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AQA A-Level Chemistry > NMR and chromatography > Flashcards

NMR and chromatography Flashcards

1
Q

Why does NMR only work with nuclei with odd mass number?
4
Odd vs even?
Magnetic field?

A
  • Nuclei with odd mass numbers (1H, C13) have spin
  • Nuclei with even mass numbers do not have magnetic properties
  • Nuclear spin induces weak magnetic fields which can be influenced by an external magnetic field
  • Hydrogens nucleus can align w/ (stable) or against (unstable) an applied field
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2
Q

How is it possible to make H flip from more to less stable alignment?
2

A
  • Supplying an exact amount of energy

- the amount of energy depends on the strength of the magnetic field

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3
Q

What is the resonance condition?

A

Energy peak when (H) flipped form more to less stable alignment on a graph

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4
Q

What solvents are used for NMR?

2

A

CCl4

CDCl3 (sometimes gives a clearer split of peaks so used for complex spectra

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5
Q

What is TMS and why is it the standard?

(1H nuclei of TMS given chemical shift value of 0)

7
Environment?
Resonance?
Cost?
Reactivity?
A

TMS: tetramthylsilane SiC4H12

  • all H and C are in the same environment so one intense peak
  • H nuclei are highly shielded due to Si’s low electronegativity so it has high resonance and strong sharp easily detected absoptions
  • non-toxic
  • inert
  • cheap
  • easy to remove from sample (low b.p)
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6
Q

What are spectra?

1

A

Resonance conditions

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7
Q

What are spectra affected by?

3

A
  • Environment of H: causes different peak shapes and chemical shifts
  • T4 affected by no. H in molecule
  • T4 affected by electronegativity of surrounding atoms
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8
Q

What is the position of the peak influenced by?

1

A

Electronegativity of surrounding atoms

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9
Q

What is the size of the peak influenced by?

1

A

Relative to number of H’s in that environment

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10
Q

What does the amount of absorptions in NMR spectra represent?
1

A

Amount of non-equivalent protons

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11
Q

What does the intensity of absorptions in NMR spectra represent?
1

A

Quantity of protons with each peak

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12
Q

What does the position of absorptions in NMR spectra represent?
1

A

Environment of each proton (electronegativity of surrounding atoms)

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13
Q

What does the splitting of absorptions in NMR spectra represent?
1

A

Amount of neighbouring inequivalent protons on C (or other atom) directly adjacent

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14
Q

What is the split with 0 inequivalent H’s?

A

Singlet

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15
Q

What is the split with 1 inequivalent H’s?

A

Doublet

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16
Q

What is the split with 2 inequivalent H’s?

A

Triplet

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17
Q

What is the split with 3 inequivalent H’s?

A

Quartet

18
Q

What is the split with 4+ inequivalent H’s?

A

Multiplet

19
Q

What stationary phase and mobile phase is used in column chromatography?

A

Stationary: powder
Mobile: solvent

20
Q

What stationary phase and mobile phase is used in paper chromatography?

A

Stationary: absorbent paper
Mobile: solvent

21
Q

What stationary phase and mobile phase is used in TLC (thin layer) chromatography?

A

Stationary: powder on glass/plastic plate
Mobile: solvent

22
Q

What stationary phase and mobile phase is used in gas chromatography?

A

Stationary: powder (sometimes) coated with liquid
Mobile: inert gas

23
Q

What is chromatography?

1

A

Process of separating substances by their relative affinity to a stationary and mobile phase

24
Q

How fast do substances with a stronger affinity for the mobile phase move?

A

More quickly

25
Q

How fast do substances with a stronger affinity for the stationary phase move?

A

More slowly

26
Q

What (type of) powders are used for stationary phase?

A

SiO2 (silica) (or Al2O3 (alumina))

-Polar powders

27
Q

What are some examples of solvents used as mobile phases?

A

Non-polar: alkanes

Polar: water, alcohols

28
Q

What type of solvent is used with a polar stationary phase and why?

3/4

A

Non-polar
For separation
-Polar molecules will move slowly as higher affinity for the polar stationary phase
-Non-polar molecules will move quickly as higher affinity for the non-polar mobile phase

29
Q

What is the Rf value?

Calculation?

A

-always less than 1

Distance moved by sample / distance moved by solvent

30
Q

How do you view results of colourless samples?

A

UV light

31
Q

What is column chromatography?

A

Large scale TLC

32
Q

How does column chromatography work?

3

A
  • Apply solvent and pass it through column
  • Sample separates into coloured bands
  • If keep passing through solvent the bottom band will reach the bottom of the column and can be collected for further testing
33
Q

How does gas chromatography work?

4

A
  • Long coiled tube packed with powder
  • Inert gas passes through tube (H2, N2, He)
  • Retention time: time to travel through tube
  • Samples are often analysed by mass spec as they leave
34
Q

What is the likely group with triplet:quartet peaks at a 3:2 ratio?

A

Ethyl group

35
Q

What is the group with singlet with intensity of 3?

A

Methyl group

36
Q

What is the group with a singlet at intensity of 6?

A

Two methyl groups on same carbon with two other non-H molecules bonded

37
Q

What is the group with triplet:triplet at 2:2 ratio of relative intensity?

A

Middle of carbon chain

CH2-CH2

38
Q

What is group with doublet:quartet at 3:1 ratio?

A

CH3-CHR2

39
Q

What is group with singlet at 9 intensity?

A

3 methyl groups on one carbon

40
Q

What is group with singlet with intention of 1?

A

Likely alcohol group

Other wise CHR3

AQA A-Level Chemistry (12 decks)

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