spectroscopy

Question 1

Infrared radiation and covalent bonds

Answer 1

Atoms in molecules are joined by covalent bonds. These bonds possess energy and vibrate naturally around a central point, the amount of vibration increases with increasing temperature. The atoms in molecules are therefore in constant motion. The bonds can absorb infrared radiation, which makes them bend or stretch more
-One type of vibration, a stretch, is a rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases. Have symmetric and asymmetric
-the second type of vibration is a bend, and results in a change in bond angle. Have symmetric and asymmetric
- any particular bond can only absorb radiation that has the same FREQUENCY as the natural frequency of the bond. The frequency values are large, so chemists use wavenumber, which is proportional to frequency
- infrared energy is only absorbed if the molecule has a permanent dipole

Question 2

the amount that a bond stretches or bends depends on

Answer 2

• 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 3

infrared radiation and atmospheric gases

Answer 3

Much of the Sun’s visible and IR radiation is relatively unaffected by atmospheric gases. This radiation passes through the atmosphere to the earths surface, where most of it is absorbed. However, some is re-emitted from the Earths surface in the form of longer- wavelength IR radiation.
Water vapour, carbon dioxide, and methane absorb this longer-wavelength IR radiation, because it has the same frequency as the natural frequency of their bonds. Eventually the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming

Question 4

uses of infrared spectroscopy

Answer 4

• used to identify pollutants in vehicle emissions in the air. Sensors detect and measure the amount of pollutants such as carbon monoxide, carbon dioxide and hydrocarbons
• used to measure alcohol levels in roadside breathalysers. Ray of infrared radiation is passes through the breath that is exhaled into the breathalyser chamber. The characteristic bonds of ethanol are detected and measured

Question 5

fingerprint region

Answer 5

The region below 1500/cm is called the fingerprint region and is unique to every molecule
- many peaks can be difficult to assign
- these peaks represent the complex vibrational interactions that occur between the different bonds within a molecule
- the value of the fingerprint region is in being able to compare the IR spectrum to a known compound from a database and coming up with an exact match
- particularly useful for identifying members of a homologous series- all members of the series will show the same type of bond present, but no two molecules will have the same fingerprint region