Question

1.You need to recrystallize a polar solute X that is contaminated with nonpolar impurity Y.If you use nonpolar solvent Q (which matches polarity of impurity Y) to carry out the recrystallization of X (take temperature of solution into consideration for these questions):

a).   How, specifically, can the impurity would be separated from solute X in this situation? __________________________________________________

b) Describe one possible advantage of using solvent Q based of the ideal characteristics for recrystallization solvents with respect to solutes (specifically solute X)? Answer should only use facts based on above information) ________________________________________________

2.Why would incomplete drying of recrystallization solid lead to poor recovery of the solid? __________________________________________________

3. While carrying out recrystallization, you accidentally bump into your cooling crystallizing solution. While none of the solution spills out of the test tube (no loss of solution), you find that you still have defeated the purpose of the crystallization process. What results from the bump and why does it occur ? ___________________________________________________________________________________________________________________________________

4.Why is it important to heat the crystal solution too strongly when dissolving the solute in the recrystallization process? _________________________________________________________

Answer 1

1. a) As impurity is non- polar than the solute, the impurities would be more soluble in the non-polar solvent, Q. The solute being much less soluble in the non-polar solvent, would tend to separate out from the solvent quickly to crystallise out. But impurities would be soluble in the solvent and hence would be removed easily from the solute.

b) Crystallisation process involves a solute which is very spararingly soluble in a particular solvent. The solute is dissolved in the solvent in hot condition and when the mixture is cooled, the solute crystallises out due to low solubility. So, here it is advantageous to use a non- polar solvent like Q to crystallise a polar solute.

2. Incomplete drying would lead to draining some solute with solvents and it results in poor recovery yield.

3. Crystallisation process is a slow process where the solidification of solute happens when the solution is settled. If you disturb the process by shaking or bumping, the crystallisation does not procced. Rather the solute starts to precipitate out.

4. We choose a solvent in which the solute is very less soluble for the purpose of crystaliisation. Initially , to dissolve the solute in the solvent, the solution needs to be heated strongly.