Question

We ran a recrystallization experiment in lab, ultimtely I know recrystallization is used for purification purposes, but this question is confusing me.

What affect did recrystallization have upon the yield of product, the appearance of the product, and its melting point? yield and MP confuse me. I wasnt given the resources to check the MP during lab so is there a generalized consenssus of wha

I know you wont be able to tell me about appearance, I have that potion but the yield and MP portion I was wondering if there is some generalized assumption I am missing for recrystalllized products? I was not given the rousources to measure the MP during lab so I assume there is a trend I am missing from recrystallization - like raising the MP because it is a purer substance.

Answer 1

Let us understand why and how we carry out Recrystalization :

Why we carry out recrystalization: In the chemical synthesis of compound we use various reagents. solvents which may not react completely and remains in the reaction mixture at the end of reaction i.e. solidification of product (ppt formation). In this process the final product get contaminated with various impurities (like unreacted reactants, solvent molecules etc).

When we weight the product not only product but adhered impurities also get weighed.

2) How we carry out recrystalization and how it purifies compound or removes impurities

i) Principle of recrystalization: The product is disolved in suitable solvent and heated to some higher temperature.

At low temperature required expected product is insoluble but on heating product get dissolved completely and possible impurities remains undisolved in hot solvent. At this elevated temperature we filter the solution by which disolved product go in filtrate and impurities remains on filter paper. On slow cooling of filtrate product recrystalized out in it's purest form.

This way impurities get separated from product

Coming to your questions:

Effect of recrystalization on yield : It's obvious that on recrystalization yield decreases as impurities (with which we weighed crude product) get removed. But it's not lowering of yield actually because crude product yield is not actual yield only.

Effect of M.P. : As compound is recrystalized in it's purest state MP of product will be a sharp point unlike for crude it;s in some range 2 or 3 oC difference.

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You are not getting recrystalized product means you need to recheck following thinks

1) Chose solvent very carefully. One have rough idea of possible functionalities in product from which we can predict it's nature like polar or non polar and accordingly chose the solvent. Remember "Like dissolves like" is the governing principle.

2) may you are not allowing filtrate recrstalize naturally. Colling should happen naturally. AT the most you can hold the out surface of testtube containing filtrate under running tap water foor some seconds.

3) try nucleation like puting very tiny peaces of neutral compounds like quartz pieces. Nucleation initiates crystal formation by providing some surface for product molecule to come together.

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