11 measurement and data processes Flashcards
what does qualitative data include?
all non numerical info obtained from observations NOT measurement
- detection of the presence but not quantity
what kind of error will repeat trials reduce?
random error
(will not reduce systematic)
where do you take the volume reading in a measuring cylinder
the bottom of the meniscus!
what is the uncertainty of an instrument?
+/- half the smallest division
uncertainty pertaining to the experimenter 2
- reaction time (time measurements)
- judgement of when an indicator changes colour
when is scientific notation used?
when the number of sf is uncertain
(hence making it like 1 x 10^3)
what are random errors caused by 4
- readability of measuring instrument
- effect of changes in surroundings eg temp var.
- insufficient data
- observer misinterpretation
in terms of experimental results whats repeatable vs reproducable
repeatable: same experimenter gets same results
reproducable: many experimenters get same results
what are systematic errors a result of
poor experimental design / procedure
eg electronic mass balance incorrectly zeroed
examples of systematic sampling 4
- measuring water volume from meniscus top not bottom
- overshooting volume of liquid added in titration = vols too high
- use of acid-base indicator whose end point doesn’t correspond to equivalence pt
- heat losses in exothermic reaction = smaller temp changes)
the smaller the systematic error, the greater the ______
the smaller the random error, the greater the ______
accuracy
precision
the smaller the value, the ______ the uncertainty
more significant
what happens to uncertainty when you add or subtract
uncertainty is the SUM of the absolute uncertainties
how to find absolute uncertainty of a multiplied/divided value
find maximum value and minus the middle value = +/- absolute uncertainty
how to find percentage uncertainty after multiplication and division?
find the absolute uncertainty as a percentage of the value
significant figures when multiplying or dividing data
same sf as least precise data
significant figures when adding or subtracting
NOT sf but dp. same dp as the one with the most dp
things to remember when plotting a graph
- title
- label axes: quantities and units
- line of best fit should show overall trend
- identify anomalies
- carefully consider inclusion of origin
how to find gradient
draw that tangent and triangle girl
systematic error best fit graph vs random error best fit graph
systematic: higher/lower than perfect, but same trend
random: slightly scattered points around the perfect line
what does the presence of an outlier suggest
some of the data may be unreliable
given ideal gas equation PV = nRT, what can be said about the pressure and volume relationship? + how would the graph look like
pressure is inversely proportional to volume
vol against pressure:
decreasing slope (steep then more gradual)
pressure against 1/volume
straight increasing line
what does the use of log scales allow for
the rearrangement of some relationships into a straight line
what is quantitative analysis?
the measurement of the QUANTITY of a substance in a mixture
what is structural analysis
description of how atoms are arranged in molecular structures
what is Infrared spectroscopy for
to identify BONDS in a molecule
what is mass spectrometry for
determine RELATIVE ATOMIC AMASS and molecular mass
how are fragmentation patterns formed in mass spectometry
ionisation process! electron from electron gun hits species and removes an electron
- energetic collision = molecule breaks up
what are fragmentation patterns for in mass spectrometry
provide evidence for structure of the compound
- the lines are possible breakdowns of the compound eg losing certain elements = certain mass lost
highest mass line in mass spectrum represents?
the Mr of the PARENT MOLECULE
- subtract other lines to find loss in mass = find out what part was lost
what is the degree of unsaturation / IHD (index of hydrogen deficiency)
in theory, how many H2 would be needed to convert the molecule (formula determined with mass spectrometry) into the corresponding saturated, non cyclic molecule
what is the frequency of radiation measured as
number of waves per centimeter / wavenumber
what waves are used in NMR
radio waves
give info abt environment of certain atoms
what effect do microwaves have on molecules
increaed rotational energy = info about bond lengths
what effect does infrared radiation have on molecules
absorbed by certain bonds = stretch / bend
what effect does visible and ultraviolet light have on molecules
can produce electronic transitions and give info about electronic energy levels
x rays are produced from electrons when…
electrons make transitions between inner energy levels
a bond will only interact with Infrared radiation IF it is ____
polar
how does a bond being polar allow it to interact with infrared radiation in IR spectroscopy
seperate areas of partial pos and neg charge = electric field of electromagnetic wave to excite vibrational energy of the molc
change in vibrational energy = change in dipole moment
intensity of absorption depends on bond polarity
what shape does O-H give in IR spec
broad trough
what shape does C=O give in IR spec
sharp spike
how can hydrogen bonding be detected in IR spec
hydrogen bonds = absorption BROADENS and shifts to lower freq
what is NMR (nuclear magnetic resonance) spectroscopy for
finding structure and shape of molecules
what is resonance and how is it formed in NMR
sample placed in electromagnet
field strength varied until radio waves have EXACT frequency needed to make the nuclei FLIP over and SPIN in OPPOSITE DIRECTION
nuclei in different ________________ produce different signals on an NMR spectrum
hydrogen environments
the signals of a H NMR are measured against the standard signal produced by?
the 12 hydrogen nuclei in tetramethylsilane (TMS)
Si in middle, 4 sides having CH3 (methyl grp)
= total 12 hydrogens
has a chemical shift of ZERO
position of NMR signal relative to the standard signal is called the ________ of the proton
chemical shift
what does the heights of H NMR spectra indicate
the relative number of protons in each environment
eg 3x as tall = 3x as many protons
why is TMR used as a standard in NMR spec?
tetramethylsilane
has 12 hydrogen nuclei all in the same environment = ONE signal
silicon has lower electronegativity = wont overlap with possible tested (like if they used C)
what does a split peak in NMR mean
the effective magnetic field is modified by the mag field of neighbouring protons (aka SPIN-SPIN coupling)
one signal in a low res NMR spec = 2 signals in a high res (doublet), explain????
each line correspodns to diff spin of neighbouring proton
what is spin-spin coupling
effective magnetic field of a particular nuclei is modified by the magnetic fields of neighbouring proton
looks like a SPLIT PEAK in a NMR spectrum
how does electronegativity affect NMR
electroneg = pulls valence electrons away from atoms = decr electron density = lower energy needed to reach resonance frequency
predict the splitting pattern produced by a neighbouring –CH2– group in a H NMR spectrum
2^2 different combinations (3 different local magnetic fields)
- both protons aligned WITH ext mag field
- one proton alighed AGAINST and one WITH (either direction = 2)
- both protons aligned AGAINST
total 4 combinations, lines with intensities 1,21
do protons bonded to same group interact with each other? NMR
NO. they are equivalent and behave as a group
do protons on non-adjacent carbons interact with e/o? NMR
NO
the O-H peak in ethanol does not split in HNMR unless?
unless the sample is PURE.
rapid exchange of protons between ethanol molc averages out possible spins
the number of lines in a HNMR spectrum = to?
total number of hydrogens
x ray diffraction constructive vs destructive interferance
constructive: waves in phase = reinforce e/o = resultant wave larger amplitude
destructive: waves out of phase by 180deg = peak aligned with another trough = cancel e/o out
what can be obtained from X ray diffraction of a solid
map of electron density
are H atoms visible on X ray diffraction electron density map?
nope! electron densities are too low
