Question 1

Why is a hot solvent used at the beginning of recrystallization?

Accepted Answer

Hot solvent is used because solubility typically increases with temperature. As temperature rises, the amount of solute that can dissolve in the solvent increases, allowing you to form a saturated solution with minimal solvent volume. This concentrated solution is essential for effective recrystallization and crystal formation during cooling.

Question 2

What happens during nucleation in the recrystallization process?

Accepted Answer

Nucleation is the first step of crystallization, where solute molecules come together to form small stable crystals called nuclei as the solution cools and solubility decreases. Additional crystal growth then occurs on these nuclei because solute molecules have greater affinity for joining existing crystals than forming new ones, leaving soluble impurities in solution.

Question 3

How does cooling rate affect crystal size and purity in recrystallization?

Accepted Answer

Rapid cooling favors formation of many nucleation sites and smaller crystals, while slow cooling induces fewer nucleation sites and larger, purer crystals. Slower cooling is preferred because it allows solute molecules to organize more efficiently into well-formed crystals with fewer impurities trapped inside.

Question 4

What characteristics should an ideal solvent have for recrystallization?

Accepted Answer

An ideal solvent should have low solubility for the desired compound at room temperature but high solubility at elevated temperatures. Impurities should be either soluble at all temperatures or insoluble even at high temperatures. The solvent should also boil well below the product's melting point and remain inert without reacting with the desired product.

Question 5

How are impurities removed during the recrystallization procedure?

Accepted Answer

Soluble impurities remain dissolved in the solvent as the desired product crystallizes during cooling. Insoluble impurities at high temperature are removed by hot gravity filtration before crystallization. After crystals form, they are filtered and washed with cold solvent to remove any remaining impurities from crystal surfaces.

Question 6

What is the relationship between solubility differences and successful recrystallization?

Accepted Answer

Successful recrystallization depends on significant differences in solubility between high and low temperatures. The greater the difference in solubility between these temperatures, the more likely the solute will precipitate from solution as it cools, forming pure crystals while impurities remain dissolved or are filtered away.

Question 7

Why is percent recovery rarely 100% in recrystallization?

Accepted Answer

Percent recovery is rarely 100% because the solubility of the compound at room temperature governs how much product actually crystallizes from solution. Some dissolved product remains in the cooled solvent and is lost during filtration, limiting the mass of recovered crystals compared to the original crude product mass.

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