Practical skills and techniques Flashcards
What does colorimetery use?
The relationship between colour intensity of a solution and the concentration of the coloured species present.
What is the use of a colorimeter?
To measure the absorbance of light of a series of standard solutions, this data is used to plot a calibration graph.
How is the concentration of the solution being tested determined?
From its absorbance 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 can the boiling point of a compound be used for?
Confirming its identity.
What is distillation used for?
Purify a compound from a mixture of liquids.
How does distillation work?
By a process of heating and cooling the liquids so that they can be separated from each other.
What does distillation use?
Different boiling points to separate the more volatile from the less volatile material.
Distillation- What happens after the mixture distils over within a certain range?
It is collected in the receiving flask.
If the liquid product is particularly volatile then?
The receiving flask should be placed in an ice/water bath and the receiver adapter should be extended well into the flask.
What does heating under reflux allow?
Heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
What happens when carrying out heating under reflux?
The reaction mixture is placed in a round bottom flask with anti bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat.
Why are anti bumping granules added?
To minimise the tendency for violent boiling.
What does vacuum filtration involve?
Carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate.
What 3 apparatus can be used during vacuum filtration?
Büchner, Hirsch or sintered glass funnel
How does vacuum filtration work?
The mixture of solid and liquid is poured through filter paper into a Büchner, Hirsch or sintered glass funnel. The solid is trapped by the filter and the liquid is drawn through the funnel into the flask below by a vacuum.
What are the steps of recrystallisation?
- dissolve impure solid gently in a minimum volume of a hot solvent.
- hot filtration of the resulting mixture to remove any insoluble impurities.
- cool the filtrate slowly to allow curtails of pure compound to form, leaving soluble impurities dissolved in the solvent.
- filter, wash and dry the pure crystals.
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 does solvent extraction involve?
Isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.
How does solvent extraction work, before extracting layers?
2 immiscible solvents form 2 layers in the separating funnel. The solute dissolves in both solvents and an equillibrium establishes between the 2 layers.
Solvent extraction- How are the layers extracted?
The lower layer is run off into a container and the upper layer is poured into a second container.
Solvent extraction - why is the process repeated?
To maximise the quantity of solute extracted.
Solvent extraction- The ratio of solute dissolved in each layer is determined by the?
Equillibrium constant (K).
The quantity of solute extracted is greater if?
A number of extractions using smaller volumes of solvent are carried out, rather than a single extraction using a large volume of solvent.
Solvent extraction- the solvent should be?
- immiscible with the liquid mixture or solution.
- one in which the solute is more soluble in than the liquid mixture or solution.
- volatile to allow the solute to be obtained by evaporation of the solvent.
- unreactive with the solute.
What is the melting point of a substance?
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 being purified confirmed?
By melting point analysis and a conparison of the experimentally determined melting point with a literature or known melting point point value.
Determination of the melting point of a compound can indicate the?
Purity of a compound.
Mp determination- What does the presence of impurities in the compound do? And why?
Lowers the mp and broadens the melting temperature range due to the disruption in intermolecular bonding in the crystal lattice
What does determination of a mixed melting point involve?
Mixing a small quantity of the product with some of the pure compound and determining the melting point.
Mixed melting point determination- The melting point value and the range of the melting temperatures can be used to determine
if the product and the pure compound are the same substance.
How will I know if the 2 substances are identical?
If the melting point is sharp and close to the expected value.
How will I know if the 2 substances are not similar? And why does this happen.
The mp will be much lower and the range much broader (due to each compound acting as an impurity of the other)
What is chromatography used for?
To seperate the components present within a mixture.
How does chromatography seperate substances?
By making use of differences in their polarity or molecular size.
How does TLC work?
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 spitted into the base (pencil) line of the chromatogram. A solvent dissolved the compounds in the spit and carries them up the chromatogram.
TLC- what does how far the compounds are carried depend on?
How soluble the compound are in the chosen solvent and how well they adhere to the plate.
TLC- what is usually required to visualise the spots on the chromatogram?
A developing agent or UV light.
TLC- Under the same conditions (temp, solvent, and saturation levels) what happens to the Rf of a compound
Always stays the same (within experimental error)
TLC - How can the identity of a compound be confirmed?
- comparing the experimentally determined Rf values with a literature or known value determined under the same conditions
OR - making a direct comparison on a TLC plate between the compound being tested and the pure substance - a co spot should be used.
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)
TLC- What does the presence of more than 1 spot show?
Impurities are present.
Why impurities lower and broaden m.p.
Impurities disrupt regular structure, weakening the IMFs holding the solid together, therefore m.p. is lower as less energy is needed to break bonds holding structure together.
