Melting Point and Recrystallization

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Recrystallization and Melting Point of Benzoic Acid
Organic Chemistry 1
10/4/12

Abstract
Recrystallization was done to remove impurities from the sample. The percent recovery of benzoic acid during recrystallization is 23.02%. The difference between the pure and impure samples was observed by comparison of melting points. It was found that impure sample had a lower and wider melting point range of 120.1-122.2 (C). The pure sample melting point range was 121.3-122.5 (C). These ranges helped determine purity by comparing the known melting point of pure benzoic acid. Introduction

Most of the organic substances recovered from organic reactions are impure and require purification to obtain the desired pure product. The extracted benzoic acid still had impurities which can be removed through the process of recrystallization. Recrystallization is a purification technique that relies on the differing solubility of compounds within a mixture separating to form a pure crystalline solid of the desired product. Often the desired compound is present in a larger quantity within the solution. Therefore, a pure sample of this compound can be crystallized from the solution while leaving the impurities dissolved within the solvent.

Recrystallization is highly dependent upon the solubility of the two mixed compounds. Solubility is the property of how well a solute dissolves in a solvent. Solubility increases with an increase in energy within the system. This is due to the increased kinetic energy of the molecules leading to more collisions of the solvent with the solute. This process is called dissociation since the molecules of the solute become separated and surrounded by molecules of solvent. Solvation occurs once the lattice has completely dissociated within the solution and no traces of solid compound remain. These steps can be explained by Gibb’s Free Energy (∆G) equation where a negative ∆G indicates a spontaneous reaction. The solvation of the lattice structure is a spontaneous reaction since the entropy and temperature increase within the system as the enthalpy becomes more negative (more exothermic). Hot water was used to dissolve the crude benzoic acid in these stages of solubility. For the recrystallization to occur properly a good solvent must be chosen. The solute must be relatively insoluble in the solvent at room temperature but much more soluble in the solvent at higher temperature. At the same time, impurities that are present must either be soluble in the solvent at room temperature or insoluble in the solvent at a high temperature. For example, if you wanted to purify a sample of Compound X which is contaminated by a small amount of Compound Y, an appropriate solvent would be one in which all of Compound Y dissolved at room temperature because the impurities will stay in solution and pass through filter paper, leaving only pure crystals behind. Also appropriate would be a solvent in which the impurities are insoluble at a high temperature because they will remain solid in the boiling solvent and can then be filtered out.

When purifying a substance with this method, the solubility of the solvent must be taken into careful consideration. It is necessary that a solvent is a poor solvent at room temperature and a very good solvent at high temperatures. As a result, polarity, the uneven distribution of electron density, must be taken into consideration. The polarity of the solvent should relatively be the opposite of the compound being dissolved. This creates a high temperature coefficient with the compound only being slightly soluble at room temperature and highly soluble at high temperatures. This allows the substance to later crystallize from the solution as it cools. The solvent also cannot be highly reactive with the solute. This will entirely defeat the purpose of dissolution since products will be created that are not the intended crystals. Additionally, the solvent should be able to dissolve either...
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