Topic 11; Measurement + data processing

Question 1

experimental errors

Answer 1

resut in a different between recorded value and generally accepted value

Question 2

random errors

Answer 2

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

Question 3

systematic errors

Answer 3

• 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

Question 4

accuracy

Answer 4

• the smaller the systematic error; the great the accuracy
• small systematic errors
• give a result close to accepted value

Question 5

precision

Answer 5

• 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

Question 6

percentage uncertanitity calculation

Answer 6

absolute uncertainty/measured value x 100

Question 7

percentage error calculation

Answer 7

accepted value- experimental value/accepted value x 100

Question 8

tip for sig. figures?

Answer 8

• whenever you manipulate data, answer should be quoted to the same number of significant figures as the least precise value

Question 9

analytical techniques

Answer 9

• qualitive analysis
• quantitve analysis
• structural analysis

Question 10

qualitive analysis

Answer 10

detection of presences but not quantity

Question 11

quantitive analysis

structural analysis

Answer 11

detection of quantity of substance

describption of molecular arragnement

Question 12

mass spectromertry

Answer 12

used to determine the relative atomic and molecular masses

- use fragmentation patterns as fingerprint techniques to identify unknown substances/molecular arrangement

Question 13

what is a molecular ion

Answer 13

(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)

Question 14

how does a mass spectrometer work?

Answer 14

1. electron gun hits incident species and removes electron
2. collision energetic; splits molecule into fragments; largest peak in mass spectrum correspond to parent ion which didn’t get fragmented
3. fragmentation pattern can provide evidence for structure of compound

Question 15

IDH

Answer 15

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

Question 16

rules for IDH

Answer 16

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

Question 17

regions on electromagnetic spectrum

Answer 17

• electromagnetic spectrum has different wavelengths and frequencies for each region

Question 18

wavelength

Answer 18

the distance between succesvie crests and troughts

Question 19

frequency

Answer 19

the number of waves which pass a point every second

Question 20

radio waves

Answer 20

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

Question 21

microwaves

Answer 21

cause molecules to increase their rotational energy; this can give information about bond lengths

Question 22

infrared radiation

Answer 22

absorved by certain bonds and cause them to stretch or bend; gives information about bonds in a molecule

Question 23

visible light (UV)

Answer 23

can produce electronic transitions and gie information about the electronic energy levels within the atom or molecule

Question 24

infrared spectroscopy

Answer 24

used to identify bonds in a molecule

Question 25

natural frequency of a chemical bond?

Answer 25

• 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

Question 26

what does IR radiation do?

Answer 26

• 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

Question 27

stretching and bonding in a polyatomic molecule?

Answer 27

• 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)

Question 28

principles of NMR

Answer 28

• 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

Question 29

H NMR

Answer 29

• 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)

Question 30

nuclear energy in NMR

Answer 30

• energy needed for the nuclei to reverse spin
  -energy needed to change orientation in magnetic field
  (energy provided by radio waves)

Question 31

higher energy spin state

Answer 31

the nucleus lines up with magnetic field against enexternal magnestic field
-nucleus spins against field

Question 32

lower energy spin state

Answer 32

nucleus has lined up WITH th emagnetic field in same direction as external magnetic field (spins parallele to field)