Question

Explain the difference in Boiling point of your unknown(Acetone) with the other solvent (Hydrogen bonding, polarity, molecular Weight, Structure branched or linear).

Compare and explain the difference in solubility of your uknown(Acetone) in water with the other solvent.

Answer 1

Ans. Note the following trends:-

I. Effect of H-bonding: Formation of intermolecular H-bonds among the molecules of solvent held the molecules attached more firmly. So, such solvent molecules can’t escape from the liquid phase liquid phase. As a result, greater is the strength and number of H-bonds, lower would be the vapor pressure of the solvent. And, Greater is the vapor pressure, lower is the boiling point.

Hence, a solvent forming H-bonds (ex- H₂O, alcohols like ethanol, etc.) would gave relatively higher boiling point than acetone that does not form H-bonds.

Greater is the force of attraction among molecules, greater is the boiling point.

II. Effect of Polarity: Greater is the polarity of solvent molecules, greater is the dipole-dipole interactions and greater would be resultant intermolecular attraction. Greater is the intermolecular attractions, lower is vapor pressure and higher would be the boiling point.

So, a solvent being molar polar than acetone would have higher boiling point.

A solvent being less polar than acetone (ex- C₂H₆) would have lower boiling point.

III. Effect of molecular weight: Greater is the molar mass, greater is intermolecular attractions due to van der Waal’s interactions and induced dipole- induced dipole interactions. Greater force of intermolecular attraction results lower vapor pressure and higher boiling point.

So, a solvent having a molar mass greater than that of acetone (but same polarity) would have relatively higher boiling point than acetone.

IV. Effect of branched or straight chain. The packing of branched-chain molecules is hindered due to stearic interactions. Whereas, the molecules with linear chain pack well.

Better is the packing of molecules, greater would be the intermolecular attractions- so, greater would be the boiling point.

So, branched chain solvents have relatively lower boiling point than their counterpart linear-chain solvents.

#B. Solubility of Acetone:

In water: Acetone is a polar molecule because of presence of highly electronegative O-atoms. The O-atom in acetone forms H-bonds with H-atoms of H₂O. However, at the same time the bulky (bulkier than H₂O) alkyl hydrocarbon chain of repels water molecules.

So, due to presence of bulkier hydrophobic portion and polar O-atom, acetone is miscible in water.   

In other solvent: Because of having hydrocarbon chain and being slightly polar, acetone is also miscible is polar organic solvents like ether, ethanol, etc.