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OCR A Level Chemistry > Chapter 29 - Chromatography and Spectroscopy > Flashcards

Chapter 29 - Chromatography and Spectroscopy Flashcards

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1
Q

What are the stationary and mobile phases of chromatography?

A

Stationary - no movement, normally a solid or liquid supported on a solid
Mobile phase - does move, normally a liquid or gas

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2
Q

What is TLC?

A

Thin layer chromatography - DISSOLVED SUBSTANCES
Uses a TLC plate (plastic sheet or glass) coated with a thin layer of a solid adsorbent substance (usually silica) - this is the stationary phase.
Separation is achieved by the relative adsorptions of substances with the stationary phase

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3
Q

What is adsorption?

A

The process by which the solid silicas holds the different substances in the mixture to it’s surface

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4
Q

What process separates components in a mixture in both gas and thin layer chromatography?

A

Adsorption

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5
Q

What is Rf, how is it calculated?

A

Retention factor
Rf = distance moved by component/distance moved by solvent front

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6
Q

What are the stationary and mobile phases of gas chromatography?

A

Stationary = high boiling liquid adsorbed onto an inert solid support in column
Mobile = inert carrier gas like helium or neon

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7
Q

How are mixtures separated in gas chromatography?

A

Dependent on their relative solubility in the liquid stationary phase, meaning they reach the detector at different times - retention time is the time taken for each component to travel through the column

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8
Q

What 2 key pieces of information can be gained for gas chromatograms?

A
  • retention time, which can be compared to known component retention times
  • peak integrations (area under each peak) used to determine concentrations of components in the sample
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9
Q

What is retention time?

A

The time between injection and emergence/detection of a component

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10
Q

How can you determine the concentration of a component in a sample (procedure)?

A
  1. prepare standard solutions of known conc of the compound being investigated
  2. obtain gas chromatograms for each standard solution
  3. plot a calibration curve of peak area against concentration
  4. obtain a gas chromatogram of the compound being investigated under same conditions
  5. use calibration curve to measure concentration of the compound
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11
Q

How can you test for an alkene functional group?

A

Add bromine water drop wise, bromine water is decolourised from orange to colourless

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12
Q

How can you test for a haloalkane?

A

Add silver nitrate and ethanol and warm to 50 degrees in a water bath
Chloro = white precip
Bromo = cream precip
Iodo = yellow precip

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13
Q

How can you test for a carbonyl functional group?

A

Add 2,4 - DNP and an orange precipitate will form

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14
Q

How can you test for an aldehyde functional group?

A

Add Tollen’s reagent and warm, silver mirror will form

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15
Q

How can you test for a primary and secondary alcohol and an aldehyde?

A

Add acidified potassium dichromate (VI) and warm in a water bath. There will be a colour change from orange to green

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16
Q

How can you test for a carboxylic acid functional group?

A

Add aqueous sodium carbonate, observe effervescence

17
Q

What is NMR spectroscopy?

A

Nuclear magnetic resonance spectroscopy

18
Q

What is resonance?

A

Property of nucleus to absorb energy when in the right combination of a strong magnetic field and radio frequency radiation and flip rapidly between electron’s 2 spin states

19
Q

What is the frequency shift in NMR measured in?

A

Chemical shift 𝛿 in units of parts per million

20
Q

What is TMS used for in NMR?

A

TMS = tetramethylsilane, used as a standard reference chemical against which all chemical shifts are measured

21
Q

How is an NMR spectrum produced?

A

Sample dissolved in solvent and placed in a narrow NMR sample tube with a small amount of TMS. Placed inside NMR spectrometer where it is spun and the sample is given a pulse of radiation, any absorptions of energy are detected and displayed on a computer screen

22
Q

What are deuterated solvents and why are they used?

A

Solvent where 1H atoms have been replaced by 2H atoms (deuterium), which produces no NMR signal in frequency ranges

23
Q

What does the number of peaks in carbon NMR tell you?

A

The number of different carbon environments within the molecule (each peak = different environment)

24
Q

What does the shift of peaks in carbon NMR tell you?

A

The types of carbon environments present within the molecule

25
Q

When do carbon atoms have the same environment in C NMR and why?

A

If they are positioned symmetrically within a molecule they have the same chemical environment. They will then absorb radiation at same chemical shift and contribute to same peak

26
Q

What does the number of peaks in proton NMR tell you?

A

The number of different proton (H) environments present

27
Q

What does the chemical shift of peaks in proton NMR tell you?

A

The types of different proton (H) environments present

28
Q

What does the relative peak areas/integration trace in proton NMR tell you?

A

The relative numbers of each type of proton (H)

29
Q

What does the spin splitting pattern in proton NMR tell you?

A

The number of non equivalent protons adjacent to a given proton

30
Q

What is the n+1 rule?

A

For a proton (H) with n protons attached to an adjacent carbon atom, the number of sub peaks in a splitting pattern = n +1

31
Q

What are the common splitting patterns/what they show?

A

No H on adjacent atoms = singlet
1 H on adjacent C atom = doublet (CH)
2 H on adjacent C atom = triplet (CH2)
3 H on adjacent C atom = quartet (methyl)
6 H on adjacent C atom = heptet (dimethyl)

32
Q

What do multiplets in proton NMR often suggest?

A

Aromatic protons

33
Q

What type of peaks do N-H and O-H groups create in proton NMR?

A

Broad peaks

34
Q

How does proton exchange identify -OH and -NH protons?

A
  1. normal proton NMR spectrum run
  2. small volume of D2O added, mixture shaken and a second spectrum run
    Deuterium exchanges and replaces OH and NH protons with deuterium atoms, which are not absorbed on chemical shift frequency, so peaks disappear
35
Q

What is a typical sequence for identification?

A

Elemental analysis (% comp by mass)
Mass spectra
Infrared spectra
NMR spectra

OCR A Level Chemistry (29 decks)

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