Analytical techniques (IR and MS)

Question 1

Why do covalent bond vibrate

Answer 1

Covalent bonds absorb IR radiation causing them to vibrate

Question 2

How does bond absorption differ

Answer 2

Different bonds absorb different frequencies of IR

Question 3

Key ideas behind the IR spectra

Answer 3

Spectrum does not tell you how many of a type of a bond (size of absorption linked to how well the bonds absorb IR radiation)

There are many different types of absorptions each which require a different amount of infrared radiation

Question 5

Describe the uses of IR spectroscopy:

Answer 5

Infrared spectroscopy is 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 unburnt hydrocarbons.This commonly occurs on motorways and in busy town centres to monitor localised pollution

• Infrared spectroscopy can be used to measure alcohol levels using roadside breathalysers:

• A ray of infrared radiation is passed through the breath that is exhaled into the breathalyser chamber. characteristic bonds of ethanol are detected and measured - the higher the absorbance of infrared radiation, the more ethanol in the person’s breath

Question 6

Explain how the absorption of greenhouse gases help scientist identify global warming as a problem

Answer 6

Greenhouse gases include carbon dioxide, water vapour, methane, and all other atmospheric gases containing atmospheric gases containing C=O, O–H and C–H bonds.

The bonds within molecules of these greenhouse gases absorb infrared radiation. This increases their kinetic energy causing the gases to heat up. As a result the temperature of the atmosphere increases, which in turn warms the earth. However, the rapid increase in carbon dioxide levels over the last hundred or so years, due to increased combustion of fossil fuels, has led to the largest increase in temperature the planet has known in the shortest time.

This is referred to as global warming.

Question 7

Explain a fingerprint region

Answer 7

The “fingerprint region” is unique for all molecules.

It is in the region of 500 and 1500 cm-1. It represents the wavenumbers absorbed by the C-C and C-O bonds as well as the vibrations of the whole molecule.

The fingerprint region is unique to each molecule, and can be used to identify a molecule by comparing it with known samples in databases.