Analytical techniques

Question 1

Describe infrared spectroscopy

Answer 1

When a beam of infra-red radiation is passed through a sample, the bonds in that sample absorb the IR energy and vibrate more with either a stretching or bending motion.

Question 2

Describe the two types of vibration of the bonds in an infra-red spectrum

Answer 2

A stretch is a rhythmic movement along the line between the two atoms so that the distance between the two atomic centres increases and decreases.
A bend results in a change in bond angle.

Question 3

Explain what a bond stretching/bending depends on (2 points).

Answer 3

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

Question 4

Explain how bonds absorb radiation

Answer 4

Bonds only absorb radiation of the same frequency as the natural frequency of the bond.

Question 5

When answering exam qus about infra-red spectra, you must… (4 points).

Answer 5

1. State the wavenumber (cm^-1) of the peak- use values given on the data sheet.
2. Write the bond and functional group for the peak.
3. State any peaks that are missing (to rule out functional groups).
4. “Peak at ‘use data sheet’ cm^-1 due to the ‘bond’ in a ‘functional group.’

Question 6

State the IR spectrum of an alcohol

Answer 6

The IR spectrum of an alcohol has an absorbance peak within the range 3200-3600cm^-1 caused by the -OH bond in an alcohol.

Question 7

State the IR sepctrum of an aldehyde/ketone

Answer 7

The IR spectrum of an aldehyde or ketone has a key absorbance peak within the range 1630-1820cm^-1 caused by the C=O bond in aldehydes and ketones.

Question 8

State the IR spectrum of a carboxylic acid

Answer 8

1. The IR spectrum of a carboxylic acid has a key absorption peak within the range 1630-1820 cm^-1, caused by the C=O bond in a carboxylic acid.
2. Carboxylic acids have a broad absorption peak within the range 2500-3300cm^-1 caused by the -OH bond in a carboxylic acid.

Question 9

Explain how using IR technology can be used to test for drink-driving (3 points).

Answer 9

1. Police will conduct a breathalyser test that indicates the alcohol level of the driver.
2. A chemical that could be used to show a colour change if alcohol is present in the driver’s breath is acidified potassium dichromate- it geos from orange to green.
3. A positive result for this test is when a peak between 3200-3600cm^-1 due to the -OH bond in alcohol.

Question 10

What is mass spectrometry used for? (3 points).

Answer 10

1. Used to determine the relative atomic mass of an element.
   Can also be used to:
2. Identify the molecular mass of an organic compound.
3. Gain further information about the structure of molecules.

Question 11

When answering questions about mass sepctrometer, you must… (2 points).

Answer 11

1. Identify the M+ peak and link the m/z value to the Mr of the molecule.
2. Identify the fragment ion for the tallest peak (this is the most abundant fragment ion).

Question 12

Explain how to tackle qus with combined techniques (4 points).

Answer 12

1. Elemental analysis- % composition data to determine the empirical formula.
2. Mass spectrometry- use of molecular ion peak from a mass spectrum to determine the Mr and thus the molecular formula; use of fragment ions to identify sections of molecule.
3. IR spectroscopy to identify bonds and determine functional groups.
4. Back to mass spectroscopy to deduce the structure by identifying fragment peaks.