chromatography and spectroscopy year 2

Question 1

chromotography

Answer 1

used to seperate individual components from a mixture of substances. All forms have a mobile and stationary phase
* stationary phase is fixed- normally a solid or liquid supported on a solid
* mobile phase moves in definite direction- normally liquid or gas
* mobile phase sweeps a mixture over stationary phase
* each component in the mixture will have different attraction for the stationary and mobile phase
* those with a greater attraction to the mobile phase than to the stationary phase will be carried futher in a given period of time
+ quick an inexpensive, small amount of substances needed

Question 2

thin layer chromotography (TLC)

Answer 2

• stationary phase is thin layer of adsorbant silica (SiO2) (or aluminia Al2O3) coated on a flat, inert solid support (glass or plastic) called the TLC plate
• mobile phase is a liquid solvent which moves vertically up the TLC plate
• different components in the mixture have different affinities for the absorbent and bind with differing strengths to its surface
• adsorption is the process by which the solid silica holds the different substances in the mixture to its surface. Seperation is achieved by the relative adsorptions of substances with the stationary phase
• analysed by working out Rf values = distance moved by components/ distance moved by solvent front. Each component is then identified by comparing Rf value with known values recorded using same solvent system and absorbent
• limitations= similar compounds have similar Rf values, unkown compounds have no Rf reference for comparison, may be difficult to find solvent that seperates all components in a mixture. If very soluble just be washed up TLC plate and if little solubility it will hardly move

Question 3

gas chromatography

Answer 3

• useful for seperating and identifying volatile organic compounds present in a mixture
• stationary phase is a high boiling point liquid adsorbed onto an inert solid support (normally long chain alkanes) OR a solid (eg silicon polymer) coating the inside surface of capillary tubing
• mobile phase is an inert carrier gas such as He or Ne
• a small amount of the volatile mixture is injected into the apparatus called a gas chromatograph. The mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase absorbed onto the solid support
• the 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 separated depending on their solubility in the liquid stationary phase. The compounds in the mixture reach the detector 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 rention time and is detected first.
• The retention time is the time taken for each component to travel through the column

Question 4

interpretation of a gas chomatogram

Answer 4

• each component is detected as a peak on the gas chromatogram
• can use retention times to identify the compounds present in a sample by comparing these retention times to known retention times for known compounds
• peak integrations (areas under each peak) can be used to determine the concentrations of components in the sample
• in some gas chromatogram instruments the outlet tube is connected to a mass spectrometer so each component can be accuratley identified
• • can find out amound of each component, only small amount of sample needed, more accurate than thin layer-more sensitive, can detect substances to trace amounts
• limitations= expensive, some training, unknown compounds have no reference Rf for comparison, portentially thousands of chemicals have same retention times and peak shapes, not all substances in sample will neccessarily be seperated and detected, only works with volatile substances, some can be hidden behind other larger peaks

Question 5

concentration of components in gas chromatograms

Answer 5

• concentration of a component in a sample is determined by comparing its peak inegration (peak area) with values obtained from standard solutions of components
• method:
• prepare standard solutions of known concentrations of the compound being investigated
• obtain gas chromatograms for each standard solution
• 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
• obtain a gas chromatogram of the compound being investigated under the same conditions
• use the calibration curve to measure the concentration of the compound

Question 6

alkene test

Answer 6

• add bromine water dropwise
• pos result: decolourised from orange to colourless

Question 7

carbonyl test

Answer 7

add 2,4- DNP
pos: orange precipitate

Question 8

aldehyde test

Answer 8

add tollens reagent AND WARM
pos: silver mirror

Question 9

primary and secondary alchohol, and aldehyde test

Answer 9

add acidified potassium dichromatie (VI) AND WARM in a water bath
positive result: colour change from orange to green

Question 10

carboxyllic acid test

Answer 10

add aqueous sodium carbonate
positive result: effervescence as CO2 gas is produced

Question 11

haloalane test

Answer 11

add silver nitrate and ethanol and warm to 50 ‘C in a water bath (needs to be aquous as H20 will react with haloalkane to allow for AgX to form)
chloroalkane: white precipitate
bromoalkane: cream precipitate
iodoalkane: yellow precipitate