Topic 7 - Modern Analytical Techniques I

Question 1

What is a mass spectrum?

Answer 1

A bar chart of relative abudnace (y) against m/z (x), used to show the relative abundance of different ions in a sample.

Question 2

What is used to produce a mass spectrum?

Answer 2

Mass spectrometer.

Question 3

What is on the y axis of a mass spectrum?

Answer 3

Relative abundance (%)

Question 4

What is on the x axis of a mass spectrum?

Answer 4

m/z (Mass to charge ratio)

Question 5

What two things can a mass spectrum be produced for?

Answer 5

• Relative isotopic abundances of one element

* Molecular samples of a compound

Question 6

What is the molecular ion on a mass spectrum?

Answer 6

• The ion of the entire molecule, with a +1 charge.

* Formed when 1 electron is removed from the molecule.

Question 7

What is the symbol for the molecular ion on a mass spectrum?

Answer 7

M+ (g)

Question 8

How do you find the relative molecular mass of a compound from its mass spectrum?

Answer 8

• Find the peak with the highest m/z value (ignoring any small M+1 peaks due to C-13)
• The m/z value is the Mr (assuming the ion has a +1 charge).

Question 9

What can cause a small peak to the right of the molecular peak on a mass spectrum?

Answer 9

A M+1 peak may be present due to the existence of C-13 isotopes, which increase the m/z by 1.

Question 10

What is the pattern of fragments on a mass spectrum called?

Answer 10

Fragmentation pattern

Question 11

What can the fragmentation pattern on a mass spectrum be used for?

Answer 11

Identifying molecules and their structure.

Question 12

On a mass spectrum, what causes the peaks at smaller m/z values than the molecular peak?

Answer 12

Bombarding electrons make some of the molecular ions to break up into fragments.

Question 13

On a mass spectrum, when the molecular ion fragments, do both products show up on the mass spectrum?

Answer 13

• No, becauses the ion will break up into an ion and a free radical.
• Only the ion will show up, not the radical.

Question 14

How might CH3CH2CH3+ fragment?

Answer 14

CH3CH2CH3+ -> CH3CH2• (radical) + CH3+ (ion)
OR
CH3CH2CH3+ -> CH3CH2+ (ion) + •CH3 (radical)

(Only the ions will show up on the mass spectrum)

Question 15

Explain how you can use a mass spectrum to work out the structure of a molecule.

Answer 15

1) Identify the fragments -> Look at the m/z values and determine what ions could have produced them
2) Piece together the fragments to give a molecule with the correct Mr -> Make sure the m/z values add up to the Mr

Question 16

Practise using the mass spectrum on pg 100 to work out the structure of the molecule.

Answer 16

Blue box, botom of pg 100 of revision guide.

Question 17

In mass spectrometry, what fragment is likely to give an m/z of 15?

Answer 17

CH3+

Question 18

In mass spectrometry, what fragment is likely to give an m/z of 29?

Answer 18

C2H5+

Question 19

In mass spectrometry, what fragment is likely to give an m/z of 43?

Answer 19

CH3CH2CH2+
OR
CH3CHCH3+

Question 20

In mass spectrometry, what fragment is likely to give an m/z of 17?

Answer 20

OH+

Question 21

How can you tell apart two molecules with the same Mr using their mass spectrum?

Answer 21

Look at the different fragments produced -> One might produce a certain fragment that the other doesn’t

Question 22

Can two molecules have the same mass spectrum?

Answer 22

• No, because each will produce different fragments

* An element’s spectrum is like a fingerprint

Question 23

Compare the mass spectrum of propanal and propanone.

Answer 23

Propanal will have a peak for a C2H5+ fragment, while propanone won’t.

(See pg 101 of revision guide)

Question 24

What are the two ways a compound can be analysed to work out what it is?

Answer 24

• Mass spectrometry

* Infrared spectroscopy

Question 25

How does infrared spectroscopy work?

Answer 25

* A beam of IR radiation is passed through a sample of a chemical * IR radiation is absorbed by the covalent bonds in their molecules, increaasing their vibrational energy * Different bonds absorb different frequencies of IR radiation * When this happens, there is a dip in the % transmittance, so there is a peak on the IR spectrum

Question 26

What two things determine the frequency of IR radiation that a bond will absorb in IR spectroscopy?

Answer 26

* The atoms the bond is between | * The type of molecule the bond is in (e.g. carboxylic acid)

Question 27

What are the two types of bond vibration in IR spectroscopy?

Answer 27

* Stretching | * Bending

Question 28

Do you need to memorise the IR spectroscopy absorption values?

Answer 28

No, they're given in the exam.

Question 29

What is on the x and y axis of an IR spectrum?

Answer 29

* x -> Wavenumber (cm^-1) | * y -> % Transmittance

Question 30

What is wavenumber on the x-axis of an IR spectrum?

Answer 30

It is just another measure of frequency (1/wavelength).

Question 31

What is used to produce an IR spectrum?

Answer 31

Infrared spectrometer

Question 32

What can an IR spectrum be used to do?

Answer 32

Identify the functional groups in a molecule.

Question 33

What do the peaks on an IR spectrum show?

Answer 33

Where IR radiation is being absorbed. | NOTE: The peaks on IR spectra are upside down.

Question 34

What is the only broad peak on an IR spectrum?

Answer 34

The OH peak -> It is smooth and wide.

Question 35

What part of an IR spectrum should you look at?

Answer 35

Above 1500cm^-1.

Question 36

What is the fingerprint region of an IR spectrum and what can it be used for?

Answer 36

* The complex region below 1500cm^-1 which contains many singl bond stretching and bending vibrations. * It is unique to each molecule -> Can be used to identify the molecule.

Question 37

When looking at an IR spectrum and you are unsure about a certain peak, what is it a good idea to do?

Answer 37

Look instead for other peaks that might be missing.

Question 38

How can computers be used along with IR spectra to identify an compound?

Answer 38

The fingerprint region of an IR spectrum can be compared with a database of fingerprint regions to identify the compound.

Question 39

Apart from identifying compounds, what can IR spectroscopy be used for?

Answer 39

Checking if a reaction has happene, by looking at the functional groups present.

Question 40

How can mass spectra and IR spectra be used together?

Answer 40

• Mass spectrum -> Used to find Mr and possible fragments • IR spectrum -> Used to find functional groups present Together, the two can be used to identify a compound exactly

Question 41

Remember to revise interpreting mass spectra and IR spectra.

Answer 41

Pgs 100-103 + Find some examples online