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Chromatography and Spectroscopy Flashcards

Rhea Sekhon Chemistry A level English 49 cards

Cards in this deck (49)

  1. 1What is chromatography?Chromatography is a type of separation technique that can be used to separate a MIXTURE of substances, dissolved in a liquid
    So, its used to separate out INDIVIDUAL components from a MIXTURE of substances
    It is also used to determine the PURITY of a sample
  2. 2What 2 phases does chromatography consist of?- The STATIONARY PHASE = this does not move and is normally a solid, or a liquid supported on a solid
    - The MOBILE PHASE = this does move and its usually a liquid or gas
  3. 3What are some applications for the use of chromatography?The analysis of:
    - drugs
    - plastics
    - flavourings
    - air samples
    - applications in forensic science
  4. 4Describe how paper chromatography works in terms of attraction to the phases and what the stationary/mobile phases are in itStationary phase = the absorbent paper/filter paper
    Mobile phase = the solvent that moves through the paper, carrying the substances with it
    The different dissolved substances in a mixture are attracted to the two phases in DIFFERENT PROPORTIONS, causing them to move at DIFFERENT RATES through the paper
    This enables chemists to interpret the PURITY of a mixture + determine what COMPONENTS make up the mixture
  5. 5Describe how thin layer chromatography works in terms of ADSORPTION and what its stationary phase isTLC is a QUICK and INEXPENSIVE analytical technique that indicates how many COMPONENTS are in a mixture
    Stationary phase = this is a SOLID ADSORBENT SURFACE eg. silica (SiO2) or aluminium oxide (Al2O3), coated on a PLASTIC SHEET/GLASS (liquid supported on a solid)
    Adsorption = the process where the solid silica (stationary phase) HOLDS a substance to its surface. Separation of a mixture is achieved because different substances in the mixture have DIFFERENT RELATIVE ADSORPTIONS to the stationary phase
    The STRONGER the adsorption of a substance in the mixture to the stationary phase, the SLOWER the substance will move up the TLC plate and so the LOWER its Rf value
  6. 6Describe what the mobile phase is in TLCThe mobile phase is a SOLVENT that moves in a definite direction (UPWARD direction for TLC)
    Separation of a mixture is achieved because different substances in the mixture have different relative SOLUBILITIES to the mobile phase
  7. 7What 2 factors allow the separation of a mixture into its components to be achieved?- Different substances in the mixture have different relative adsorptions to the stationary phase
    - Different substances in the mixture have different relative solubilities to the mobile phase
  8. 8Describe the process used to carry out a TLC- On a TLC plate, draw the base line in PENCIL about 1cm from one end of the plate
    - Using a capillary tube, spot a small amount of solution of the sample onto the base line of the plate
    - Prepare a chromatography tank with the solvent in it. Ensure the solvent depth is LOWER than the base line
    - Leave the TLC plate to form a chromatogram
    - Mark the solvent front with a pencil (up to where the solvent travelled)
    - Allow the plate to dry
    - If the separation isn't visible to the naked eye, dip the TLC into a CHEMICAL eg. iodine, potassium permanganate or ninhydrin or use a UV LAMP if the spots are visible under UV light
  9. 9Why must be baseline be drawn in pencil?To prevent it from dissolving into the solvent and being carried up the TLC plate
  10. 10Why does the solvent depth have to be lower than the baseline?So that the solvent doesn't wash the components in the sample sport off the TLC plate
  11. 11Why are TLC plates developed in a SOLVENT TANK? (4 reasons)- To prevent the potentially HAZARDOUS SOLVENT from EVAPORATING and becoming a health hazard
    - To make a SATURATED ATMOSPHERE of the solvent
    - To SLOW the rate of wicking
    - To ensure MAXIMUM SEPARATION
  12. 12How can you interpret a TLC plate using Rf values?Rf value = a quantity that shows how far a chemical has moved up a chromatogram, compared to the solvent front
    Rf = distance moved by the component/distance moved by the solvent
    Data tables will LIST Rf values, allowing for the IDENTIFICATION of substances in a sample
    The Rf values are DIFFERENT if the PHASES ARE CHANGED eg. using a different SOLVENT or GEL
