Practical skills and techniques

Question 1

What does colorimetery use?

Answer 1

The relationship between colour intensity of a solution and the concentration of the coloured species present.

Question 2

What is the use of a colorimeter?

Answer 2

To measure the absorbance of light of a series of standard solutions, this data is used to plot a calibration graph.

Question 3

How is the concentration of the solution being tested determined?

Answer 3

From its absorbance by referring to the calibration curve.

Question 4

The concentration of coloured species in the solution being tested must…

Answer 4

Lie in the straight line section of the calibration graph.

Question 5

What can the boiling point of a compound be used for?

Answer 5

Confirming its identity.

Question 6

What is distillation used for?

Answer 6

Purify a compound from a mixture of liquids.

Question 7

How does distillation work?

Answer 7

By a process of heating and cooling the liquids so that they can be separated from each other.

Question 8

What does distillation use?

Answer 8

Different boiling points to separate the more volatile from the less volatile material.

Question 9

Distillation- What happens after the mixture distils over within a certain range?

Answer 9

It is collected in the receiving flask.

Question 10

If the liquid product is particularly volatile then?

Answer 10

The receiving flask should be placed in an ice/water bath and the receiver adapter should be extended well into the flask.

Question 11

What does heating under reflux allow?

Answer 11

Heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.

Question 12

What happens when carrying out heating under reflux?

Answer 12

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.

Question 14

Why are anti bumping granules added?

Answer 14

To minimise the tendency for violent boiling.

Question 15

What does vacuum filtration involve?

Answer 15

Carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate.

Question 16

What 3 apparatus can be used during vacuum filtration?

Answer 16

Büchner, Hirsch or sintered glass funnel

Question 17

How does vacuum filtration work?

Answer 17

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.

Question 18

What are the steps of recrystallisation?

Answer 18

• 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.

Question 19

The solvent for recrystallisation is chosen so that the compound being purified is?

Answer 19

Completely soluble at high temperatures and only sparingly soluble at lower temperatures.

Question 20

What does solvent extraction involve?

Answer 20

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

Question 21

How does solvent extraction work, before extracting layers?

Answer 21

2 immiscible solvents form 2 layers in the separating funnel. The solute dissolves in both solvents and an equillibrium establishes between the 2 layers.

Question 22

Solvent extraction- How are the layers extracted?

Answer 22

The lower layer is run off into a container and the upper layer is poured into a second container.

Question 23

Solvent extraction - why is the process repeated?

Answer 23

To maximise the quantity of solute extracted.

Question 24

Solvent extraction- The ratio of solute dissolved in each layer is determined by the?

Answer 24

Equillibrium constant (K).

Question 25

The quantity of solute extracted is greater if?

Answer 25

A number of extractions using smaller volumes of solvent are carried out, rather than a single extraction using a large volume of solvent.

Question 26

Solvent extraction- the solvent should be?

Answer 26

- 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.

Question 28

What is the melting point of a substance?

Answer 28

The temperature range over which the solid first starts to melt, to when all of the solid has melted.

Question 29

How can the identity of a pure compound being purified confirmed?

Answer 29

By melting point analysis and a conparison of the experimentally determined melting point with a literature or known melting point point value.

Question 30

Determination of the melting point of a compound can indicate the?

Answer 30

Purity of a compound.

Question 31

Mp determination- What does the presence of impurities in the compound do? And why?

Answer 31

Lowers the mp and broadens the melting temperature range due to the disruption in intermolecular bonding in the crystal lattice

Question 32

What does determination of a mixed melting point involve?

Answer 32

Mixing a small quantity of the product with some of the pure compound and determining the melting point.

Question 33

Mixed melting point determination- The melting point value and the range of the melting temperatures can be used to determine

Answer 33

if the product and the pure compound are the same substance.

Question 34

How will I know if the 2 substances are identical?

Answer 34

If the melting point is sharp and close to the expected value.

Question 35

How will I know if the 2 substances are not similar? And why does this happen.

Answer 35

The mp will be much lower and the range much broader (due to each compound acting as an impurity of the other)

Question 36

What is chromatography used for?

Answer 36

To seperate the components present within a mixture.

Question 37

How does chromatography seperate substances?

Answer 37

By making use of differences in their polarity or molecular size.

Question 38

How does TLC work?

Answer 38

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.

Question 39

TLC- what does how far the compounds are carried depend on?

Answer 39

How soluble the compound are in the chosen solvent and how well they adhere to the plate.

Question 40

TLC- what is usually required to visualise the spots on the chromatogram?

Answer 40

A developing agent or UV light.

Question 41

TLC- Under the same conditions (temp, solvent, and saturation levels) what happens to the Rf of a compound

Answer 41

Always stays the same (within experimental error)

Question 42

TLC - How can the identity of a compound be confirmed?

Answer 42

- 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.

Question 43

A pure substance, when spotted and developed on a TLC plate should appear as?

Answer 43

A single spot (some impurities may not be visible by TLC analysis)

Question 44

TLC- What does the presence of more than 1 spot show?

Answer 44

Impurities are present.

Question 45

Why impurities lower and broaden m.p.

Answer 45

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.