C7: Chromatography
Unlock all answers in this set
Unlock answersquestion
Why is chromatography an important analytical technique?
answer
it allows chemists to separate substances in complex mixtures.
question
What does Qualitative analysis mean?
answer
Any method of identifying the chemicals in a sample. It describes properties and characteristics without expressing numbers (colour)
question
What does Quantitative analysis mean?
answer
Working out how much of a particular chemical there is in a sample. Quantitative data has numbers.
question
What must analysis be carried out on an why?
answer
A sample that represents the bulk of a material being tested. This means the results from the analysis can be used to draw conclusions about the rest of the substance.
question
What are many analytical techniques based on?
answer
Samples in solution.
question
What do standard procedures cover?
answer
-Collecting samples -Storing samples -Preparing samples for analysis.
question
What is Chromatography used for?
answer
To find out what unknown mixtures are made up of. Done by comparing the mixture to a known reference sample.
question
What happens in chromatography?
answer
Substances are separated as they travel in a mobile phase which passes through a stationary phase.
question
What are the stationary and mobile phases in paper chromatography?
answer
-Stationary phase is paper -Mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent.
question
What is an example of an organic solvent
answer
Propanone
question
What is there for each chemical in the sample?
answer
There is a dynamic equilibrium between the stationary phase and the mobile phase.
question
What does separation by chromatography depend on?
answer
Components being distributed between the mobile and stationary phases. (How strongly attracted the chemicals are to the mobile and the stationary phases)
question
What does Thin Layer Chromatography use for the stationary and mobile phases?
answer
-Stationary phase: A plate of glass (flat, unreactive surface). -Mobile Phase: Silica/Aluminia (thin layer of an inert substance)
question
What are the advantages of TLC over paper chromatography?
answer
-mobile phase moves more quickly through stationary -mobile phase moves more evenly through stationary -there is a range of absorbencies for the stationary phase -TLC tends to produce more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - easier to analyse.
question
When and what are locating agents used for?
answer
Locating agents are used in thin layer and paper chromatography to show test materials if they're colourless.
question
How do locating agents work?
answer
They bind to the chemicals in the spots. Sometimes, another chemical is then added, which reacts with the locating agent to produce a coloured spot, or the chromatogram is put under ultraviolet light and the locating agent glows to show where the spots are.
question
What is the movement of a substance during chromatography, relative to the movement of the solvent, measured by?
answer
calculating its retardation factor (Rf)
question
What is the equation for the Rf?
answer
Rf= Distance travelled by substance/distance travelled by solvent
question
What are the stationary and mobile phases in Gas Chromatography?
answer
-Stationary Phase: very thin layer of an inert liquid on an inert solid support (beads of silica packed into a long thin tube) -Mobile Phase: An inert gas (helium)
question
What is GC used for?
answer
It is used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography.
question
Why is GC better at separating comple mixtures than thin-layer or paper chromatography?
answer
It is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.
question
How is Gas Chromatography carried out?
answer
1.Mixture to be analysed injected in stream carrier gas. 2.As it passes along the long thin tube it separates into the different substances. Substances with a greater attraction for mobile phase reach the detector at the end of the tube more quickly. Substances with a greater attraction for stationary phase move more slowly through column.
question
What is an example of where GC is used?
answer
Gas chromatography is used to detect banned substances in urine samples from athletes.
question
What is the retention time?
answer
The amount of time that a substance takes to pass through the column (tube).
question
What is the number of compounds in a mixture represented by?
answer
The number of peaks.