    Alternatively, CONTROL SPOTS can be run on the same TLC plate and a DIRECT COMPARISON can be made between components to identify the unknown ones, based on the control plates you have run, when you know exactly what the chemical is
  13. 13Describe 3 applications of TLC1. CHECK THE PURITY OF A SAMPLE (after recrystallisation, the purity of the sample can be confirmed using TLC - compare the separated components from the TLC to that of the PURE PRODUCT. You can also confirm the purity of a recrystallised product by measuring its MELTING POINT using a MELTING POINT APPARATUS and comparing it to a DATA BOOK VALUE)
    2. TO DETERMINE THE EXTENT OF A CHEMICAL REACTION (you can run TLC plates and locate the starting material eg. if after 10 mins has passed and the starting material of the reaction was still present on the chromatogram, it is still present in the reaction mixture so you know the reaction is incomplete and you need to CONTINUE the chemical reaction, completing another TLC plate after a further time period, until the starting material spot disappears from the reaction mixture sample)
    3. Determining the best MOBILE PHASE/SOLVENT to purify an impure product using FLASH COLUMN CHROMATOGRAPHY
  14. 14What is gas chromatography used to separate?Gas chromatography is used to separate VOLATILE ORGANIC COMPOUNDS in a mixture
    The gas chromatogram is often connected to a MASS SPECTROMETER so that the separate parts of the mixture can undergo FURTHER ANALYSIS
  15. 15What are the two main phases in gas chromatograohy?Stationary phase = a solid/liquid coating on a COILED TUBE (adsorption for the liquid); the coating is usually a hydrocarbon with a HIGH boiling point
    Mobile phase = an inert carrier gas eg. helium/nitrogen/neon
  16. 16How do the components move through the gas chromatogram?The components will SLOW DOWN as they interact with the liquid stationary phase in 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 separated depending on their SOLUBILITY in the liquid stationary phase
    The compounds in the mixture will reach the detector at the end of the gas column at different times, depending on their interactions with the stationary phase in the column
    The compound retained in the column for the SHORTEST time has the LOWEST retention time and is detected first
  17. 17What is retention time?This is the time it takes for a component to pass through the column, from the column inlet to the detector
    This value can be compared to known values to identify the component
  18. 18How do we interpret retention times?The MORE VOLATILE a substance, the FASTER it will travel through the mobile phase and reach the detector
    So, the SHORTER its retention times
    Retention time is also shorter if the compound has a LOWER SOLUBILITY in the stationary phase
  19. 19What does gas chromatography produce, for analysis?It produces a chart of ABSORPTION (y-axis) against TIME (x-axis)
    Time = retention time
  20. 20How can RELATIVE concentrations of components be determined from gas chromatography charts?The area under each peak on the chart is PROPORTIONAL to the CONCENTRATION of each component
  21. 21Describe how gas chromatography calibration curves can be used to determine the EXACT concentration of a component?To determine the EXACT concentration of a component in a sample, the peak area of the component has to be compared with peak areas from STANDARD SOLUTIONS (solutions of KNOWN concentrations) of the PURE component
    1. Prepare standard solutions of the component of interest
    2. Obtain gas chromatograms of each of the standard solutions, under the SAME CONDITIONS
    3. Plot a calibration curve of RELATIVE PEAK AREA (y-axis) and CONCENTRATION (x-axis) for the standard solutions
    4. A gas chromatogram of the sample being investigated is then run, under the SAME conditions
    5. The relative peak area of the component is MATCHED to a corresponding concentration on the calibration curve
  22. 22Describe the test for alkenesReagent: bromine water (add drop-wise)
    Positive test: bromine water is decolourised from orange to colourless
  23. 23Describe the test for haloalkanesReagent: aqueous silver nitrate and ethanol (warm to 50 degrees in a water bath)
    Positive test:
    Chloroalkane = white precipitate
    Bromoalkane = cream precipitate
    Iodoalkane = yellow precipitate
  24. 24Describe the test for phenolsReagent: pH indicator and carbonate
    Positive test: phenols are WEAKLY ACIDIC so will turn pH litmus paper red
    Phenols are not reactive enough to react with carbonates so NO REACTION
    In exam answers, include BOTH TESTS for phenols!
