Year 2 Analysis

Question 1

What is the stationary phase in chromatography?

Answer 1

This is the phase that is fixed in place, in TLC the stationary phase is the paper. In gas chromatography, the stationary phase is the liquid which the mixture components have different attractions to the stationary phase

Question 2

What is the mobile phase in chromatography?

Answer 2

This moves in a definite direction, this could be an inert gas in gas chromatography and the solvent in TLC

Question 3

How is the Rf value calculated?

Answer 3

Distance moved by component / Solvent front

Question 4

How can TLC be used to determine the purity of a mixture?

Answer 4

The number of spots shown on the chromatogram in the line of that mixture is the number of compounds in that mixture. This is because each compound in a mixture has different solubility in different solvents. If substances have similar Rf values then they’re likely to have similar structures. Rf values can be compared to data book values to identify what compounds have been produced

Question 5

What is the chart produced to display gas chromatography results?

Answer 5

A graph of adsorption against time, the time taken to adsorb can be used to identify the compound by comparing with data book values. The adsorption allows the person running the test to determine how much of that substance is in the original mixture by calculating the area under the peak

Question 6

What is the test for an alkene function group (unsaturated)

Answer 6

Add a few drops of bromine water to the sample and shake well. If the bromine water decolourises then an addition reaction has occurred between an alkene and Br2 and the compound was unsaturated

Question 7

What is the test for a haloalkane?

Answer 7

Add aqueous silver nitrate and ethanol to the sample, if a precipitate forms then a haloalkane was present. - - —– White (milk) precipitate means a silver chloride was formed and implies a chloroalkane

• Cream precipitate silver bromide, bromoalkane
• Yellow (butter) precipitate formed, silver iodide, iodoalkane
• The ethanol provides a hydroxide ion which undergoes hydrolysis with the haloalkane to produce a halide ion and an alcohol, this is why the AgNO3 is able to form precipitates with the different halogens

Question 8

What is the test for an aldehyde/ketone?

Answer 8

Brady’s reagent (2,4-DNP) is added to the sample, an orange precipitate is formed if an aldehyde or ketone is present

Question 9

What is the test for an aldehyde?

Answer 9

• Tollen’s reagent is added and if a silver mirror is produced the an aldehyde is present
• K2Cr2O7/H+ is added, if it turns from orange to green when an aldehyde is present, with a ketone there is no change

Question 10

What is the test for aliphatic carboxylic acids?

Answer 10

pH probe, universal indicator paper or a reactive metal. Using the pH probe and universal indicator paper the pH check that pH is that of a weak acid. When a reactive metal is added, effervescence will be observed as H2 gas is produced

Question 11

What is the test for phenols?

Answer 11

UI paper and pH probe to check pH is that of a weak acid eg <7, no reaction with carbonate ion whereas a carboxylic acid would react eg. Na2CO3 + 2CH3COOH → 2CH3COO-Na+ + CO2 + H2O

Question 12

What is the test for an alcohol?

Answer 12

Acidified potassium dichromate, colour change from orange to green in primary alcohols as alcohol is oxidised to aldehyde or carboxylic acid. Secondary alcohol will produce the same colour change as they’re oxidised to a ketone. Tertiary alcohol will produce no colour change as they can’t be oxidised

Question 13

Why are H1 and C13 used for NMR?

Answer 13

They have a property known as spin, compounds containing these isotopes are put in a large magnetic field the direction of spin of these nuclei aligns with the direction of the magnetic field. Energy in the form of radio waves is used to flip the spins and change their direction. The amount of energy required to change the spin depends on the environments that the atom is in. All values collected are compared to a sample.

Question 14

What is the reference chemical for proton and carbon NMR?

Answer 14

TMS (tetramethylsilane), (CH3)4Si. The spectrometer makes a scale on the x-axis relative to TMS. TMS is given the value 0ppm and all other values on the x-axis are compared to it

Question 15

Why is TMS a good standard material to use for NMR?

Answer 15

• It contains both C and H meaning it can be used for both carbon and proton NMR
• Produces one sharp signal as all the hydrogens are all in the same environment and there is also only one carbon environment
• It is non-toxic
• Low boiling point and is volatile meaning it can be easily removed from the sample
• It is inert meaning it is unlikely to react with the chemical that is being investigated

Question 16

What organic solvents are used in NMR?

Answer 16

CHCl3, however we use CDCl3 which has the deuterium isotope of hydrogen which doesn’t have the spin property meaning it won’t affect the spectrum

Question 17

In carbon-13 NMR, what is a carbon environment?

Answer 17

A carbon that is bonded to different groups to other carbons. If there is a different carbon bonded to the same groups then it counts as the same environment

Question 18

In proton NMR what is a hydrogen environment?

Answer 18

Hydrogens bonded to an atom where that atom is bonded to different groups than other atoms. For example CH3CH2CH2CH3, has 2 hydrogen environments as both CH2 groups are bonded to CH3 and CH2CH3 and both CH3 groups are bonded to CH2CH2CH3.

Question 19

In high resolution proton NMR,what does it mean if a peak is split into n number of peaks.

Answer 19

The adjacent carbon has n-1 hydrogens on it

Question 20

Does OH produce any splitting of peaks?

Answer 20

No, the Oh group and the hydrogens on any adjacent carbon don’t interact to produce any splitting of peaks