AH Chemistry - Researching Chemistry - (f) - Practical Skills and Techniques

What does colorimetry do?

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 do when plotting a calibration graph?

A colorimeter or spectrophotometer is used to measure the absorbance of light of a series of standard dilutions, and this data is used to plot a calibration graph.

What must the concentration of the coloured species in the solution being tested do?

The concentartion 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. It can be used to purify a compound by separating it from less volatile substances in the mixture. The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm it’s identity.

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What does heating under reflux allow for?

Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.

How is heating under reflux carried out?

Heating under reflux is carried out by:

• Placing the mixture in a round-bottomed flask with anti-bumping granules.

• The round-bottomed flask is fitted with a condenser.

• The flask is then heated using an appropriate source of heat.

What end does the water come into the condenser?

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What does vacuum filtration involve?

Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster mean of separating a precipitate from a filtrate. A Büchner funnel, a Hirsch, or sintered glass funnel can be used during vacuum filtration.

What is the process of recrystallisation for purification of an impure solid?

The process of recrystallisation for purification of an impure solid is:

• 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.

How is the solvent for 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 does solvent extraction involve?

Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.

What is the process of solvent extraction using a separating funnel?

The process of solvent extraction using a separating funnel is:

• Two immiscible solvents form two layers in the separating funnel. The solute dissolves in both solvents and an equilibrium establishes between the two layers. 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.

• The process is repeated to maximise the quantity of solute extracted.

When is the quantity of solute extracted greater while using a separating funnel?

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.

What should the solvent be in solvent extraction?

In solvent extraction, the solvent should be:

• 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

In melting point and mixed melting point determination, what is the melting point of a substance?

In melting point and mixed melting point determination, 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 using melting point?

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.

What does the presence of impurities in a compound do to the melting point, and why is this?

The presence of impurities in a compound lowers the melting point and broadens its melting temperature range due to the disruption in intermolecular bonding in the crystal lattice.

What does determination of a mixed melting point involve, and what can this be used to determine?

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 is chromatography?

Chromatography is a technique used to separate components present within a mixture.

How does chromatography separate substances?

Chromatography separates substances by making use of their polarity or molecular size.

How is thin layer chromatography (TLC) carried out?

Thin layer chromatography (TLC) is carried out by:

• Using 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. 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.

• A developing agent or ultraviolet light is normally required to visualise the spots on the chromatogram.

How are R_f values calculated?

R_f values are calculated using:

Under the same conditions (temperature, solvent, and saturation levels) what does the compound always have?

Under the same conditions (temperature, solvent, and saturation levels) the compound always has the same R_f value (within experimental error).

Using chromatography, how can the identity of a compound be confirmed?

Using chromatography, the identity of a compound can be confirmed by:

• Comparing the experimentally determined R_f 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 thin layer chromatography used to assess?

Thin layer chromatography is used to assess the purity of substances.

What result should be expected for a pure substance when spotted and developed on a TLC plate?

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).

What does the presence of more than one spot in thin layer chromatography show?

The presence of more than one spot in thin layer chromatography shows that impurities are present.