Topic 11 - Measurement & Data Processing

Question 1

uncertainty in analog scales

Answer 1

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

Question 2

uncertainty in digital scales

Answer 2

the smallest scale division on the scale

e.g. when measuring weight and the reading says 10.00g, use 10.00 ± 0.01g

Question 3

sources of uncertainty

Answer 3

• instrument readings
• uncertainties in judging (especially with qualitative data)
• errors

Question 4

significant figures

Answer 4

• all the figures involved in the reading

e. g. 10.00 has 4 significant figures as the .00 denotes the level of uncertainty

Question 5

types of errors

Answer 5

• random

- systematic

Question 6

causes of random errors

Answer 6

• readability of measuring instrument
• effects of changes in surrounding (e.g. temperature variations)
• insufficient data
• misinterpreted readings

Question 7

how to reduce the probability of random errors

Answer 7

repeated trials

Question 8

repeatable results

Answer 8

if the experimenter can duplicate the experiment and observes the same results

Question 9

reproducible results

Answer 9

if different experimenters can duplicate the experiment and observe the same results

Question 10

causes of systematic errors

Answer 10

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

Question 11

how to reduce the probability of systematic errors

Answer 11

careful experimental design

Question 12

accuracy

Answer 12

smaller systematic error = higher accuracy

• small systematic errors
• gives result close to accepted value

Question 13

precision

Answer 13

smaller random uncertainties = greater precision

• small random errors
• reproducible

Question 14

graph setup (variables)

Answer 14

x-axis: independent variable

y-axis: dependent variable

Question 15

drawing a line of best fit

Answer 15

• should pass as close as possible to many data points
• doesn’t have to pass through all
• can be used for extrapolation

Question 16

applications of extrapolation of line of best fit

Answer 16

• absolute zero value can be found

- by extrapolating the vol/temp graph for an ideal gas

Question 17

interpolation

Answer 17

the assumption that the trend line applies between 2 points

Question 18

recognizing errors with a graph

Answer 18

presence of outliers indicate that data may not be reliable

Question 19

types of analysis

Answer 19

• qualitative
• quantitative
• structural

Question 20

infrared spectroscopy

Answer 20

used to identify the bonds in a molecule

Question 21

mass spectrometry

Answer 21

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

Question 22

nuclear magnetic resonance spectroscopy (NMR)

Answer 22

• 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

Question 23

mass spectrometry: fragmentation patterns

Answer 23

• 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

Question 24

degree of unsaturation

Answer 24

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

Question 25

effect of radio waves on molecules

Answer 25

- 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

Question 26

effect of microwaves on molecules

Answer 26

- cause molecules to increase their rotational energy | - gives info about bond lengths

Question 27

effect of IR (infrared radiation) on molecules

Answer 27

- 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

Question 28

effect of visible light/UV light on molecules

Answer 28

- produces electronic transitions | - gives info about energy levels in a molecule

Question 29

information given by X-rays

Answer 29

- 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

Question 30

why can IR only be absorbed by polar bonds?

Answer 30

- 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

Question 31

effect of IR on polyatomic molecules

Answer 31

- 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)

Question 32

resonance (in NMR)

Answer 32

phenomenon in NMR in which the nuclei flips over and spins in the opposite direction.

Question 33

higher energy spin state

Answer 33

- nucleus lines up with magnetic field against the external field - spins against the field

Question 34

lower energy spin state

Answer 34

- nucleus lines up with magnetic field parallel to the external field - spins parallel to the field

Question 35

how does NMR work?

Answer 35

- a sample is placed in an electromagnet | - the field strength is varied until radio waves have the exact frequency needed to cause resonance

Question 36

NMR hydrogen spectrum

Answer 36

- 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

Question 37

standard signal used by NMR

Answer 37

signal produced by the 12 H nuclei in tetramethylsilane (TMS)

Question 38

chemical shift

Answer 38

the position of the NMR signal relative to the standard signal

Question 39

NMR - effect of chemical environment on chemical shifts

Answer 39

- 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

Question 40

why is TMS used as the standard signal for NMR?

Answer 40

- 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

Question 41

typical NMR spectrum of an organic compound

Answer 41

- 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

Question 42

spin-spin coupling

Answer 42

- 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