Modern analytical chemistry Flashcards
(29 cards)
Reasons for using analytical techniques
Faster
cheaper
more accurate and precise
experiments can be repeated quickly
Structure of compound found using variety of methods/diff observation singularly or in combination
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Describe electromagnetic spectrum
Infrared light - low energy, long wavelengths, Red, Orange
towards Ultraviolet - high energy, short wavelengths, Blue-Purple
Middle section = visible region
Diff b/n absorption and emission spectra
how each is produced
Emission Spectra - Hot, excited gaseous sample of an element that emits light which can be split through a prism to produce a characteristic emission spectrum
Coloured lines on a black background, converging at the high energy end
Absorption Spectra -
light passed through a cold, unexcited gaseous sample of an element
Electrons within atoms of the sample will absorb light
The remaining light is passed through a prism to produce a characteristic absorption spectrum
Coloured background with black lines
Atomic and molecular processes in which absorption of energy takes place
Vibrations
rotations
electronic transitions
symmetric and asymmetric stretching vibrations
Bending vibrations - IN-plane rocking in-plane scissoring out-of-plane wagging (near/near) out-of-plane twisting (near/far)
Operating principles of double-beam IR spectrometer
diagram is “sufficient”
Light source produced by IR source
Source beam split into separate beams
Travels through sample and reference (control)
E.g. In aqueous sample
Water/solvent used as reference
Allows cancellation of effect of water
segmented rotating disc alternates which two beams pass to the detector
Prism acts as monochromator, selecting correct wavelength of light
Detector compares signlas from both sample and reference
Determines how much absorbance due to sample
How info from IR spectrum is used to identify bonds
Wavenumber -
Higher bond enthalpy = more energy = higher wavenumber
More bonds = higher bond enthalpy = higher wavenumber
Larger mass = lower frequency of vibration = lower energy = lower wavelength
Particular functional groups have different absorbance bands
Analyse IR spectra of organic compounds
Can identify bonds present, but not always functional groups
Determine molecular mass of a compound from molecular ion peak
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Analyse fragmentation patterns in a mass spectrum to find structure of a compound
Examples: Mr-15 Loss of Ch3 Mr -17 Loss of OH Mr - 29 loss of CHO or C2H5 Mr - 31 loss ofCH2O Mr- 45 loss of COOH
Only picks up radicals (particle with one unpaired electron)
Tallest peak = base peak
Often this is the parent molecular ion (molecule minus e-)
Deduce structure of a compound given information from its HNMR spectrum
Number of diff H environments
relative number of H in each environent
Familiar with word and diagram of spectrum, including integration trace
Outline how NMR is used in body scans
Protons in water molecules within human cells can be detected by MRI, giving 3D view of organs in the body
State uses of AA spectroscopy
Identfication of metals in water, blood, soils and food
Principles of AA
Energy absorbed by a metal as electrons are excited is measured.
Use of Fuel atomiser monochromatic light source monochromatic detector readout
Hollow Cathode Lamp - Monochromatic light source. Emits light of a particular wavelength to be absorbed by the element.
Fuel atomiser - Acetylene-air mixture or nitrous oxide - acetylene mixture (hotter flame)
Vapourised and atomised (metal ions to metal atoms)
Some light absorbed by sample
Monochromator and slit - Remaining non-absorbed light is passed through a monochromator. Isolates one wavelength that is strongly absorbed by element of interest.
Monochromatic detector - Measures the intensity of this wavelength.
Determine concentration of a solution from calibration curve
Measure absorbance of unknown
Use standard curve of that metal to determine the concentration of the unknown
State reason for using chromatography
Used to separate components of a mixture
Identified and then [thing in sample] determined
Mainly used for organic compounds
Principles of chromatography
All Chromatography techniques involve adsorption on a stationary phase and dissolution into the mobile phase
Use of paper chromatography, TLC and column chromatography
Paper -
Thing layer of water bound to a piece of high quality absorbent paper
TLC -
A thin layer of absorbent material (Silica/aluminaA) spread onto glass or plastic plate
Both stationary phases are polar in paper/tlc
Capillary action pulls liquid up
Rf value - Retardation factor
Distance moved by component/distance moved by solvent
Column chromatography - Stationary phase, solid/solid that has been thinly coated in viscous liquid and packed into glass/plastic column
Sample applied to top of column, solvent (mobile phase) is dripped through the column.
Tap at bottom alloows solvent to elute from column at same rate as enters.
Separation occurs same as paper/tlc just components can be collected.
Describe effect of different ligands on splitting of d orbitals in transition metals
CN->NH3>H2O>Cl-
Factors that affect colour of transition metals
Nature of metal ion - charge/ionic radii influences distance/attraction between ligand and ion
Charge of transition metal ion - Greater charge, greater attraction, INCREASED splitting of d-orbitals
Size of ligand - Smaller ligand, closer lone pairs can get to d-orbital of ion. repulsion INCREASES splitting of d-orbitals
Shape of complex ion - d-orbitals of transition metal ion in a tetrahedral shape complex split differently to those in octahedral complex.
Organic molecules containing a double bond absorb UV radiation
Conjugated and delocalised systems
Inc. Arenes, alkenes and chlorophyll
Effect of conjugation of double bonds in organic molecules on the wavelength of absorbed light
Energy difference between molecular orbitals decreases, and the wavelengths of light absorbed increase.
As conjugation increases, differences between molecular orbital energies decreases and the light that is absorbed enters the visible range.
Predict whether a molecule will absorb UV or visible radiation
Difference in energy levels small: More conjugation = visible radiation
Energy difference in levels large: double bonds/triple bonds = UV radiation
