Infrared Spectroscopy

Question 1

what can infrared radiation do to bonds in a molecule if it is absorbed

Answer 1

it can stretch or bend the bonds

Question 2

what does stretching and bending mean in this case

Answer 2

• stretching changes the length of the bond

- bending changes the bond angle

Question 3

what would happen to a bond between hydrogen and carbon if it were to absorb infrared radiation

Answer 3

• the bond would increase in length (stretch)

- the angle between the hydrogen atom and another would increase (bend)

Question 4

what are the three factors the amount of energy absorbed by a molecule depends on

Answer 4

• the length of the bond
• the strength of the bond
• the mass of each atom involved in the bond

Question 5

why do molecules such as H2 and Cl2 not absorb infrared radiation

Answer 5

• because they a non-polar

- the absorption of IR is linked to changes in the polarity of the molecules

Question 6

what is an important fact to remember about how molecules absorb IR when tying to understand infrared spectroscopy

Answer 6

the bonds in the molecules absorb radiation from some parts of the spectrum but not from others

Question 7

how are the axes of an IR spectroscopy graph labelled

Answer 7

• the y axis is labeled as the transmittance
• going from 0 to 100% from the origin
• the x axis is the wavenumber measured per cm (cm-1)1

Question 8

what needs to be understood about the nature of a transmittance variable on a graph

Answer 8

• 100% transmittance means none of the IR is absorbed and passes through
• 0% mean all of his has been absorbed
• meaning youre looking for sharp drops in the graph to see when IR has been absorbed

Question 9

what is the wavenumber more commonly known as

Answer 9

frequency

Question 10

why is it considered to be the frequency

Answer 10

• because the wavenumber is the reciprocal of the wavelength
• which is what frequency is (f = v / Y)

Question 11

would the numerical values (taken as independent) on the x axis be increasing or decreasing from the origin and why

Answer 11

• decreasing
• because with the cm-1 unit a decrease in the numerical value would indicate an increase in frequency
• like 4000cm-1 being less than 500cm-1 and 500cm-1 is ‘quicker’

Question 12

what does an infrared absorption spectrum graph consist of, simply

Answer 12

• a series of near horizontal lines close to 0%

- making a pattern of dips and troughs at specific wavenumbers

Question 13

what are the lines called

Answer 13

• absorptions

- or peaks if the graph is turned upside down

Question 14

what do weak and strong intensities (of absorption) imply about the transmittance values

Answer 14

• weak intensities will have high transmittance values

- strong intensities will have low transmittance values

Question 15

what will be given to you in order to figure out what kind of compound a substance from the infrared spectrum

Answer 15

• a table with a range of wavenumbers (which would have their own smaller wavenumber ranges within them)
• then the type of bond corresponding to the wavenumber of the IR it would absorb
• followed by the functional group the bond is from in order to identify what compound it is

Question 16

how would this info and the spectrum be used to deduce what compound the substance is

Answer 16

• you would identify the wavenumbers on the graph where a significant amount of absorption occurs
• then match the values with the ranges given by the wavenumber section of the table
• you would finally use the corresponding bond and what functional group to determine what kind of compound it is

Question 17

why is it important to have the functional group column rather than just the bonds

Answer 17

• because you can have two of the same bonds that come from different functional groups
• due to complexities within those different types of molecules
• they would also have different wavenumber absorption ranges

Question 18

as an example what two functional groups contain the O-H bond

Answer 18

• alcohol (3750-3200)

- carboxylic acid (3300-2500)