Moduel 6 Chapter 29

Question 1

What is chromatography

Answer 1

It is a technique used to separate components with similar physical properties from a mixture.

Question 2

How are the different substances separated in chromatography

Answer 2

The mobile phase is passed over a stationary phase and each compound has a different affinity for each phase.

Question 3

What is a phase

Answer 3

A phase is a physically distinct form of a substance

Question 4

What does the stationary phase do in chromatography

Answer 4

The stationary phase slows compound movement and the more interactions there are the more it is slowed down
—> allowing compounds to move at different rates

Question 5

What does a solid stationary phase do

Answer 5

Separated molecules by adsorption as molecules bind on to the surface
—> stationary adsorption is slower

Question 6

What does a liquid stationary phase do

Answer 6

Separates by relative solubility as molecules dissolve into the phase
—> greater solubility = slower

Question 7

What is the stationary phase in thin layer chromatography

Answer 7

A stationary phase of silica gel (SiO2) or alumina (Al2O3) is cooled onto a plate of glass or plastic

Question 8

What is adsorption

Answer 8

The process that occurs when a gas or liquid or solute is held to the surface of a solid.

Question 9

What is the process of thin layer chromatography being done

Answer 9

A sample is added to a starting point on the plate which is then placed into a liquid solvent which will get drawn up.
—> at the end the height of the solvent is marked - the solvent front

Question 10

How is separation achieved in thin layer chromatography

Answer 10

By the relative adsorptions of substances with the stationary phase

Question 11

What is an Rf value

Answer 11

It shows how far a component has moved relative the solvent front

Question 12

What is the formula for calculating the Rf value

Answer 12

Rf = distance moved by component/distance moved by solvent front

Question 13

How can the Rf value be used to identify components

Answer 13

As it will always have the same Rf value, so by comparing it to values in a database of Rf values of substances using that solvent you can identify the compound.

Question 14

What happens when compounds have similar rf values

Answer 14

A suitable,e solvent is needed to separate the two separate compounds.

Question 15

What is the stationary phase in gas chromatography

Answer 15

Solid silicone polymers or a liquid long chain alkane, is coated inside a capillary tube.

Question 16

What is the mobile phase in gas chromatography

Answer 16

AN inert unreactive gas such as He or N2

Question 17

How is the sample inserted into the gas chromatogram

Answer 17

It is injected and vaporised before being pushed through the capillary column by a carrier gas.

Question 18

What does the detector do in gas chromatography

Answer 18

The detector at the end records how long it took for a components to pass through the column which is known as the retention time.

Question 19

What is the area below a peak indicate in gas chromatography

Answer 19

The area below a peak is proportional to the concentration

Question 20

What is the retention time used for, however what are problems with this

Answer 20

To help identify the component but, chemicals can have the same retention time, peak shape and detector response, and peaks can be hidden by others with the same retention time but a greater concentration.

Question 21

What is used at the same time as gas chromatography to improve the results

Answer 21

Mass spectrometry is often run at the same time

Question 22

Hollow do you work out the concentration of a substance using gas chromatography

Answer 22

Compare its peak integration area with values, these are obtained from a, standard solution of the component.

Question 23

What are the steps for making a calibration curve to work out the concentration of a substance using gas chromatography

Answer 23

1. Prep SS - known conc
2. Obtain gas chromatograms
3. Plot calibration curve of peak area against concentration = external calibration, convert area into conc
4. Obtain chromatogram of compound under same conditions
5. Use calibration curve to measure the corn of the compound.

Question 24

What is the chemical shift in HNMR and CNMR

Answer 24

The position at which a nucleus absorbs energy along the NMR spectrum is known as the chemical shift.

Question 25

What is the chemical shift relative to

Answer 25

A reference signal

Question 26

What is the substance that is used for the reference signal in NMR

Answer 26

A TMS molecule which has 12 equivalent protons which give it a distinct NMR signal which is given as the value of 0ppm.

Question 27

Why is TMS used as the substance for the reference signal in NMR

Answer 27

As it is unreactive and its volatility allows it to be removed from the sample at the end.

Question 28

What is the chemical shift of the different carbons or hydrogens in NMR determined by

Answer 28

The amount of shift is determined by chemical environment particularly nearby electronegative atoms.

Question 29

DRAW TMS

Question 30

what is a property of the nucleus

Answer 30

Nucleus also have a property called nuclear spin

Question 31

When does nuclear spin become significant

Answer 31

If there is an off number of nucleons

Question 32

How is NMR normally carried out and why

Answer 32

In solution to avoid unused signals deuterated (in which the ordinary isotope of hydrogen has been replaced with deuterium) solvents are used.

Question 33

What does D represent

Answer 33

The hydrogen isotope deuterium.

Question 34

What is a feature of deuterium

Answer 34

It has two nucleons and it has no spin and no single on an NMR spectra.

Question 35

How much of all carbon atoms are the 13C isotope

Answer 35

13C makes up around 1.1.% of all carbon atoms and as it has an odd number of nucleons it has a residual magnetic spin.

Question 36

What does the chemical shift indicate

Answer 36

The chemical environment of carbon atoms present.

Question 37

When are carbon atoms in the same environment

Answer 37

When they are positioned symmetrically, they will have the same environment.

Question 38

What is a feature of the nucleus of the 1H isotope

Answer 38

it has a single proton and so has a spin

Question 39

What is a feature of the 1H isotope

Answer 39

It has a relative abundance of 99.99% so less material is needed to be analysed compared to 13C

Question 40

What do the number of peaks indicate and the chemical shift indicate in proton NMR

Answer 40

number of peaks = number of environments Chemical shift indicate = type of proton environment

Question 41

What are features of protons that are equivalent and protons that aren’t

Answer 41

Protons that are equivalent will absorb at the same chemical shifts, increasing the size of the peak. Protons of different types have different chemical shifts

Question 42

What is an integration trace

Answer 42

A measurement of the relative area under a peak, which is related to the number of protons.

Question 43

What does the integration trace being displayed on a spectra provide

Answer 43

a ratio of protons in each environment

Question 44

Why are some signals split

Answer 44

Due to spin-spin coupling from interactions of protons on adjacent carbons —> under the influence of the external negative fields

Question 45

What is the number for splits caused as a result of

Answer 45

It is linked to the number of protons present on the adjacent carbons

Question 46

What can be used to identify the number of hydrogen atoms that are present on an adjacent carbon

Answer 46

The splitting pattern of a peak into sub-peaks provides evidence for the number of adjacent protons using n-1

Question 47

What is a feature of aromatic protons

Answer 47

They are expected to absorb in the range of 6.2-8.0ppm Splitting occurs however it’s difficult to interpret.

Question 48

If you see a splitting pattern what does this indicate

Answer 48

There has to be another As splitting patterns occur in pairs With each proton splitting the signal of another

Question 49

What is a feature of the hydroxyl and amino groups in proton NMR

Answer 49

They often show up as a broad singlet due to hydrogen bonding, this occurs in alcohols, phenols, CA, amides, amino acids They also don’t split or cause splitting in adjacent protons too

Question 50

Why are hydroxyl and amino groups difficult to identify

Answer 50

As they have chemical shifts over a large range and, the values are varied based on the conc of the solvent used, broad signals, no splitting pattern, not involved in spin-spin coupling too.

Question 51

How can identification of the -OH and -NH groups be done

Answer 51

Using heavy water D2O

Question 52

Why is heavy water used

Answer 52

As it doesnt show up on the spectra It is able to exchange the H in an -OH group or -NH group for a D R-OH + D2O <-> R-OD + HOD The addition of the D removes the spin from the group, thus removing it from the spectra.