Practical Skills and Techniques Flashcards
What is distillation used for
Distillation is used for identification and purification of organic compounds
What can the boiling point of a compound confirm
The boiling point of a compound determined by distillation is one of the physical properties that can be used to confirm its identity
How does distillation purify the compound
Distillation can be used to purify a compound by separating it from less volatile substances in the mixture
Refluxing
Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping
What is the procedure of 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 up using an appropriate source of heat
Vacuum filtration
Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate
Which flasks can be used during vacuum filtration
Buchner
Hirsch
Sintered glass funnel
What is used to draw the filtrate through the filter paper
Uses a vacuum pump to draw the filtrate through the filter paper at much greater speed
What is used with a vacuum flask
A Buchner or Hirsch funnel is used with a vacuum flask which are designed to cope with increased pressures involved
What is the melting point of a substance
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
What 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
What can determination of the melting point of a compound give an indication of
Determination of the melting point of a compound can give an indication of the purity of a compound. The presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the disruption in the intermolecular binding in the crystal lattice
What does the determination of a mixed melting point involve
Determination of a mixed melting point involves mixing a small quantity of the product with some of the pure compound and determining the melting point. The melting point value and the range of the melting temperature can be used to determine if the product and the pure compound are the same substance.
What does colorimetry use the relationship between
Colorimetry uses the relationship between colour intensity of a solution and the
concentration of the coloured species present.
What is a colorimeter or spectrophotometer used to measure
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 being tested determined
The concentration of the solution being tested is determined from its absorbance and by
referring to the calibration curve.
Where must the concentration of coloured species in the solution being tested lie
The concentration of coloured species in the solution being tested must lie in the straight
line section of the calibration graph.
What technique is chromatography
Chromatography is a technique used to separate the components present within a mixture.
How does chromatography separate substances
Chromatography separates substances by making use of differences in their polarity or molecular size.
What type of silica is used and what is it spread over in thin layer chromatography
Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic.
What is spotted onto the chromatogram
A small sample of the mixture being tested is spotted onto the base (pencil) line of the chromatogram.
What dissolves the compounds in the spot
A solvent dissolves the compounds in the spot and carries the compounds up the chromatogram. 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 required to visualise spots on the chromatogram
A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.
Rfvalues can be calculated:
distance travelled by the sample
R, = distance travelled by the solvent
What does a compound always have the same of under the same conditions
Under the same conditions (temperature, solvent, and saturation levels) a compound always has the same R, value (within experimental error).
The identity of a compound can be confirmed by:
• comparing the experimentally determined R, 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 should appear on a TLC plate
A pure substance, when spotted and developed on a TLC plate, should appear as a single spot. The presence of more than one spot shows that impurities are present.
