Chemcial Analysis Flashcards
How does infrared sprectroscopy work?
- Shines a range of IR frequencies, one at a time, through a sample of organic compound
- At some frequencies, the energy will be absorbed
- A detector on the other side of the sample will show that some are being absorbed, whilst others aren’t
- Different energies correspond to different functional groups
What happens when the energy of IR is absorbed by a bond?
The bond vibrates more
What IR radiation can molecules absorb if they have covalent bonding?
It can absorb IR radiation at the same frequency as the natural frequency of the bond
What are the axis on an absorption spectrum?
- Horizontal axis shows the wavenumber of the radiation absorbed in cm^-1
- Vertical axis shows transmittance, which is the percentage of radiation that passes through the sample
What is the shape of the peak on an IR spectrum of an alcohol?
- Absorbance peak is curved and broad
- Within range of 3200-3600com-1 (data sheet)
- Peak represents O-H group!
What is the shape of the peak of an IR spectrum of an aldehyde or ketone?
- Absorbance peak is sharp and thin
- Close to 1700cm-1 (data sheet)
- Peak represents C=C!
What is the shape of the peak of an IR spectrum of a carboxylic acid?
- One absorbance peak broad and pointy between 1630-1820cm-1
- Represents O-H group!
- Second absorbance peak sharp and thin near 1700cm-1
- Represents C=O!
What are the advantages of IR spectroscopy?
- Small sample size
- Sample is easy to prepare
How to answer an exam Q based on IR spectroscopy?
- Mention whether peak is sharp or broad
- Mention range of peak
- Refer to functional group represented
- Identify organic compound
What are the real life applications of IR spectroscopy?
- Breathalysers
- Monitor gases and air pollution
What are the main three greenhouse gases?
- CO2
- CH4
- H20 (water vapour)
What is the normal function of greenhouse gases in the greenhouse effect?
- Absorb IR radiations emmited from Earth’s surface at the same frequency as the natural frequency of their bonds
- Then re-emit IR
What are the bonds responsible for absorbing IR in greenhouse gases?
CO2-> C=O
CH4 -> C-H
H20 -> O-H
How does mass spectrometry work?
- Separates atoms according to their mass
- Shows relative numbers of different atoms/fragments present
- Before atoms can be detected, they are converted into positively charged ions then separated by deflection
What is the m/z value?
- Mass to charge ratio
- It is numerically equal to the mass of the ion
What does the height of a peak on a mass spectrum show?
-Height of each peak measures the relative abundance of the ions that gives rise to that peak
Which peak represents the molecular mass for the whole molecule?
Clearest peak FURTHEST TO THE RIGHT
Which species aren’t well detected by the mass spectrometer and why?
Radicals because they re not accelerated or deflected by the mass spectrometer
How does the analysis of fragments work in mass spectrometry?
- When an organic compound passes through the spectrometer its molecules are broken into positively charged fragments
- Each +ve fragment gives a corresponding line in the mass spectrum
- So, we can work out its mass from its position
- By piecing together fragments, we can deduce the structure of the parent molecule
What happens when a molecular ion fragments?
A positive ion and a radical form
What are the real-life uses of mass spectrometry?
- Monitoring air pollution
- MOT emission testing
How to write an equation to show the formation of a molecular ion
Compound —–> Molecular ion + Electron (e^-)
