Recrystallization of Benzoic Acid

Topics: Solubility, Chemistry, Temperature Pages: 6 (1920 words) Published: December 9, 2010
The objective of this experiment was to observe multi-step purification of benzoic acid after performing the extraction from a mixture containing benzoic acid, cellulose, and methyl orange. Recrystallization was done to remove impurities from the sample. The amount of BA recovered during recrystallization is much less than the extracted amount of BA. The difference between the pure and impure samples was observed by comparison of melting points. It was found that impure samples will have a lower and wider melting point range. The experiment performed was important because it provides important knowledge about the chemical nature and reactivity’s of various compounds that can be used in everyday life. Introduction

Now that the desired compound is extracted, it still contains impurities. These impurities can be removed through a process called recrystallization. Recrystallization is a method for purifying solid compounds which are frequently the products of organic reactions. This theory is based on three important concepts. The first is that a compound is always more soluble in hot solvents than in cold solvents. Secondly, molecules have unique solubility properties. Lastly, a growing crystal will only accept similar entities into its lattice. This is related to the crystal lattice theory which states crystal formation is anti-entropic based on the equation ∆G =∆H-T∆S. There are three stages of solubility: collision, dissociation, and solvation. In order to raise the probability of collision between the solute and solvent, heat is added to the system. When this occurs, the solute will attack the crystalline structure of the solute and start dissociation. As dissociation continues until all the molecules are free, solvation occurs.

Choosing the right solvent is important when considering recrystallization. The solute must have a high temperature coefficient in the solvent: it must be soluble at high temperatures and insoluble at low temperatures, satisfying the first principle stated above. Also the boiling point of the solvent must be lower than the melting point of the solute. This will ensure that the solute dissolves in the solvent and does not melt. The solvent must also be inert to prevent chemical interactions with the solute. Impurities must either be readily dissolved in the solvent or be insoluble. The solvent must also be volatile, and finally, it would be beneficial to the experimenter if the solvent is relatively inexpensive.

Once the color and most impurities are removed, the crystals can be formed in the solution through a process called nucleation. Nucleation can be induced by slowly cooling the solution to room temperature. It is important to do this slowly so that small solute resembling impurities do not enter the crystal lattice, as described by the third principle of recrystallization. Chemists often add already pure crystals of the solute to the solution in order to provide a pre-formed lattice for other molecules to enter. This is called seeding. Scratching the glass container in which the solution is contained with a glass stir rod will also induce nucleation on microscopic glass particles. The concept of scratching is similar to that of seeding. Finally, nucleation can be induced by cooling the solution farther in an ice bath; however, this is only a last resort option due to its least efficiency to bring forth crystals. The crystallized solute can then be collected by vacuum filtration. There is a difference observed in the physical properties of the pure compound and the impure mixture. One of the tests of purity is melting point. Melting point is an intrinsic, or intensive, property; the value is independent of the quantity of the substance. Melting point is the temperature at which a substance changes physical state from solid to liquid. Because the substance will have a certain shape and surface area exposure, there will be a time interval in which the...
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