Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Chemistry Alevel OCR > 6.3 Analysis > Flashcards

6.3 Analysis Flashcards

(75 cards)

Start Studying
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Define retention time ?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the equation for an Rf value?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

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

A

-how many chemicals make up the mixture

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

In TLC what will similar compounds have ?

A

-similar Rf values

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is GC (gas chromatography ) used for?

A

-to separate volatile components in a mixture

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the mobile phase in gas chromatography ?

A

-an unreactive carrier gas such as helium or nitrogen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What will a GC produce?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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 .

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
What does the area under a peak in GC not tell you?
-it does not give you the exact concentration
26
What is the equation for % of component in mixture?
%of component in mixture= Area of peak ——————————- Total area of all peaks
27
What is the equation for % of component in mixture?
%of component in mixture= Area of peak ——————————- Total area of all peaks
28
What is a calibration curve used for?
-used in gas chromatography to confirm concentrations of components
29
How do you create a calibration curve?
-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
What is NMR?
-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
How does NMR work?
-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
When does nuclear spin occur ?
-it occurs if an atomic nucleus has an odd number of nucleons (protons and neutrons)
33
What does the energy required to change the alignment of nucleons in NMR tell us?
-the energy required to change the alignment depends on the environment the atom is in and can be compared to a standard
34
How many carbon environments are in this diagram?
-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
How many hydrogen environments are in this diagram?
-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
What is TMS ?
-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
What is the internal standard for carbon and proton NMR?
-TMS (tetramethylsilane)
38
What is the formula for TMS?
- (CH3)4Si
39
Why is TMS a good internal standard?
-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
What is chemical shift?
- 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
What effects chemical shift values?
-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value
42
What effects chemical shift values?
-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value
43
Why can you not use normal organic solvents in NMR?
-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
What solvents are used in NMR?
-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
What is an example of a deuterated solvent used in NMR?
-CDCl3 , heavy trichlomethane
46
What does the number of peaks on a carbon-13 NMR tell us?
-peaks tell us about the number of different carbon environments in a molecule
47
What does the number of peaks on a carbon-13 NMR tell us?
-peaks tell us about the number of different carbon environments in a molecule
48
What information does carbon-13 NMR give us?
-gives information about the number of carbon environments in a molecule -but not the ratio of atoms in each environment
49
In carbon-13 NMR carbon atoms in the same environment are known as …..
-equivalents ->they are bonded to the same atoms and feel the same magnetic field in the NMR spectrometer
50
How does electron shielding affect the chemical shift value?
-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
What should we look for when determining the structure of a cyclic compound from a carbon-13 NMR?
-look for symmetry to which atoms are in different environments
52
Define equivalent protons
-hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer
53
What do the number of peaks on proton NMR’s tell you?
-the number of different hydrogen environments
54
What does the area under each signal tell you in proton NMR?
-it gives the ratio of atoms in each environment -this is often shown as an integration trace
55
What does proton NMR spectroscopy tell you?
-how many different hydrogen environments there are -how many hydrogens in each environment in the sample
56
How can you use the numbers above the peaks in proton NMR ?
-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
How do you work out the relative numbers of each type of proton present from an integration trace?
-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
What is a splitting pattern?
-peaks on the proton NMR that split into smaller peaks -it is caused by the spin- spin coupling of neighbouring protons
59
What causes a splitting pattern?
-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
What rule can be used to predict the number of peaks in the splitting pattern?
n+1 rule -n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon
61
What rule can be used to predict the number of peaks in the splitting pattern?
n+1 rule -n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon
62
Explain what a singlet peak, a doublet peak , a triplet peak and a quartet peak mean on a proton NMR spectrum?
-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
Why can it be difficult to identify -OH and -NH protons?
-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
Why does D2O not produce a signal in a proton NMR spectrum?
-it does not have an odd number of nucleons
65
How is D2O (heavy water) used to identify -OH and -NH protons?
-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
Compare carbon-13 and proton NMR?
-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
Compare carbon-13 and proton NMR?
-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
68
What is the test used to identify an alkene group? Describe the observations
-add a few drops of bromine water to the sample and shake well -> if the bromine water decolourises then the compound contains an alkene group
69
What is the test to identify haloalkanes? Describe the observations
-add aqueous silver nitrate and ethanol ->white precipitate means a chloroalkane is present ->cream precipitate means a bromoalkane is present ->yellow precipitate means an iodoalkane is present
70
What is the test to identify a carbonyl compound? Describe the observations
-add 2,4-DNP ->a yellow or orange precipitate is formed when a ketone or aldehyde is present
71
What are two tests to identify an aldehyde functional group?
-add Tollen’s reagent ->a silver mirror is formed if an aldehyde is present -add acidified potassium dichromate ->the orange solution turns green
72
What is the test to identify primary and secondary alcohols? What are the observations?
-add acidified potassium dichromate ->colour change from orange to green as first and aldehyde is formed and then a carboxylic acid ->secondary alcohols will be oxidised to a ketone - and will also produce a colour change from orange to green ->tertiary alcohols cannot be oxidised in this way so there is no observable change
73
What is the test to identify carboxylic acids?
-add CO3^2- (e.g. a metal carbonate- calcium carbonate) ->if carboxylic acid is present effervescence will occur
74
How do you identify phenols? What are the observations?
-phenols will have a weak acidity- you can check the pH is that for a weak acid by using a pH probe or universal indicator ->there will be no reaction when a carbonate ion is added (no effervescence)
75
B

Chemistry Alevel OCR (13 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions