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Chemistry OCR A Level > Module 6.3 > Flashcards

Module 6.3 Flashcards

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1
Q

What is chromatography?

A

A technique used to separate mixtures of compounds

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2
Q

List several types of chromatography

A

Gas Chromatography
Thin Layer Chromatography (TLC)
High Performance TLC

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3
Q

What is the name for the different phases in chromatography?

A

Mobile and stationary

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4
Q

If the stationary phase is non- polar, and the molecules are polar, what phase are the molecules more likely to bind to?

A

Mobile phase (travels further)

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5
Q

If the stationary phase is polar, and the molecules are polar, what phase are the molecules more likely to bind to?

A

Stationary phase (Doesn’t travel as far)

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6
Q

Give the method of TLC

A

1) Draw a pencil line near the bottom of the plate and put a spot of the mixture to be separated on the line
2) Dip the bottom of the plat (NOT including the spot) into a solvent (usually ethanol)
3) As the solvent spreads on the plate, the different substances in the mixture move with it (but at different distances so they separate out)
4) When the solvent’s nearly reached the top of the plate, take the plate out and mark the distances that the solvent has moved (Solvent front) in pencil

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7
Q

What is meant by the solvent front?

A

The distance that the solvent has moved

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8
Q

Define the term Rf value

A

(insert)

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9
Q

What is the formula to work out Rf value?

A

(Distance travelled by spot/Distance travelled by solvent)

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10
Q

How can we work out what was in the mixture using an Rf value?

A

Compare it to other Rf values from a table of known values which comes from a TRUSTED source

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11
Q

How does separation work in TLC?

A

Through ADSORPTION

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12
Q

In TLC, if a substance is strongly absorbed will it move slowly or fast?

A

Slowly

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13
Q

In TLC, if a substance is weakly absorbed will it move slowly or fast?

A

Fast

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14
Q

During TLC, what external variables must we keep constant and why?

A

Temperature - Because it can alter how far our mixture moves

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15
Q

How can you confirm your unknown identified compound using TLC?

A

We can confirm out unknown compound by running a TLC of a pure sample of the known substance ALONGSIDE our unknown compound. If we get identical Rf values, we can confirm our result to be accurate

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16
Q

List the key points of TLC

A
  • Ethanol is the mobile phase
  • Silica gel or Aluminium powder is the stationary phase
  • Separation via ADSORPTION
  • When discussing TLC, we talk about the Rf value.
  • Similar chemicals have similar Rf values
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17
Q

What is the method of Gas Chromatography?

A

1) The sample to be analysed is injected into a stream of gas, which carries it through a coiled tube coated with a viscous liquid (e.g. oil) or a solid
2) We then allow time for the mixture to dissolve in the oil or onto the solid. This then evaporates back into ga and then redissolves as they travel through the tube.
3) We monitor the time taken for the substances to pass through the coiled tube and reach the detector (retention time)
4) We use our retention time and compare it to other retention times from a table of known values, which comes from a TRUSTED source, to identify our product

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18
Q

Define retention time

A

The time taken for a substance to pass through a coiled tube and reach the detector

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19
Q

How does separation work in GC?

A

SOLUBILITY

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20
Q

In GC, are the compounds that bind most to the mobile phase are eluted first or last?

Eluted = Given out

A

Eluted first

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21
Q

On a Gas chromatogram, what does the area of the peak show?

A

The relative amount of the compound

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22
Q

How can you confirm your unknown identified compound using GC?

A

We can confirm our unknown compound by running a TLC of a pure sample of the known substance alongside our unknown compound. If we get identical retention times , we can confirm out result to be accurate

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23
Q

What factors affect retention time?

A

Solubility
Boiling point of the substance to be tested
Temperature of the gas chromatography instrument

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24
Q

Explain how a highly soluble substance affects retention time

A

Highly soluble substance = More time spent dissolved = Longer time to go through to tube to the detector = Higher retention time

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25
Q

Explain how a higher boiling point of the substance to be tested affects retention time

A

If the substance has a higher boiling point, it will spend more time condensed as a liquid in the tube than as a gas. This means that it will take longer to travel through the tube

Higher BP = Higher retention time

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26
Q

Explain how the temperature of the gas chromatography instrument affects retention time

A

The higher the temperature of the gas chromatography instrument means that the substance will spend more time evaporated in the gas and so it will move along the tube quickly. It shortens the retention time for ALL substances in the tube.

Higher temperature of the GC instrument = Shorter Retention Time.

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27
Q

What do gas chromatograms show?

A

A series of peaks a the time where the detector senses something other than the carrier gas leaving the tube (another compound)

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28
Q

What can gas chromatograms be used to do?

A

Identify substances within a sample and their relative proportions

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29
Q

In a gas chromatogram, what does each peak represent?

A

A substance, with a particular retention time.

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30
Q

In a gas chromatogram, the area under each peak is?

A

Proportional to the relative amount of each substance

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31
Q

In a gas chromatogram, just because a peak is the tallest, does it mean that it is the most abundant substance?

