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Chemistry Alevel OCR > 6.3 Analysis > Flashcards

6.3 Analysis Flashcards

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1
Q

What is an Rf value?

A

-a comparison between how far a component has moved compared to the solvent in thin layer chromatography

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2
Q

Define retention time ?

A

-the time taken for a component to travel from the inlet to the detector in a gas chromatograph

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3
Q

What is the stationary phase in chromatography?

A

-it is fixed in place
-it can be a liquid (e.g. in some gas chromatography systems where the mixture components have different attractions to each phase )
-it can also be a solid (e.g. in thin layer chromatography where the components of the mixture are absorbed by different amounts)

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4
Q

What is the mobile phase in chromatography ?

A

-moves in a definite direction
-e.g. an inert gas in gas chromatography (GC) or a liquid solvent in thin layer chromatography (TLC)

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5
Q

How does separation occur in chromatography?

A

-by each chemical in a mixture having a different attraction to each phase
-e.g. a chemical that is very attracted to the mobile phase but not the stationary phase will travel a long way up, but a chemical that is very attracted to the stationary phase will not travel far

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6
Q

What is thin layer chromatography used for ?

A

-to analyse organic compounds

E.g. to check the purity of a sample or to determine the extent of a chemical reaction

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7
Q

What is the stationary phase in TLC (thin layer chromatography )?

A

-a thin piece of inert material
-e.g. glass covered with an absorbent chemical such as aluminium oxide

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8
Q

What is the mobile phase in TLC (thin layer chromatography)?

A

-usually an organic solvent
-the solvent will move in one direction vertically up the plate and develop the chromatogram

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9
Q

Why is a lid sometimes used in TLC?

A

-as the lid reduces the rate at which the solvent is absorbed and allows for a better separation

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10
Q

What is the equation for an Rf value?

A

Rf value= distance moved by component
——————————————————-
distance moved by solvent

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11
Q

What do the number of spots on the plate in TLC tell you?

A

-how many chemicals make up the mixture

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12
Q

How can you use Rf values to identify substances in a sample when doing thin layer chromatography ?

A

By either:
-using data tables that list Rf values for a wide variety of chemicals
-or control spots can be run on the same TLC plate and a direct comparison can be made

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13
Q

When doing thin layer chromatography what should you be careful of when using data tables to identify a substance?

A

-the Rf values are different if the phases are changed (e.g if a different solvent is used) so it is important to use the correct table for the mobile and stationary phases in your experiment.

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14
Q

In TLC what will similar compounds have ?

A

-similar Rf values

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15
Q

What is a limitation of TLC (thin layer chromatography)?

A

-chemicals that a very attracted to the mobile phase may not be separated using this method

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16
Q

What is GC (gas chromatography ) used for?

A

-to separate volatile components in a mixture

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17
Q

Why is a gas chromatograph often connected to a mass spectrometer?

A

-so that the separate parts of the mixture can undergo further analysis

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18
Q

What is the stationary phase in gas chromatography?

A

-a solid or liquid coating on a coiled tube
-this coating is normally a hydrocarbon with a high boiling point

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19
Q

What is the mobile phase in gas chromatography ?

A

-an unreactive carrier gas such as helium or nitrogen

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20
Q

What will a GC produce?

A

-a chart of absorption against time
-where the x-axis is retention time

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21
Q

What can the retention time of a chemical tell you?

A

-this value can be compared to known values to identify the component as each component will take a different amount of time to travel through the column and reach the detector

-be careful as retention times for the same chemical will be different when different phases are used so use the correct data table

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22
Q

What do the different parts of a GC charts show?

A

-retention time-can be compared to known values to identify the component

-each peak represents a different substance each substance has a different retention time

-the area under the absorption peak tells us the amount of each substance or percentage of a component in a mixture, the larger the area the more substance .

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23
Q

i) Which compound is present in the greatest quantity?

ii) Which compounds were present in equal amounts?

iii) Which compound had the strongest interaction with the stationary phase?

A

i) D (the larger the relative size of the peak, the greater the quantity of that substance present)

ii) B and C (the peak sizes are equal)

iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)

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24
Q

i) Which compound is present in the greatest quantity?

ii) Which compounds were present in equal amounts?

iii) Which compound had the strongest interaction with the stationary phase?

A

i) D (the larger the relative size of the peak, the greater the quantity of that substance present)

ii) B and C (the peak sizes are equal)

iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)

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25
Q

What does the area under a peak in GC not tell you?

A

-it does not give you the exact concentration

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26
Q

What is the equation for % of component in mixture?

A

%of component in mixture= Area of peak
——————————-
Total area of all peaks

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27
Q

What is the equation for % of component in mixture?

