17.2 Infrared spectroscopy

Question 1

What happens when bonds absorb infrared radiation

Answer 1

Makes them bend or stretch more

Question 2

What is a stretch

Answer 2

A rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

Question 3

What is a bend

Answer 3

Results in change in bond angle

Question 4

What does the amount that a bond stretches or bends depend on

Answer 4

The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
The strength of the bond - stronger bonds vibrate faster than weaker bonds.

Question 5

What is the scale called wavenumber

Answer 5

Proportional to frequency

Frequency values of a bond

Question 6

The vibrations of most bonds are observed in the IR wavenumber range of…

Answer 6

…200cm^-1 to 4000cm^-1

Question 7

What is some Infrared radiation re-emitted from the Earth as

Answer 7

Longer-wavelength Infrared radiation

Question 8

The greenhouse effect and longer wavelength IR radiation

Answer 8

Greenhouse gases absorb this as it has the same frequency as the natural frequency of their bonds
Vibrating bonds in these molecules re-emit this energy as radiation which increases the temperature of the atmosphere close to the Earth’s surface

Question 9

What are the three most abundant greenhouse hases

Answer 9

Water vapour
Carbon dioxide
Methane

Question 10

Infrares spectroscopy to identify functional groups in organic molecules

Answer 10

1. Sample under investigation placed inside IR spectrometer
2. Beam of IR in range 200-4000cm^-1 passed through sample
3. Molecule absorbs some IR frequencies and emerging beam of radiation analysed to identify frequencies that have been absorbed by sample
4. IR spectrometer usually connected to computer that plots graph of transmittance against wavenumber

Question 11

What is the fingerprint region of a spectrum

Answer 11

Contains unique peaks which can be used to identify particular molecule under investigation
Using computer software or comparing to booklets of published spectra

Question 12

WHAT DO YOU NEED TO KNOW HOW TO IDENTIFY (FUNCTIONAL GROUPS) FROM AN IR SPECTRUM IN THE EXAM

Answer 12

O-H groups in alcohols
C=O group in aldehydes, ketones and carboxylic acids
COOH group in carboxylic acids

Question 13

What region do all organic compounds produce a characteristic peak at from the presence of C-H bonds

Answer 13

2850 to 3100cm^-1

OFTEN CONFUSED WITH O-H peak in alcohols

Question 14

What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the O-H bond

Answer 14

3200-3600cm^-1

Question 15

What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the C-O bond

Answer 15

1000-1300cm^-1

Question 16

What is the range for the absorbance peak in the IR spectrum of an aldehyde or ketone caused by the C=O bond

Answer 16

1630-1820cm^-1

Usually around 1700cm^-1

Question 17

What is the range for the absorbance peak in the IR spectrum of a carboxylic acid caused by the C=O bond

Answer 17

1630-1820cm^-1

Usually around 1700cm^-1

Question 18

What is the range for the broad peak in the IR spectrum of a carboxylic acid caused by the O-H group

Answer 18

2500-3330cm^-1

Question 19

Why isn’t the C-O peak always reliable in the IR spectrum of a carboxylic acid

Answer 19

Because it is in the fingerprint region

Question 20

How can pollutants be identified

Answer 20

By their IR spectral fingerprints
Remote sensors analyse IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons

Question 21

How do IR-based breathalysers work

Answer 21

Pass a beam of IR radiation through captured breath in sample chamber and detect IR absorbance of compounds in breath
Bonds present in ethanol are detected
More IR absorbed, higher reading, more ethanol in breath

Question 22

What does a typical sequence for identification include

Answer 22

Elemental analysis
Mass spectrometry
Infrared spectroscopy
Once you have empirical formula and molecular mass, can determine molecular formula

Question 23

What is elemental analysis

Answer 23

Use of percentage composition data to determine empirical formula

Question 24

What is mass spectrometry

Answer 24

Use of the molecular ion peak from mass spectrum to determine molecular mass; use of fragment ions to identify sections of molecule

Question 25

What is infrared spectroscopy

Answer 25

Use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule