hi any you help me with this plz

Describe the scientific principles behind the key steps in the preparative methods for substances in terms of Recrystallisation of Benzoic Acid

any help would be really appreciated.... :o:

Reply 1

chembob

Original post by HAZROckx

Describe the scientific principles behind the key steps in the preparative methods for substances in terms of Recrystallisation of Benzoic Acid

any help would be really appreciated.... :o:

Chemical products are often contaminated with impurities. One method for purifying chemicals, recrystallization, takes advantage of the differences in the solubilities of the desired products and impurities, and the tendency for the slow formation of crystals to exclude impurities from the crystalline solid.

Benzoic acid, the chemical that you are purifying, can be impure do to the presence of other ionic solids. Benzoic acid can be separated from ionic solids because the materials have different solubilities in water. At room temperature, benzoic acid is not particularly soluble in water, whereas ionic solids are. Thus, adding water to the impure Benzoic acid will dissolve the ionic solid, and nearly all of the benzoic acid will remain undissolved. If the suspension were filtered, much of the impurity would be washed away; however, the resulting product wouldn’t be as pure as a recrystallized product.

To take advantage of the extra purity garnered by crystallization, all of the benzoic acid must be dissolved. On the other hand, the benzoic acid needs to come back out of solution to crystallize. Simply adding room-temperature water cannot accomplish this goal; once the benzoic acid is dissolved there would be no reason for it crystallize. Since benzoic acid is
much more soluble in hot water as compared to cold, small amounts of hot water are added to dissolve the benzoic acid. If the hot, saturated, aqueous solution of benzoic acid is cooled, the solution becomes supersaturated, and the supersaturated solution crystallizes.

Does that help at all?