A

NO

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32
Q

What TWO formula do we use to find the percentage composition of each component in a mixture?

A

First formula is : Area = 1/2 (b x h)

Second formula is: Percentage composition = (Area of each peak/Total area of all products) x 100

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33
Q

In GC, what is the stationary phase and what is the mobile phase?

A

Stationary phase = Gas/oil in the coil

Mobile phase = Inert gas (N2 or He)

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34
Q

What do external calibration curves help us find?

A

Concentration

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35
Q

How do we create an external calibration curve?

A

1) Create a series of standard solutions of different concentrations of analyte (the substance you’ve chosen to detect for your calibration
2) One by one, inject your standard solutions into a GC instrument and record the result
3) Calculate the area under the peak of each response for the standard solution.
4) Plot these values on a graph of area vs concentration
5) Join up your points to create an external calibration curve.

36
Q

When making an external calibration curve, why is it a good idea, to run a blank?

A

A blank is a solution containing all the solvents and regeants when making the standard solution, but NO ANALYTE,. By subtracting the response of the bank from each of the responses of our standard solutions, we can find a CORRECTED PEAK VALUE (a value that takes into account the effect reagents and solvents on the peak areas).

37
Q

List the key points of GC

A
  • Inert gas (e.g. N2 or He) is the mobile phase
  • Gas/Oil in the coil is the stationary phase
  • Separation via SOLUBILITY
  • When discussing TLC, we talk about the Retention time
  • The area of retention time tells us the relative amount of the substance
  • Number of peaks = Number of chemcials
38
Q

What are alternatives to TLC and GC?

A

High peformance TLC, or GC with Mass Spectroscopy

39
Q

How can you test for an : Alkene

What is observed?

A

Add BROMINE WATER to the unknown substance into a test tube and shake.

Observation = Bromine water decolourises from orange to colourless

40
Q

How can you test for a : Haloalkane

What is observed?

A
  • Add ETHANOL and AQUEOUS SILVER NITRATE to the unknown substance in a test tube and heat.

Observation = Precipitates forms (Colour depends on the halogen)

Chloroalkane = White ppt
Bromoalkane = Cream ppt
Iodoalkane = Yellow ppt
41
Q

How can you test for a : Phenol

TWO Individual tests, two individual observations

A

Test one) Add sodium hydroxide to the unknown substance in a test tube and shake.

Observation one = The solid will dissolve and a colourless solution of sodium salt will form

Test two) Add sodium carbonate solution to the unknown substance in a test tube.

Observation two = NO reaction whatsoever

Key note = Another thing you should know is that Phenol is weakly acidic (pH of about 5-6)

42
Q

How can you test for a : Carbonyl group

What is observed?

A

Dissolve Brady’s reagent into sulphuric acid, water and methanol. Add this to the unknown substance

Observation = Bright orange precipitate forms

43
Q

How can we make Brady’s reagent?

A

Dissolve 1cm^3 of sulphuric acid, 2cm^3 of water and 5cm^3 of methanol into a test tube.

44
Q

How can you test for a : Aldehyde

What is observed?

A

Mix Tollens regeant and the unknown substance in a test tube. Place the test tube in a hot water bath and wait.

Observation = Silver mirror forms

45
Q

How do we make Tollens Reagent?

A

Mix silver nitrate and sodium hydroxide in a test tube (brown ppt forms). Then add dilute ammonia until the brown ppt dissolves completely (now you have formed TOLLENS REAGENT.

46
Q

How can you test for a : 1st ,2nd and 3rd degree alcohols

What is observed?

A

Add the alcohol to acidified potassium dichromate. Reflux the mixture and watch for a colour change

Observation :
1st degree alcohol = carboxylic acid
2nd degree alcohol = Ketone
3rd degree alcohol = No reaction

47
Q

How can you test for a : Carboxylic acid

What is observed?

A

Add solid sodium carbonate to the unknown substance in a test tube and pass the gase produced through limewater in a 2nd test tube.

Observation = Limewater goes cloudy and Fizzing occurs.

48
Q

What is NMR?

A

NMR is a technique used to find out the structure of compounds based upon the environments that specific atoms are in.

49
Q

What forms do NMR come in?

A

Carbon - 13 NMR

Hydrogen - 1 NMR

50
Q

What is carbon-13 NMR?

A

Carbon-13 NR is a form of NMR that gives us infomration, about the number of carbon atoms that are in a molecule, and the environments that they are in

51
Q

What does carbon-13 NMR tell us about?

A
  • The number of carbon environments in a molecule
52
Q

What is H NMR?

A

H NMR is a form of NMR that gives us information about the number of hydrogen atoms that are in a molecule and the environments that they’re in.

53
Q

What is the method of NMR?

A

1) A sample of the compound is placed in a strong magnetic field and is exposed to a range of different frequencies of radiowaves
2) The nuclei of certain atoms within the molecule adsorb energy from the radiowaves,
3) The amount of energy that a nuclei absorbs at each frequency will depend on the ENVIRONMENT that it is in
4) Information is extracted from the NMR spectra given out and then we can piece them together to work out the structure of the molecule.

