Topic 11; Measurement + data processing Flashcards
experimental errors
resut in a different between recorded value and generally accepted value
random errors
caused by;
- readiability of measuring instrument
- effects of change sin surrounds (temperature or ai currents)
- insufficient data
- observe misinterpreting reading
- errors can be reduced by repeated measurement; imporatnt to have repetable and reproducible experiments
example; experimenter approximating reading
systematic errors
- as a result of poor experimental deisgn or procedure
- can NOT be reduced by repeating experiment
examples;
- measuring volume of water form top rather than bottom of meniscus
- overshooting volume of a liquid delivered in a titration
- useing an acid-base indicitator that doesn’t correspond to the equivalence point of the titration
- heat loss in exothermic reaction
accuracy
- the smaller the systematic error; the great the accuracy
- small systematic errors
- give a result close to accepted value
precision
- the smaller the random uncertainties, the greater the precision
- small random errors
- reproducible in repeated trials
- repeated measurements are all in a close value to one another
percentage uncertanitity calculation
absolute uncertainty/measured value x 100
percentage error calculation
accepted value- experimental value/accepted value x 100
tip for sig. figures?
- whenever you manipulate data, answer should be quoted to the same number of significant figures as the least precise value
analytical techniques
- qualitive analysis
- quantitve analysis
- structural analysis
qualitive analysis
detection of presences but not quantity
quantitive analysis
structural analysis
detection of quantity of substance
describption of molecular arragnement
mass spectromertry
used to determine the relative atomic and molecular masses
- use fragmentation patterns as fingerprint techniques to identify unknown substances/molecular arrangement
what is a molecular ion
(parent ion) is formed when a molecule loses one electron but remains otherwise unchanged;
parent ions can break into smaller ions in mass spectrometer
(causing fragmentation pattern)
how does a mass spectrometer work?
- electron gun hits incident species and removes electron
- collision energetic; splits molecule into fragments; largest peak in mass spectrum correspond to parent ion which didn’t get fragmented
- fragmentation pattern can provide evidence for structure of compound
IDH
index of hydrogen deficiency; provides a measure of the structure of a molecule once its formula is known
-measure of how many hydrogen molecules would be needed in theory to convert the molecule to the corresponding saturated, non-cylcic molecule
rules for IDH
in normal molecules; 2n+2 in alkene; 2n in alkyne; 2n-2 -ignore oxygen and sulfur - subtract the number of nitrogens
general formula; 2C+2+N-H-X/2 where N= # Nitrogen H= # of Hydrogens X= number of halogens
regions on electromagnetic spectrum
- electromagnetic spectrum has different wavelengths and frequencies for each region
wavelength
the distance between succesvie crests and troughts
frequency
the number of waves which pass a point every second
radio waves
can be absorbed by certain nuclei; causing them to reverse their spin
- they are used in NMR and can give info about the environemnt of certain atoms
microwaves
cause molecules to increase their rotational energy; this can give information about bond lengths
infrared radiation
absorved by certain bonds and cause them to stretch or bend; gives information about bonds in a molecule
visible light (UV)
can produce electronic transitions and gie information about the electronic energy levels within the atom or molecule
infrared spectroscopy
used to identify bonds in a molecule
natural frequency of a chemical bond?
- chemical bonds vibrate and bend at natural frequences
- diatomic molecules (HCl, HBr, HI) can only vibrate when the bond stretches
- more complex molecules have bending so a range of frequencies is present
what does IR radiation do?
- energy exictes molecules and makes them vibrate with greater amplitude
- bonds will only interact if polar; presence of partial postiive and negative charges allows electric field of waves to exicte VIBRATIONAL energy
- intensity of absorbtion depends on polarity of bond
stretching and bonding in a polyatomic molecule?
- water strectches + bends as a whole
- three modes of vibration possible ( two types of symmetric, assymetric)
- carbon dioxide; four modes of vibration ( symmetric stretch in IR is inatctive as there is no change in dipole omvement; equal bond dipoles)
- it has a symmetric stretch, an assymetric stretch and two symettric bends (only assymetrical stretches or symetric bonds create temporary dipoles)
principles of NMR
- used to show chemical environement of certain isotopes in a molecule + give structural info
- uses radio waves
- causes atoms with odd # of protons to spin like tiny bar magnets
- place them in an external magnetic field and see if they line up against or with it; leads to two nuclear energy levels
H NMR
- hydrogen nuclei in different chemical environments have different chemical shifts
- hydrogen nuclei; asct as spies to give information about their position with organic compounds
- position of NMR signal relative to chemical shift of proton (charateristic chemical shifts)
nuclear energy in NMR
- energy needed for the nuclei to reverse spin
-energy needed to change orientation in magnetic field
(energy provided by radio waves)
higher energy spin state
the nucleus lines up with magnetic field against enexternal magnestic field
-nucleus spins against field
lower energy spin state
nucleus has lined up WITH th emagnetic field in same direction as external magnetic field (spins parallele to field)
