M4 B6 Analytical Techniques
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What happens to the bonds in a sample when a beam of infra-red radiation is passed through?
The bonds in that sample absorb the IR energy and vibrate more, either with a stretching or bending motion
Bonds absorb different frequencies of radiation, what frequency is this?
Bonds will only absorb radiation of the same frequency as the natural frequency of the bond.
We use wavenumber instead of frequency to measure radiation. What are the units for wavenumber?
cm⁻¹
Radiation from the sun is absorbed by the planet itself, however, greenhouse gases absorb reflected radiation. What are the 3 most plentiful greenhouse gases in the atmosphere?
• Carbon dioxide (CO2) • Methane (CH4) • Water vapour (H2O)
When greenhouse gases absorb IR radiation, the bonds vibrate, what happens to the energy causing them to vibrate?
The vibrating bonds re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface, LEADING TO GLOBAL WARMING
What does infrared spectroscopy allow us to identify?
• Allows us to identify a functional group in a molecule
How does infrared spectroscopy work? (3)
• A sample of the molecule is placed inside an IR spectrometer. • A beam on IR radiation is passed through it • The beam that gets passed THROUGH the sample (not absorbed) is detected, and a graph is plotted on a computer.
What are the x and y axis of an IR spectra graph?
X- wavenumber (cm⁻¹) Y- Transmission (%) (of IR radiation)
When answering questions about infrared spectra, you must follow 3 criteria What are they?
• State the wavenumber (WITH UNITS) of the peak • Write the bond and functional group for the peak • State any peaks that are missing (to rule out functional groups)
What are 2 applications (uses) of infrared spectroscopy in real life?
• Identification of atmospheric pollutants (like CO or NO from vehicle emissions) • To test for drink driving using a breathalyser to measure ethanol in the breath
What does mass spectrometry allow us to determine?
The relative atomic mass of an element
What can mass spectrometry be used for? (2)
• Identify the molecular mass of an organic compound • Gain further information about the structure of molecules
What happens to a sample when it is put in a mass spectrometer?
• It loses an electron to form a positive ion • This is known as the molecular ion ( [M+] )
On a mass spectrometry graph, what is the x axis?
X- m/z which means ‘mass to charge ratio’ - Since the ions detected are all +1, the m/z value represents the mass of the ion.
(Fill in the word) In the mass spectrometer, some molecular ions break down into smaller pieces called….
Fragments (fragment ions)
When answering questions about mass spectra, there are 2 criteria to remember What are the criteria?
• Identify the M+ peak and link the m/z value to the Mr of the molecule • Identify the fragment ion for the tallest peak (this is the most abundant fragment ion)
When determining the structure of an organic molecule, chemists get information from a variety of sources. What are these sources? (4)
• % composition data (determine empirical formula) • IR spectroscopy to determine functional groups • Mass spectrometry to determine the Mr and therefore molecular formula • Identifying fragment peaks from Mass spectrometry (deduce structure)