Question

1. Explain why a packed fractionating column (steel wool inside the glass tube) is more efficient than an unpacked one for distillation? Explain in detail.

2. If 3 spots on a TLC, plates have the following Rfs, A= 0.43, B=0.89, and C=0.53 what would be the order that the spots came off a column? What would happen if you increased the polarity of the solvent?

Answer 1

1.

When the difference in boiling points of two miscible liquids is less than 30 K, the vapours formed can never be pure. Hence, a fractionating column is inserted between the flask and condenser. The column consists of a number of obstructions due to which the cooling area for the hot vapours increases. When vapours of both the liquid rise in the column, they come into contact with large area of obstructions. Vapours of liquid having lower boiling point rise up the column and pass over into the condenser, whereas the vapours of liquid having higher boiling point are condensed in the column itself and condensed liquid trickles down back into the distillation flask. Thus two vapours are separated.

2.

The order of Rf values is A= 0.43, C=0.53, and B=0.89

A has minimum Rf value. Hence, it will have highest affinity for stationary phase. Hence, it will be eluted last.

B has maximum Rf value. Hence, it will have lowest affinity for stationary phase. Hence, it will be eluted first.

Hence, the order that the spots came off a column will be B (first), C and A (last)

If you increased the polarity of the solvent, the order of elution will be same but the separation will be less efficient.

This is because, the affinity for mobile phase increases with increase in solvent polarity.