A

%of component in mixture= Area of peak
——————————-
Total area of all peaks

28
Q

What is a calibration curve used for?

A

-used in gas chromatography to confirm concentrations of components

29
Q

How do you create a calibration curve?

A

-prepare standard solutions (known concentration) of compound under investigation

-obtain gas chromatogram for each standard solution and measure the area under the curve

-plot calibration curve (where the peak area is plotted against concentration)

-use this graph to find out the concentration of the sample

30
Q

What is NMR?

A

-a non-destructive analytical technique
-can confirm the formula mass of an organic molecule and is used to determine the structure of a chemical

31
Q

How does NMR work?

A

-The nucleus of some atoms, e.g. ¹H and ¹³C have a property known as nuclear spin
-This creates a weak magnetic field, NMR spectroscopy analyses how these small magnetic fields react when placed in a much larger magnetic field
-When these nuclei are placed in a much larger magnetic field they can either align themselves with a low -energy state or they can oppose it with a higher -energy state
-When electromagnetic radio waves are fired at them the energy from these waves are absorbed and causes the nuclear spins to flip and change direction e.g. nuclear spin changes from a low -energy state to a high-energy state
-These absorptions are called resonances and are recorded on the NMR spectrum

32
Q

When does nuclear spin occur ?

A

-it occurs if an atomic nucleus has an odd number of nucleons (protons and neutrons)

33
Q

What does the energy required to change the alignment of nucleons in NMR tell us?

A

-the energy required to change the alignment depends on the environment the atom is in and can be compared to a standard

34
Q

How many carbon environments are in this diagram?

A

-here there are 3 carbon environments
-the green C (CH2Br) that is bonded to CH2CH3
-the purple C (CH2) that is bonded to CH2Br(CH3)
-the red C (CH3) that is bonded to CH2(CH2Br)

35
Q

How many hydrogen environments are in this diagram?

A

-here there is 1 hydrogen environment, as the 4 red hydrogens are in the same environment and they are all equal distance from electronegative Br atoms

36
Q

What is TMS ?

A

-also called tetramethylsilane
-used in both proton and carbon NMR
-it is used as a reference chemical to compare the values from the sample
-the spectrometer makes a relative scale (chemical shift) for the x-axis of the spectrum
-the TMS reference peak is given the value 0ppm and all other peaks are placed on the x-axis compared to it

37
Q

What is the internal standard for carbon and proton NMR?

A

-TMS (tetramethylsilane)

38
Q

What is the formula for TMS?

A
  • (CH3)4Si
39
Q

Why is TMS a good internal standard?

A

-contains both carbon and hydrogen so can be used for both proton and carbon NMR
-produces one sharp single as hydrogen atoms are in a single environment, also there is only one carbon atoms so it produces only one signal in carbon-13 NMR
-non toxic
-low boiling point and is volatile so can be easily removed from sample
-inert so unlikely to react with any chemical that is being investigated

40
Q

What is chemical shift?

A
  • is the x-axis variable on an NMR spectrum that compares the frequency of NMR absorption with the frequency of the reference peak of TMS
    -has the symbol delta δ
    -measured in ppm (parts per million)
    -TMS (the internal standard) is given 0ppm and is on the far right of the axis
41
Q

What effects chemical shift values?

A

-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value

42
Q

What effects chemical shift values?

A

-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value

43
Q

Why can you not use normal organic solvents in NMR?

A

-in NMR spectroscopic analysis the molecule is first dissolved in a solvent
-because this technique is used to investigate organic compounds organic solvents like trichloromethane are needed
-however these chemicals contain carbon and hydrogen which would produce signals that would swamp the spectrum and make it unreadable

44
Q

What solvents are used in NMR?

A

-hydrogen has 3 isotopes but only ¹H has the property of spin and is active in NMR
-therefore the deuterium isotope 2H is used as it is not NMR active as it does not have the property of spin, it is shown by using a ‘D’ in the formula
-deuterium solvents are often called heavy as deuterium has a greater atomic mass then the ¹H isotope
-therefore trichloromethane CHCl3 (also called chloroform) would become heavy trichloromethane ,CDCl3

45
Q

What is an example of a deuterated solvent used in NMR?

A

-CDCl3 , heavy trichlomethane

46
Q

What does the number of peaks on a carbon-13 NMR tell us?

A

-peaks tell us about the number of different carbon environments in a molecule

47
Q

What does the number of peaks on a carbon-13 NMR tell us?

A

-peaks tell us about the number of different carbon environments in a molecule

48
Q

What information does carbon-13 NMR give us?

