved in each layer is determined by the equilibrium constant, K. The lower layer is run off into a container and the upper layer is poured into a second container. This process is repeated to
maximise the quantity of solute extracted.
what are the qualities needed in a solvent for solvent extraction
immiscible with the liquid mixture or solution (usually water)
one in which the solute is more soluble in than the liquid mixture or solution (usually water)
volatile to allow the solute to be obtained by evaporation of the solvent
unreactive with the solute
melting point and mixed melting point
The melting point of a substance is the temperature range over which the solid first starts to melt, to when all of the solid has melted.
how can the identity of a pure compound be confirmed
The identity of a pure compound can be confirmed by melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value.
