Answer following two questions COMPLETELY. Questions are based on a lab experime

Question

Answer following two questions COMPLETELY. Questions are based on a lab experiment, which lab details are down below the questions.

- Why was the gravity filtration done while the solution was hot? (what would have happened if we let the solution cool first?)
- What caused the precipitation of benzoic acid prior to vacuum filtration?

CRYSTALLIZATION

One of the most useful and frequently used ways of purifying compounds is recrystallization. Chemical reactions, especially organic reactions, seldom lead to one product; therefore, some form of purification is usually necessary. For solid products, crystallization from solution is often used and throughout the year every solid product you make will be recrystallized. The procedures may

appear to vary, but there are five basic steps applicable in every case. These are:

Dissolve the product to be recrystallized in hot solvent

Filter off the impurities

Cool to room temperature or below

Filter to isolate the crystals from the recrystallizing solvent

Wash with cold solvent

There are several points to be considered for each of these steps.
1. Dissolve in hot solvent. The material to be purified is placed in a minimal amount of warm solvent and brought to a boil. If all the compound does not dissolve, more solvent is added in small amounts until the compound has just dissolved at the boiling point of the solvent.

The choice of solvent is very important and the following factors must be considered if you do not know the required solvent for a recrystallization:

a)

b)

The solvent chosen must dissolve large quantities of the material to be purified at high temperatures, but only small amounts at low temperatures.

The impurities you wish to remove must behave in one of two ways. Either they must be very soluble in the solvent at all temperatures so that they do not interfere in the crystal formation of the desired compound at low temperatures; or, they must be insoluble at high temperatures and hence can be removed by filtration at high temperatures while the desired product is still in solution.

15

c)

d)

16 The desired solvent should be chemically inert so that no reaction will take place during

recrystallization.
It is desirable that the solvent be fairly volatile so that the excess solvent can be removed

easily when drying the purified crystals.
When choosing a solvent a good general rule, apart from the considerations mentioned, is

that "like dissolves like". This means that if the compound is polar, a polar solvent should be tried first, or if it is non-polar, a non-polar solvent should be tried. It is sometimes impossible to find just one solvent which will fulfill all these requirements and it becomes necessary to use a mixture of two or more solvents. Later in the year you will encounter a few situations of this sort. Sometimes compositions are so similar that no suitable solvent or solvent system can be found and other purification techniques, e.g. column or thin-layer chromatography, must be used.

2. Filter off impurities. This should be done at high temperatures while the desired product is still in solution. A gravity filtration is usually done at this point. Hot solutions are seldom filtered with suction in the laboratory because suction causes rapid evaporation and cooling of the solution which leads to troublesome premature crystallization in the funnel.

The impurities to be removed at this point might be either insoluble impurities (chemical or mechanical, e.g. dirt, paper, etc.) which were present in the original mixture, or other material added to assist in purification (e.g. charcoal) which had been added to the mixture in Step 1 to remove coloured or resinous impurities which could be removed only by adsorption on the charcoal. Only small amounts of these reagents are used to avoid excessive adsorption of the desired compound. 3. Cool to form crystals. This is probably the most critical step after choice of solvent. The impurities have now been removed or are in solution. In general a slower cooling gives better crystals. A rapid cooling and crystallization tends to trap (or occlude) the impurities in the crystals during formation. The objective of the cooling is to deposit the desired product as pure crystals with any soluble impurities remaining dissolved in solution. The lower the temperature to which the

17 solution is cooled, the more of the desired product will be crystallized; however, at some point the impurities may also begin to separate from solution. Frequently the solution is cooled to room temperature and a "crop" of crystals is collected. The filtrate may then be cooled to a lower temperature and a second "crop" of crystals is collected. A determination of the melting point of each crop will reveal their purity. The best temperature for recovering the crystals will vary from

compound to compound.
The size of the crystals which separate will vary with the rate of cooling and the amount that

the solution is agitated. Rapid cooling and stirring tend to give smaller crystals while slow cooling of an undisturbed solution tends to give larger crystals. Generally, either very large or very small crystals are undesirable. Large crystals tend to occlude the solution. When these crystals are dried by evaporation of the solvent, the impurities in the solution will remain in the crystals. On the other hand, if the crystals are too small, significant amounts of impurities may be adsorbed because of the large surface resulting from such small crystals. A medium crystal size is preferable.