A

-gives information about the number of carbon environments in a molecule
-but not the ratio of atoms in each environment

49
Q

In carbon-13 NMR carbon atoms in the same environment are known as …..

A

-equivalents
->they are bonded to the same atoms and feel the same magnetic field in the NMR spectrometer

50
Q

How does electron shielding affect the chemical shift value?

A

-the lower the electron shielding (e.g. if the carbon is near an electronegative element) the higher the chemical shift

-the higher the electron shielding the lower the chemical shift

51
Q

What should we look for when determining the structure of a cyclic compound from a carbon-13 NMR?

A

-look for symmetry to which atoms are in different environments

52
Q

Define equivalent protons

A

-hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer

53
Q

What do the number of peaks on proton NMR’s tell you?

A

-the number of different hydrogen environments

54
Q

What does the area under each signal tell you in proton NMR?

A

-it gives the ratio of atoms in each environment
-this is often shown as an integration trace

55
Q

What does proton NMR spectroscopy tell you?

A

-how many different hydrogen environments there are
-how many hydrogens in each environment in the sample

56
Q

How can you use the numbers above the peaks in proton NMR ?

A

-the number above the peaks tells you the ratio of the areas under the peak
-this allows you to work out the relative numbers of each type of proton present from the relative peak areas
E.g. if the numbers are 1 and 3 it will be a 3:1 ratio

57
Q

How do you work out the relative numbers of each type of proton present from an integration trace?

A

-the height ratio of the trace corresponds to the area ratio
-so measure the vertical parts of the trace, then write the lengths down and use these numbers to come up with a ratio

58
Q

What is a splitting pattern?

A

-peaks on the proton NMR that split into smaller peaks
-it is caused by the spin- spin coupling of neighbouring protons

59
Q

What causes a splitting pattern?

A

-organic chemicals in the NMR spectrometer experience a large magnetic field
-the protons on neighbouring carbon atoms also affect the magnetic field
-this affects the alignment of the proton and causes a split in the signal
-different numbers of adjacent protons will cause different numbers of splits

60
Q

What rule can be used to predict the number of peaks in the splitting pattern?

A

n+1 rule
-n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon

61
Q

What rule can be used to predict the number of peaks in the splitting pattern?

A

n+1 rule
-n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon

62
Q

Explain what a singlet peak, a doublet peak , a triplet peak and a quartet peak mean on a proton NMR spectrum?

A

-a SIGNLET peak means there are 0 hydrogens on neighbouring (adjacent) carbon

-a DOUBLET peak means there is 1 hydrogen on adjacent carbon

-a TRIPLET peak means there are 2 hydrogens on adjacent carbon

-a QUARTET peak means there are 3 hydrogens on adjacent carbon

63
Q

Why can it be difficult to identify -OH and -NH protons?

A

-peaks can appear over a wide range of chemical shift values depending on the solvent used and the concentration of the sample
-the signals are often broad
-there usually no splitting pattern

64
Q

Why does D2O not produce a signal in a proton NMR spectrum?

A

-it does not have an odd number of nucleons

65
Q

How is D2O (heavy water) used to identify -OH and -NH protons?

A

-a proton NMR spectrum is run
-a small amount of D2O is added to the sample solution and is shaken vigorously as D2O and CDCl3 are immiscible
-a second proton NMR is run and any peak due to -OH or -NH protons disappear , as the deuterium in D2O exchanges with the H present in -OH and -NH (by proton exchange)
-the two spectra can be compared to show the absorptions due to -OH and -NH

66
Q

Compare carbon-13 and proton NMR?

A

-both use TMS as an internal standard
-the number of peaks on a carbon-13 NMR tells you the number of different carbon environments, the number of peaks on a proton NMR tells you the number of different proton environments
-the chemical shift range is much smaller for proton NMR
-a much smaller sample size can be used for proton NMR
-unlike carbon NMR proton NMR gives information on the number of protons on adjacent carbons
-the peak area on a carbon-13 NMR gives information on the relative number of carbon atoms of each type , the peak area on a proton NMR gives information on the relative number of protons of each type

67
Q

Compare carbon-13 and proton NMR?

A

-both use TMS as an internal standard
-the number of peaks on a carbon-13 NMR tells you the number of different carbon environments, the number of peaks on a proton NMR tells you the number of different proton environments
-the chemical shift range is much smaller for proton NMR
-a much smaller sample size can be used for proton NMR
-unlike carbon NMR proton NMR gives information on the number of protons on adjacent carbons
-the peak area on a carbon-13 NMR gives information on the relative number of carbon atoms of each type , the peak area on a proton NMR gives information on the relative number of protons of each type

Chemistry Alevel OCR (9 decks)

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