Chapter 17 - Spectroscopy

Question 1

Mass spectrometry definition

Answer 1

• measures mass-to-charge ratio (m/z)

Question 2

5 stages of mass spectrometry

Answer 2

1) vaporisation - need vaporised sample to move through the machine
2) ionisation - molecules bombarded with electrons, some collide with enough force to knock off an electron - forms +1 charge
3) acceleration - ions accelerated through electric field to have same kinetic energy
4) deflection - ions passed through magnetic field & will deflect at different rates - in proportion to mass & charge
5) detection - beams of ions are detected electrically at different times depending on amount of deflection

Question 3

M⁺ peak

Answer 3

Molecular ion peak - usually the molecular mass of the ion with only one electron lost

Question 4

M⁺¹ peak

Answer 4

• often little peak after M⁺ peak as 1.1% of carbon isotopes are C-13, so it increases the mass by one

Question 5

What is shown on mass spectrometry graph

Answer 5

• shows fragment ions - made up of a radical and cation (positive ion)
• only positive ions show on graph
• graph is relative abundance against m/z
• M⁺ peak & M⁺¹ peak are right at the end of the graph

Question 6

What affects the abundance of the cations formed

Answer 6

• alkyl groups - as the number of groups increases (gets closer to tertiary) the more stable it is so the more abundant it will be
• elements such as nitrogen and oxygen, can donate lone pair to cation to make it more stable

Question 7

Infrared spectroscopy

Answer 7

Way of identifying functional groups present in organic molecules

Question 8

How does infrared spectroscopy work

Answer 8

1) range of IR waves passed through organic compound
2) some absorbed in varying amounts by different bond types
3) detector on other side can tell which frequencies have been absorbed and by how much

Question 9

How to tell difference between O-H (alcohol) & O-H (carboxylic acid)

Answer 9

• Carboxylic acid is broad
• slightly different wavelength range
• carboxylic acid peak will be more shallow (towards the top)

Question 10

IR radiation on earth

Answer 10

• most absorbed at earth’s surface - some re-emitted in form of longer wavelength IR
• this is absorbed by water vapour, carbon dioxide & methane (greenhouse gases)
• causes bonds to vibrate more
• this re-emits as radiation causes an increase in temperature near earth’s surface causing global warming

Question 11

Fingerprint region

Answer 11

• below 1500, has unique peaks
• can identify molecule from this using published spectra
• often has lots of peak so hard to identify (a-level)

Question 12

IR & covalent bonds

Answer 12

• covalent bonds vibrate around central point
• if they absorb IR, they can bend/stretch
• stretch is rhythmic movement along line between atoms so distance increases/decreases
• bending can cause an increase/decrease in bond angle

Question 13

IR uses

Answer 13

1) remote sensors analyse IR spectra of car emissions to detect/measure carbon dioxide, carbon monoxide and hydrocarbons to monitor pollution
2) breathalysers - pass beam of IR radiation through captured breath in sample chamber - detects IR absorbance of compounds in breath, detects ethanol bonds
- as absorption increases so does reading and ethanol consumed