C8 Paper Chromatography (page 154) Flashcards
What is Chromatography?
it is an analytical (analiyze) method used to separate the substances in a mixture. You can then use it to identify the substances.
There are different types of Chromatography, but they all have two phases, what are the two phases?
A Mobile Phase - where the molecules can move. This is always a liquid or a gas.
A stationary phase - where the molecules can’t move. This can be a solid or a really thick liquid.
During a chromatography experiment, the substances in the sample constantly move between the mobile and the stationary phases, what is formed?
an equilibrium (balanced) is formed between the two phases.
The mobile phase move through the statoinary phase, and everything dissolved in the mobile phase moves with it., how quickly do the chemical move?
It depends on how it’s distributed between the two phases - whether it spends more time in the mobile phase or the stationary phase.
What happens when the chemicals that spend more time in the mobile phase than the stationary phase?
the chemicals that spend more time in the mobile phase than the stationary phase will move further
What happens to the components in a mixture when in the stationary phase?
The components in a mixture will normally separate through the stationary phase, so long as all components spend different amounts of time in the mobile phase. The number of spots may change in different solvents as the distribution of the chemical will change depending on the solvent. A pure substance will only ever form one spot in any solvent as there is only one substance in the sample.
During paper chromatography what phase is the chromatography paper (often filter paper)?
the paper chromatography is the stationary phase.
During paper chromatography what stage if the mobile phase?
the mobile phase is the solvent. (e.g. ethanol or water).
The amount of time the molecules spend in each phase depends on two things, what are they?
How soluble they are in the solvent.
How attracted they are to the paper.
(the method for carrying out paper chromatography is on pg 100).
Molecules whith a higher solubility in the solvent, and which are less attracted to the paper, will spend more or less time in the mobile phase?
they will spend more time in the mobile phase, and they’ll be carried further up the paper.
The result of Chromatography analysis is called what?
Chromatogram.
You can calculate the Rr value for each chemical. what is the Rr value?
it is the Ratio between the distance travelled by the dissolved substance the solute, and the distance travelled by the solvent.
The further through the stationary phase a substance moves, the larger the Rr Value.
what formula is used to calculate the Rr value?
distance travelled by substance (B) - (this is the distance from the baseline to the centre of the spot)
Rr = ___________________________________
Distance travvelled by solvent (A)
see diagram on page 154.
Why is Chromatography often carried out?
To see if a certain substance is present in a mixture.
To do this, you must run a pure sample of that substance (a reference) alongside the unknown mixture. If the Rr values of the references and one of the spots in the mixture match, the substance may be present (although you haven’t yet proved they’re the same).
The Rr value is dependent on what? and why?
it is dependent on the solvent - if you change the solvent the Rr value for the substance will change.
You can test both the mixture and the reference in a number of different solvents. If the Rr value of the reference compound matches the Rr value of one of the spots in the mixture in all the solvents, then it’s likely the reference compound is present in the mixture.
If the spots in the mixture and the spot in the reference only have the same Rr value in some of the solvents, then the reference compound isn’t present in the mixture.
