Analytical Techniques

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

1. Bond strength
2. Bond length
3. Mass of each atom in the 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

Is fragmentation predictable? Why?

Answer 7

No, because it can happen anywhere in the molecule

Question 8

In a mass spectrometry how is a molecular ion represented?

Answer 8

It is the peak with the highest mass/charge ratio.

Question 9

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

Answer 9

• cheap
• small quantities of samples required

Question 10

What is the main disadvantage of using mass spectrometry?

Answer 10

The sample is completely destroyed.