54
Q

What is meant by a carbon environment?

A

The position of a carbon in a compond

55
Q

What is meant by the chemical shift value?

A

The position a peak appears at on a NMR spectra

56
Q

Carbon atoms are deshielded by electron withdrawing groups such as oxygen. If a carbon was deshielded by an OH bond where would it lie on a 13C NMR Spectra, closer to the left or closer to the right?

A

Closer to the LEFT

57
Q

In terms of environments, what can lines of symmetry do?

A

Cause different carbon atoms to be in the same environment

58
Q

What must we look at when assigning a carbon to be in its own environment or in a similar environment to other carbon atoms?

A

The whole compound and symmetry (try it as if it was chirality)

59
Q

In NMR, the number of peaks = ?

A

Number of environments

60
Q

What is the peak position?

A

The peak position is a way of letting us know what structures are in the compound. For example. a peak at 50-90 on a carbon-13 NMR spectrum is for a C-O bond whereas a peak at 2-3 on a hydrogen NMR spectrum is for a H-C=O bond

61
Q

Before linking certain parts in a spectrum to a structure from the data sheet, what must we do?

A

Look at the axis (this allows us to rule many structures our, e.g. the H NMR chemical shift value only goes up to …. Therefore, it means that the unknown compound is not a …..

62
Q

What do hydrogen environments tell us about?

A

The number and type of different environments there are in a compound

63
Q

What does the relative peak area tell us?

A

It tells us how many hydrogens are in each environment compared to each other.

64
Q

How is the relative peak area given?

A

In its LOWEST whole number ratio

65
Q

In H NMR, what does the relative peak area tell us?

A

The relative umber of hydrogen atoms in each environment.

66
Q

Given a ratio for H environments of 6:2:10 give the relative peak area

A

3:1:5

67
Q

What does an integration trace us?

A

It shows us how many hydrogen atoms there are in that environment

68
Q

What is meant by the effect of spin-spin coupling?

A

When the split pattern formed by a hydrogen environment is dependent on the number of hydrogen atoms that are bonded to neighbouring carbons.

69
Q

What is the spin-spin coupling formula?

A

n+1

Where n = the number of hydrogens on adjacent carbons.

70
Q

How many parts is a CH3 peak split into and why??

A

3 because there are 2 hydrogens on the adjacent carbon

71
Q

How many parts is a CH2 peak split into and why??

A

4 because there are 3 hydrogens on the adjacent carbon

72
Q

When is a singlet produced?

A

When there are NO hydrogen atoms on adjacent carbons.

73
Q

A peak that is not split is called a ?

A

Singlet

74
Q

A peak that is split into 2 is called a ?

A

Doublet

75
Q

A peak that is split into 3 is called a ?

A

Triplet

76
Q

A peak that is split into 4 is called a ?

A

Quartet/Quadruplet

77
Q

A peak that is split into 5 or more is called a ?

A

Multiplet

78
Q

What splitting pattern do NH an OH groups produce and why?

A

Singlet - This is because they are continuously undergoing proton exchange.

79
Q

What spitting pattern does Benzene produce?

A

Multipliet.

80
Q

Explain the use of TMS as the standard for chemical shift measurements

A

TMS is an internal standard (reference point) for both carbon-13 NMR and H NMR.

TMS produces a single absorption peak because all of its carbon and hydrogen atoms are in the SAME environment.

TMS is a good internal standard because :

  • Inert (won’t react
  • Non -toxic
  • Contains both carbon and hydrogen so it can be used for both types of NMR
  • Produces a SHARP clear peak
  • Produced at a lower frequency than everything else
  • Low boiling point and is volatile so we can easily remove it from our sample
81
Q

Why is TMS (Tetramethylsilane) a good internal standard?

A
  • Inert (won’t react
  • Non -toxic
  • Contains both carbon and hydrogen so it can be used for both types of NMR
  • Produces a SHARP clear peak
  • Produced at a lower frequency than everything else
  • Low boiling point and is volatile so we can easily remove it from our sample
82
Q

Is the TMS peak an environment?

A

NO. IT IS NOT!!!

83
Q

Why do we use deuterated solvents such as CDCl3 in NMR?

A

They don’t adsorb the radiowave energy in the process of NMR, so they don’t add peaks to the spectrum.

84
Q

What is the most commonly used deuterated solvent?

A

CDCl3

85
Q

How can we identify NH an OH peaks? (What is used? Outline the process)

A

Using D2O

1) We run TWO spectra of the molecule (one with D2O and one without D2O)
2) If an OH or NH proton is present, it’ll swap with the deuterium making the peak DISAPPEAR. This is because deuterium doesn’t absorb the radio wave energy.

86
Q

Why do we not use H2O as the solvent in NMR?

A

It will add peaks to the spectrum

Chemistry OCR A Level (15 decks)

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