Analysis

Question 1

What is an Rf value in chromatography?

Answer 1

A comparison between how far a component has moved compared to the solvent in thin layer chromatography.

Question 2

What is the stationary phase in chromatography?

Answer 2

Where the component is fixed in place.

Question 3

What is the mobile phase in chromatography?

Answer 3

Where the component moves in a definite direction.

Question 4

What is one-way thin layer chromatography?

Answer 4

A quick and easy technique that can be used to check the purity of a sample or to determine the extent of a chemical reaction. The stationary phase is a thin piece of inert material and the organic phase is often an organic solvent.

Question 5

What is gas chromatography?

Answer 5

A technique used to separate volatile components in a mixture. The gas chromatograph is often connected to a mass spectrometer so that the separate parts of the mixture can undergo further analysis. The stationary phase is a solid or liquid coating on a coiled tube and the mobile phase is an unreactive carrier gas.

Question 6

What is the retention time in a gas chromatograph?

Answer 6

The time taken for a component to travel from the inlet to the detector.

Question 7

How do you test for alkenes?

Answer 7

Add a few drops of bromine water to the sample and shake it, if it decolourises then the compound is unsaturated.

Question 8

How do you test for haloalkanes?

Answer 8

Add aqueous silver nitrate and ethanol, if a precipitate forms you can identify the halogen…
Chloride: white
Bromide: cream
Iodide: yellow

Question 9

How do you test for carbonyls?

Answer 9

Add Brady’s reagent and a yellow or orange precipitate is formed if a ketone or aldehyde is present.

Question 10

How do you test for aldehydes?

Answer 10

Add Fehling’s reagent and the blue solution will produce a dark red precipitate OR add Tollen’s reagent to from a silver mirror OR add acidified potassium dichromate to turn the orange solution green.

Question 11

How do you test for aliphatic carboxylic acids?

Answer 11

Add a reactive metal or metal carbonate and and there should be effervescence and use a pH probe to check it has the pH of a weak acid.

Question 12

How do you test for phenols?

Answer 12

Use universal indicator or a pH probe to check that it has a weak acid pH.

Question 13

How do you test for alcohols?

Answer 13

Add acidified potassium dichromate and the colour should change from orange to green as an aldehyde is formed, though tertiary alcohols cannot be oxidised this way so no colour change would be observed.

Question 14

What is TMS?

Answer 14

The internal standard for carbon and proton NMR.

Question 15

What is chemical shift?

Answer 15

The scale that compares the frequency of NMR absorption with the frequency of the reference peak of TMS.

Question 16

What is deuterium?

Answer 16

An isotope of hydrogen that does not produce a signal in the proton NMR spectrum.

Question 17

What are equivalent protons?

Answer 17

Hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer.

Question 18

Why can it me difficult to -OH and -NH protons?

Answer 18

• Peaks appear over a wide range of chemical shift values
• The signals are often broad
• There is usually no splitting pattern

Question 19

How is heavy water used for NMR spectra of -OH and -NH?

Answer 19

• An NMR spectrum is run
• A small amount of heavy water is added to the sample solution and is shaken
• A second NMR spectrum is run and any peak due to -NH or -OH disappears