Infrared spectroscopy

Question 1

How does infrared spectroscopy work?

Answer 1

When we shine a beam of IR radiation through a sample of the substance some of the bonds will absorb energy.
A bond absorbs IR energy at the same frequency as its bonds vibrate.
The radiation that leaves the sample is missing the same frequencies as the bonds in the sample vibrate at.

Question 2

What is the unit of frequency used in infrared spectroscopy?

Answer 2

reciprocal centimeters cm-1

Question 3

The fingerprint region

Answer 3

Each substance has a unique IR fingerprint region

Question 4

Fingerprinting (below 1500 cm-1)

Answer 4

Complicated and contains many signals
Is unique for every compound - hence fingerprint
Can be used to check if a compound is pure

Question 5

State the functional group peaks and their respective wave number (above 1500)

Answer 5

O-H alcohol = 3230-3550cm-1
O-H acid = 2500-3000 cm-1
C=O = 1680-1750 cm-1
C=C = 1620-2680 cm-1

Question 6

With reference to infrared spectroscopy
explain why carbon dioxide, methane and
water vapour all contribute to global
warming.

Answer 6

These gases and others in the atmosphere absorb infrared radiation emitted by the Earth that might otherwise escape into space.

The reason that these gases have such an effect is due to their structures and the bonds they contain. The vibrational energies of the O - H bonds in water, the C - O bonds in carbon dioxide and the C - H bonds in methane correspond to energies in the infrared region.

When infrared radiation passes through these gases, energy is absorbed and these bonds stretch and the bond angles in the molecules change (bending). When
collisions between molecules occur, the energy absorbed by these molecules can be a transferred to other molecules as kinetic energy.

This process results in an increase in the average energy of the molecules in the atmosphere and, hence, global warming.