Often crystallization is very slow and you must wait, even a number of days, or you may induce crystallization by scratching to form a seed crystal, or by introducing a seed crystal and allowing the solution to stand without disturbance.
1. Filter to separate. Once the desired product has crystallized it is necessary to remove it from the solvent or "mother liquor". Since there are large amounts of crystals at this stage, filtration is best accomplished by suction filtration, e.g. Hirsch funnel for small quantities or a Buchner funnel for larger quantities.
2. Wash with cold solvent. Once the crystals are in the funnel and they are somewhat dry, wash them with 2 or 3 small (in order not to dissolve the crystals) portions of cold solvent to remove all of the recrystallization solvent that contains the undesired impurities. Allow the crystals to dry somewhat with suction and then finish drying in the air. You should always take a melting point before and after a recrystallization to check on the effectiveness of the procedure and compare these

18 melting points with the melting point quoted in the literature. Weighing the solid before and after

recrystallization gives one a direct measure of the efficiency of recovery.
In the filtration of the hot solutions by gravity, greater efficiency is achieved if a fluted filter

paper is employed.

Direction for Fluting Filter Paper

To make a fluted filter paper from an ordinary filter paper, follow the directions below.
1. Fold the filter paper in half to form a semi-circle.
2. Fold in half again
3. Open to a semi-circle and fold each edge to meet in the centre thus dividing each quadrant in

two.
NOTE: All of the creases thus far are in the same direction as noted by the dotted lines.

Creases in the opposite direction will be indicated by unbroken lines.
4. Divide each section in half by folding in the opposite direction, thus producing a "fan" effect. 5. Open.

Experimental Procedure

Place 1 g of crude benzoic acid in a 250 mL Erlenmeyer flask. Add 75 mL of distilled water and a boiling chip. Heat the flask containing the benzoic acid and the water on a hot plate until boiling commences. If most of the acid has not dissolved in 5 minutes (min), remove the flask from the hot plate, and add more water in 5 mL quantities (up to 10 mL) until all of the benzoic acid has dissolved. After each addition the solution should be brought to the boiling point once again. Cool the solution slightly and add, WITH CARE, a spatula tip of decolourizing charcoal. Reheat the solution on the hot plate and allow to boil gently for approximately 20 min. In order to prevent crystallization in the funnel, just prior to the hot filtration, replace the water lost by evaporation during the boiling period by adjusting the solution volume to the original level and add another 5 mL of water. However, make sure your solution is still hot (almost boiling) when you start to filter.

Filter the hot solution by pouring it through a fluted filter paper which is placed in a powder funnel supported above a 250 mL beaker. The filtration must be performed carefully so that no charcoal passes into the solution. When all of the material has filtered through the paper, wash the filter paper by passing 2-3 mL of boiling water through the paper into the beaker. This should remove all traces of benzoic acid, which may have crystallized on the filter paper during the filtration. Prepare for this filtration in advance while the benzoic acid is boiling with the charcoal. Then allow the beaker containing the filtrate to cool to room temperature, followed by cooling briefly in an ice bath.

Collect the crystals in a Buchner funnel by suction.

Wash with two 5 mL portions of ice-cold water by sucking the wash liquid through the crystals in the Buchner funnel and leave the crystals in the funnel for 5 min while the suction is maintained. Weigh the product and determine the melting point. Determine the percentage of recovered benzoic acid. (Note: it should not be 100% or more.) Hand in the product.

Explanation / Answer

- Why was the gravity filtration done while the solution was hot? (what would have happened if we let the solution cool first?)

In crystallization we know that the amount of solute that can be dissolved by a solvent increases with temperature. We use solvents at their boiling point because the solute has a greatly increased solubility in the solvent, so a much smaller quantity of hot solvent is needed than when the solvent is at room temperature. At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected.

- What caused the precipitation of benzoic acid prior to vacuum filtration?

It was precipitated because the benzoic acid is not soluble at the solvent at room temperature.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.