Topic 11 - Measurement & Data Processing Flashcards
uncertainty in analog scales
half of the smallest division visible on the scale
e.g. when measuring the volume on water on a measuring cylinder divided with a scale of 4ml, use ± 2ml
uncertainty in digital scales
the smallest scale division on the scale
e.g. when measuring weight and the reading says 10.00g, use 10.00 ± 0.01g
sources of uncertainty
- instrument readings
- uncertainties in judging (especially with qualitative data)
- errors
significant figures
- all the figures involved in the reading
e. g. 10.00 has 4 significant figures as the .00 denotes the level of uncertainty
types of errors
- random
- systematic
causes of random errors
- readability of measuring instrument
- effects of changes in surrounding (e.g. temperature variations)
- insufficient data
- misinterpreted readings
how to reduce the probability of random errors
repeated trials
repeatable results
if the experimenter can duplicate the experiment and observes the same results
reproducible results
if different experimenters can duplicate the experiment and observe the same results
causes of systematic errors
poor experimental design/procedure
e. g. measuring the volume of water from the top of the meniscus instead of the bottom
e. g. using an acid–base indicator whose end point does not correspond to the equivalence point of the titration
how to reduce the probability of systematic errors
careful experimental design
accuracy
smaller systematic error = higher accuracy
- small systematic errors
- gives result close to accepted value
precision
smaller random uncertainties = greater precision
- small random errors
- reproducible
graph setup (variables)
x-axis: independent variable
y-axis: dependent variable
drawing a line of best fit
- should pass as close as possible to many data points
- doesn’t have to pass through all
- can be used for extrapolation
applications of extrapolation of line of best fit
- absolute zero value can be found
- by extrapolating the vol/temp graph for an ideal gas
interpolation
the assumption that the trend line applies between 2 points
recognizing errors with a graph
presence of outliers indicate that data may not be reliable
types of analysis
- qualitative
- quantitative
- structural
infrared spectroscopy
used to identify the bonds in a molecule
mass spectrometry
used to:
- determine relative atomic and molecular masses
- can also be used to identify unknown substances or as evidence for atomic arrangements in a molecule (like a fingerprint)
- thus one can derive a molecule’s molecular formula
nuclear magnetic resonance spectroscopy (NMR)
- used to show the chemical environment of certain isotopes in a molecule
- nucleis of atoms with odd numbers of protons have the ability to spin and behave like tiny bar magnets
- they will line up with an applied field when placed in a magnetic field
- this leads to 2 energy states (higher/lower)
- radio waves provide energy needed for the nuclei to reverse their spin and change their orientation in a magnetic field
- gives vital info about structures
- as the samples are unchanged, NMR is non-invasive
mass spectrometry: fragmentation patterns
- ionization step involves shooting an e- at the incident species and removing an e- from the species
- the collision may be so energetic that it causes the species to break apart
- some parent ions can pass through unscathed
- but individual ions produced as a result of the breakup can also be detected
- a chemist can piece the fragments together based on the data of their mass
- to form a picture of the complete molecule
degree of unsaturation
AKA Index of Hydrogen Deficiency (IHD)
- measure of how many molecules of H are required to convert the molecule to the corresponding saturated, non-cyclic molecule
- provides a useful clue for the structure of a molecule once its formula is known
effect of radio waves on molecules
- can be absorbed by certain nuclei
- causing them to reverse their spin
- used in NMR (nuclear magnetic resonance spectroscopy)
- gives info about the environment of certain atoms
effect of microwaves on molecules
- cause molecules to increase their rotational energy
- gives info about bond lengths
effect of IR (infrared radiation) on molecules
- a chemical bond is like a spring
- bonds vibrate & bend depending on bond strength and masses of atoms
- light atoms vibrate more than heavier atoms
- the more bonds there are (single, double, triple, etc), the higher the frequency of vibration
- IR can be absorbed by polar bonds to cause them to vibrate at a greater amplitude
- thus causing stretching/bending
- gives info about bonds in a molecule
effect of visible light/UV light on molecules
- produces electronic transitions
- gives info about energy levels in a molecule
information given by X-rays
- X-rays are produced when e-s make transitions between energy levels
- they have wavelengths of the same order of magnitude as inter-atomic distances in crystals
- so they produce diffraction patterns providing direct evidence of molecular and crystal structure
why can IR only be absorbed by polar bonds?
- polar bonds have the presence of separate areas of partial +tive/-tive charges
- this allows the electric field (due to the electromagnetic nature of IR waves) to excite the vibrational energy of the molecule
- thus providing a corresponding change in the dipole moment of the molecule
effect of IR on polyatomic molecules
- bonds should be considered as a whole rather than looking at individual bonds
- water molecules are v-shaped and vibrate at 3 fundamental frequencies
- each of the 3 changes result in a change of dipole which can be detected with IR spectroscopy
- symmetric linear molecules (e.g. CO2) have 4 modes of vibration, of which only 3 are IR active (as the symmetric stretch has no changes in dipole moment)
resonance (in NMR)
phenomenon in NMR in which the nuclei flips over and spins in the opposite direction.
higher energy spin state
- nucleus lines up with magnetic field against the external field
- spins against the field
lower energy spin state
- nucleus lines up with magnetic field parallel to the external field
- spins parallel to the field
how does NMR work?
- a sample is placed in an electromagnet
- the field strength is varied until radio waves have the exact frequency needed to cause resonance
NMR hydrogen spectrum
- peaks in NMR graphs gives us info about hydrogen environments
- 1 peak = 1 hydrogen environment
- the areas under the peaks correspond to the ratio of H-1 isotopes in each H environment
standard signal used by NMR
signal produced by the 12 H nuclei in tetramethylsilane (TMS)
chemical shift
the position of the NMR signal relative to the standard signal
NMR - effect of chemical environment on chemical shifts
- e-s shield the nucleus from the full effects of the external magnetic field
- differences in e- distribution produce differing energy separations between the 2 energy spin states
- H-1 NMR (proton NMR) is generally used as it’s present in all organic molecules
- they can give info about their position in a molecule
why is TMS used as the standard signal for NMR?
- there’s only 1 hydrogen environment
- so only 1 signal is recorded
- and Si is less electronegative than C
- so TMS absorbs radio waves in a different region from the region of absorption of H nuclei attached only to C
- therefore its signals won’t overlap with signals under investigation
- it’s also chemically inert and soluble in most organic solvents
- since it has a low b.pt it can also be easily removed from the mixture
typical NMR spectrum of an organic compound
- general misconception that it consists only of single peaks
- but a sensitive high-res NMR machine can tell that the single peaks are actually split into multiple peaks
spin-spin coupling
- the phenomenon in which peaks are split in an NMR spectrum
- occurs due to the modification of the magnetic field by neighboring protons
- thus altering the effect experienced by particular nuclei
