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A LEVEL - Chemistry (Inorganic) > Topic 7 - Modern Analytical Techniques I > Flashcards

Topic 7 - Modern Analytical Techniques I Flashcards

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1
Q

What is a mass spectrum?

A

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

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2
Q

What is used to produce a mass spectrum?

A

Mass spectrometer.

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3
Q

What is on the y axis of a mass spectrum?

A

Relative abundance (%)

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4
Q

What is on the x axis of a mass spectrum?

A

m/z (Mass to charge ratio)

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5
Q

What two things can a mass spectrum be produced for?

A
  • Relative isotopic abundances of one element

* Molecular samples of a compound

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6
Q

What is the molecular ion on a mass spectrum?

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

* Formed when 1 electron is removed from the molecule.

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7
Q

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

A

M+ (g)

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8
Q

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

A
  • 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).
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9
Q

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

A

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

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10
Q

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

A

Fragmentation pattern

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11
Q

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

A

Identifying molecules and their structure.

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12
Q

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

A

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

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13
Q

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

A
  • No, becauses the ion will break up into an ion and a free radical.
  • Only the ion will show up, not the radical.
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14
Q

How might CH3CH2CH3+ fragment?

A

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

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

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15
Q

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

A

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

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16
Q

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

A

Blue box, botom of pg 100 of revision guide.

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17
Q

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

A

CH3+

18
Q

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

A

C2H5+

19
Q

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

A

CH3CH2CH2+
OR
CH3CHCH3+

20
Q

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

A

OH+

21
Q

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

A

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

22
Q

Can two molecules have the same mass spectrum?

A
  • No, because each will produce different fragments

* An element’s spectrum is like a fingerprint

23
Q

Compare the mass spectrum of propanal and propanone.

A

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

(See pg 101 of revision guide)

24
Q

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

A
  • Mass spectrometry

* Infrared spectroscopy

25
Q

How does infrared spectroscopy work?

A
  • 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
26
Q

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

A
  • The atoms the bond is between

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

27
Q

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

A
  • Stretching

* Bending

28
Q

Do you need to memorise the IR spectroscopy absorption values?

A

No, they’re given in the exam.

29
Q

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

A
  • x -> Wavenumber (cm^-1)

* y -> % Transmittance

30
Q

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

A

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

31
Q

What is used to produce an IR spectrum?

A

Infrared spectrometer

32
Q

What can an IR spectrum be used to do?

A

Identify the functional groups in a molecule.

33
Q

What do the peaks on an IR spectrum show?

A

Where IR radiation is being absorbed.

NOTE: The peaks on IR spectra are upside down.

34
Q

What is the only broad peak on an IR spectrum?

A

The OH peak -> It is smooth and wide.

35
Q

What part of an IR spectrum should you look at?

A

Above 1500cm^-1.

36
Q

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

A
  • 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.
37
Q

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

A

Look instead for other peaks that might be missing.

38
Q

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

A

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

39
Q

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

A

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

40
Q

How can mass spectra and IR spectra be used together?

A

• 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

41
Q

Remember to revise interpreting mass spectra and IR spectra.

A

Pgs 100-103 + Find some examples online

A LEVEL - Chemistry (Inorganic) (21 decks)

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