  25. 25Describe the test for the carbonyl groupReagent: 2,4 DNP
    Brady's reagent = 2,4 DNP + methanol + sulfuric acid
    Positive test: orange precipitate
  26. 26Describe the test for aldehydesReagent: Tollen's Reagent
    Positive test: silver mirror precipitate forms
  27. 27Describe the test for primary/secondary alcohols and aldehydesReagent: K2Cr2O7/H2SO4 (+ warm in a water bath)
    Positive test: colour change from orange to green
  28. 28Describe the test for carboxylic acidsReagent: add aqueous sodium carbonate
    Positive test: effervescence (CO2 evolved)
  29. 29What is NMR spectroscopy and how is it used?NMR spectroscopy is an analytical technique that is used to detect different ENVIRONMENTS surrounding 1H and 13C atoms in molecules
    This enables us to determine the STRUCTURE of an unknown compound, in addition to using IR spectroscopy and mass spectrometry
    A sample of a compound is placed into an MMR spectrometer and an NMR spectrum is created
  30. 30NMR spectroscopy is reliant on a nucleus having the property of 'spin'. What is this?The nucleus has the property of NUCLEAR SPIN
    When nuclei have spin, their atoms have their own magnetic fields and can behave like MAGNETS
    Nuclei with an ODD NUMBER OF NUCLEONS have spin and can be detected from NMR spectra
    13C and 1H have an odd number of protons so can be detected on NMR spectra
  31. 31What is TMS (tetramethylsilane)?This is a REFERENCE COMPOUND
    The signal from the carbon/hydrogen atoms in TMS is defined as having a chemical shift of 0 ppm (parts per million)
    So, TMS is used as the standard reference chemical against which all chemical shifts are measured
  32. 32What is chemical shift?An NMR spectrum has a RELATIVE FREQUENCY SCALE, called a CHEMICAL SHIFT, on the x-axis of the spectrum
    The amount of chemical shift is determined by chemical environment, especially the presence of nearby electronegative atoms (greater the chemical shift, the more to the LEFT the peak will be, on the spectrum)
  33. 33What is the purpose of using deuterated solvents eg. CDCl3 when running an NMR spectrum?Molecules of most common solvents contain carbon and hydrogen atoms, which will produce a signal in both 13C and 1H NMR spectra. This can't be used because it will interfere with the analysis of the compound
    A deuterated solvent is usually used, in which the 1H atoms have been replaced by 2H atoms (DEUTERIUM,D)
    Deuterium produces NO NMR signal in the frequency ranges used in 1H and 13C NMR spectroscopy, so this means analysis of the compound isn't interfered with
    Deuterated trichloromethane, CDCl3 is commonly used as a solvent, but this will still produce a peak in a carbon-13 NMR spectrum. The computer usually filters out this peak before displaying the spectrum
  34. 34What 2 bits of information can be obtained from carbon-13 NMR spectra?1. The NUMBER of different carbon environments (equal to the number of peaks present in the NMR spectrum)
    2. The TYPES of carbon environment present. This is linked to the CHEMICAL SHIFT value of each peak
  35. 35Give some examples of commonly used solvents in (proton) NMR- CCl4
    Deuterated solvents:
    - CDCl3
    - C6D6
  36. 36What reference compound is used to calibrate the sample and give reasons WHY this is usedTo calibrate the sample, a small quantity of TMS is added to samples as this produces a single, internal standard to which other peaks are compared
    Reasons its used:
    - It gives only ONE signal
    - Its NON-TOXIC
    - Its INERT
    - It has a LOW BOILING POINT (26 degrees) and so can be EASILY REMOVED from the sample afterwards
    - It gives a signal that is FURTHER RIGHT than most of the signals from organic compounds
  37. 37What 4 pieces of information can be obtained from proton NMR spectra?1. The NUMBER of proton environments, from the number of peaks
    2. The TYPES of proton environment, from the chemical shift
    3. The RELATIVE NUMBERS of each proton type, from integration traces/ratio numbers of relative peak areas
    4. The number of non-equivalent protons ADJACENT to a given proton, from the spin-spin splitting pattern
  38. 38High resolution vs low resolution proton NMR spectra and spin-spin coupling interactionsIn a HIGH resolution NMR spectrum, it's possible to see that some signals are made from a CLUSTER of peaks. This is known as a SPLITTING PATTERN, caused by the proton's spin interacting with the spin states of NEARBY protons, that are in DIFFERENT environments
