Please help me answer this question;
The goal of the recrystallization procedure is to obtain purified material with a maximized recovery. For each of the following, explain why this goal would be adversely affected.
a) In the solution step, an unnecessarily large volume of solvent is used.
b) Crystallization is accelerated by immediately placing the flask of hot solution in ice water.
In recrystallizations, one uses the minimum amount of solvent to dissolve the material in a HOT solution, then cools the solution to obtain the crystals. The idea is that the solute is less soluble in the cold solution than in the hot solution.
a. If one uses too much solvent, there is just that much more loss when the solution is cooled. More solvent means more of the solute will dissolve. In real practice, one can have so much solvent that ALL of the product is lost.
b. If the solution is suddenly cooled, for example with an ice bath, the solute precipitates VERY FAST and that entraps both solvent and some of the impurities into the crystal structure (not necessarily inTO the crystal structure but into the spaces (interstices) within the crystal itself). When I was in school they called this co-precipitation. In fact, a large part of my dissertation was on co-precipitation (but I wanted it). I don't know what they call it in today's world.
. A student performed a recrystallization of phthalic acid and used 35 mL of water as solvent.
Assuming no loss of solvent to evaporation, calculate the loss of product in the filtrate based on
the solubility of phthalic acid
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To answer this question, we need to understand the recrystallization procedure and how its various steps affect the goal of obtaining purified material with a maximized recovery.
a) In the solution step, an unnecessarily large volume of solvent is used:
In the recrystallization process, a solute is dissolved in a solvent at an elevated temperature (above the solute's melting point). This step is important to effectively remove impurities from the solute. However, using an unnecessarily large volume of solvent can adversely affect the goal of maximizing recovery for several reasons:
1. Dilution: By adding excessive solvent, the concentration of the solute decreases, which reduces the driving force for the solid to crystallize out of the solution. This can result in a lower yield as more solute may remain in the solution, instead of forming crystals.
2. Increased impurity retention: The larger volume of solvent can also increase the solubility of impurities, making it more difficult to remove them during subsequent steps like filtration or cooling. This can lead to impurities being retained in the final product, decreasing its purity.
To maximize recovery, it is important to use the minimum amount of solvent required to dissolve the desired amount of solute completely. This can be achieved by gradually adding the solvent while heating and stirring until the solute dissolves.
b) Crystallization is accelerated by immediately placing the flask of hot solution in ice water:
In the recrystallization procedure, after dissolving the solute, the solution is allowed to cool slowly to facilitate the formation of pure crystals. However, immediately cooling the solution by placing the flask in ice water can adversely affect the goal of maximizing recovery because:
1. Formation of small crystals: Rapid cooling promotes the formation of smaller crystals. Smaller crystals have a larger surface area compared to larger ones and are more prone to trapping impurities during crystallization. This can reduce the purity of the final product.
2. Formation of impure amorphous solids: Immediate cooling can hinder the formation of well-defined crystalline structures and result in the formation of amorphous solids. These amorphous solids may contain higher levels of impurities, leading to decreased purity of the final product.
To maximize recovery, it is generally recommended to cool the hot solution slowly at room temperature or through controlled evaporation. This allows for the formation of larger, well-defined crystals with fewer impurities, resulting in a higher yield and purity of the final product.