Assigning Unlabelled Liquids from Boiling Points and Intermolecular Forces
A laboratory assistant finds four unlabelled bottles in a storeroom. The inventory lists four possible compounds: butane, propanone, propan-1-ol and propane-1,3-diol. At 20 °C, three are liquids and one is a gas that condenses at -0.5 °C. The measured boiling points are -0.5 °C, 56 °C, 97 °C and 214 °C. All four are colourless. Bottle A (bp 56 °C) is miscible with water and does not react with sodium. Bottle B (bp 97 °C) is miscible with water and releases a gas when treated with sodium. Bottle C (bp 214 °C) is viscous, miscible with water, and also reacts with sodium, producing about twice as much gas per mole as B under the same conditions. Bottle D (bp -0.5 °C) is insoluble in water and does not react with sodium. A fifth white solid E melts at 801 °C, does not conduct electricity when solid, but conducts when molten and in aqueous solution.
1. Match A–D to the four compounds and explain the boiling-point order using intermolecular forces.
Read the worked answer
D is butane: it has the lowest boiling point (-0.5 °C), is insoluble in water and does not react with sodium. Its molecules are nonpolar and interact only by weak London dispersion forces.
A is propanone: bp 56 °C, miscible with water, no reaction with sodium. Propanone has a polar C=O group, so dipole–dipole attractions are present, but it has no O–H bond to donate a hydrogen bond.
B is propan-1-ol: bp 97 °C, miscible with water, and it reacts with sodium because it has an O–H group. Hydrogen bonding raises its boiling point well above that of propanone.
C is propane-1,3-diol: bp 214 °C, viscous, and it gives about twice the gas volume per mole because it has two O–H groups. Two hydrogen-bonding sites per molecule give a more extensive hydrogen-bond network than one O–H group, so the boiling point is much higher.
2. Explain why propanone is miscible with water while butane is not. Why is propane-1,3-diol more viscous than propan-1-ol?
Read the worked answer
Water molecules are strongly hydrogen-bonded. Propanone can accept hydrogen bonds from water at the carbonyl oxygen, and its polar C=O group also has dipole–dipole interactions with water, so mixing is energetically favourable. Butane is nonpolar and cannot form hydrogen bonds with water; dissolving it would disrupt the water–water hydrogen-bond network without replacing those interactions, so it is essentially insoluble.
Propane-1,3-diol has two O–H groups per molecule, so it can form more hydrogen bonds than propan-1-ol. This stronger and more extensive intermolecular network resists flow more strongly, giving higher viscosity as well as a higher boiling point.
3. Identify the type of bonding in solid E and explain its electrical behaviour in the solid, molten and aqueous states.
Read the worked answer
E is an ionic solid. Its melting point of 801 °C is typical of a strong ionic lattice, such as NaCl. In the solid state the ions are held in fixed positions in the lattice, so there are no mobile charge carriers and the solid does not conduct.
When melted, the lattice breaks down and the ions become mobile, so the molten liquid conducts electricity. When dissolved in water, the ions separate and are hydrated, giving a conducting aqueous solution. This contrast between non-conducting solid and conducting melt/solution is the standard test for ionic bonding.