Why should activated carbon be used during a recrystallization? Under which circ

Question

Why should activated carbon be used during a recrystallization?

Under which circumstances is it wise to use a mixture of solvents to carry out a recrystallization?

Why is gravity filtration rather than suction filtration used to remove sus- pended impurities and charcoal from a hot solution?

Why is a fluted filter paper used in gravity filtration?

Why is the final product from the recrystallization process isolated by vacuum filtration rather than gravity filtration?

Why should you never use a beaker for recrystallization?

2. What is the effect of an insoluble impurity, such as sodium sulfate, on the observed melting point of a compound?

3. Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove that the test tubes contain three different chemical compounds?

4. One of the most common causes of inaccurate melting points is too rapid heat- ing of the melting point bath. Under these circumstances, how will the observed melting point compare to the true melting point?

5. Strictly speaking, why is it incorrect to speak of a melting point?
6. What effect would the incomplete drying of a sample (for example, the incom-

plete removal of a recrystallization solvent) have on the melting point?

7. Why should the melting point sample be finely powdered?

You have an unknown with an observed melting point of 90

Explanation / Answer

1) activated charcoal is a wonderful absorbant. activated charcoal is carbon that has been treated with oxygen. leaving the carbon in a highly porous state. crystalization can remove impurities from the solution but it doesn't remove all of the impurities, the charcoal removes the impurities during crystalization so that the crystals forming are as pure as possible.

2) when both the product and impurity are dissolved in the first solvent and you want to separate the impurity from the product you can add a second solvent (immiscible with the other solvent) in which only the product (or only the impurity) is soluble in. This way you can easily separate the impurity from the product.

So the circumstance is when you want to have impurities in solution and want to separate them out.

3) Under suction a hot solution is likely to boil. Then you'd be sucking lots of vapor rather than liquid. By evaporating the liquid, this would also decrease the solution temperature and increase the concentration of the solute, which could cause some of the solute to precipitate out of solution (which may or may not be desirable

4) Fluted filter paper is used when we have to separate a liquid and a solid, keeping liquid and discarding the solid. The arrangement of folds in the fluted filter paper will allow the liquid to pass through it very quickly and give a lot of surface area on which to collect the solid impurity. Fluted filter paper is folded in a special manner to increase the surface area that comes in contact with the solution being filtered. This makes gravity filtration more efficient.

5) When you are doing a filtration, the speed by which the liquid phase moves through the filter depends on the difference of pressure on the two sides of the filter.

During gravity filtration, the pressure below the filter is equal to the atmospheric, while above it is equal to the atmospheric + that generated because of the solution's weight. As the solvent (actually it is a saturated solution, but anyway) moves through the filter, the weight of the solution decreases, the difference in the pressure decreases and thus the rate of filtration also decreases. At some point you will still have some residual solvent but the difference in the pressures is going to be too small to "encourge" the solvent to pass through the filter-at least within a reasonable time period.

When you are using a vaccum, you decrease the pressure from the lower side of the filter, keeping a big pressure difference which makes the solvent pass through the filter quickly, regardless of the amount of solution above the filter.

This way you minimize the residual amount of solvent in your solid product, but also avoid losses due to temperature increase like mentioned in the answer above.

[2]

1) Never say never. It's not ideal because it allows too much solvent to boil off when you're heating, so it's hard to keep track of the volume you have in there. Erlenmeyer flasks are a better bet, and you can loosely sit a watch glass on the top which will allow some of the vapor to condense and fall back in. I've even done whole reflux setups for things that required long boiling to dissolve. Still, I've used beakers before, out of laziness. They're also good sometimes to get a "second crop" of crystals. After you funnel your first batch, you boil off some more solvent, and some more crystals will come out, though usually not as clean as the first.

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