Analytical Techniques

Question 1

principle behind IR SPECTROSCOPY

Answer 1

• bonds in a molecule absorb IR radiation causing bonds to vibrate more and absorb energy
• different bonds in molecules will absorb infrared radiation at characteristic wavenumbers (frequencies) and will stretch/bend

Question 2

what do the peaks in IR spectrum show

Answer 2

the frequency at which the bonds absorb IR (energy)

Question 3

what can IR SPECTROSCOPY be used for

Answer 3

• to identify the functional groups in a molecule
• to monitor gases causing air pollution (CO and NO from car emissions)
• in breathalysers to measure ethanol in breath

Question 4

what’s the region below 1500cm-1 called and what’s it used for

Answer 4

• fingerprint region
• unique to a particular compound
• can be compared to a database spectra for known compounds to help identify unknown compounds

Question 5

to prove what a molecule is…

Answer 5

• you state the peaks present that indicate which bonds are present
• you state the absence of peaks that ensure its not a different molecule

Question 6

what does the degree of vibration of bonds depend on

Answer 6

• bond strength ( stronger bonds vibrate at higher freq)
• bond length
• mass of atoms ( heavier atoms vibrate at lower freq)

Question 7

why can greenhouse gasses absorb IR quite well, causing global warming

Answer 7

• C=O, O-H and C-H bonds absorb IR at a frequency that matches the bonds natural frequency causing them to bend/ stretch more than normal

Question 8

what are governments doing to address global warming

Answer 8

implementing policies to limit the emission of greenhouse gasses such as methane, water vapour and CO2

Question 9

what’s mass SPECTROMETRY used for

Answer 9

• to determine molar mass of a compound

Question 10

what happens to compounds when they enter a mass spectrometer

Answer 10

• compound loses e- to form +ve ion ( molecular ion, M+)

Question 11

what’s an M peak

Answer 11

• molecular ion peak
• clear peak with largest m/z ratio (it’s at the far right)
• m/z value is equal to the relative molecular mass

Question 12

what’s an M+1 peak and why does it arise

Answer 12

• much shorter peak than M+
• arises due to C-13 isotope

Question 13

what’s fragmentation of organic molecules

Answer 13

• molecular ion breaks down into cations which are detected by the spectrometer, causing many peaks
• these peaks will be at lower m/z values

Question 14

m/z value of the fragment CH3+

Answer 14

15

Question 15

m/z value of the fragment CH3CH2+

Answer 15

29

Question 16

m/z value of the fragment CH3CH2CH2+

Answer 16

43

Question 17

m/z value of the fragment CH3CH2CH2CH2+

Answer 17

57

Question 18

m/z value of the fragment OH+

Answer 18

17