Chromatography and Spectroscopy

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  • Created on: 25-04-14 00:04
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Chromatography and Spectroscopy ­ Answers
1. When some inorganic compounds are heated in a Bunsen flame, heat energy is converted into light energy:
The heat from the Bunsen promotes an electron in a metal to a higher energy level
This is an unstable state
The electron spontaneously drops back to a lower level and radiation is given off
This radiation is in the form of UV or visible light
If this light is observed in a spectrometer, bright lines are seen at different frequencies due to the
different energy levels that the electrons were promoted to. The energy of a spectral line is the
difference between the energy of the electron in the higher state, E2, and that in the lower state, E1.
2. The colours given by group 1 and group 2 cations can be used to show their presence in a compound:
Lithium burns with a red or magenta flame
Sodium burns with a yellow flame
Potassium burn with a lilac flame
Rubidium burns with a red flame
Calcium burns with an orange-red flame
Strontium burns with a crimson or red flame
Barium burns with a pale green flame
3. X-rays have high energy per photon and have many uses, particularly in modern medicine:
Bone X-rays: X-rays are strongly absorbed by atoms with a high atomic number. Bones contain calcium ions
and absorb some X-rays, but soft tissue is mainly carbon, oxygen and hydrogen and does not absorb X-rays.
Use of this is made when taking an X-ray picture of a suspected fracture etc.
CT scans: Barium has a large atomic number and is opaque to X-rays. Taking X-ray pictures of the human gut
after the patient has eaten a barium sulphate meal can show up blockages in the digestive system.
Angiogram: Analyses the state of the arteries around the heart. A solution of a compound containing
several iodine atoms is injected into the blood around the heart via a catheter introduced into an artery in
the patient's groin. The heart area is then X-rayed and any narrowing of the arteries detected.
4. UV radiation is high energy. When it is absorbed by a molecule, a bond is broken. For example, ozone absorbs UV
rays that provide the energy for the reaction:
O3 O2 + O·
5. When visible light is absorbed by a molecule or ion, either:
A bond is broken: For example, the energy of blue light is sufficient to cause a chlorine molecule to break
into two radicals and initiate the free-radical chain substitution reaction of chlorine with alkanes: Cl2 2Cl·
A bonding electron is moved to a higher energy level: The light energy of a particular band of frequencies
is absorbed by transition metals and an electron is moved from the lower level of the split d-orbitals to the
upper level. The colour of the ion is complementary colour to that absorbed. On collision with another
molecule, the energy is released as heat. The electron returns to its lower level and is then able to absorb
more light.
6. Absorption of infrared radiation causes a bond to stretch or bend, thus gaining visible energy. A particular bend
or stretch is only infrared active if it is accompanied by a change in dipole moment. Thus oxygen and nitrogen,
along with all other diatomic elements do not absorb infrared radiation; therefore they are not greenhouse
7. Carbon dioxide absorbs infrared radiation. Oxygen is more electronegative than carbon, so the carbon atom in
carbon dioxide is + and the oxygen atom is -. However, the molecule is linear, so the dipoles cancel out and the
molecule is non-polar. There are three ways that the molecule can vibrate:
Asymmetrical stretching - cause a change in dipole moment from zero and are infrared active.
Symmetrical stretching ­ Does not cause a change in dipole moment and so does not absorb IR.
Bending - cause a change in dipole moment from zero and are infrared active.
Therefore carbon dioxide absorbs infrared radiation and is a greenhouse gas.
8. Electromagnetic radiation in the infrared region causes bonds to vibrate by bending or stretching. Each bond
vibrates at a particular frequency and absorbs the light at that frequency. This means that by looking at which
frequencies are absorbed it is possible to determine which bonds are present. The frequency is usually
expressed as the wave number, which is the reciprocal of the wavelength in units of cm-1. The actual value of the
frequency depends on the neighbouring atoms and groups, so the absorption due to the stretching of a
particular bond occurs over a range of frequencies, e.g. the C=O bond absorbs at around 1700cm-1, but the actual
value depends on the other atoms attached to the C=O group.

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The region below about 1300cm-1 is known as the fingerprint region. It shows a complex series of peaks that
depends on the exact compound being analysed. Computer analysis of the fingerprint region can be used to
identify a pure unknown organic substance.
10.…read more

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The number of different peaks represents the number of different environments of the hydrogen ions
For example, a molecule of propanal, CH3CH2CHO has three different peaks:
The peak caused by the hydrogen atom on the CHO group
The peak caused by the two hydrogen atoms in the CH2 group that is next to the CHO group
The peak caused by the hydrogen atoms in the CH3 group
The height of each peak indicates the ratio of the different hydrogen ion environments to each other, for…read more

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In chromatography, the sample is dissolved in a solvent and washed through a stationary phase by a mobile
phase called the eluent. The competition between the sample molecules adsorbed by the stationary phase and
dissolved by the eluent results in separation.
29. How fast the compounds get carried up the plate depends on two things:
How soluble the compound is in the solvent. This will depend on how much attraction there is between the
molecules of the compound and those of the solvent.…read more


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