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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 is chromatography used for?

A

Seperate individual components from a mixture of substances

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2
Q

What are the 2 phases for all forms of chromatography?

A
  • Stationary phase
  • Mobile phase
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3
Q

What is the stationary phase?

A

Does not move and is normally a solid or a liquid supported on a solid

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4
Q

What is the mobile phase?

A

Does move, and is normally a liquid or a gas

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5
Q

What is a TLC plate made of?

A

Made of a plastic sheets or glass, coated with thin layer of solid adsorbent substance - usually silica

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6
Q

What is adsorption in TLC?

A

Process by which the solid silica (absorbent) holds the different substances in the mixture to its surface. Seperation is achieved by the relative adsorptions of substances with the stationary phase (silica).

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7
Q

How are thin layer chromatograms analysed?

A

Calculate Rf value for each component and compare it with known values recorded using the same solvent system and adsorbent

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8
Q

How do you calculate retention factor (Rf) for each component in TLC?

A

Rf = distance moved by the component/ distance moved by the solvent front

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9
Q

What is gas chromatography useful for?

A

Seperating and identifying volatile organic compounds present in a mixture

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10
Q

What are the 2 phases in gas chromatography?

A

stationary - high boiling liquid adsorbed onto an inert solid support.

Mobile phase - An inert carrier gas such as helium or neon

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11
Q

What happens in gas chromatography?

A

Small amount of volatile mixture is injected into the gas chromatgraph. The mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase adsorbed onto the solid support. Components slow down as they interact with the liquid stationary phase inside the column - the more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column. The components of the mixture are seperated depending on their solubility in the liquid stationary phase. The compounds in the mixture rech the detector at different times

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12
Q

What is the retention time?

A

Time taken for a component to travel through the column

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13
Q

How can retention times be used to identify components present in the sample?

A

Compare to retention times for known compounds

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14
Q

What does a gas chromatogram look like?

A
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15
Q

What does a peak integration show?

A

Area under peak

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16
Q

What can peak integrations for gas chromatograph be used to determine?

A

Concentrations of components in the sample

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17
Q

Describe the procedure for determining the concentration of a component in a sample by comparing its peak integration with values obtained from standard solutions of the component

A
  1. ) Prepare standard solutions of known concentrations of the component being investigated
  2. ) Obtain gas chromatograms for each standard solution
  3. ) Plot a calibration curve of peak area against concentration. This is called external calibration and offers a method for converting a peak area into a concentration.
  4. ) Obtain a gas chromatogram of the compound being investigated under the same conditions
  5. ) Use the calibration curve to measure the concentration of the compound
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18
Q

What’s the qualitative test for an alkene?

A
  • Add bromine water drop-wise to sample and if it is an alkene, bromine water decolourises from orange to colourless
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19
Q

What is the test for the haloalkane functional group?

A
  • Add silver nitrate and ethanol to sample and warm to 50°C in a water bath.

If chloroalkane is present - white precipitate forms

If bromoalkane is present - cream precipitate forms

If iodoalkane is present - yellow precipitate forms

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20
Q

What is the test for a carbonyl functional group?

A

Add 2,4-dinitrophenylhydrazine (2,4-DNP) to sample. If carbonyl is present orange precipitate forms

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21
Q

What is the test for the aldehyde fuctional group?

A

Add tollens’ reagent and warm sample. If aldehyde is present silver mirror forms

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22
Q

What is the test for primary alcohols, secondary alcohols and aldehydes?

A

Add acidified pottasium dichromate (VI) and warm in a water bath. If these functional groups are present the colour will change from orange to green.

23
Q

What is the test for a carboxylic acid functional group?

A

Add aqueous sodium carbonate and if carbylic acid is present effervesence will occur.

24
Q

What is the test for the phenol functional group?

A

Blue litmus paper will turn red in presence of a phenol as phenols are weak acids.

To distinhuish between a carboxylic acid and a phenol, react with sodium carbonate - as phenols don’t react.

25
Q

What is NMR spectroscopy?

A

Nuclear magnetic resonance spectroscopy

Uses combination of a strong magnetic field and radio frequnecy radiation to analyse organic compounds. The nuclei of some atoms absorb this radiation - energy is then measured and recorded

26
Q

What are the 2 types of NMR spectrocopy?

A

H-1 (proton) spectroscopy and C-13 spectroscopy

27
Q

What is resonance for NMR?

A

When the nucleus absorbs energy and rapidly flips between the 2 spin states

28
Q

What is an atom’s environment?

A

all the groups that the atom is connected to - going right along the molecule

29
Q

What is chemical shift?

A

Energy absorbed by nuclei in different envioroments relative to a standard substance

30
Q

How does nuclei environement change chemical shift?

