Topic 7/19 Modern Analytical Techniques 1/2

Question 1

What happens when a molecule absorbs infrared radiation?

Answer 1

It makes the covalent bond vibrate more in a stretching or bending motion

Question 2

What factors affect the amount of vibration of a bond?

Answer 2

What factors affect the amount of vibration of a bond?

Question 3

How does infrared spectroscopy work?

Answer 3

Every bond has a unique vibration frequency in the infrared region of the EM spectrum
Bonds absorb radiation that has the same frequency as their frequency of vibration
Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group

Question 4

What do the peaks on an infrared spectrum represent?

Answer 4

Absorbance of energy from the infrared radiation

Question 5

What are the 2 peaks that must be present to identify a substance
as carboxylic acid?

Answer 5

● Very broad peak at 2500 - 3300 cm -1 → O-H group
● Sharp peak at 1680 - 1750 cm -1 → C=O group

Question 6

What happens inside a mass spectrometer?

Answer 6

● Organic compound is vaporised and passed through the
spectrometer
● Some molecules lose an electron and forms molecular ions
● Excess energy from the ionisation makes the bonds vibrate
more
● Vibration causes bond to weaken
● Molecular ion splits by fragmentation

Question 7

What is the symbol of molecular ion?

Answer 7

M +

Question 8

Is fragmentation predictable? Why?

Answer 8

No, because it can happen anywhere in the molecule

Question 9

In a mass spectrometry how is a molecular ion represented?

Answer 9

It is the peak with the highest mass/charge ratio

Question 10

The molecular mass of the molecular ions is equal to what?

Answer 10

Relative molecular mass of the compound

Question 11

Will the molecular ion peaks of two isomers of the same compound be same or different?

Answer 11

Same

Question 12

What are the advantages of using mass spectrometry? (2)

Answer 12

● Cheap
● Small quantities of samples required

Question 13

What is the main disadvantage of using mass spectrometry?

Answer 13

The sample is completely destroyed

Question 14

How does infrared spectroscopy work?

Answer 14

Every bond has a unique vibration frequency in the infrared region of the EM spectrum
Bonds absorb radiation that has the same frequency as their frequency of vibration
Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group

Question 15

What do the troughs on an infrared spectrum show?

Answer 15

The frequencies where radiation has been absorbed - match to table to find out which bonds they represent

Question 16

What is the fingerprint region?

Answer 16

Area of the infrared spectrum below wavenumber of 1500cm -1
Many peaks, caused by complex vibrations of the whole molecule. Unique to every compound, so can be used to identify compounds

Question 17

How is the fingerprint region used to identify compounds?

Answer 17

The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately