WM 3, 4, 5 Flashcards
what does it mean that the energy possessed by molecules is quantised
- molecules must take a small number of definite energy values rather than any energy value
what is the frequency and wavelength range of infrared spectroscopy
- 10^14 - 10^13
- 2.5 - 15 micro meters
how do you calculate the speed of light and what is each component measured in
speed of light (m/s) = wavelength (m) x frequency (s^-1)
what is the direct measure of frequency using wavelength?
what does it equal?
- 1/lander
- wavenumber
what happens as wave number increases
- wavelength decreases
- frequency increases
how do diatomic hydrogen molecules vibrate
- they only vibrate by stretching, where the atoms pull apart then pish together again
in the case of hydrogen diatomic molecules, what does the vibrational infrared absorption of each of them correspond to
- corresponds to the molecules changing from their lowest energy level to to the next higher energy level, where the vibrations are more vigorous
why are the frequencies of the absorptions different for each molecule
- the energy needed to excite a vibration is dependent on the strength of the bond holding the atoms together
- the weaker the bonds, the less energy required
give examples of vibrational modes of more complex molecules
- rocking
- scissoring
- twisting
- wagging
why are some absorptions on the absorption spectrum intense whilst others are weaker
- the strongest absorptions arise when there is a large change of bond polarity associated with the vibration
- the more polar bonds give more intense absorptions than the non polar
how does hydrogen bonding affect absorption spectra and what is the evidence too prove it
- when a molecule that contains an OH bond e.g. ethanol is in its gas state, there is little hydrogen bonding occurring
- so there is a sharp peak at the OH wave number
- but when it is in its liquid state and there is a lot of hydrogen bonding occurring
- the peak at the OH is a lot fatter, longer and broader and shifts a lower wave number
what is the fingerprint region, and describe it
- it is the region below the wave number of 1500
- characteristic to the particular molecule
- quite complex
give an example where the finger print region would be useful
- when you need to compare two different spectra to see if they are of the same compound
what type of compounds can the finger print region be useful towards, and how
- aromatic compounds
- display a complex pattern in the finger print region
- can be identified by comparing their infrared spectra with reference spectra
what is mass spectrometry used for
- the atomic mass of elements
- relative abundance of isotopes in an elements
what is the base peak
- highest point in a mass spectrum
- always 100%
- is what the intensity of the rest of the values are based on
what does the heaviest ion correspond to
the molecular mass of the molecule minus 1
why are there multiple peaks with an m/z value below the molecular mass of the unfragmented compound
- the M+ ion fragments into smaller ions and the mass spectrometer detects and analyses them
- the way the parent ion breaks down is what is characteristic to the compound
why does tiny a peak often appear at M+1
(just after the peak with the highest mass)
- because a small amount of carbon exists as carbon-13 instead of carbon-12
- if a molecule has incorporated one of these carbon 13 atoms, then the molecular mass will be M+1
what causes the rest of the peaks with a wave number lower than M
- due to fragmentation of the ion of the parental molecule
- which is the one with the highest mass
what is green chemistry
- developing chemicals and processes that are sustainable and as environmentally friendly as possible
what are the explanations for the principles:
1. better atom economy
2. prevention of waste products
3. less hazardous chemical synthesis
- more of the feedstock is incorporated into the product and production of less waste products
- prevention of waste production is better than treating and disposing of waste
- use of less hazardous chemicals in chemical reactions
what are the explanations for the principles of:
4. design safer chemical products
5. use safer solvents
6. lower energy usage
- less toxic and hazardous chemical products
- minimise the use of organic solvents
- use of lower temperature and pressure
what are the explanations for the principles of:
7. lower renewable feedstocks
8. reduce reagents used and the number of steps
9. use catalysts and more selective catalysts
- use renewable feedstocks instead of depleting natural resources
- as they can generate waste
- because they generally reduce energy usage and waste products