A

nucleus is shielded from the effects of the magnetic field by surrounding electrons, so nuclei in a molecule will feel different magnetic fields depending on their environments. Nuclei in different environments will absorb different amounts of energy at different frequencies

31
Q

What is the standard substance in NMR?

A

Tetramethylsilane (TMS) - (CH3)4Si

has chemical shift at 0ppm

32
Q

What is the purpose of CDCl3 as a solvent in NMR spectroscopy?

A

produces no NMR signal in the frequency ranges used in 1H spectroscopy and computer can filter out peak in 13C spectroscopy before displaying the spectrum

33
Q

What 2 pieces of information about a molecule can be found from carbon-13 NMR?

A

number of different carbon environments - from the number of peaks

types of carbon environment - from the chemical shift

34
Q

What are the 3 steps used to interpret a C-13 spectrum?

A
  1. ) Count the number of peaks in the spectrum - this is equal to the number of carbon environments in the molecule
  2. ) use chemical shift values in data booklet to work out what type of carbon environment is causing each peak.
  3. ) Use this information to figure out the structure of the molecule.
35
Q

What does a proton Nmr spectrum tell you?

A

Number of different environments - from the number of peaks

types of proton environment present - from the chemical shift

relative numbers of each type of proton - from integration traces/ ratio of relative peak areas

numer of non-equivalent protons adjacent to a given proton

36
Q

What are a non-equivalent protons/hydrogens?

A

Protons that have a different type of proton environment, and therefore absorb at different chemical shifts

37
Q

How many proton environments are there in butanoic acid? How many peaks will there be on the proton NMR spectrum?

A

4 environments, therefore 4 peaks

Each CH2 group is connected to different groups on one side, so therefore the protons in the two CH2 groups are non-equivalent

38
Q

How many proton environments are in butanedioic acid? How many peaks will there be?

A

2 proton environments. 2 peaks

2 CH2 groups are in the same environment

39
Q

What is spin-spin splitting?

A

splitting of a main peak into sub-peaks giving indication of number of non-equivalent protons on the adjacent carbon

40
Q

How do you work out the number of adjacent protons from spin-spin splitting pattern

A

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

41
Q

What is this splitting pattern callled?

A

singlet

42
Q

What is this splitting pattern called?

A

Doublet

43
Q

What is this splitting pattern called?

A

Triplet

44
Q

What is this splitting pattern called?

A

Quartet

45
Q

Why does spin-spin splitting happen in pairs?

A

Each proton splits the signal of the other. Makes it very easy to spot a structural feature when analysing a molecule

Several very common splitting pairs e,g. CH3CH2 triplet/quartet combination

46
Q

What is the purpose of D2O as a solvent in proton NMR spectroscopy?

A

O-H and N-H peaks no longer appear

47
Q

How does adding D2O as a solvent make the OH and NH proton peaks dissapear

A

Deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms. For example, the following equilibrium is set up with methanol: CH3OH + D2O CH3OD + HOD

so spectrum is being run on CH3OD. As deuterium does not absorb in this chemical shift range, the OH peak disappears.

48
Q

How does the OH and NH peak disappearing allow for OH and NH protons to be identified?

A

Proton NMR spectrum is run as normal first, then D2O is added and second spectrum is run and as OH and NH peaks disappear they can be identified.

49
Q

What is the way to lay out a 5+ marker on spectroscopy for full marks?

A

1st column: ‘Peak’
2nd column: ‘ Chemical shift, ppm’
3rd: ‘Peak Area,’ no. of protons for chem environment
4th: ‘Type of Proton’ from data sheet
5th: ‘Splitting Pattern’
6th: ‘No. of Adjacent protons’

50
Q

What are the steps in predicting a NMR spectrum from a given structure

A
  1. ) Draw out the structure
  2. ) Identify the number of chemical environemnts - gives amount of peaks
  3. ) Using data sheet predict the chemical shifts

for proton NMR:

  1. ) Predict relative peak heights from the number of each type of proton
  2. ) Predict the splitting pattterns from the number of each type of proton
51
Q

What is the sequence for identification of the structure of an organic molecule?

A

Elemental analysis - use of percentage composition by mass to determine the empirical formula f a compund

Mass spectra - Use of molecular ion peak to determine the molecular mass and fragment ions to determine parts of the molecule. The molecular formula can then be determined from the empirical formula and molecular mass

Infrared spectra - use of absorption peaks to identify bonds present and functional group

NMR spectra - To determine the number and types of carbon and hydrogen atoms from the chemical shifts of peaks and the order of atoms within molecules from splitting patterns

52
Q

Why would there be some uncertainty about using gas chromatography alone to predict the number of compounds in mixture?

A

Some compounds may have the same retention time

53
Q

How does gas chromatography seperate compounds in a mixture?

A

Relative solubilities in the stationary phase.

More soluble compound in the stationary phase has a longer retention time

OCR A-level Chemistry (30 decks)

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