Chemistry Notes I

Part one of my Chemistry Notes. Some info is omitted, I thought it was basic/easy so left it out. 

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  • Created by: Daniel
  • Created on: 19-03-12 10:30
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Chromatography
Phase: a physically distinctive form of a substance
Chromatography is an analytical technique that separates components in a mixture between a mobile
phase and a stationary phase.
The stationary phase interacts with the components in the mixture, slowing them down.
This allows different compounds to flow over the stationary phase at different speeds, separating
the components.
In TLC:
The stationary phase is a solid and the mobile phase is liquid
In Gas Chromatography:
The stationary phase is liquid or solid on a solid support, and the mobile phase is a gas.
Adsorption is the process which holds molecules of a gas or liquid to the surface of a solid.
As the mobile phase passes over the solid phase, some of the molecules are adsorbed.
TLC
Stationary Phase: Thin layer of adsorbent such as silica gel coated on a sheet of glass.
Mobile phase is a liquid solvent.
Sometime UV light is needed to see some components of the sample.
Rf value:
Distance moved by component / Distance moved by solvent front
Limitations:
Similar compounds have similar Rf values
Unknown compounds have no reference value for comparison
Some components may `hide' behind others
Gas Chromatography
Stationary phase is a thin layer of solid or liquid coated on tubes. The tubes are heated. The mobile
phase is an inert gas which moves through the column. The sample is vaporised and carried along by
the gas. The stationary phase slows some components down.
Rt is the time for a component to pass from the inlet to the detector.
Different compounds have different retention times
The area under the peak is the amount of a compound in a sample

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Limitations:
Sample may have chemicals with the same retention time and peak shape
Substance may hide behind another
Unknown compounds have no retention times to compare to
Uses of GC-MS
Forensics
Space Probes
Airport security
Environmental analysis
NMR
NMR involves interaction of materials with low energy radio waves.
Chemical shift:
Scale measured in ppm, relative to the TMS signal. TMS has 12 protons in the same environment
and these give rise to a single, sharp peak. TMS has a shift of 0.…read more

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Spin-spin coupling gives info about adjacent protons
-OH and NH protons are difficult to identify:
Peaks have a wide range of shift values
Signals are broad
No splitting
D2O or CDCl3 is used as it does not produce a signal. The D swaps with the H in NH and OH bonds so
no peak appears.
NMR is the same as MRI scans in medicine, but does not include the Nuclear part as people don't like
the word.…read more

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Benzene
Kekule said that Benzene had alternating double bonds. This cannot be true, however, because:
If C=C bonds were present then benzene would react like alkenes.
Each C=C bond would react with bromine water.
To fix this Kekule said that two forms of benzene existed, both with alternating C=C bonds. The
structure would change so rapidly that bromine couldn't react with it.
The bonds in Benzene are between the lengths of C-C bonds and C=C bonds.…read more

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Phenols
Phenol is benzene with an OH group attached.
When dissolved in water, phenol forms a weak acidic solution by losing H+ from the ­OH group:
C6H6 + aq C6H5O- + H+
When a metal reacts with phenol, hydrogen gas and a salt is produced.
Phenols are used as antiseptic as they kill bacteria.
Bromine reacts much more readily with phenol than benzene because:
A lone pair of electrons in O is drawn into the benzene ring.…read more

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