Recrystallization and Melting Points Determinantion of Benzoic Acid

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Experiment 2&3 Recrystallization and Melting Points Determinantion of Benzoic Acid

Abstract
In experiment 2, recrystallization was used to purify the crude benzoic acid extracted from a mixture during experiment 1. Then a percent recovery for this recrystallization process was calculated. In experiment 3, the melting point ranges of the crude and purified benzoic acid were both determined by using Melt Temp Apparatus.

Introduction
Recrystallization is a purification technique for non-volatile solids. The general idea of recrystallization is rely on the solubility of target product (as well as other impurities) and the temperature of solvent. The target material is dissolved in a hot solvent (in this case, hot water) and then solvent is slowly cooled down. Due to the solubility of a dissolved material decreases with decreasing temperature, the solution will become oversaturated and the solid will separate from the solution as it is cooled. Thus the mixture (crude benzoic acid) from experiment 1 can be separated. By looking into more details, the solution should be cooled down slowly after dissolvable solid mixture dissolved in solvent to ensure the purity of desired product. According to crystal lattice theory, when solid forms, it blocks impurities to form solid; thus recrystallization is possible. Solubility is crucial in this experiment. There are three stages of solubility. Collision is when solid just added in solvent and nothing happens during collision stage; dissociation is when solvent attacks solids and part of the solids come off and mix with solvent; the last stage is solvation when solids completely dissolve in solvent thus two combines together. In addition, picking a good solvent determines the success of this lab. The solvent needs to have high temperature coefficient so it can be heated to a high temperature. Then the solvent needs to have a boiling point less than the melting point of solid. This is crucial because solids might melt before solvent even boils, which cannot be recrystalized. In terms of solubility towards impurities, solvent needs either easily dissolve impurity or not at all. At last, the solvent needs to be relatively volatile and cheap. In terms of Gibb’s free energy theory (ΔG=ΔH-TΔS), which means free energy=net enthalpy (energy)-temperature * net entropy (order). From the equation, TΔS needs to be larger than ΔH in order to make ΔG smaller. Therefore, in this experiment, letting the solution cool down can cause the energy losing of the system, in turn making ΔH smaller. Also, the transition from liquid to solid gains order, which is ΔS, which goes up. Both mechanisms work can result a negative value of ΔG, and that’s exactly what’s required in recrystallization. When recrystallized benzoic acids are collected, they are being tested for physical properties such as melting points along with the original crude benzoic acids. Physical properties are defined as properties that can be observed/measured without changing the substance’s identity. There are three basic features about physical property. Intensive, which means intrinsic property, is same regardless of quantity of that substance. Such physical properties include melting point, boiling point, and density, and so on so forth. Extensive, as known as extrinsic physical property, depends on substance’s quantity; examples are moles and volume. Also, there are always units on physical properties such as gram, liter, and degree. Physical changes distinguish themselves with chemical changes by the following. There are no covalent bonds broken, no composition change, and phase change is involved. All energy absorbed/ released causes phase changes. There are three stages of phase changes, and all of them are able to go back and forth: solid melts down to liquid, and liquid boils/ vaporizes to gas, which in turn is able to condensate back to liquid, and liquid freezes into solid. Melting point is one of many intrinsic...
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