    This provides information about the NUMBER OF PROTONS bonded to ADJACENT carbon atoms
    In low resolution NMR spectra, the peaks aren't split up so you can't work out the number of protons bonded to adjacent carbons
  39. 39Splitting - the n+1 ruleThe splitting of a main peak into sub-peaks is called SPIN-SPIN COUPLING or SPIN-SPIN SPLITTING
    The number of subpeaks is one greater than the number of adjacent protons causing the splitting
    So, for a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern is n+1
  40. 40Name the 5 splitting patterns- Singlet (no H on adjacent atoms)
    - Doublet (adjacent CH)
    - Triplet (adjacent CH2)
    - Quartet (adjacent CH3)
    - Multiplet (more than 3 H's on adjacent atom)
  41. 41Complicated splitting patterns: Multiplets (heptets)In CH(CH3)2, the CH proton has 6 protons on the adjacent carbon atoms, giving the HEPTET (7) splitting pattern
    In CH(CH3)2, the two CH3 groups are in the SAME environment
    Sometimes, the adjacent protons may have DIFFERENT ENVIRONMENTS. In molecules such as CH3CH2CH2COOH, the central -CH2- would be split DIFFERENTLY by the CH3 and CH2 protons
    The resulting splitting pattern would then show as a MULTIPLET, due to it being of 2 different splitting patterns
    More advanced analysis allows multiplets to be analysed
  42. 42Analysing aromatic protons on proton NMR spectraAromatic protons are expected to absorb in the range 6.2-8.0ppm
    Splitting DOES occur but this can be difficult to interpret
    You are only expected to interpret aromatic protons as GROUPS of protons, often forming one or more MULTIPLETS
  43. 43Splitting always occurs in pairsIn an NMR spectrum, if you see one splitting pattern, there must always be another because splitting patterns occur in PAIRS, because each proton splits the signal of ANOTHER. This can be helpful in spotting a STRUCTURAL FEATURE when analysing a molecule
  44. 44What's anomalous about hydroxyl and amino protons?OH and NH protons are NOT usually involved in spin-spin coupling (no splitting pattern)
    This can make assigning OH and NH protons difficult
    OH and NH peaks can occur at almost ANY chemical shift and this means its hard to identify which peaks are them
    Also, the signals are often BROAD
    This can be solved using PROTON EXCHANGE
    OH functional groups: alcohol, phenols, carboxylic acids
    NH functional groups: amides, amines, amino acids
  45. 45Describe what comprises proton exchangeChemists have devised a technique called PROTON EXCHANGE for identifying -OH and -NH protons
    1) A proton NMR spectrum is run as normal
    2) A small volume of deuterium oxide, D2O, is added, the mixture is SHAKEN and a SECOND spectrum is run
    Deuterium exchanges and REPLACES the OH and NH protons in the sample, with deuterium atoms eg:
    CH3OH + D2O -> CH3OD + HOD
    So, the second spectrum is essentially being run on CH3OD. As deuterium does NOT absorb in this chemical shift range, the OH peak DISAPPEARS
    You can then COMPARE the two spectra to identify the peak(s) on the first spectrum that represented OH and NH protons, as this peak would have disappeared on the second spectrum
  46. 46How does integration work on proton NMR spectra?In a carbon-13 NMR spectrum, the peak area is NOT directly related to the number of carbon atoms responsible for the peak
    But, in a proton NMR spectrum, the RATIO of the relative AREAS under each peak gives the ratio of the NUMBER OF PROTONS responsible for each peak
  47. 47When analysing a proton NMR spectrum, draw a table out with 5 subheadings to help you What are these subheadings?1. Chemical shift
    2. Peak area
    3. Splitting pattern
    4. Proton type
    5. Adjacent groups
  48. 48What 3 factors make OH and NH peaks difficult to interpret?- They can occur over a wide range of different chemical shift values
    - The signals are often broad
    - There is usually no splitting pattern
  49. 49What splitting pattern is seen for protons adjacent to OH or NH groups?A SINGLET peak
    This is because a OH/NH group NEVER causes a split in a signal

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A level Chemistry Flashcards on Chromatography and Spectroscopy Flashcards, created by Rhea Sekhon on 18/04/2022.
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